NAVAL AIR TRAINING COMMAND
NAS CORPUS CHRISTI, TEXAS
CNATRA P-476 (05-22)
REFERENCE BOOK
HELICOPTER FUNDAMENTALS AND
BASIC AERODYNAMICS
TH-73A
2022
DEPARTMENT OF THE NAVY
CHIEF OF NAVAL AIR TRAINING
250 LEXINGTON BLVD SUITE 179
CORPUS CHRISTI TX 78419-5041
CNATRA P-476
N714
4 May 2022
CNATRA P-476
Subj: REFERENCE BOOK, HELICOPTER FUNDAMENTALS AND BASIC
AERODYNAMICS, TH-73A
1. Distribution. This document is issued for information, standardization of instruction, and
guidance to all instructors and student aviators within the Naval Air Training Command.
2. Purpose. This document shall be used as an explanatory aid to support the Flight Training
curriculum.
3. Cancellation. None.
4. Action. This document is effective on receipt. Recommendations for changes shall be
submitted via the electronic Training Change Request (TCR) form located on the Chief Naval
Air Training (CNATRA) Web site.
T. P. ATHERTON
By direction
Releasability and distribution:
This instruction is cleared for public release and is available electronically only via Chief of
Naval Air Training Issuances Website, https://www.cnatra.navy.mil/pubs-pat-pubs.asp.
REFERENCE BOOK
FOR
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
TH-73A
Q-2A-0015
P-476
iii
LIST OF EFFECTIVE PAGES
Dates of issue for original and changed pages are:
Original…0…04 May 22 (this will be the date issued)
TOTAL NUMBER OF PAGES IN THIS PUBLICATION IS 196 CONSISTING OF THE
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CHANGE SUMMARY
The following changes have been previously incorporated into this document.
CHANGE
NUMBER
REMARKS/PURPOSE
The following interim changes have been incorporated into this change/revision:
INTERIM
CHANGE
NUMBER
REMARKS/PURPOSE
ENTERED BY
v
DATE
SAFETY/HAZARD AWARENESS NOTICE
This course does not require any special safety precautions other than those normally found on
the flight lines.
vi
INTRODUCTION
Every aviator has a toolbox of items collected and mastered over the years. Among other things,
that toolbox probably contains emergency procedures, operating limitations, regulations, or
instructions, and an assortment of lessons learned.
A mastery of basic aerodynamic principles is an important tool for the professional aviator,
especially when it comes to the challenging rigors of rotary-wing flight in military aviation.
Understanding the fundamentals of aerodynamics could save your life. Time after time, mishap
reports attribute either a lack of understanding or the inappropriate application of aerodynamic
principles as a causal factor of the mishap.
The exposure in flight training to these principles and their application is only the first step in the
mastery of the essential concepts. Every profession requires continuing education. As iron
sharpens iron, we learn from one another. It is the responsibility of aviators to continually
sharpen their skills in order to maintain the edge that may affect mission accomplishment or
crew survival.
Periodic review of both systems and aerodynamic course material will provide for the greatest
retention and immediate recall. Each time aviators re-read the text, they will glean some new
fact or relationship to improve their overall understanding, even before setting foot in a new
aircraft.
A survey of pilots of the Aviation Safety Officer Course reveals that few of the aerodynamic
principles necessary to investigate a mishap were retained from flight training. Each Aviation
Safety Officer (ASO) completes twenty lecture hours of rotary-wing aerodynamics in the course.
It is clear from this course that every individual is not only capable of, but also highly motivated
toward, making basic aerodynamic principles an integral part of their toolbox. Their goal is not
to develop mishap investigation skills, but rather to develop mishap avoidance skills they can
share.
Therefore, the goal of this text is to present the principles of helicopter aerodynamics in a
straightforward, comprehensible manner, such that both the newest student naval aviator and the
crustiest old instructor pilot may have at their disposal a concise, accurate reference. The best
available tool, however, is only of use to the craftsman who develops and maintains a level of
expertise to make its use second nature.
SCOPE
This reference book provides a broad understanding of helicopter heritage and history, basic
helicopter fundamentals, and aerodynamic principles. It is intended to be a reference fleet pilots
can use as a single source document, not just as a flight school reference book. It includes a
great deal of information as well as a great list of reference materials.
This reference book was produced through the collective effort of Training Air Wing Five flight
instructors and academic instructors. It is a work in progress and will be updated periodically.
Please report any errors in the reference list, the interpretation of the reference materials, or the
information as presented to the Training Air Wing Five Academic Training Department.
vii
ASSUMPTIONS
1.
This reference book serves as an introductory text that serves as an adequate, stand-alone,
ready reference for military helicopter pilots. A recommended reading list provides options for
those with greater curiosity.
2.
Students should concurrently complete the Interactive Courseware (ICW) associated with
this book.
3.
Students are not required to have an engineering background for mastery of the basic
concepts in this document.
CHANGE RECOMMENDATIONS
Change recommendations to this publication may be submitted by anyone to the Commander,
Training Air Wing FIVE, using the Training Change Request (TCR) process, which improves
training curricula and its associated training publications. This includes all personnel involved at
every level of flight training. A TCR can be submitted online
(https://www.cnatra.navy.mil/tip.asp) or by submitting a form to the squadron or wing
standardization personnel. Remember, no TCR is too small!
viii
TABLE OF CONTENTS
LIST OF EFFECTIVE PAGES .................................................................................................. iv
CHANGE SUMMARY ................................................................................................................. v
SAFETY/HAZARD AWARENESS NOTICE .......................................................................... vi
INTRODUCTION....................................................................................................................... vii
TABLE OF CONTENTS ............................................................................................................ ix
LIST OF FIGURES ..................................................................................................................... xi
LIST OF TABLES ..................................................................................................................... xiv
100.
101.
102.
103.
104.
105.
106.
107.
- INTRODUCTION TO HELICOPTER FUNDAMENTALS ................ 1-1
INTRODUCTION .................................................................................................. 1-1
LEARNING OBJECTIVES.................................................................................... 1-1
HELICOPTER ROLES AND MISSIONS ............................................................. 1-1
EARLY ROTORCRAFT DEVELOPMENT ......................................................... 1-2
TYPES OF ROTORCRAFT ................................................................................... 1-5
MAIN ROTOR HEAD CONFIGURATIONS ....................................................... 1-8
ANTI-TORQUE SYSTEMS ................................................................................ 1-11
HELICOPTER COMPONENTS .......................................................................... 1-14
200.
201.
202.
203.
204.
205.
206.
207.
- THE ATMOSPHERE .............................................................................. 2-1
INTRODUCTION .................................................................................................. 2-1
LEARNING OBJECTIVES.................................................................................... 2-1
ATMOSPHERIC PROPERTIES ............................................................................ 2-1
STANDARD ATMOSPHERIC PARAMETERS .................................................. 2-4
ALTITUDE ............................................................................................................. 2-6
Q-CODES ............................................................................................................... 2-7
ALTIMETER SETTINGS ...................................................................................... 2-7
DENSITY ALTITUDE CHARTS .......................................................................... 2-9
300.
301.
302.
303.
304.
305.
306.
- THEORIES OF LIFT .......................................................................... 3-1
INTRODUCTION .................................................................................................. 3-1
LEARNING OBJECTIVES.................................................................................... 3-1
VECTOR ANALYSIS ............................................................................................ 3-1
AIRCRAFT REFERENCE SYSTEM .................................................................... 3-3
FORCES ACTING ON THE BLADE ................................................................... 3-4
FORCES ACTING ON THE HELICOPTER ........................................................ 3-6
LIFT THEORIES .................................................................................................. 3-14
400.
401.
402.
403.
404.
- AIRFOILS ............................................................................................... 4-1
INTRODUCTION .................................................................................................. 4-1
LEARNING OBJECTIVES.................................................................................... 4-1
AIRFOILS............................................................................................................... 4-1
ROTOR BLADE ANGLES .................................................................................... 4-5
RELATIVE WIND ................................................................................................. 4-9
ix
500.
501.
502.
503.
504.
505.
506.
507.
- POWERED FLIGHT ............................................................................... 5-1
INTRODUCTION .................................................................................................. 5-1
LEARNING OBJECTIVES.................................................................................... 5-1
HOVERING FLIGHT ............................................................................................ 5-1
VERTICAL FLIGHT.............................................................................................. 5-9
FORWARD FLIGHT ............................................................................................. 5-9
SIDEWARD FLIGHT .......................................................................................... 5-20
REARWARD FLIGHT ........................................................................................ 5-20
TURNING FLIGHT ............................................................................................. 5-21
600.
601.
602.
603.
604.
605.
606.
- AUTOROTATION...................................................................................... 6-1
INTRODUCTION ..................................................................................................... 6-1
LEARNING OBJECTIVES....................................................................................... 6-1
AUTOROTATIONAL FLIGHT ............................................................................... 6-1
ENERGY MANAGEMENT CONCEPTS ................................................................ 6-2
AERODYNAMIC FORCES THAT AFFECT AUTOROTATION ......................... 6-4
PHASES OF AUTOROTATION ............................................................................ 6-12
HEIGHT-VELOCITY DIAGRAM ......................................................................... 6-18
700.
701.
702.
703.
704.
705.
706.
707.
- HELICOPTER PERFORMANCE PLANNING ............................... 7-1
INTRODUCTION ..................................................................................................... 7-1
LEARNING OBJECTIVES....................................................................................... 7-1
HELICOPTER PERFORMANCE FACTORS.......................................................... 7-1
POWER PERFORMANCE ....................................................................................... 7-2
PARASITE POWER ................................................................................................. 7-9
HOVER PERFORMANCE ..................................................................................... 7-11
CLIMB PERFORMANCE ...................................................................................... 7-12
CRUISE PERFORMANCE ..................................................................................... 7-13
800.
801.
802.
803.
804.
805.
- FLIGHT PHENOMENA...................................................................... 8-1
INTRODUCTION ..................................................................................................... 8-1
LEARNING OBJECTIVES....................................................................................... 8-1
FLIGHT ENVELOPE/V-N DIAGRAM ................................................................... 8-1
MOMENTS AND CENTER OF GRAVITY ............................................................ 8-4
DYNAMIC ROLLOVER .......................................................................................... 8-5
ROTOR DYNAMICS ................................................................................................ 8-8
900.
901.
902.
903.
TAIL ROTOR CONSIDERATIONS ....................................................... 9-1
INTRODUCTION ..................................................................................................... 9-1
LEARNING OBJECTIVES....................................................................................... 9-1
TAIL ROTOR AERODYNAMICS ........................................................................... 9-1
TAIL ROTOR MALFUNCTIONS............................................................................ 9-4
APPENDIX A - GLOSSARY ................................................................................................... A-1
APPENDIX B - RECOMMENDED READING LIST .......................................................... B-1
x
LIST OF FIGURES
Figure 1-1 C4 Autogiro ............................................................................................................. 1-2
Figure 1-2 PCA-2 Autogiro ...................................................................................................... 1-3
Figure 1-3 Gyroplane Laboratorie .......................................................................................... 1-3
Figure 1-4 First Practical Helicopters ..................................................................................... 1-4
Figure 1-5 Sikorsky R-4 Hoverfly ........................................................................................... 1-4
Figure 1-6 Bell 47 ...................................................................................................................... 1-5
Figure 1-7 Lockheed Cheyenne ............................................................................................... 1-6
Figure 1-8 Tilt-rotor Aircraft .................................................................................................. 1-7
Figure 1-9 Future Rotary-Wing Concepts .............................................................................. 1-7
Figure 1-10 Semi-rigid Main Rotor Head Systems ................................................................ 1-8
Figure 1-11 Fully Articulated Main Rotor Heads .................................................................. 1-9
Figure 1-12 Rigid Rotor Head Systems ................................................................................... 1-9
Figure 1-13 CH-47 Chinook Helicopter Tandem Rotor Head System .............................. 1-10
Figure 1-14 Synchropter Rotor Head Systems ..................................................................... 1-11
Figure 1-15 Coaxial Rotor Head Systems ............................................................................. 1-11
Figure 1-16 Conventional Tail Rotor .................................................................................... 1-12
Figure 1-17 Single Main Rotor Head Configuration with Fenestron Tail ........................ 1-12
Figure 1-18 NOTAR Configuration ...................................................................................... 1-13
Figure 1-19 Electrically Distributed Anti-Torque System .................................................. 1-13
Figure 1-20 Major Components of a Conventional Tail Rotor Helicopter ....................... 1-14
Figure 1-21 Basic Engine and Transmission Schematic ...................................................... 1-15
Figure 1-22 Basic Main Rotor Components ......................................................................... 1-15
Figure 1-23 Basic Anti-torque System Components ............................................................ 1-16
Figure 1-24 Helicopter Controls ............................................................................................ 1-17
Figure 1-25 Cyclic Inputs ....................................................................................................... 1-18
Figure 1-26 Collective Inputs ................................................................................................. 1-19
Figure 1-27 Anti-Torque Pedal Inputs.................................................................................. 1-20
Figure 1-28 Basic Helicopter Throttle Control .................................................................... 1-20
Figure 2-1
Figure 2-2
Figure 2-3
Figure 2-4
Figure 2-5
Figure 2-6
Static Pressure Change with Altitude................................................................... 2-2
Molecular Energy and Air Temperature ............................................................. 2-3
Relative Humidity .................................................................................................. 2-4
Air Temperature Changes, Indicated Altitude Vs True Altitude ...................... 2-9
DA Chart ............................................................................................................... 2-10
Dew Point Correction Chart ............................................................................... 2-11
Figure 3-1
Figure 3-2
Figure 3-3
Figure 3-4
Figure 3-5
Figure 3-6
Figure 3-7
Figure 3-8
Resultant Vector Using the Tip to Tail Method .................................................. 3-2
Force and Velocity Vectors on a Helicopter Blade.............................................. 3-2
Force Vectors on a Helicopter in Flight ............................................................... 3-3
Aircraft Reference System..................................................................................... 3-4
Area of Blade .......................................................................................................... 3-4
Profile of an Airfoil ................................................................................................ 3-5
Aerodynamic Force ................................................................................................ 3-6
Forces Acting on a Helicopter in Forward Flight ............................................... 3-7
xi
Figure 3-9 Production of Lift ................................................................................................... 3-7
Figure 3-10 Load Factor Vs Angle of Bank (AOB) Diagram ............................................... 3-9
Figure 3-11 Form Drag ........................................................................................................... 3-11
Figure 3-12 Skin Friction ....................................................................................................... 3-12
Figure 3-13 Induced Drag ...................................................................................................... 3-12
Figure 3-14 Drag Curve.......................................................................................................... 3-13
Figure 3-15 Bernoulli’s Principle .......................................................................................... 3-14
Figure 3-16 Venturi Effect ..................................................................................................... 3-15
Figure 3-17 Magnus Effect ..................................................................................................... 3-16
Figure 3-18 Induced Velocity and Momentum Theory ....................................................... 3-17
Figure 3-19 Blade Element Theory ....................................................................................... 3-18
Figure 4-1 Airfoil Aerodynamic Terms .................................................................................. 4-2
Figure 4-2 Airfoil Types ........................................................................................................... 4-3
Figure 4-3 Blade Element Diagram ......................................................................................... 4-4
Figure 4-4 Angle of Incidence .................................................................................................. 4-5
Figure 4-5 Angle of Attack ....................................................................................................... 4-6
Figure 4-6 Blade Stall at Increased AOA ............................................................................... 4-6
Figure 4-7 Blade Twist.............................................................................................................. 4-7
Figure 4-8 Angular Position of Main Rotor Blades ............................................................... 4-8
Figure 4-9 Radial Position of Main Rotor Blades .................................................................. 4-9
Figure 4-10 Relative Wind ....................................................................................................... 4-9
Figure 4-11 Horizontal Component of Relative Wind......................................................... 4-10
Figure 4-12 Rotational Relative Wind .................................................................................. 4-10
Figure 4-13 Induced Flow ...................................................................................................... 4-11
Figure 4-14 Resultant Relative Wind .................................................................................... 4-12
Figure 4-15 Indicated Airspeed Effects on Relative Wind .................................................. 4-13
Figure 4-16 Induced Flow during Transition to Forward Flight ....................................... 4-14
Figure 4-17 In Ground Effect ................................................................................................ 4-15
Figure 4-18 Out of Ground Effect ......................................................................................... 4-16
Figure 5-1 No-Wind Hover ...................................................................................................... 5-2
Figure 5-2 Translating Tendency ............................................................................................ 5-3
Figure 5-3 Coning ..................................................................................................................... 5-5
Figure 5-4 Coning Vs Main Rotor Disk Area Due to Low Rotor RPM ............................... 5-6
Figure 5-5 Changes in Angular Momentum due to Flapping ............................................... 5-7
Figure 5-6 Gyroscopic Precession............................................................................................ 5-8
Figure 5-7 Forward Cyclic Input............................................................................................. 5-8
Figure 5-8 Forces on a Helicopter in Forward Flight ............................................................ 5-9
Figure 5-9 Power Vs Airspeed Chart .................................................................................... 5-10
Figure 5-10 Airflow in Forward Flight ................................................................................. 5-11
Figure 5-11 Dissymmetry of Lift ........................................................................................... 5-13
Figure 5-12 Effect of Flapping ............................................................................................... 5-14
Figure 5-13 Counteracting Blowback ................................................................................... 5-15
Figure 5-14 Airflow with Minimal Headwind ...................................................................... 5-16
Figure 5-15 Airflow just Prior to Effective Translational Lift ........................................... 5-16
xii
Figure 5-16
Figure 5-17
Figure 5-18
Figure 5-19
Figure 5-20
Translational Thrust .......................................................................................... 5-17
Transverse Flow Effect ...................................................................................... 5-19
Sideward Flight .................................................................................................. 5-20
Rearward Flight ................................................................................................. 5-21
Turning Flight .................................................................................................... 5-22
Figure 6-1 Airflow in Powered Flight Vs Autorotation ......................................................... 6-2
Figure 6-2 Rotor Blade Regions in Autorotation ................................................................... 6-5
Figure 6-3 Rotor Disk Regions in Autorotation During Forward Flight ............................ 6-7
Figure 6-4 RPM Response to Small RPM Variations............................................................ 6-9
Figure 6-5 Force Vectors After Power Loss – Reduced Collective .................................... 6-13
Figure 6-6 Force Vectors in Autorotative Steady-State Descent ........................................ 6-14
Figure 6-7 Autorotational Glide Characteristics Chart – Rates of Descent Vs Airspeed 6-15
Figure 6-8 Blade Element and Thrust During Steady-State Auto and Flare .................... 6-17
Figure 6-9 Generic Height-Velocity Diagram ...................................................................... 6-18
Figure 6-10 TH-73A Height-Velocity Diagram .................................................................... 6-20
Figure 7-1 Jet Thrust Vs Turboshaft Power .......................................................................... 7-3
Figure 7-2 Power Required Curve .......................................................................................... 7-4
Figure 7-3 Optimum Airspeeds ............................................................................................... 7-5
Figure 7-4 Maximum Range Airspeed Adjustment for Winds ............................................ 7-6
Figure 7-5 Induced Power Requirements ............................................................................... 7-9
Figure 7-6 Parasite Power Requirements ............................................................................. 7-10
Figure 7-7 Decrease in Excess Power as Airspeed Decreases ............................................. 7-11
Figure 7-8 Rate of Climb Vs Best Angle of Climb ............................................................... 7-12
Figure 7-9 Power Required Vs Airspeed .............................................................................. 7-13
Figure 7-10 Example of RPM Vs Fuel Flow ......................................................................... 7-14
Figure 7-11 Maximum Range Altitude Vs Gross Weight ................................................... 7-15
Figure 7-12 Maximum Range Airspeed Vs Gross Weight .................................................. 7-17
Figure 8-1 V-n Diagram for Fixed-Wing Aircraft ................................................................. 8-2
Figure 8-2 AH-64 Apache V-n Diagram ................................................................................. 8-3
Figure 8-3 Dynamic Rollover ................................................................................................... 8-6
Figure 8-4 Slope Takeoff or Landing Upslope Roll ............................................................... 8-7
Figure 8-5 Slope Takeoff or Landing Downslope Roll .......................................................... 8-8
Figure 8-6 Ground Vortex........................................................................................................ 8-9
Figure 8-7 Critical Mach Number ......................................................................................... 8-11
Figure 8-8 Stall Region of the Retreating Blade .................................................................. 8-12
Figure 8-9 Rotor Tip Vortex .................................................................................................. 8-13
Figure 8-10 Mast Bumping .................................................................................................... 8-16
Figure 9-1
Figure 9-2
Figure 9-3
Figure 9-4
Figure 9-5
Figure 9-6
Tail Rotor Unbalanced Force ................................................................................ 9-1
Translating Tendency ............................................................................................ 9-2
Vertical Stabilizer ................................................................................................... 9-3
Weathervaning Effect ............................................................................................ 9-4
Tail Rotor Vortex Ring State ................................................................................ 9-8
Tail Rotor AOA Reduction in a Right Crosswind .............................................. 9-9
xiii
LIST OF TABLES
Table 2-1
Table 2-2
ISA Conditions at Mean Sea Level ................................................................... 2-5
Standard Atmosphere Table ............................................................................. 2-6
Table 6-1
Table 6-2
Helicopter Energy .............................................................................................. 6-3
Energy Management During Autorotation ..................................................... 6-4
Table 8-1
Vibration Analysis Indications and Causes ................................................... 8-10
xiv
INTRODUCTION TO HELICOPTER FUNDAMENTALS
100. INTRODUCTION
This chapter introduces the heritage, history, applications, and unique characteristics of
rotorcraft, which are inherently more complex than conventional airplanes, but inefficient and
more difficult to fly in comparison. In addition, the mechanical complexity of rotorcraft means
they are generally more expensive to operate than almost any other type of aircraft.
101. LEARNING OBJECTIVES

Recall the history of rotorcraft.

Identify common helicopter operational capabilities.

Describe the aerodynamics of rotorcraft.

Identify helicopter rotor configurations.

Identify helicopter flight controls.
102. HELICOPTER ROLES AND MISSIONS
Rotorcraft performance shortcomings and higher operating expenses are offset by their unique
capabilities and flying characteristics, such as vertical takeoff and landing, the ability to hover
and the ability to operate from areas unsuitable for other forms of transport. The unique
operating environment of rotorcraft, flying them is a multi-sensory experience, requires
advanced flight training and a high level of situational awareness. A robust knowledge of
rotorcraft design and aerodynamic characteristics is needed to allow the pilot to safely operate
the aircraft to its full potential.
Although all helicopters must follow the laws of physics, their size and configuration can vary
tremendously. Helicopters are engineering marvels capable of accomplishing a wide range of
missions due to their inherent flexibility. Unique flying characteristics allow them to operate to
and from remote areas where airstrips are not available, and in operational and tactical scenarios
where airplanes are unsuitable. The ability to hover permits them to deploy or pick up personnel
and/or cargo almost anywhere. Improved or unimproved Landing Zones (LZs), ship flight
decks, buildings, helipads, oil rigs, mountain peaks, and even oceans are feasible landing sites.
If the surface is unsuitable for landing, many helicopters can transfer cargo and passengers while
hovering by using a hoist or external lift capabilities.
Military transport helicopters excel in inserting and extracting troops, delivering ammunition and
food, evacuating wounded, recovering crews of downed aircraft, and even recovering the
downed aircraft itself. Armed helicopters are ideally suited for attacking ground, sea, and
undersea targets and escorting transport helicopters. Because of their agility and
INTRODUCTION TO HELICOPTER FUNDAMENTALS
1-1
CHAPTER ONE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
maneuverability, helicopters can avoid enemy detection by flying at low altitude and using
terrain masking and unmasking techniques. Helicopters can be equipped with cameras capable
of seeing across the electromagnetic spectrum, and sensors capable of detecting radio signals and
underwater acoustic signatures.
Their flying capabilities make them an ideal search and rescue vehicle. Many lives have been
saved because helicopters can get to places that would otherwise be difficult or impossible to
reach. It is estimated that over three million lives have been saved by helicopters in both
peaceful and wartime operations since the first person was rescued from the sea in 1944.
Firefighting, newsgathering, training, powerline patrol, aerial photography/videography, and film
production are some other major application where helicopters have been used.
103. EARLY ROTORCRAFT DEVELOPMENT
The word helicopter is adapted from the French hélicoptère, coined by Gustave de Ponton
d’Amécourt in 1861 and linked to the Greek words helix/helikos (spiral or turning) and pteron
(wing). Even though the concept of using a rotating wing to achieve flight can be traced back to
Leonardo da Vinci’s helix wing sketch, its practical history begins in the early 20th century
alongside early fixed-wing powered flight development.
Until the 1940s, the development of rotorcraft was focused on overcoming the unique
aerodynamic and control issues inherent to them. Development progress took place at a slower
pace than fixed-wing aviation due to both the complexity of using a rotating wing to generate lift
and the limited power of early engines. When compared with fixed-wing aircraft, rotorcraft
require a greater power to weight ratio, and had to withstand unique airframe stress and
vibration. Early rotorcraft lacked sufficient power to fly and once they could fly, they were
uncontrollable in all but a static hover. Today’s rotorcraft operate successfully because of a
tremendous number of innovations to overcome these obstacles.
Figure 1-1 C4 Autogiro
1-2
INTRODUCTION TO HELICOPTER FUNDAMENTALS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER ONE
The autogiro concept represented an early stage in the evolution of rotorcraft. In 1923, Juan de
la Cierva built and flew the first successful autogiro, the C4. See Figure 1-1. He was the first to
recognize that an unpowered main rotor can hold an aircraft in the air if the aircraft is pushed or
pulled through the air by a separate propeller. His flapping-blade design has been used in all
rotorcraft since.
Figure 1-2 PCA-2 Autogiro
In 1930, Harold F. Pitcairn created the Pitcairn Aircraft Company and sold the first Americanmanufactured autogiro, the PCA-2. See Figure 1-2. The PCA-2 was the first rotary-wing
aircraft to achieve type certification in the United States, the first aircraft to land on the White
House lawn, and the first rotorcraft to fly across the continental United States. The U.S. Navy
evaluated a PCA-2 with the aircraft carrier USS Langley (CV-1) in 1931 and accomplished the
first rotary-wing craft operations from a ship at sea.
Figure 1-3 Gyroplane Laboratorie
INTRODUCTION TO HELICOPTER FUNDAMENTALS
1-3
CHAPTER ONE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
In 1935, Louis Bréguet and Rene Dorand built the Gyroplane Laboratoire (Figure 1-3)
configured with a coaxial rotor head design and recognized as the first rotorcraft to meet the
performance capabilities expected of a true helicopter.
Figure 1-4 First Practical Helicopters
Other rotorcraft developed in the 1930s include the German Focke-Wulf Fw 61, the Weir W-5 in
the United Kingdom, and the Sikorsky VS-300 in the United States (Figure 1-4).
Figure 1-5 Sikorsky R-4 Hoverfly
In an attempt to encourage and fund the construction of helicopter prototypes, the U.S. Congress
passed the Dorsey-Logan Act of 1940. In 1943, the Sikorsky R-4 Hoverfly (Figure 1-5) became
the first rotorcraft to enter mass production and the first helicopter in U.S. military service with
both the Army and the Navy.
1-4
INTRODUCTION TO HELICOPTER FUNDAMENTALS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER ONE
Figure 1-6 Bell 47
In 1946, the Bell 47, based on the research of Arthur Young, was the first helicopter to be
certified in the United States. See Figure 1-6. It was used both commercially and in the military.
To learn more about the history of helicopters, explore the collection of documents, squadron
histories, personal biographies, sea stories, and videos on the Naval Helicopter Association
Historical Society (NHAHS) website (https://www.nhahistoricalsociety.org). The naval
helicopter history timeline provides an excellent reference of milestones in naval helicopter
development. Additionally, there are many examples of the challenges that faced early rotorcraft
pioneers and the innovations that were developed to overcome those challenges.
104. TYPES OF ROTORCRAFT
The Federal Aviation Administration (FAA) defines two classes of rotorcraft: helicopters and
gyroplanes. A rotorcraft is any heavier than air aircraft that generates lift by one or more rotors.
Although the FAA does not considered them true rotorcraft, powered lift (tilt-rotor) aircraft share
many characteristics with rotorcraft.
1.
Helicopters
Helicopters are rotorcraft that use one or more engine-driven rotors with two or more blades to
generate thrust and control horizontal, lateral, and vertical movement.
With all rotorcraft, thrust from a rotor is aligned with a line drawn perpendicular to the rotor
blades’ tip path, and a conventional helicopter propels itself by tilting the rotor in order to
generate a horizontal thrust component. Unlike fixed-wing aircraft, which are supported in flight
by the dynamic reaction of the air against the wings, helicopters derive their source of lift from
the rotor blades. With some helicopters, a nominal amount of lift is generated during forward
flight by airflow around the fuselage.
2.
Gyroplanes
Gyroplanes obtain lift from an un-driven rotor. Horizontal thrust is generated by a propeller, but
the main rotor spins freely. The main rotor is tilted away from the direction of flight causing air
INTRODUCTION TO HELICOPTER FUNDAMENTALS
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CHAPTER ONE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
to flow up through it. Aerodynamically, the rotor system of a gyroplane in flight operates like a
helicopter rotor during an engine-out autorotative descent. Since the rotor needs to be pulled
through the air to generate lift, gyroplanes cannot fly in a stationary hover. Simple gyroplanes
use ground roll during takeoff to spin up the rotor and generate lift. Variants that are more
complex use engine power to accelerate the rotors on the ground allowing them to perform a
jump takeoff using stored energy.
3.
Gyrodynes
Gyrodynes are a hybrid type of rotorcraft that obtain lift from a power-driven main rotor, but
unlike helicopters, maintain their main rotor disk parallel to the direction of flight, and are
propelled forward by a conventional propeller. The gyrodynes offer high-speed potential, but
with the penalties of increased complexity, weight, and control difficulty.
Figure 1-7 Lockheed Cheyenne
The Lockheed Cheyenne, with an anti-torque tail rotor and pusher propeller on the tail, is an
early example of a military gyrodyne. See Figure 1-7. Some gyrodyne manufacturing
companies replaced the traditional tail rotor concept with a side-mounted propeller capable of
countering torque when hovering, while also providing thrust for forward flight.
4.
Tilt-rotor Aircraft
Convertiplanes, commonly known as tilt-rotor aircraft, fall under the powered lift category, but
are not considered rotorcraft. Their designs mainly consist of two prop-rotors on rotating
nacelles mounted on the ends of a wing. By rotating the nacelles, the two prop-rotors are able to
provide propulsion or generate lift. In forward flight, thrust from the two prop-rotors powers the
aircraft while lift is developed by the wing. In a hover, lift is generated solely by the prop-rotors.
Yaw control in hover is provided by differentially tilting one prop-rotor forward and one
backwards. The diameter of the prop-rotors typically means that tilt-rotor aircraft cannot land in
forward flight configuration without the prop-rotors striking the ground.
1-6
INTRODUCTION TO HELICOPTER FUNDAMENTALS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER ONE
Tilt-rotor aircraft have several operational advantages over traditional helicopters: faster speeds,
reduced vibration levels, higher efficiency, and greater range. Disadvantages include a reduction
in hover performance and lift capability. The Bell-Boeing Osprey V-22, Leonardo AW-609, and
Bell V-280 Valor are examples of tilt-rotor aircraft (Figure 1-8).
Bell-Boeing Osprey V22
Leonardo AW-609
Bell V-280 Valor
Figure 1-8 Tilt-rotor Aircraft
5.
Future Rotary-Wing Concepts
The design of future military and commercial rotary-wing aircraft continue to evolve as
manufacturers look to expand aircraft performance and capabilities. The ultimate goal is to
create agile and maneuverable aircraft that fly faster, farther, and carry more payload
(passengers-cargo-weapons), while maintaining vertical takeoff and landing capability. Aviation
companies have started developing several next generation rotorcraft to satisfy these goals.
These include the Sikorsky-Boeing SB-1 Defiant, Sikorsky S-97 Raider, Bell 360 Invictus, and
the Airbus Helicopters RACER (Figure 1-9).
Figure 1-9 Future Rotary-Wing Concepts
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105. MAIN ROTOR HEAD CONFIGURATIONS
In order to function safely, helicopter main rotor blades must be allowed to change pitch, move
up and down (flap), and move fore and aft (lead and lag). There are three types of main rotor
head designs used in helicopters semi-rigid, fully articulated, and rigid. The designs vary and
have different capabilities and limitations, but each allows the blades to move independently
while withstanding the tremendous stresses applied to helicopter blades in flight.
1.
Semi-Rigid
The semi-rigid rotor head system (Figure 1-10) is only found on aircraft with two rotor blades.
The blades are connected using a teetering hinge so that as one blade flaps up, the opposite blade
flaps down. Due to the rotor head design, the blades have minor fore and aft in plane movement
(lead and lag) which is absorb by the main rotor hub. Each blade can also rotate about its axis to
change blade pitch. Semi-rigid systems are simple but are limited to two blade systems.
Figure 1-10 Semi-rigid Main Rotor Head Systems
2.
Fully Articulated
A fully articulated rotor head system (Figure 1-11) is found on aircraft with more than two
blades with the ability to move each individual blade in three directions. In this design, hinges
allow each blade to move fore and aft in plane (lead and lag) and flap up and down independent
of the other blades. Each blade can also rotate about its axis to change blade pitch.
1-8
INTRODUCTION TO HELICOPTER FUNDAMENTALS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER ONE
Figure 1-11 Fully Articulated Main Rotor Heads
3.
Rigid
The rigid rotor head system (Figure 1-12) is a rare design, but potentially offers the best
properties of both the fully articulated and semi-rigid rotors. In this design, the blade roots are
rigidly attached to the rotor hub. The blades do not have hinges to allow lead-lag or flapping.
Instead, the blades accommodate these motions by using elastomeric bearings. Elastomeric
bearings are molded, rubber-like materials that are bonded to the appropriate parts. Instead of
rotating like conventional bearings, they twist and flex to allow proper movement of the blades.
Figure 1-12 Rigid Rotor Head Systems
4.
Contra-Rotating Rotor Configurations
The additional configurations handle torque reaction using contra-rotating rotor head systems.
Helicopters with a contra-rotating rotor configuration have no need for the tail rotor since the
torque generated by the main rotors negate each other. However, helicopters with a contrarotating rotor configuration have a tail fin or fins for directional stability during forward flight.
INTRODUCTION TO HELICOPTER FUNDAMENTALS
1-9
CHAPTER ONE
a.
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Tandem Rotor Configuration
A tandem rotor configuration (Figure 1-13) uses two synchronized rotors turning in
opposite directions at the opposite ends of a long hull. The CH-47 Chinook is a good
example of a helicopter with a tandem rotor configuration. This configuration has the
advantage of offering increased lifting capability with shorter blades due to the
overall large disk area. Since there is no tail-rotor to drive, more engine power is
available to generate lift. The elongated fuselage required to accommodate two main
rotors provides a long cabin with ample load space.
Figure 1-13 CH-47 Chinook Helicopter Tandem Rotor Head System
b.
Synchropter
The Synchropter (intermeshing rotor head configuration) uses two contra-rotating
main rotors, which mesh so closely that the shafts can be driven by the same gearbox.
Meshing is achieved by tilting the shafts outwards so that the blades of one rotor can
pass over the rotor head of the other. The German Kolibri (Flettner Fl 282) of World
War II, a small anti-submarine warfare helicopter, was the first successful machine to
use this concept. During the Cold War, American Charles Kaman (Kaman Aircraft)
adopted the synchropter principle and produced the famous HH-43 Huskie, which
became the definitive firefighting and rescue helicopter of its time. It was used by the
U.S. Air Force, Navy, and Marine Corps. A modern synchropter model is the
Kaman K-MAX, which has been used by the U.S. Marine Corps for unmanned cargo
operations. See Figure 1-14.
1-10 INTRODUCTION TO HELICOPTER FUNDAMENTALS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER ONE
Figure 1-14 Synchropter Rotor Head Systems
c.
Coaxial Configuration
Coaxial helicopters (Figure 1-15) are another type of contra-rotation main rotor
configuration. This configuration places both rotors one above the other on a
common shaft and drives them in opposite directions around the same axis of
rotation. The main advantage of coaxial helicopters is the absence of a tail rotor so
the machine can be much more compact, a crucial factor in Naval Aviation, where
everything must be squeezed into limited hangar space. Nikolai Kamov in the USSR
put the idea into production and the coaxial configuration became a noted feature of
helicopters produced by the Russian Kamov design bureau.
Kamov Ka-50
Kaman Ka-29
Figure 1-15 Coaxial Rotor Head Systems
106. ANTI-TORQUE SYSTEMS
There are several different anti-torque systems (tail rotor configurations) on current military and
commercial rotary-wing aircraft. These systems provide anti-torque force to counteract the main
rotor torque and are used for yaw control. Anti-torque system designs include the conventional
tail rotor, Fenestron, No Tail Rotor (NOTAR), and innovative Electrically Distributed AntiTorque (EDAT).
INTRODUCTION TO HELICOPTER FUNDAMENTALS
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CHAPTER ONE
1.
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Conventional Tail Rotor
The conventional tail rotor design (Figure 1-16) is an open tail rotor installed at the end of the
tail boom that provides anti-torque force and yaw control.
H-60
Figure 1-16 Conventional Tail Rotor
2.
Fenestron Design
The Fenestron design (Figure 1-17) includes an enclosed tail rotor installed at the end of the tail
boom that provides anti-torque and yaw control.
AS-365 Dauphin
Fenestron
Figure 1-17 Single Main Rotor Head Configuration with Fenestron Tail
3.
No Tail Rotor
In a NOTAR (No Tail Rotor) system, the tail rotor is replaced with a ducted fan inside the tail
boom (Figure 1-18). The fan generates a high volume of low-pressure air, which exits through
two slots on the right side of the tail boom. This creates a boundary layer flow of air called the
1-12 INTRODUCTION TO HELICOPTER FUNDAMENTALS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER ONE
Coanda Effect. The result is that the tail boom becomes a wing, flying in the downwash of the
rotor head system. The high pressure on the left side of the tail boom generates up to 60 percent
of the anti-torque required in a hover. The remaining anti-torque force and directional yaw
control is gained through a vented, rotating drum at the end of the tail boom, called the direct jet
thruster.
MD-500
Figure 1-18 NOTAR Configuration
4.
Electrically Distributed Anti-Torque
The Electrically Distributed Anti-Torque (EDAT) system (Figure 1-19) is currently being
developed by Bell that uses four ducted, variable-RPM fans instead of the conventional tail rotor
head system. These fans are electrically powered by a generator that operates independently
from the main rotor. Thrust changes are determined not by blade pitch, but by how fast they are
spinning.
Figure 1-19 Electrically Distributed Anti-Torque System
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CHAPTER ONE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
107. HELICOPTER COMPONENTS
Helicopters contain many systems and components, but these can generally be broken down into
a number of major areas. Figure 1-20 displays the major drive and control components of a
conventional tail rotor helicopter.
To understand helicopter aerodynamics and the correlation between power available and power
required, pilots should be familiar with helicopter engine power, the rotor mechanics and
components, and flight controls.
Figure 1-20 Major Components of a Conventional Tail Rotor Helicopter
1.
Engines
The powerplant provides the mechanical power to turn the rotors during powered flight and run
the accessory systems. See Figure 1-21. Gas turbine and piston engines are the most common
power sources in modern rotorcraft. Since engines turn much faster than the rotor shaft, a
transmission with a reduction gear system must be used when connecting the engine to the main
and tail rotors.
1-14 INTRODUCTION TO HELICOPTER FUNDAMENTALS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER ONE
Figure 1-21 Basic Engine and Transmission Schematic
2.
Main Rotor and Anti-torque Systems
The main rotor system (Figure 1-22) and the anti-torque system (Figure 1-23) both generate
thrust, lift, and drag. These systems are also used to control the helicopter. The most common
anti-torques system is the conventional tail rotor system.
Figure 1-22 Basic Main Rotor Components
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CHAPTER ONE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 1-23 Basic Anti-torque System Components
3.
Flight Controls
A conventional tail rotor helicopter is controlled by changing the pitch of the main and tail rotor
blades. Changing the pitch modifies the thrust, lift, and drag forces produced by the blades and
rotors. An increase or decrease in the required thrust will determine the amount of power the
engine must provide: more thrust requires more power, less thrust requires less power.
Helicopters have three primary flight controls: cyclic, collective, and anti-torque pedals (Figure
1-24). The pilot can control the magnitude of the main rotor thrust with the collective pitch
lever, the direction of the main rotor thrust vector with the cyclic stick, and the magnitude of the
tail rotor thrust with the anti-torque pedals.
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER ONE
Figure 1-24 Helicopter Controls
The cyclic stick works in two dimensions forward and aft or left and right. If the cyclic is
pushed in any direction, the rotor tip path plane tilts the same way. Based on cyclic movement,
individual blades change pitch depending on their position along their revolution around the
mast, causing the main rotor system blades to produce unequal lift/thrust (Figure 1-25). This
results in the helicopter moving in the direction of cyclic input. This control is called a cyclic
because it varies the pitch of the main rotor blades cyclically throughout each revolution of the
main rotor system.
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CHAPTER ONE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 1-25 Cyclic Inputs
The collective changes the pitch angle of all the main rotor blades collectively (i.e., all at the
same time) and regardless of their positions. Therefore, if a collective input is made, all the
blades change equally, increasing or decreasing total lift/thrust, resulting in the helicopter
increasing or decreasing altitude or airspeed (Figure 1-26).
1-18 INTRODUCTION TO HELICOPTER FUNDAMENTALS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER ONE
Figure 1-26 Collective Inputs
Anti-torque pedals control aircraft yaw around the vertical axis by altering the pitch of the tail
rotor blades, as depicted in Figure 1-27. Application of the pedal in either direction increases or
reduces the thrust produced by the tail rotor, causing the nose to yaw in the direction of the
applied pedal. In forward flight a pedal input puts the helicopter in longitudinal trim (centers the
ball). In a hover, the pedals enable the pilot to turn 360 degrees around the vertical axis.
INTRODUCTION TO HELICOPTER FUNDAMENTALS
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CHAPTER ONE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 1-27 Anti-Torque Pedal Inputs
The throttle (Figure 1-28) controls the power produced by the engine. Helicopter rotors are
generally designed to turn at a constant speed or RPM. In helicopters that do not have a fuel
control unit, the throttle setting will have to be adjusted whenever the power demand changes so
that rotor RPM remains constant. To avoid manual adjustment of the throttle with every
collective manipulation and to lower pilot workload, almost all modern helicopters (including the
TH-73A) are equipped with automatic fuel control systems. In modern helicopters, the pilot
places the throttle in the flight position prior to takeoff. The throttle will remain in the flight
position during all normal operations. Unlike a manual throttle, the pilot is not required to adjust
the throttle, except in the case of an emergency. In single engine and some multi-engine
helicopters, the throttle control is a twist grip mounted on the collective. In most multi-engine
helicopters, the throttles are controlled using levers or mode switches for each engine mounted
on the overhead or center panel.
Figure 1-28 Basic Helicopter Throttle Control
1-20 INTRODUCTION TO HELICOPTER FUNDAMENTALS
THE ATMOSPHERE
200. INTRODUCTION
This chapter provides a review of basic physics and atmospherics relevant to helicopter
aerodynamics. Helicopter pilots should be particularly concerned with the characteristics of the
lower atmosphere because they directly affect the day-to-day, and sometimes minute-to-minute,
performance of a helicopter.
Although atmospheric properties such as altitude, temperature, and humidity affect fixed-wing
aircraft performance, they are much more crucial to a helicopter. For example, on a cool, dry
day at sea level, a CH-53E would be able to lift a 16,000-pound external load with a full tank of
15,000 pounds of fuel. However, if a CH-53E attempted to lift that same external load on a hot,
humid day from an airfield at 3,000 feet, it might only be able to carry 5,000 pounds of fuel. As
a result, the CH-53E would only be able to carry the load for about an hour and a half, instead of
the three and half hours afforded by a full tank of gas.
Due to the changes in helicopter performance based on atmospheric conditions, helicopter pilots
must be well versed in atmospheric properties and how they affect helicopters. This chapter is
meant as a review of some atmospheric basics. For more information, review the Aerodynamics
Student Guide (NAVAVSCOLSCOM-SG-200) from Naval Introductory Flight Evaluation
(NIFE).
201. LEARNING OBJECTIVES

Describe the composition of the atmosphere.

Describe the international standard atmosphere parameters and the temperature lapse
rate.

Describe the different types of altitudes.

Describe Q-code altitudes.

Describe the relationship between air density and performance.

Describe performance charts and calculations.
202. ATMOSPHERIC PROPERTIES
The atmosphere is composed of approximately 78 percent nitrogen (N2), 21 percent oxygen (O2),
and 1 percent other gases by volume, which includes argon and carbon dioxide. Air is
considered a uniform mixture of these gases and will be examined as a whole rather than as
separate gases.
THE ATMOSPHERE 2-1
CHAPTER TWO
1.
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Static Pressure
Static Pressure (PS) is the force each air particle exerts on those around it. On a more
macroscopic scale, ambient PS (14.7 psi at sea level on a standard day) is equal to the weight of a
column of air over a given area. The force of PS acts perpendicularly to any surface with which
the air particles collide. As altitude increases, less air is above you, so the weight of the column
of air decreases (Figure 2-1). Thus, atmospheric PS decreases with an increase in altitude at a
rate of approximately 1.0 inch of mercury (inHg) per 1,000 feet, near the Earth’s surface. This
means that the standard pressure lapse rate is 1,000 feet of PA for each inHg.
Figure 2-1 Static Pressure Change with Altitude
2.
Air Density
Air density () is the total mass of air particles per unit of volume. The distance between
individual air particles increases with altitude resulting in fewer particles per unit volume.
Therefore, air density decreases with an increase in altitude.
3.
Density Ratio
Density ratio (σ) is the ratio of the density of air at a specific altitude to that of the standard
altitude (sea level).
2-2
THE ATMOSPHERE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
4.
CHAPTER TWO
Temperature
Temperature (T) is a measure of the average kinetic energy of air particles. As temperature
increases, particles begin to move and vibrate faster, increasing their kinetic energy, as seen in
Figure 2-2. Air temperature decreases linearly with an increase in altitude at a rate of
approximately two degrees Celsius (ºC) or 3.57 degrees Fahrenheit (ºF) per 1,000 feet up through
36,000 feet Mean Sea Level (MSL). This is called the standard or adiabatic lapse rate. Above
36,000 feet lies the isothermal layer where air is at a constant temperature of -56.5 °C.
Figure 2-2 Molecular Energy and Air Temperature
5.
Humidity
Humidity is the amount of water vapor in the air. As humidity increases, water molecules
displace an equal number of air molecules. Since water molecules have less mass (H2O,
Molecular Weight [MW] 18) than air (N2, MW 28; and O2, MW 32) and occupy approximately
the same volume, the overall mass in a given volume decreases. Therefore, as humidity
increases, air density decreases. Compared to dry air, the density of air at 100 percent humidity
is 4 percent less.
6.
Viscosity
Viscosity () is a measure of the air’s resistance to flow and shearing. Air viscosity can
determine its tendency to either stick to a surface or how easily it flows past it. For liquids, as
temperature increases, viscosity decreases. Recall that the oil in your car flows better or gets
thinner when the engine gets hot. Just the opposite happens with air viscosity increases with an
increase in temperature.
THE ATMOSPHERE 2-3
CHAPTER TWO
7.
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Relative Humidity
Relative Humidity (RH) is the amount of water vapor in the air, expressed as a percentage of the
maximum amount that the air could hold at a given temperature (Figure 2-3). It is measured in
grams per cubic meter (g/m3) and expressed as a percent. In a more practical way, it is how
much water vapor is in this air versus how much could possibly get into the air at this
temperature. Warm air can possess more water vapor (moisture) than cold air, so with the same
amount of absolute/specific humidity, air will have a higher RH if the air is cooler, and a lower
RH if the air is warmer.
Figure 2-3 Relative Humidity
8.
Dew Point
Dew point provides a measure of the actual amount of water vapor in the air and is the
temperature to which air must be cooled to become saturated with water vapor. When cooled
further, the airborne water vapor will condense to form a liquid. Dew point spread refers to the
difference between the dew point and air temperature. A difference of less than 3 °C (5 °F) will
increase the probability of visible moisture forming in the air.
203. STANDARD ATMOSPHERIC PARAMETERS
The atmospheric layer where most flying is done is an ever-changing environment. Temperature
and pressure vary with altitude, season, location, time, and even solar sunspot activity. It is
impractical to take all of these into consideration when discussing aircraft performance. In order
to disregard these atmospheric changes, an engineering baseline was developed and called the
International Standard Atmosphere (ISA). ISA is a set of reference conditions with average
values of air properties as a function of altitude, as seen in
Table 2-1. Unless otherwise stated, any discussion of atmospheric properties in this publication
will assume standard atmospheric conditions.
2-4
THE ATMOSPHERE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
MEAN SEA LEVEL
Pressure
Temperature
Air Density
Relative Humidity
CHAPTER TWO
STANDARD CONDITION
1013.2 hPa (29.92 inHg, 1013.2 mbar)
15 °C (59 °F)
1.225 kg/m3 (0.0765 lb/ft3)
0%
Table 2-1 ISA Conditions at Mean Sea Level
Standard temperatures at a given altitude can be found on a standard atmosphere table
(Table 2-2). If the table is not available, the standard temperature at any altitude can be found
using the standard temperature change with altitude (also called the adiabatic lapse rate). A
standard atmosphere assumes the temperature at sea level is 15 °C and decreases 2 °C every one
thousand feet. So, to calculate the temperature at any altitude, multiply the altitude (in
thousands) by 2 (the lapse rate), and then subtract the product from 15. For example, at 4,000
feet, the standard temperature is about 7 °C, or 15 − (2 × 4).
STANDARD ATMOSPHERE
ALTITUDE (FT)
PRESSURE (INHG)
(ºC)
(ºF)
0
101325
59
15
1000
97716.6
55.4338
13.0188
2000
94212.9
51.8677
11.0376
3000
90811.7
48.3015
9.0564
4000
87510.5
44.7354
7.0752
5000
84307.3
41.1692
5.094
6000
81199.6
37.603
3.1128
7000
78185.4
34.0369
1.1316
8000
75262.4
30.4707
-0.8496
9000
72428.5
26.9046
-2.8308
10000
69681.7
23.3384
-4.812
11000
67019.8
19.7722
-6.7932
12000
64440.9
16.2061
-8.7744
13000
61942.9
12.6399
-10.7556
14000
59523.9
9.07376
-12.7368
15000
57182
5.5076
-14.718
16000
54915.2
1.94144
-16.6992
17000
52721.8
-1.62472
-18.6804
THE ATMOSPHERE 2-5
CHAPTER TWO
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
18000
50599.8
-5.19088
-20.6616
19000
48547.6
-8.75704
-22.6428
20000
46563.3
-12.3232
-24.624
Table 2-2 Standard Atmosphere Table
204. ALTITUDE
There are several methods used to determine the height of an aircraft above the surface.
2-6

True altitude is the actual height above Mean Sea Level (MSL). Elevations of
airports, mountaintops, towers, and other obstructions are given in true altitude.

Absolute altitude is the actual height Above Ground Level (AGL) at a point is space.
Radar altimeter measures this altitude. Absolute altitude can also be calculated by
subtracting the terrain elevation from the true altitude. Weather reports, such as the
Terminal Area Forecast (TAF), Automatic Terminal Information Service (ATIS),
Automated Airport Weather Station (AAWS), and Automatic Surface Observation
System (ASOS) all report cloud ceilings in AGL. In addition, instrument approach
charts give the Height Above Touchdown (HAT), Height Above Airport (HAA), and
Threshold Crossing Height (TCH) in AGL.

Pressure Altitude (PA) is the true altitude corrected for non-standard pressures. It is
the indicated altitude when an altimeter is set to 29.92 inHg (1013 hectopascal [hPa]
or Millibar [mbar]). It is primarily used in aircraft performance calculations and in
high-altitude flight (above 18,000 feet in the U.S.). PA rarely equals true altitude.

Density Altitude (DA) is the PA corrected for nonstandard temperature. Since air
density decreases with an increase in temperature and/or altitude there will be a
negative impact on aircraft performance. The effect of increased DA on aircraft
performance must be accounted for and is used for takeoff, hover, and landing
performance calculation.

Indicated altitude is the altitude on the altimeter when the local altimeter setting is
set by the pilot. This is expressed as inHg in the United States or as mbar or hPa in
other regions of the world. On the ground when the altimeter is set to the airport
elevation, the indicated altitude should be the same as the true altitude and the setting
in barometric altimeter should match the current airfield altimeter setting. While
flying, indicated altitude will differ from true altitude unless the aircraft altimeter
setting is continually updated to match the local altimeter settings. For safety of
flight, it is critical that the pilot properly sets the altimeter and periodically checks for
changes.
THE ATMOSPHERE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER TWO
205. Q-CODES
Q-codes are three-letter codes used to reference altimeter settings and are more commonly used
in other parts of the world but can still be encountered in the United States. For example, Notice
to Airman (NOTAM) are still transmitted using Q-codes. The Flight Information Handbook
(FIH) contains all the Q-code definitions. The following are a few examples:

QNH is the barometric pressure as reported by an airport or station. This is the
barometric pressure setting most often used in the United States below 18,000 feet.

QNE is the PA (altimeter set to 29.92).

QFE is a setting at an airfield to read height above ground at that location (the
altimeter would read zero at the airfield surface).
206. ALTIMETER SETTINGS
Checking for changes in altimeter setting during cross-country flights or as weather changes in
the local area is vital. Air traffic controllers, automated weather reporting sources, and some
electronic kneeboard applications provide the current altimeter setting at airfields and weather
stations near the route of flight.
When station pressure or ambient temperature decreases, an altimeter set at the previous
condition will display the same indicated altitude (no change in the altimeter reading), but the
true altitude of the aircraft will be lower. The aircraft is flying lower than indicated. The only
way to minimize this error is to adjust the altimeter setting. The opposite occurs when the station
pressure or temperature rises.
THE ATMOSPHERE 2-7
CHAPTER TWO
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 2-4 Pressure Station Changes, Indicated Altitude Vs True Altitude
Figure 2-4 shows the following example: if the pilot sets the altimeter to the station pressure of
29.92 inHg (standard atmosphere) and climbs to 1,000 feet MSL, the aircraft would be at
pressure of 28.92 inHg. If the aircraft flies into a high pressure system of 30.92 inHg and the
pilot does not adjust the altimeter setting, the altimeter would read the same altitude of 1,000 feet
MSL but the aircraft true altitude would be 2,000 feet MSL (the indicated altitude lower than the
true altitude). If the flight then proceeds into a low-pressure area of 28.92 inHg, and the pilot
again did not update the altimeter setting, the altimeter would continue to indicate the initial
altitude of 1,000 feet MSL but this time the true altitude would be zero feet (indicated altitude
higher than the true altitude.)
2-8

From Low Pressure to High Pressure – True Altitude Increases

From High Pressure to Low Pressure – True Altitude Decreases

From Cold to Hot Temperature – True Altitude Increases

From Hot to Cold Temperature – True Altitude Decreases (Figure 2-4)
THE ATMOSPHERE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER TWO
Figure 2-4 Air Temperature Changes, Indicated Altitude Vs True Altitude
207. DENSITY ALTITUDE CHARTS
By finding ambient PA (setting 29.92 in the barometric altimeter) and noting the current Outside
Air Temperature (OAT), an aviator can determine DA. If you know the OAT and humidity, and
you can determine the PA, you can derive the DA.
Typically, aviators use a chart to determine DA for the ambient PA and temperature, as in Figure
2-5. First, the pilot dials 29.92 into the altimeter to determine the current PA. Enter the chart at
the bottom at the appropriate OAT and plot vertically upward to intersect the current PA
depicted on the diagonals. From this point, read laterally to the left to determine the DA (not
corrected for humidity). In the example depicted, a temperature of 6 °C and 2,400 feet PA,
results in 2,000 feet DA.
THE ATMOSPHERE 2-9
CHAPTER TWO
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 2-5 DA Chart
Another method for estimating DA is to use the rule of thumb equation. The rule of thumb
merely requires an accurate understanding of standard temperature at altitude. Remember that
the temperature at sea level for a standard day is 15 °C. With an average lapse rate of 2 °C per
1,000 feet MSL, the standard temperature at altitude can be easily determined (i.e., 5 °C at 5,000
feet MSL and -5 °C at 10,000 feet MSL).
𝑫𝑨 = (𝑶𝑨𝑻 − 𝑰𝑺𝑨) × 𝟏𝟐𝟎 + 𝑷𝑨
In this equation, OAT is the ambient temperature in degrees Celsius. PA is the altitude. ISA is
the standard temperature at PA in degrees Celsius. The ISA can be determined from the ISA
chart or calculated by subtracting two times the current altitude in thousands of feet from 15 C
2-10 THE ATMOSPHERE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER TWO
(the standard temperature at sea level).
For example:
PA = 6,000 feet
OAT 13 C
ISA @ 6,000 feet = 15 − (2 × 6) = 3C
DA = (13-3) × 120 + 6,000 = 7,200 feet DA
Changes in the water vapor content, or humidity, can also greatly affect the density of the air. As
humidity increases, water molecules with less mass, but approximately the same volume as air
molecules, displace the denser air molecules to make the same overall volume containing less
actual mass. Thus, an increase in humidity leads to a decrease in air density and, therefore, an
increase in DA.
One way to adjust calculations for humidity is to use a higher temperature that might be
associated with lower density in performance charts. This fictional quantity is known as virtual
temperature, and is defined as OAT corrected for RH. In the same way that wind chill is applied
to a cold day’s temperature to reflect how the wind affects the human body, a virtual temperature
correction may be applied to a temperature measurement to reflect the effect of humidity on the
air’s density. The dew point temperature correction chart accepts dew point and temperature,
and then yields virtual temperature and DA. See Figure 2-6.
Figure 2-6 Dew Point Correction Chart
THE ATMOSPHERE 2-11
CHAPTER TWO
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Moisture in the air can be slightly beneficial in controlling engine temperature, but reduced air
density decreases the amount of lift the rotor system can produce. For this reason, the overall
effect of humidity degrades helicopter performance.
2-12 THE ATMOSPHERE
THEORIES OF LIFT
300. INTRODUCTION
This chapter provides a review of basic mechanics and atmospheric principles relevant to
helicopter aerodynamics by introducing useful concepts about the principles of helicopter
aerodynamics and helicopter flight. Mechanics is the study of the motion of bodies under the
actions of forces and of how objects interact with the forces applied to them. Using the flight
controls, the pilot changes the helicopter path and flight parameters by shifting the forces applied
to it. By understanding some basic mechanics, the pilot can predict what control inputs will be
necessary to maneuver the helicopter while remaining in control of it. When necessary, review
the API Fundamentals of Aerodynamics Student Guide.
301. LEARNING OBJECTIVES

Review basic aerodynamics terminology.

Define vector analysis.

Define the aircraft reference system.

Identify forces acting on an aircraft.

Define classic aerodynamic principles.

Define general theories of lift.
302. VECTOR ANALYSIS
Vector analysis is a means to represent various interacting forces.
A scalar is a quantity that describes only magnitude (e.g., time, speed, temperature, density, or
volume). It is expressed as a single number including units. In aerodynamics, scalars are mostly
used to describe the characteristics of the atmosphere.
A vector is a quantity that describes both magnitude and direction. It commonly represents
displacement, velocity, acceleration, or force. Vectors are represented as arrows. The direction
and length of the arrow represent the direction and magnitude of the vector. Vectors may be
added by placing the tail of each succeeding vector on the head (or tip) of the one preceding it,
and drawing the resultant vector from the tail of the first to the tip of the last, as in Figure 3-1.
This new vector is the resulting magnitude and direction of all the original vectors working
together.
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CHAPTER THREE HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 3-1 Resultant Vector Using the Tip to Tail Method
Conversely, a vector may be deconstructed in two or more component vectors that lie in a
desired plane of motion or direction. See Figure 3-2.
Figure 3-2 Force and Velocity Vectors on a Helicopter Blade
Vectors are largely used to describe the aerodynamic forces, speeds, and accelerations that act on
a helicopter, its rotors, and its blades (Figure 3-3).
3-2
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER THREE
Figure 3-3 Force Vectors on a Helicopter in Flight
303. AIRCRAFT REFERENCE SYSTEM
An aircraft (helicopter or fixed-wing) reference system (Figure 3-4) consists of three mutually
perpendicular axes intersecting at a single point, called the Center of Gravity (CG).

The CG is the point at which all weight is concentrated and at which all forces are
measured. Theoretically, the aircraft will balance if suspended at the CG. When in
flight, the aircraft will rotate about the CG, so all moments will be resolved around it
as well. The CG will move as fuel burns, weapons are expended, or cargo shifts.

The longitudinal axis passes from the nose to the tail of the aircraft. Rotation around
the longitudinal axis is roll, or lateral control.

The lateral axis passes from wingtip to wingtip. Rotation around the lateral axis is
pitch, or longitudinal control.

The vertical axis passes vertically through the CG. Rotation around the vertical axis
is yaw, or directional control. As an aircraft moves through the air, the axes move
with it. Therefore, the movement of the aircraft can be described by the movement of
its CG.
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CHAPTER THREE HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 3-4 Aircraft Reference System
304. FORCES ACTING ON THE BLADE
Gravity acting on the mass of an object creates a force called weight. The rotor blade in Figure
3-5 weighs 100 pounds. It is 20 feet long (span) and is 5 feet wide (chord). Accordingly, its
surface area is 100 square feet.
Figure 3-5 Area of Blade
The blade is perfectly balanced on a pinpoint stand, as seen in Figure 3-6, looking at it from the
blade tip (the airfoil view). The goal is for the blade to defy gravity and stay exactly where it is
when we remove the stand. If nothing is done before removing the stand, the blade will simply
fall to the ground.
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER THREE
Figure 3-6 Profile of an Airfoil
Every object in the atmosphere is surrounded by a gas that exerts a static pressure of 14.70
pounds per square inch (2,116 pounds per square foot) at sea level. That pressure is exerted
equally all over the blade (top and bottom) and, therefore, does not create any useful force on the
blade. A difference of a single pound of static pressure differential per square foot of blade
surface will have a force equal to the blade’s weight (100 pounds of upward pressure opposite
100 pounds of downward weight).
Total pressure consists of static pressure and, if the air is moving, dynamic pressure (a pressure
in the direction of the air movement). If dynamic pressure is increased, the static pressure will
decrease. Due to the design of the airfoil, the velocity of the air passing over the upper surface
will be greater than that of the lower surface, leading to higher dynamic pressure on the upper
surface than on the lower surface. The higher dynamic pressure on the upper surface lowers the
static pressure on the upper surface. The static pressure on the bottom will now be greater than
the static pressure on the top and the blade will experience an upward force. This upward force
is the primary source of lift that allows any aircraft with an airfoil to fly. With just the right
amount of air passing over the blade, the upward force will equal one pound per square foot.
This upward force is equal to, and acts opposite to, the blade’s weight of 100 pounds in Figure
3-6. Under these conditions, if the stand were removed, the blade would remain in the same
position.
The force created by air moving over an object (or moving an object through the air) is called
aerodynamic force (Figure 3-7). Accordingly, by moving the air over an airfoil we can change
the static pressures on the top and bottom, thereby generating a useful aerodynamic force. The
portion of the aerodynamic force that is measured perpendicular to the air flowing around the
airfoil is called lift and is used to oppose weight. Drag is the portion of aerodynamic force that is
measured as the resistance created by an object passing through the air (or having the air passed
over it). Drag acts in a streamwise direction with the wind passing over the airfoil and retards
forward movement.
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CHAPTER THREE HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 3-7 Aerodynamic Force
305. FORCES ACTING ON THE HELICOPTER
Once a helicopter leaves the ground, it is acted upon by four aerodynamic forces thrust, drag, lift,
and weight. Understanding how these forces work and knowing how to control them with the
use of power and flight controls are essential to flight.
The four aerodynamic forces acting on the helicopter (Figure 3-8) are defined as follows:
3-6

Lift opposes the downward force of weight and is produced by the dynamic effect of
the air acting on the airfoil. Lift acts perpendicular to the flightpath. It opposes
weight and acts vertically upward through the blade center of pressure.

Weight is the combined load of the aircraft, the crew, the fuel, and the cargo or
baggage. Weight pulls the aircraft downward because of the force of gravity. It
opposes lift and acts vertically downward through the aircraft’s CG.

Thrust is the force produced by the rotor. It opposes or overcomes the force of drag.
As a rule, it acts parallel to the longitudinal axis.

Drag is a rearward, retarding force caused by the disruption of airflow over the wing,
rotor, fuselage, and other protruding objects. Drag opposes thrust and acts rearward
parallel to the relative wind.
THEORIES OF LIFT
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER THREE
Figure 3-8 Forces Acting on a Helicopter in Forward Flight
1.
Lift
Lift is generated when an object changes the direction of flow of a fluid or when the air is forced
to move by the object passing through it, as depicted in Figure 3-9. When the object and air
move relative to each other and the object turns the airflow in a direction perpendicular to that
flow, the force required to do this work creates an equal and opposite force that is lift. The
object may be moving through stationary air, or air may be flowing past a stationary object.
These two are effectively identical in principle; it is only the frame of reference that differs.
Figure 3-9 Production of Lift
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CHAPTER THREE HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
The lift generated by an airfoil depends on the following factors:

Speed of the airflow

Density of the air

Total area of the segment or airfoil

Angle of Attack (AOA) between the air and the airfoil
The AOA is the angle at which the airfoil meets the oncoming airflow (or vice versa). Again, in
the case of a helicopter, the object is the rotor blade (airfoil) and the fluid is the air. Lift is
produced when a mass of air is deflected, and it always acts perpendicular to the resultant
relative wind. A symmetric airfoil must have a positive AOA to generate positive lift. At a zero
AOA, no lift is generated. At a negative AOA, negative lift is generated. A cambered or
nonsymmetrical airfoil may produce positive lift at zero, or even small negative AOA.
The basic concept of lift is simple, however, the details of how the relative movement of air and
airfoil interact to produce the turning action that generates lift are complex. In any case causing
lift, the flow meeting the leading edge of the object is forced to split over and under the object.
The sudden change in direction over the object causes an area of low pressure to form behind the
leading edge on the upper surface of the object. In turn, due to this pressure gradient and the
viscosity of the fluid, the flow over the object is accelerated down along the upper surface of the
object. At the same time, the flow forced under the object is rapidly slowed or stagnated causing
an area of high pressure. This also causes the flow to accelerate along the upper surface of the
object. The two sections of the fluid each leave the trailing edge of the object with a downward
component of momentum.
2.
Weight
Normally, weight is thought of as being a known, fixed value, such as the weight of the
helicopter, fuel, and occupants. To lift the helicopter off the ground vertically, the rotor disk
must generate enough lift to overcome or offset the total weight of the helicopter and its
occupants. Newton’s first law states, “Every object in a state of uniform motion tends to remain
in that state of motion unless an external force is applied to it.” In this case, the object is the
helicopter, whether in a hover or on the ground, and the external force applied to it is lift, which
is accomplished by increasing the pitch angle of the main rotor blades. This action forces the
helicopter into a state of motion. Without increasing lift, the helicopter would remain either on
the ground or at a hover.
Aerodynamic loads can also influence the weight of the helicopter. This weight is known as
apparent weight. When you bank a helicopter while maintaining a constant altitude, the load
factor (G load) increases. The load factor is the actual load on the rotor blades at any time,
divided by the normal load or gross weight (weight of the helicopter and its contents). Any time
a helicopter flies in a constant altitude curved flightpath, the load supported by the rotor blades is
greater than the total weight of the helicopter. The tighter the curved flightpath, the steeper the
bank, or the more rapid the flare or pullout from a dive is, the greater the load supported by the
3-8
THEORIES OF LIFT
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER THREE
rotor. Therefore, the load factor must also be greater. See Figure 3-10
Figure 3-10 Load Factor Vs Angle of Bank (AOB) Diagram
To overcome this additional load factor, the helicopter must be able to produce more lift. If
excess engine power is not available, the helicopter either descends or an increase in aircraft
pitch attitude and subsequent reduction in airspeed is required to maintain the same altitude. The
load factor and, hence, apparent gross weight increase, is relatively small in banks up to 30
degrees. Even so, under the right set of adverse circumstances, such as high DA, turbulent air,
high gross weight, or poor pilot technique, sufficient or excess power may not be available to
maintain altitude and airspeed. Pilots must take all of these factors into consideration throughout
the entire flight from the point of ascending to a hover to landing. Above 30 degrees of bank, the
apparent increase in gross weight rises rapidly. At 30 degrees of bank or pitch, the apparent
gross weight and actual load on the rotor disk increase is only 16 percent, but at 60 degrees it is
twice the aircraft gross weight.
For example, if the weight of the helicopter is 1,600 pounds, the weight supported by the rotor
disk in a 30 degree bank at a constant altitude would be 1,856 pounds (1,600 + 16 percent [or
256]). In a 60 degree bank, it would be 3,200 pounds. In an 80 degree bank, it would be almost
six times as much, or 8,000 pounds. It is important to note that each rotor blade must support a
percentage of the gross weight. In a two-bladed system, each blade of the 1,600-pound
helicopter as stated above would have to lift 50 percent or 800 pounds. If this same helicopter
had three rotor blades, each blade would have to lift only 33 percent, or 533 pounds. One
additional cause of large load factors is rough or turbulent air. The severe vertical gusts
produced by turbulence can cause a sudden increase in AOA, resulting in increased rotor blade
loads that are resisted by the inertia of the helicopter.
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CHAPTER THREE HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Each type of helicopter has its own limitations that are based on the aircraft structure, size, and
capabilities. Regardless of how much weight a helicopter can carry or the engine power it may
have, all helicopters are all susceptible to aerodynamic overloading. If the pilot attempts to push
the performance envelope, the consequences can be fatal. Pilots need to truly understand the
capabilities of the helicopter under any and all circumstances and plan never to exceed the flight
envelope for any situation.
3.
Thrust
Thrust, like lift, is generated by the rotation of the main rotor disk. In a helicopter, thrust can be
forward, rearward, sideward, or vertical. The pilot flies the helicopter by controlling the amount
of thrust the main rotor disk produces, via collective inputs, and by directing the thrust, via cyclic
inputs.
The thrust produced by the main rotor disk depends on how much engine power the rotor can
absorb. The amount of engine power the main rotor can absorb is a function of the main rotor
solidity ratio. The thrust produced by the main rotor disk depends on how much engine power
the rotor can absorb. The amount of engine power the main rotor can absorb is a function of the
main rotor solidity ratio.
The solidity ratio is the ratio of the combined area of the plan form of all the blades to the disk
area. In other words, the blade area divided by disk area.
If the solidity ratio of the main rotor is increased, the rotor can absorb more power from the
engine. If it can absorb more power, it can produce more lift, so the helicopter’s performance
increases. The disadvantage is that more blade surface area equates to more profile drag, and
therefore more power is required to turn the blades at the same RPM.
Although adding more rotor blades to increase the solidity ratio creates a smoother ride, it
increases the overall weight of the rotor system and requires a larger rotor head, along with its
associated increase in parasite drag. It also reduces the distance between blade tips, which
means the blades are operating in more of the tip vortices from the preceding blade.
Helicopters designed for high speed or high maneuverability require a main rotor with a higher
solidity ratio.
Many helicopter accidents are caused by the rotor disk being overloaded. Simply put, pilots
attempt maneuvers that require more lift than the rotor disk can produce, or more power than the
helicopter’s powerplant can provide. Trying to land with a nose high attitude along with any
other unfavorable condition (i.e., high gross weight or wind gusts) may end in disaster.
The tail rotor also produces thrust. The amount of tail rotor thrust is controlled by using the antitorque pedals which controls the helicopter’s yaw.
4.
Drag
The force that resists the movement of a helicopter through the air is called drag. In order to turn
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER THREE
the rotor, the engine must overcome drag. Drag always acts parallel to the relative wind. Total
drag is composed of three types of drag profile, induced, and parasite.
a.
Profile Drag
Profile drag develops from the resistance of the blades rotating through the air, and it
is not related to the production of lift. It does not change significantly with the
airfoil’s AOA but increases moderately when airspeed increases. Profile drag is
composed of form drag (the shape of the airfoil) and skin friction (smoothness of the
airfoil surface). Form drag results from the turbulent wake caused by the separation
of airflow from the surface of a structure.
Figure 3-11 shows how the shape of an object will change the amount of form drag it
generates. The amount of drag is related to both the size and shape of the structure
that protrudes into the relative wind.
Figure 3-11 Form Drag
Skin friction is caused by surface roughness (Skin friction is caused by surface
roughness (see Figure 3-12). Even though the surface appears smooth, it may be
quite rough when viewed under a microscope. A thin layer of air clings to the rough
surface and creates small eddies that contribute to drag.
THEORIES OF LIFT
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CHAPTER THREE HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 3-12 Skin Friction
b.
Induced Drag
Induced drag (Figure 3-13) is generated by the airflow circulation around the rotor
blade as it creates lift. The high-pressure area beneath the blade joins the lowpressure area above the blade at the trailing edge and at the rotor tips. This causes a
spiral, or vortex, which trails behind each blade whenever lift is being produced.
These vortices deflect the airstream downward in the vicinity of the blade, creating an
increase in downwash. Therefore, the blade operates in an average relative wind that
is inclined downward and rearward near the blade. Because the lift produced by the
blade is perpendicular to the relative wind, the lift is inclined aft by the same amount.
The component of lift that is acting in a rearward direction is induced drag.
Figure 3-13 Induced Drag
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As the air pressure differential increases with an increase in AOA, stronger vortices
form, and induced drag increases. Because the blade’s AOA is usually lower at
higher airspeeds, and higher at low speeds, induced drag decreases as airspeed
increases and increases as airspeed decreases. Induced drag is the major cause of
drag at lower airspeeds.
c.
Parasite Drag
Parasite drag is present any time the helicopter is moving through the air. This type
of drag increases with airspeed. Non-lifting components of the helicopter, such as the
cabin, rotor mast, tail, and landing gear, contribute to parasite drag. Any loss of
momentum by the airstream, due to such things as openings for engine cooling,
creates additional parasite drag. Because of its rapid increase with increasing
airspeed, parasite drag is the major cause of drag at higher airspeeds. Parasite drag
varies with the square of the velocity; therefore, doubling the airspeed increases the
parasite drag four times.
d.
Total Drag
As airspeed increases, parasite drag increases, while induced drag decreases. Curve
A in Figure 3-14 shows that parasite drag is very low at slow airspeeds and increases
with higher airspeeds. Parasite drag goes up at an increasing rate at airspeeds above
the midrange.
Profile drag remains relatively constant throughout the speed range with some
increase at higher airspeeds. Curve B in Figure 3-14 shows the profile drag curve.
Combining all drag forces results in a total drag curve. Curve D in Figure 3-14 shows
the total drag curve and represents the sum of the three curves in the chart. The low
point (E) on the total drag curve shows the airspeed at which drag is minimized. This
is the point where the lift-to-drag ratio is greatest and is referred to as L/DMAX. At
this speed, the total lift capacity of the helicopter, when compared to the total drag of
the helicopter, is most favorable. This is an important factor in helicopter
performance.
Figure 3-14 Drag Curve
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CHAPTER THREE HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
306. LIFT THEORIES
The purpose of lift theory is to explain how lift is created by an object moving through air.
Propel anything fast enough and lift will be generated. The critical factor is the magnitude and
direction of lift. Lift can be generated in any direction and it may not be sufficient to overcome
the weight of the object, or it may be negative lift. Depending upon the shape of the object, it
might also be creating an enormous amount of drag and expending great quantities of energy
while generating lift.
Several theories developed during the last two hundred years have attempted to explain the
generation of lift by an airfoil. These theories are Pressure Distribution, Circulation Theory,
Momentum Theory, and Blade Element Theory. Each theory has a set of specific constraints and
approximations, which limits its applicability. There will be situations where one method is
more convenient to use than another and other situations that demand one specific method of
understanding. Bernoulli’s principle, the Venturi principle, and Newton’s third law of motion
are the principles on which these theories of lift are based.
1.
Bernoulli’s Principle
Bernoulli’s principle describes the relationship between internal fluid pressure and fluid velocity.
It is a statement of the law of conservation of energy and helps explain why an airfoil develops
an aerodynamic force. The concept of conservation of energy states that energy cannot be
created or destroyed, and the amount of energy entering a system must equal the amount of
energy leaving the system. In aerodynamics, the energy referred to is dynamic pressure (kinetic
energy) of the air where more velocity equals more kinetic energy, and the static air pressure
(potential energy). The proportion of these energies may vary, but the total energy remains
constant inside the system.
A simple tube containing a fluid with a constricted portion near the center of its length illustrates
this principle. The mass of flow per unit area (cross-sectional area of tube) is the mass flow rate.
In Figure 3-15, the flow entering into the tube is constant, neither accelerating nor decelerating.
The mass flow rate through the tube must be the same at Stations 1, 2, and 3. The rate at which
mass enters the tube is equal to the rate at which mass leaves the system (continuity principle).
If the cross-sectional area at any one of these stations in the tube is reduced (Station 2), the fluid
velocity must increase to maintain a constant mass flow rate while moving the same amount of
fluid through a smaller area. The continuity of mass flow causes the air to move faster through
the narrower portion. In other words, fluid speeds up in direct proportion to the reduction in
area. The narrow portion of the tube is called a venturi.
Figure 3-15 Bernoulli’s Principle
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The Bernoulli principle (Ptotal = Pdynamic + Pstatic) states that the increase in velocity will
increase the stream wise dynamic pressure. Since the total pressure in the tube must remain
constant, the static pressure in the venturi section will decrease. Venturi effect is the term used
to describe this phenomenon.
2.
Venturi Principle
While the amount of total energy within a closed system (the tube) does not change, the form of
the energy may be altered. The pressure of flowing air may be compared to energy in that the
total pressure of flowing air always remains constant unless energy is added or removed. Fluid
flow pressure has two components: static and dynamic pressure. Static pressure is the force per
unit area acting on a surface. Dynamic pressure of flow is the component of pressure existing
because of the movement of fluid. The sum of these two pressures is total pressure. As fluid
flows through the venturi, dynamic pressure increases as velocity increases, causing a decrease
in static pressure.
In Figure 3-16, the bottom half of the constricted area of the tube resembles the top half of an
airfoil. Note in the image PSF = Pounds Square Feet. Even with the upper portion of the tube
removed, air accelerates over the curved area because the upper air layers restrict the flow, just
as the top half of the venturi section of the tube does. This acceleration causes decreased static
pressure above the curved portion and creates a pressure differential. The decrease of pressure
occurring on the top of the airfoil generates lift, causing it to rise.
Figure 3-16 Venturi Effect
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CHAPTER THREE HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
3.
Newton’s Third Law of Motion
Additional lift is generated by an airfoil’s lower surface as air striking it is deflected downward.
According to Newton’s third law of motion, “for every action there is an equal and opposite
reaction.” The air that is deflected downward also produces an upward (lifting) force on the
airfoil.
Under most flying conditions, the impact pressure and the deflection of air from the lower
surface of the rotor blade provides a comparatively small percentage of the total lift. The
majority of lift is the result of decreased pressure above the blade, rather than the increased
pressure below it.
4.
Pressure Distribution Theory
Pressure distribution theory evolves from the principle of continuity, and the principle of
conservation of energy as applied to fluid dynamics (Bernoulli’s principle). Putting the
Bernoulli principle together with the continuity principle, the area decreases, the velocity
increases, and as the velocity increases the static pressure decreases.
If static pressure decreases over the top of a wing more so than over the bottom, the wing will be
lifted up. This differential in pressure is accounted for by the continuity principle and the
Bernoulli principle. It is the method of choice when describing the mechanics of lift by the
pressure distribution theory.
5.
Circulation Theory
Circulation theory, also known as the Kutta-Joukowski Theorem, is a method for describing flow
over a spinning cylinder. If a non-rotating cylinder is placed in a flow field, it will produce no
lift. When the cylinder is rotated, however, it induces a rotational or circulatory flow, and there
is a distinct change in the streamlines and pressure distributions. If the cylinder rotates in the
direction of the airflow, the air next to the surface of the cylinder is sped up on the top and
slowed down on the bottom by the relative motion of the cylinder’s surface. The difference in
flow speeds causes a pressure difference between the top and bottom, with the result being a net
lifting force perpendicular to the relative velocity. This lifting force is termed the Magnus effect,
as see in Figure 3-17. A practical application of this lift theory can be found on helicopters
equipped with a NOTAR anti-torque system.
Figure 3-17 Magnus Effect
3-16 THEORIES OF LIFT
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER THREE
6.
Momentum Theory
The Momentum Theory of lift relies on Newton’s laws of motion regarding air as it passes over
an airfoil and the reaction of the airfoil to the motion of the air. In a hover, this theory states that
a specific amount of air above the rotor system is accelerated to a certain velocity at a certain
distance below the rotor generating a force as a result of Newton’s second law, “force equals
mass times acceleration” (F=ma).
Specifically, the theory shows that given an initial velocity (V0) of zero above the rotor system,
the air is accelerated downward to a particular induced velocity (Vi). The mass of air is further
accelerated to twice the induced velocity at approximately one rotor diameter distance below the
rotor (Figure 3-18).
Figure 3-18 Induced Velocity and Momentum Theory
Induced power is the portion of total power used to accelerate air downward and create lift. In
equilibrium, the lift generated by the rotor is equal to aircraft weight, so the induced power
required to hover is then a function of aircraft weight.
7.
Blade Element Theory
The blade element theory allows for a greater fidelity in understanding the action and reaction of
individual blades within a rotor disk. The forces acting on a blade vary span wise moving out
from the hub. The basis of blade element theory is to take a very small portion of the rotor blade
(Δr) and determine the forces acting on it.
Airflow conditions at the blade element in Figure 3-19 show that the blade sees a combination of
linear velocity (sometimes called linear flow) and downward induced velocity as components of
relative wind. The blade element diagram breaks down the components of each force acting on
the blade.
THEORIES OF LIFT
3-17
CHAPTER THREE HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 3-19 Blade Element Theory
3-18 THEORIES OF LIFT
AIRFOILS
400. INTRODUCTION
This chapter presents the aerodynamics of airfoils. Pilots must possess a thorough knowledge of
the forces acting on the airfoils of a helicopter and how they change through different regimes of
flight. Helicopter flight procedures are designed around the interaction between the rotor blades
and the surrounding environment. These forces and interactions are most commonly displayed
on a blade element diagram.
401. LEARNING OBJECTIVES

Describe the components, characteristics, and function of an airfoil.

Describe the components, terms, and characteristics of the rotor hub and blades.

Identify the characteristics and flight dynamics of the blade elements.

Identify the controls and characteristics of rotor blade pitch angles.

Describe the concepts and characteristics of relative wind.

Identify the effects airspeed has on rotor blades and resultant relative wind.

Describe the characteristics of ground effect and its influence on airflow.
402. AIRFOILS
Airfoil characteristics are expressed by a variety of aerodynamic features, such as coefficients of
lift and drag, the pitching moment, the zero-lift angle, the lift-drag ratio, and airfoil design
qualities, such as airfoil shape, span, twist, taper, thickness, and aspect ratio. Any aerodynamic
quality peculiar to a particular airfoil, especially to an airfoil section or profile, usually is defined
at a specified AOA. Airfoil characteristics are part of the design process.
Helicopters are able to fly due to aerodynamic forces produced when air passes around an airfoil.
An airfoil is any surface producing more lift than drag when passing through the air at a suitable
angle. Although airfoils are most often associated with the production of lift, airfoils are also
used for stability, control, and thrust or propulsion. Certain airfoils, such as rotor blades,
combine some of these functions. The main and tail rotor blades of the helicopter are airfoils,
and air is forced to pass around the blades by rotating the blades. In some conditions, parts of
the fuselage, such as the vertical and horizontal stabilizers, can become airfoils. Airfoils are
carefully structured to accommodate a specific set of flight characteristics.
AIRFOILS
4-1
CHAPTER FOUR
1.
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Airfoil Terminology
Figure 4-1 Airfoil Aerodynamic Terms
4-2

Blade span: The length of the rotor blade from center of rotation to tip of the blade

Hub: The attaching point for the root of the blade on the mast and the axis about
which the blade rotates

Tip: The farthest outboard section of the rotor blade

Root: The inner end of the blade and the point that attaches to the hub

Twist: The change in blade incidence from the root to the outer blade

Chord line: An infinitely long, straight line which passes through the leading and
trailing edges of the airfoil.

Chord: The length of the chord line from the leading edge to the trailing edge; it is
the characteristic longitudinal dimension of the airfoil section.

Mean camber line: A line drawn halfway between the upper and lower surfaces of
the airfoil, as depicted in Figure 4-1. The chord line connects the ends of the mean
camber line. Camber refers to curvature of the airfoil and subsequent curvature of the
mean camber line. The shape of the mean camber is important for determining
aerodynamic characteristics of an airfoil section. Maximum camber (displacement of
the mean camber line from the chord line) and its location help to define the shape of
the mean camber line. The location of maximum camber and its displacement from
AIRFOILS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER FOUR
the chord line are expressed as fractions or percentages of the basic chord length. By
varying the point of maximum camber, the manufacturer can tailor an airfoil for a
specific purpose. The profile thickness and thickness distribution are important
properties of an airfoil section.
2.

Leading edge: The front edge of an airfoil

Trailing edge: The rearmost edge of an airfoil

Center of pressure: The point along the chord line of an airfoil through which all
aerodynamic forces are considered to act. Since pressures vary on the surface of an
airfoil, an average location of pressure variation is needed. As the AOA changes,
these pressures change, and the center of pressure moves along the chord line.
Airfoil Types
a.
Symmetrical Airfoil
The symmetrical airfoil has identical upper and lower surfaces, as seen in Figure 4-2.
The mean camber line and chord line are the same on a symmetrical airfoil, and it
produces no lift at zero AOA. Most light helicopters incorporate symmetrical airfoils
in the main rotor blades.
b.
Nonsymmetrical Airfoil (Cambered)
The nonsymmetrical airfoil has different upper and lower surfaces, with a greater
curvature of the airfoil above the chord line than below, as seen in Figure 4-2. The
mean camber line and chord line are different. The nonsymmetrical airfoil design can
produce useful lift at zero AOA. A nonsymmetrical design has advantages and
disadvantages. The advantages are more lift production at a given AOA than a
symmetrical design, an improved lift-to-drag ratio, and better stall characteristics.
The disadvantages are that the center of pressure travels up to 20 percent of the chord
line (creating undesirable torque on the airfoil structure) and greater production costs.
Figure 4-2 Airfoil Types
AIRFOILS
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CHAPTER FOUR
c.
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Blade Element Diagram
The blade element diagram (Figure 4-3) is a representation of all the forces acting on
a blade. The following terms and definitions describe the relationships among all the
forces to enable a better understanding of blade aerodynamics as a whole.
Figure 4-3 Blade Element Diagram
4-4

Linear velocity is the horizontal component of relative wind. It is always parallel to
the tip path plane.

Induced velocity is the vertical component of relative wind. It is always
perpendicular to the tip path plane.

Relative wind represents all the wind the blade experiences. It is the sum of the
induced velocity and linear velocity.

Lift is the force produced as the blade is subject to relative wind. Lift is always
perpendicular to relative wind.

Induced drag is the horizontal portion of the lift vector. It is always perpendicular
and proportional to induced velocity.

Profile drag is the drag caused by the shape of the blade and friction between the
AIRFOILS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER FOUR
blade skin and the air. It is always parallel to the relative wind.

Aerodynamic force, sometimes called the Total Aerodynamic Force (TAF), is the
combination of all of the lift and drag forces on the blade. It is the vector sum of lift
and profile drag.
403. ROTOR BLADE ANGLES
There are two angles that enable a rotor disk to produce the lift required for a helicopter to fly the
Angle of Incidence (AOI) and the Angle of Attack (AOA).
1.
AOI/Blade Pitch Angle
AOI, commonly called the blade pitch angle, is the angle between the chord line of a blade and
the tip path plane (Figure 4-4). For fixed airfoils, such as vertical fins or horizontal stabilizer,
AOI is the angle between the chord line of the airfoil and a selected reference plane of the
helicopter.
Figure 4-4 Angle of Incidence
The AOI is a mechanical angle rather than an aerodynamic angle and is sometimes referred to as
blade pitch angle. In the absence of induced flow, AOA and AOI are the same. When induced
flow or airspeed modifies the relative wind, the AOA is different from the AOI. Collective input
and cyclic feathering change the AOI. A change in the AOI changes the AOA, which changes
the coefficient of lift, thereby changing the lift produced by the airfoil.
2.
AOA
AOA is the angle between the chord line and the relative wind (Figure 4-5). An increase in
induced velocity will result in a decrease in AOA. A decrease in AOA results in a decrease in
lift and an increase in induced drag. An increase in linear velocity will result in an increase in
profile drag, but as linear velocity increases, the AOA and lift both increase (provided induced
velocity remains constant or decreases).
AIRFOILS
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CHAPTER FOUR
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
The AOA is the angle between the airfoil chord line and relative wind. It is an aerodynamic
angle and not easy to measure. It can change with no change in the blade pitch angle.
Figure 4-5 Angle of Attack
When the AOA is increased, air flowing over the airfoil is diverted over a greater distance,
resulting in an increase of air velocity and more lift. As the AOA is increased further, it becomes
more difficult for air to flow smoothly across the top of the airfoil. At this point, the airflow
begins to separate from the airfoil and enters a burbling or turbulent pattern. The turbulence
results in a large increase in drag and loss of lift. Increasing the AOA increases lift until the
critical AOA is reached. Any increase in the AOA beyond this point will result in the airfoil
stalling and a rapid decrease in lift (Figure 4-6).
Figure 4-6 Blade Stall at Increased AOA
Several factors may change the rotor blade AOA, including wind, the movement of the blades as
they rotate, and pilot control inputs. Pilots adjust AOA through control manipulation of the pitch
angle of the blades. If the pitch angle is increased, the AOA increases; if the pitch angle is
reduced, the AOA is reduced. Blade feathering allows these changes. Feathering is the rotation
4-6
AIRFOILS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER FOUR
of the blade about its longitudinal axis by collective and cyclic inputs, causing changes in blade
pitch angle.
Collective feathering changes the blade pitch angle equally and in the same direction on all rotor
blades simultaneously. This action changes AOA, which changes Coefficient of Lift (CL), and
affects overall lift of the rotor disk.
Cyclic feathering changes the blade’s pitch angle, and consequently the AOA, differentially
around the rotor disk and creates differential lift. Aviators use cyclic feathering to control the
attitude of the rotor disk. Doing so changes the lift (thrust) vector generated by the rotor but
does not change the amount of net lift the rotor disk is producing.
AOA changes do not come only from cyclic and collective inputs. Airspeed, rate of climb, rate
of descent and flapping also change the blade AOA.
3.
Blade Twist
Because of the lift differential due to differing rotational relative wind values along the blade, the
blade should be designed with a twist to alleviate internal blade stress and distribute the lifting
force more evenly along the blade. Blade twist (Figure 4-7) provides higher pitch angles at the
root where velocity is low and lower pitch angles nearer the tip where velocity is higher. This
increases the induced air velocity and blade loading near the inboard section of the blade.
Figure 4-7 Blade Twist
The angular position of the main rotor blades when viewed from above as they rotate about the
vertical axis of the mast (Figure 4-8) is measured from the helicopter’s longitudinal axis, and
usually referenced from the nose of the helicopter.
AIRFOILS
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CHAPTER FOUR
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 4-8 Angular Position of Main Rotor Blades
The radial position of a segment of the blade is the distance from the hub as a fraction of the total
distance (Figure 4-9).
4-8
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER FOUR
Figure 4-9 Radial Position of Main Rotor Blades
404. RELATIVE WIND
Knowledge of relative wind is essential for an understanding of aerodynamics and its practical
flight application for the pilot. Relative wind (Figure 4-10) is the airflow relative to an airfoil.
Movement of an airfoil through the air creates relative wind. Relative wind moves in a direction
parallel, but opposite of the movement of the airfoil.
Figure 4-10 Relative Wind
AIRFOILS
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CHAPTER FOUR
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
There is a horizontal and a vertical component to wind passing a rotor blade. The horizontal
component (Figure 4-11) is caused by the blades turning plus movement of the helicopter
through the air. This is also called linear flow.
Figure 4-11 Horizontal Component of Relative Wind
The vertical component is caused by the air being forced down through the rotor blades plus any
movement of the air relative to the blades caused by the helicopter climbing or descending,
commonly called induced flow.
1.
Rotational Relative Wind
The rotation of rotor blades as they turn about the mast produces rotational relative wind. The
term rotational refers to the method of producing relative wind. Rotational relative wind Figure
4-12) flows opposite the physical flightpath of the airfoil, striking the blade at 90 degrees to the
leading edge and parallel to the tip path plane of rotation. It is constantly changing in direction
during rotation. Rotational relative wind velocity is highest at blade tips, decreasing uniformly
to zero at the axis of rotation (center of the mast).
Figure 4-12 Rotational Relative Wind
4-10 AIRFOILS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
2.
CHAPTER FOUR
Induced Flow (Downwash)
At flat pitch, air leaves the trailing edge of the rotor blade in the same direction it moved across
the leading edge; no lift or induced flow is being produced. As blade pitch angle increases, the
rotor disk induces a downward flow of air through the rotor blades creating a downward
component of air that is added to the rotational relative wind. Because the blades are moving
horizontally, some of the air is displaced downward. The blades travel along the same path and
pass a given point in rapid succession. The rotor blade action changes the still air to a column of
descending air, therefore, each blade has a decreased AOA due to the downwash. This
downward flow of air (Figure 4-13) is called induced flow (downwash). It is most pronounced at
a hover under no-wind conditions.
Figure 4-13 Induced Flow
3.
Resultant Relative Wind
The resultant relative wind in a hover (Figure 4-14) is rotational relative wind modified by
induced flow. The relative wind in a hover is inclined downward at some angle and opposite the
effective flightpath of the individual blade, rather than the physical flightpath of the helicopter as
a whole. The resultant relative wind also serves as the reference plane for development of lift,
drag, and TAF vectors on the airfoil.
AIRFOILS
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CHAPTER FOUR
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 4-14 Resultant Relative Wind
When the helicopter is in horizontal motion, airspeed further modifies the resultant relative wind.
Airspeed generates a relative wind that changes the resultant relative wind. Airspeed is added to,
or subtracted from, the rotational relative wind depending on whether the blade is advancing or
retreating in relation to helicopter movement, as depicted in Figure 4-15.
4-12 AIRFOILS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER FOUR
Figure 4-15 Indicated Airspeed Effects on Relative Wind
Airspeed also modifies induced flow. Generally, the downward velocity of induced flow is
reduced. The pattern of air circulation through the disk changes when the aircraft is in horizontal
motion. As the helicopter gains airspeed, the addition of forward velocity results in decreased
induced flow velocity and an increase in AOA (Figure 4-16). This change results in improved
efficiency and additional lift production from a given blade pitch setting.
AIRFOILS
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CHAPTER FOUR
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 4-16 Induced Flow during Transition to Forward Flight
4.
In Ground Effect
In Ground Effect (IGE) is the increased efficiency of the rotor disk caused by airflow
interference when near the ground (Figure 4-17). As the helicopter approaches the ground, the
downwash begins to strike the ground. This causes the air pressure or density to increase, which
acts to decrease the downward velocity of air. Ground effect decreases the induced drag, causing
4-14 AIRFOILS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER FOUR
the resultant relative wind to be more horizontal and the lift vector to be more vertical. These
conditions allow the rotor disk to be more efficient.
Figure 4-17 In Ground Effect
Since the induced flow velocity decreases, the AOA encountered at a specific blade pitch
increases. Since the AOA required to hover is the same whether IGE or Out of Ground Effect
(OGE), the pilot can use a lower pitch angle to create the same amount of lift. The closer the
helicopter is to the ground, the greater the ground effect will be. Maximum ground effect is
achieved when hovering over smooth hard surfaces. When hovering over surfaces such as tall
grass, trees, bushes, rough terrain, and water, maximum ground effect is reduced. For most
helicopters, ground effect increases rotor efficiency up to a height of about one rotor diameter
(measured from the ground to the rotor disk).
5.
Out of Ground Effect
The benefit of placing the helicopter near the ground is lost above IGE altitude. Above this
altitude (approximately one rotor diameter), the power required to hover remains nearly constant,
given similar conditions (such as wind), as in Figure 4-18. Induced flow velocity increases,
resulting in a decrease in AOA for a specific blade pitch and a decrease in lift. The pilot will
need to use a greater blade pitch angle to maintain the same AOA as in IGE hover. The
increased blade pitch angle also creates more induced drag. As a result, HOGE requires more
power than HIGE.
AIRFOILS
4-15
CHAPTER FOUR
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 4-18 Out of Ground Effect
4-16 AIRFOILS
POWERED FLIGHT
500. INTRODUCTION
This chapter introduces powered flight analysis and associated effects. Powered flight regimes
in a rotorcraft include hovering, vertical, forward, turning, sideward, and rearward flight.
501. LEARNING OBJECTIVES

Recall the aerodynamic phenomena that affect rotorcraft during powered flight.

Define the aerodynamic phenomena that affect rotorcraft during hovering flight.

Define the aerodynamic phenomena that affect rotorcraft during forward flight.

Define the aerodynamic phenomena that affect rotorcraft during sideward, rearward,
and turning flight.
502. HOVERING FLIGHT
Hovering is the most challenging part of flying a helicopter. This is because a helicopter
generates its own gusty air while in a hover, which acts against the fuselage and the rotor system.
The result is constant control inputs and corrections by the pilot to keep the helicopter where it is
required to be. Despite the complexity of the task, the control inputs in a hover are simple. The
cyclic is used to eliminate drift in the horizontal plane, controlling forward, backward, right, and
left movement or travel. The collective is used to maintain altitude. The anti-torque pedals are
used to control nose direction or heading. It is the interaction of these controls that makes
hovering difficult, since an adjustment in any one control requires an adjustment of the other
two, creating a cycle of constant correction.
During hovering flight, a helicopter maintains a constant position over a selected point, usually a
few feet above the ground. The ability of the helicopter to hover comes from a balance between
the lift component of the main rotor thrust, which is the force developed by the main rotors to
overcome gravity (the aircraft weight) and the horizontal component of the main rotor thrust,
which acts horizontally to accelerate or decelerate the helicopter in the desired direction. Pilots
direct the thrust of the rotor disk by using the cyclic to tilt the rotor disk plane relative to the
horizon in order to move the helicopter or to compensate for the wind and hold a position.
During a stationary hover in a no-wind condition, all opposing forces (lift, thrust, drag, and
weight) are in balance: they are equal and opposite (Figure 5-1).
POWERED FLIGHT
5-1
CHAPTER FIVE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 5-1 No-Wind Hover
While hovering, the amount of main rotor thrust can be adjusted to maintain a desired hovering
height. This is done by moving the collective and thus changing the pitch of the main rotor
blades, and hence the blade’s AOA. However, changing the AOA also changes the lift and drag
produced by the rotor blades. If AOA increases, so do lift and drag. As drag increases, the
engines must produce more power to maintain a constant rotor speed.
The weight that must be supported during hover flight is the total weight of the helicopter, its
onboard fuel, occupants, and cargo. If the amount of lift is greater than the actual weight, the
helicopter accelerates upwards until the lift force equals the weight of the helicopter; if lift is less
than weight, the helicopter accelerates downward.
The drag affecting a hovering helicopter is mainly induced drag incurred while the blades are
producing lift. There is, however, some profile drag on the blades as they rotate through the air
and a small amount of parasite drag from the non-lift-producing surfaces of the helicopter, such
as the rotor hub, cowlings, and landing gear.
An important consequence of producing thrust is torque. As discussed earlier, Newton’s third
law states, “for every action there is an equal and opposite reaction.” Therefore, as the engine
turns the main rotor disk in a counterclockwise direction, the helicopter fuselage wants to turn
clockwise. The amount of torque is directly related to the amount of engine power being used to
turn the main rotor disk. As power changes, the torque changes.
To counteract this torque-induced turning tendency, an anti-torque system is incorporated into
helicopter designs. The pilot can vary the amount of thrust produced by the anti-torque system
in relation to the amount of torque produced by the engine. As the engine supplies more power
to the main rotor, the anti-torque system must produce more thrust to overcome the increased
torque effect. This control change is accomplished using anti-torque pedals.
5-2
POWERED FLIGHT
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
1.
CHAPTER FIVE
Translating Tendency
During hovering flight, a single main rotor helicopter tends to move in the direction of tail rotor
thrust. This lateral (or sideward) movement is called translating tendency. For helicopters
configured with a counterclockwise rotating main rotor, the translating tendency may be
overcome by using one of the following designs:

The main transmission can be mounted at a slight angle to the left (when viewed from
behind) so that the rotor mast has a built-in tilt to oppose the tail rotor thrust by
generating a lateral thrust component of the main rotor thrust vector.

Flight controls can be rigged so that the rotor disk is tilted to the left slightly when the
cyclic is centered.
When either of these two methods is used, the tip path plane is tilted slightly to the left while in a
hover.
If the transmission is mounted so the rotor shaft has no lateral tilt with respect to the fuselage (as
in the TH-73A), the pilot must apply a small amount of left cyclic to counteract the translating
tendency (Figure 5-2). This will cause the helicopter to hang left skid low in the hover. If
proper control inputs are used, the helicopter will leave the ground right skid first and land left
skid first. The opposite is true for helicopters with rotor disks that turn clockwise when viewed
from above.
Figure 5-2 Translating Tendency
In forward flight, the tail rotor continues to push to the right, and the helicopter is at a small
POWERED FLIGHT
5-3
CHAPTER FIVE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
angle from the wind when the rotors are level and the ball is in the middle. This is called
inherent sideslip. For some larger helicopters, the tail rotor is mounted on the vertical stabilizer
to correct this inherent sideslip and to eliminate some of the tilting in a hover. By mounting the
tail rotor on top of the vertical stabilizer, the anti-torque is more in line with the horizontal plane
of torque (main rotor tip path plane), resulting in less airframe-tilt-caused tail rotor thrust. In
addition, having the tail rotor higher off the ground reduces the risk of objects coming in contact
with the blades, but at the cost of increased weight and complexity.
2.
Pendular Action
Since the fuselage of a helicopter with a single main rotor, is suspended from a single point, it is
free to oscillate longitudinally or laterally in the same way a pendulum would. This pendular
action can be exacerbated by over controlling; therefore, control movements should be smooth
and not exaggerated.
In normal flight regimes, forces acting on the fuselage tend to align it with the main rotor.
Consequently, in forward flight the nose tends to pitch down, matching the main rotor. To avoid
an uncomfortable nose-down attitude at high airspeed, helicopters are equipped with a horizontal
stabilizer on the tail boom. The horizontal stabilizer is essentially an upside-down wing. As
relative wind across the stabilizer increases, it creates downforce that pushes the tail down and
the nose up. Pendular action allows the fuselage and the main rotor to maintain slightly different
attitudes.
In rearward flight, the horizontal stabilizer continues to act as an upside-down wing pressing the
tail downward. Normally, with the helicopter mostly into the wind, the horizontal stabilizer
simply experiences a smaller headwind component as the helicopter begins rearward travel.
When rearward flight ground speed equals the wind speed, the helicopter is merely hovering in a
no-wind condition. However, if the rearward flight speed exceeds the wind speed or the
helicopter is hovering with a strong tailwind, the horizontal stabilizer may push the tail down.
This places the helicopter in danger of striking the tail on the ground. Pilots must use great care
to ensure tail clearance when conducting rearward slides or hovering with a tailwind.
Pendular action is most pronounced in semi-rigid rotor systems because it is a true pendulum. In
a fully articulated system, such as the TH-73A, the blade attachment points are offset from the
center of rotation. This causes the centrifugal force pulling out on the blades to be transferred to
the fuselage. As a result, the fuselage tends to follow the rotor attitude more closely than in a
semi-rigid rotor system.
3.
Coning
The rotation of the rotor disk creates centrifugal force (outward inertial force), which tends to
pull the blades straight outward from the main rotor hub; the faster the rotation, the greater the
centrifugal force, the slower the rotation, the smaller the centrifugal force. This force gives the
rotor blades their rigidity and, in turn, the strength to support the weight of the helicopter. The
maximum centrifugal force generated is determined by the maximum operating rotor RPM.
5-4
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER FIVE
As lift on the blades is increased (in a takeoff, for example), two major forces are acting at the
same time—centrifugal force acting outward, and lift acting upward. The result of these two
forces is that the blades assume a conical path instead of remaining in the plane perpendicular to
the mast. This can be seen in any helicopter when it takes off; the rotor disk changes from flat to
a slight cone shape (Figure 5-3). The heavier the aircraft, the greater the coning angle. A
heavier helicopter requires more lift to take off into a hover. However, the centrifugal force
remains the same for a constant RPM. Therefore, the greater the lift, the more it will counteract
the centrifugal force and cause the blades to cone.
Figure 5-3 Coning
If the rotor RPM is allowed to go too low (below the minimum power-on rotor RPM, for
example), the centrifugal force becomes smaller and the coning angle becomes much larger
(Figure 5-4).
POWERED FLIGHT
5-5
CHAPTER FIVE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 5-4 Coning Vs Main Rotor Disk Area Due to Low Rotor RPM
As the coning angle increases, the rotor disk area decreases. As a result, the lift provided by the
main rotor decreases and an un-commanded descent may develop, especially when the power
required is close to the maximum that the engine can produce. If this occurs, the pilot must resist
the impulse to continue to raise the collective because greater blade pitch will only exacerbate
the problem. Assuming that the helicopter engine throttle is in its full-open flight position and
that the engine is working properly, the only way to recover is to regain main rotor RPM by
lowering the collective or decreasing aircraft weight by jettisoning external loads or dumping
fuel.
4.
Coriolis Effect
The Coriolis Effect is also referred to as the law of conservation of angular momentum. It states
that the value of angular momentum of a rotating body does not change unless an external force
is applied. In other words, a rotating body continues to rotate with the same rotational velocity
until some external force is applied to change the speed of rotation. Angular momentum is the
moment of inertia (mass times distance from the center of rotation squared) multiplied by the
speed of rotation.
Changes in angular velocity, known as angular acceleration and deceleration, take place as the
mass of a rotating body is moved closer to or farther away from the axis of rotation. An
5-6
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
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excellent example of this principle in action is a figure skater performing a spin on ice skates.
The skater begins rotation on one foot, with the other leg and both arms extended. The rotation
of the skater’s body is relatively slow. When a skater draws both arms and one leg inward, the
moment of inertia (mass times radius squared) becomes much smaller and the body is rotating
almost faster than the eye can follow. This is caused by the conservation of angular momentum
(moment of inertia decreases–the speed of rotation increases.)
The rotor blade rotating about the rotor hub possesses angular momentum. As the rotor begins to
cone, the mass of the blade moves closer to the rotor hub and the diameter of the rotor disk
shrinks. Due to conservation of angular momentum, the blades increase speed. As the blades
rotate around the hub in maneuvering flight, the mass of the blades will move toward and away
from the hub. So, throughout a single rotation, a blade will accelerate and decelerate. This
change in velocity is called lead and lag. Rotor heads are designed to allow or minimize the lead
and lag motion in order to reduce the stress on the blade attachment point. The TH-73A has
elastomeric bearing build into the rotor head that allow the blades to move fore and aft (as well
as up and down). Each blade also has a hydraulic damper to smooth out the lead-lag motion.
See Figure 5-5.
Figure 5-5 Changes in Angular Momentum due to Flapping
5.
Gyroscopic Precession
The spinning main rotor of a helicopter acts like a gyroscope. As such, it has the properties of
gyroscopic action, one of which is precession. Gyroscopic precession (Figure 5-6) is the
resultant action or deflection of a spinning object when a force is applied to this object. This
action occurs approximately 90 degrees in the direction of rotation from the point where the
force is applied (or 90 degrees later in the rotation cycle).
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 5-6 Gyroscopic Precession
Examine a two-bladed rotor disk to see how gyroscopic precession affects the movement of the
tip path plane. Moving the cyclic increases the pitch angle of one rotor blade with the result of a
greater lifting force being applied at that point in the plane of rotation. This same control
movement simultaneously decreases the pitch angle of the other blade the same amount, thus
decreasing the lifting force applied at that point in the plane of rotation. The blade with the
increased pitch angle tends to flap up; the blade with the decreased pitch angle tends to flap
down. Because the rotor disk acts like a gyro, the blades reach maximum deflection at a point
approximately 90 degrees later in the plane of rotation. Figure 5-7 illustrates the result of a
forward cyclic input. The retreating blade pitch angle is increased, and the advancing blade pitch
angle is decreased, resulting in the tip path plane tilting forward. The maximum deflection takes
place 90 degrees later when the blades are at the rear and front, respectively.
Figure 5-7 Forward Cyclic Input
5-8
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In a rotor disk using three or more blades, the movement of the cyclic changes the pitch angle of
each blade an appropriate amount so that the result is the same.
503. VERTICAL FLIGHT
Hovering is actually an element of vertical flight. Increasing the pitch angle of the rotor blades
while keeping their rotation speed constant generates additional lift and the helicopter ascends.
Decreasing the pitch causes the helicopter to descend. In a no-wind condition in which lift and
thrust are less than weight and drag, the helicopter descends vertically. If lift and thrust are
greater than weight and drag, the helicopter ascends vertically.
504. FORWARD FLIGHT
In steady forward flight, with no change in airspeed or vertical speed, the four forces of lift,
thrust, drag, and weight must be in balance. Once the tip path plane is tilted forward, the main
rotor thrust is also tilted forward. This resultant main rotor thrust can be resolved into two
components: lift acting vertically upward and thrust acting horizontally in the direction of flight.
In addition to lift and thrust, there is weight (the downward acting force) and drag (the force
opposing the motion of the blades and airframe through the air).
In straight-and-level, unaccelerated forward flight, lift equals weight and thrust equals drag. If
lift exceeds weight, the helicopter accelerates vertically until the forces are in balance. If thrust
is less than drag, the helicopter slows down until the forces are in balance. See Figure 5-8.
Figure 5-8 Forces on a Helicopter in Forward Flight
As a helicopter initiates a move forward, it begins to lose altitude because lift is lost as thrust is
diverted forward. As the helicopter begins to accelerate from a hover, the rotor disk becomes
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
more efficient due to translational lift. The result is excess power over the power required to
hover. Continued acceleration causes an even larger increase in airflow through the rotor disk up
to a maximum determined by drag and the engine’s limit of power (Figure 5-9), and more
efficient flight. In order to maintain unaccelerated flight, the pilot must understand that with any
changes in collective (power) or cyclic movement, the helicopter begins either to climb or to
descend. Once straight-and-level flight is obtained, the pilot should make note of the power
(torque setting) required and not make major adjustments to the flight controls.
Figure 5-9 Power Vs Airspeed Chart
1.
Airflow In Forward Flight
Airflow across the rotor disk in forward flight varies from airflow at a hover. In forward flight,
air flows opposite the aircraft’s flightpath. The velocity of this airflow equals the helicopters
forward speed. Because the rotor blades turn in a circular pattern, the velocity of airflow across
a blade depends on the position of the blade in the plane of rotation, its rotational velocity, and
the airspeed of the helicopter. Therefore, the airflow meeting each blade varies continuously as
the blade rotates. The highest velocity of airflow occurs over the right side (3 o’clock position)
of the helicopter (the advancing blade in a rotor disk that turns counterclockwise) and decreases
to rotational velocity over the nose. It continues to decrease until the lowest velocity of airflow
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CHAPTER FIVE
occurs over the left side (9 o’clock position) of the helicopter (retreating blade). As the blade
continues to rotate, velocity of the airflow then increases to rotational velocity over the tail. It
continues to increase until the blade is back at the 3 o’clock position.
The advancing blade in Figure 5-10 (position A) moves in the same direction as the helicopter.
The velocity of the air meeting this blade equals rotational velocity of the blade plus wind
velocity resulting from forward airspeed. The retreating blade (position C) moves in a flow of
air moving in the opposite direction of the helicopter. The velocity of airflow meeting this blade
equals rotational velocity of the blade minus wind velocity resulting from forward airspeed. The
blades (positions B and D) over the nose and tail move essentially at right angles to the airflow
created by forward airspeed. The velocity of airflow meeting these blades equals the rotational
velocity. This results in a change in velocity of airflow all across the rotor disk and a change to
the lift pattern of the rotor disk.
Figure 5-10 Airflow in Forward Flight
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5-11
CHAPTER FIVE
a.
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Advancing Blade
As the relative wind speed of the advancing blade increases, the blade gains lift and
begins to flap up. It reaches its maximum up flap velocity at the 3 o’clock position,
where the wind velocity is the greatest. This up flap creates a downward flow of air
and has the same effect as increasing the induced flow velocity by imposing a
downward vertical velocity vector to the relative wind, which decreases the AOA.
b.
Retreating Blade
As relative wind speed of the retreating blade decreases, the blade loses lift and
begins to flap down. It reaches its maximum down flap velocity at the 9 o’clock
position, where wind velocity is the least. This down flap creates an upward flow of
air and has the same effect as decreasing the induced flow velocity by imposing an
upward vertical velocity vector to the relative wind, which increases the AOA.
2.
Dissymmetry of Lift
Dissymmetry of lift is the differential (unequal) lift between advancing and retreating halves of
the rotor disk caused by the different wind flow velocity across each half, as depicted in Figure
5-11. This difference in lift would cause the helicopter to be uncontrollable in any situation
other than hovering in a calm wind. There must be a means of compensating, correcting, or
eliminating this unequal lift to attain symmetry of lift.
When the helicopter moves through the air, the relative airflow through the main rotor disk is
different on the advancing side from the retreating side. The relative wind encountered by the
advancing blade is increased by the forward speed of the helicopter, while the relative wind
speed acting on the retreating blade is reduced by the helicopter’s forward airspeed. Therefore,
because of the relative wind speed, the advancing blade side of the rotor disk produces more lift
than the retreating blade side.
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CHAPTER FIVE
Figure 5-11 Dissymmetry of Lift
If this condition were allowed to exist, a helicopter with a counterclockwise rotating main rotor
would roll to the left because of the difference in lift. In reality, the main rotor blades flap to
equalize lift across the rotor disk. Articulated rotor heads, usually with three or more blades,
incorporate a horizontal hinge (flapping hinge) to allow the individual rotor blades to flap up and
down as they rotate. A semi-rigid rotor head (two blades) utilizes a teetering hinge, which
allows the blades to flap as a unit. When one blade flaps up, the other blade flaps down. In a
rigid rotor head the blades bend to allow flapping.
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 5-12 Effect of Flapping
In Figure 5-12, the rotor blade reaches the advancing side of the rotor disk (A), it reaches its
maximum up flap velocity. When the blade flaps upward, the angle between the chord line and
the resultant relative wind decreases. This decreases the blade’s AOA, which reduces the
amount of lift produced by the blade. At position (C), the rotor blade is now at its maximum
down flapping velocity. Due to down flapping, the angle between the chord line and the
resultant relative wind increases. This increases the blade’s AOA and thus the amount of lift
produced by the blade.
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
3.
CHAPTER FIVE
Blowback
Flapping solves the problem of dissymmetry of lift, but introduces the problem of blowback. As
airflow over the blades increases during the transition to forward flight, increasing lift on the
advancing portion of the rotor disk causes the blades to flap up, and the retreating side’s
decreasing lift causes the blades to flap down. Since phase lag causes maximum displacement to
occur 90 degrees after the maximum applied force, the maximum up displacement occurs at the
12 o'clock position and maximum down displacement occurs at the 6 o’clock position. The net
effect is that with increasing forward speed, the rotor disk tilts aft. This tendency to tilt aft with
increasing speed is called blowback.
Pilots must correct for this tendency by maintaining a constant rotor disk attitude that will move
the helicopter through the speed range in which blowback occurs. If the nose is permitted to
pitch up while passing through this speed range, the aircraft will slow down and may also tend to
roll to the right. To correct for this tendency, the pilot must continuously move the cyclic
forward as velocity of the helicopter increases until the transition to forward flight is complete.
Figure 5-13 depicts blowback during a transition to forward flight. As the pilot begins the
transition, he or she moves the cyclic forward, causing the rotor disk to tilt forward (dotted line).
As airspeed increases, dissymmetry of lift increases, and the main rotors begin to flap. The
cyclic nature of the flapping causes the rotor disk to tilt aft (solid line).
Figure 5-13 Counteracting Blowback
4.
Translational Lift
Improved rotor efficiency resulting from increasing the relative wind is called translational lift.
The efficiency of the hovering rotor disk is greatly improved with each knot of incoming wind
gained by the horizontal movement of the aircraft or surface wind. As the incoming wind
produced by aircraft movement or surface wind enters the rotor disk, turbulence and vortices are
left behind and the flow of air becomes more horizontal (Figure 5-14). In addition, the tail rotor
becomes more aerodynamically efficient during the transition from hover to forward flight.
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 5-14 Airflow with Minimal Headwind
5.
Effective Translational Lift
While transitioning to forward flight at about 10 to 15 knots, the helicopter begins to go through
Effective Translational Lift (ETL), where the rotor blades become more efficient as forward
airspeed increases (Figure 5-15).
Figure 5-15 Airflow just Prior to Effective Translational Lift
Between 16 and 24 knots, the rotor disk completely outruns the recirculation of old vortices and
begins to work in relatively undisturbed air. The flow of air through the rotor disk is more
horizontal, which reduces induced flow and drag, and correspondingly increases the angle of
attach and lift. The additional lift available at this speed makes the rotor disk operate more
efficiently. This increased efficiency continues with increased airspeed until the best rate of
climb airspeed is reached, and total drag is at its lowest point.
As speed increases from a hover, translational lift becomes more effective, the nose pitches up,
and the aircraft rolls to the right. The combined effects of dissymmetry of lift, gyroscopic
precession, and transverse flow effect cause this tendency. It is important to understand these
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER FIVE
effects and anticipate correcting for them. When the helicopter is transitioning through ETL, the
pilot needs to apply forward and left lateral cyclic input to maintain a constant rotor-disk attitude.
6.
Translational Thrust
Translational thrust (Figure 5-16) occurs when the tail rotor becomes more aerodynamically
efficient during the transition from hover to forward flight. As the tail rotor works in
progressively less turbulent air, this improved efficiency produces more anti-torque thrust,
causing the nose of the aircraft to yaw left (with a main rotor turning counterclockwise) and
forces the pilot to apply right pedal (decreasing the AOA in the tail rotor blades) in response. In
addition, increased airflow over the vertical stabilizer will provide additional anti-torque to aid
the tail rotor.
Figure 5-16 Translational Thrust
7.
Induced Flow
As the rotor blades rotate, they generate what is called rotational relative wind. This airflow is
characterized as flowing parallel and opposite the tip-path plane and striking perpendicular to the
rotor blade’s leading edge. This rotational relative wind is used to generate lift. As rotor blades
produce lift, air is accelerated over the foil and projected downward. Anytime a helicopter is
producing lift, it moves large masses of air vertically and down through the rotor disk. This
downwash or induced flow can significantly change the efficiency of the rotor disk. Rotational
relative wind combines with induced flow to form the resultant relative wind. As induced flow
increases, resultant relative wind becomes less horizontal. Since AOA is determined by
measuring the difference between the chord line and the resultant relative wind, as the resultant
relative wind becomes less horizontal, AOA decreases.
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CHAPTER FIVE
8.
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Transverse Flow Effect
As the helicopter accelerates in forward flight, induced flow drops to near zero at the forward
disk area and increases at the aft disk area. These differences in lift between the fore and aft
portions of the rotor disk produces the transverse flow effect. This increases the AOA at the
front disk area causing the rotor blade to flap up and reduces AOA at the aft disk area causing
the rotor blade to flap down. Because the rotor acts like a gyroscope, maximum displacement
occurs 90 degrees in the direction of rotation. The result is a tendency for the helicopter to roll
slightly to the right as it accelerates through approximately 20 knots or if the headwind is
approximately 20 knots.
Transverse flow effect is recognized by increased vibrations of the helicopter at airspeeds around
12 to 15 knots and can be produced by forward flight or from the wind while in a hover (Figure
5-17). This vibration happens at an airspeed just below ETL on takeoff and after passing
through ETL during landing. The vibration happens close to the same airspeed as ETL because
that is when the greatest lift differential exists between the front and rear portions of the rotor
system. As such, some pilots confuse the vibration felt by transverse flow effect with passing
through ETL. To counteract transverse flow effect, a cyclic input to the left may be needed.
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CHAPTER FIVE
Figure 5-17 Transverse Flow Effect
9.
Retreating Blade Stall
The combination of blade flapping and slower relative wind acting on the retreating blade
normally limits the maximum forward speed of a helicopter. At a high forward speed, the
retreating blade stalls because of a high AOA and slow resultant relative wind. This situation is
called retreating blade stall and is evidenced by a nose pitch up, vibration, and a rolling
tendency, usually to the left in helicopters with counterclockwise blade rotation.
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Pilots can avoid retreating blade stall by not exceeding the never-exceed speed. This speed is
designated VNE and is indicated on a placard and marked on the airspeed indicator by a red line.
505. SIDEWARD FLIGHT
In sideward flight (Figure 5-18), the tip path plane is tilted in the desired sideward direction.
This tilts the total lift-thrust vector sideward. In this case, the vertical or lift component is still
straight up and weight straight down, but the horizontal thrust component now acts sideward
with drag acting to the opposite side.
Figure 5-18 Sideward Flight
Sideward flight can be a very unstable condition due to the parasitic drag of the fuselage
combined with the lack of horizontal stabilizer for that sideward flight. Increased altitudes help
with control, but the pilot must always scan in the direction of flight. Movement of the cyclic in
the intended direction of flight causes the helicopter to move and controls the ground speed and
the ground track. However, the collective and pedals are key to successful sideward flight. Just
as in forward flight, the collective keeps the helicopter from contacting the ground and the pedals
help maintain the correct heading. Even in sideward flight, the tail of the helicopter should
remain behind you. Inputs to the cyclic should be smooth and controlled, and the pilot should
always be aware of the tip path plane in relation to the ground.
Contacting the ground with the skids during sideward flight will most likely result in a dynamic
rollover event before the pilot has a chance to react. Extreme caution should be used when
maneuvering the helicopter sideways to avoid such hazards.
506. REARWARD FLIGHT
For rearward flight (Figure 5-19), the tip path plane is tilted rearward, which, in turn, tilts the
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER FIVE
main rotor thrust vector rearward. Drag now acts forward with the lift component straight up
and weight straight down.
Figure 5-19 Rearward Flight
Pilots must be aware of the hazards of rearward flight. Because of the position of the horizontal
stabilizer, the tail end of the helicopter tends to pitch downward in rearward flight, increasing the
probability of contacting the ground with the tail. Another factor to consider in rearward flight is
skid design. Most helicopter skids are not turned upward in the back, and any contact with the
ground during rearward flight can put the helicopter in an uncontrollable position leading to tail
rotor contact with the ground. Pilots must do a thorough scan of the area before attempting to
hover rearward, looking for obstacles and terrain changes. Slower airspeeds and a higher than
normal hover altitude can help mitigate the risks associated with rearward flight.
507. TURNING FLIGHT
In forward flight, the rotor disk is tilted forward, which also tilts the main rotor thrust forward.
When the helicopter is banked, the rotor disk is tilted sideward resulting in lift being separated
into two components. Lift acting upward and opposing weight is called the vertical component
of lift. Lift acting horizontally and opposing inertia (centrifugal force) is the horizontal
component of lift (centripetal force).
As the Angle of Bank (AOB) increases (Figure 5-20), the total lift force is tilted more toward the
horizontal, thus causing the rate of turn to increase because more lift is acting horizontally.
Since the resultant lifting force acts more horizontally, the effect of lift acting vertically is
decreased.
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 5-20 Turning Flight
To compensate for this decreased vertical lift, the AOA of the rotor blades must be increased in
order to maintain altitude. The steeper the AOB is, the greater the rotor blade AOA required to
maintain altitude. Thus, with an increase in bank and a greater AOA, the resultant lifting force
increases, and the rate of turn is higher. Collective pitch controls the pitch of the main rotor
blades which, along with other factors, determines the overall AOA in the rotor disk. Simply
put, collective pitch must be increased in order to maintain altitude and airspeed while turning.
5-22 POWERED FLIGHT
AUTOROTATION
600. INTRODUCTION
This chapter introduces the conceptual and aerodynamic principles involved during
autorotational flight. In a helicopter, an autorotative descent is a power-off maneuver in which
the engine is disengaged from the main rotor disk and the rotor blades are driven solely by the
upward flow of air through the rotor. In other words, the engine is no longer supplying power to
the main rotor.
601. LEARNING OBJECTIVES

Define autorotational flight.

Recall powered flight energy management concepts.

Define the aerodynamic forces that affect autorotation.

Identify autorotation performance variables.

Define the phases of autorotation.

Identify the purpose of Height-Velocity (H-V) diagrams.
602. AUTOROTATIONAL FLIGHT
At the instant of engine failure, the main rotor blades are producing lift and thrust from their
AOA and velocity. By lowering the collective (which must be done immediately in case of an
engine failure), lift and drag are reduced, and the helicopter begins an immediate descent, thus
producing an upward flow of air through the rotor disk. This upward flow of air through the
rotor disk (Figure 6-1) provides sufficient thrust to maintain rotor RPM throughout the descent.
The tail rotor is driven by the main rotor transmission during autorotation, so heading control is
maintained with the anti-torque pedals as in normal flight.
AUTOROTATION
6-1
CHAPTER SIX
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 6-1 Airflow in Powered Flight Vs Autorotation
Autorotation is further defined as the state of flight where the main rotor disk of a helicopter is
being turned by the action of air moving up through the rotor rather than engine power driving
the rotor. In normal, powered flight, air is drawn into the main rotor disk from above and
exhausted downward, but during autorotation, air moves up into the rotor disk from below as the
helicopter descends.
Autorotation is permitted mechanically by a freewheeling unit—a clutch mechanism that allows
the rotor system to continue turning even if the engine is not running. If the engine fails, the
freewheeling unit automatically disengages the engine from the rotor system allowing it to rotate
freely. It is the means by which a helicopter can be landed safely in the event of an engine
failure.
603. ENERGY MANAGEMENT CONCEPTS
During an autorotation, the pilot’s main task is to manage the energy stored in the helicopter and
land safely. Poor energy management could lead to hard landings and aircraft mishaps. Among
all the variables affecting autorotation, rotor RPM is the most critical. Poor control of rotor RPM
is the primary cause of hard landings and main rotor over-speeds. Autorotation is primarily
performed by maintaining a good outside scan, keeping the landing spot in sight, and managing
helicopter energy.
Energy is the ability to do work and work may be performed on a body to change its energy (
Table 6-1). Some energy-work correlations are reversible, but some are not. Burning fuel to run
an engine is not a reversible process. The energy released during the engine’s combustion
process allows the engine to do work; however, fuel energy cannot be restored by doing work on
the engine. Wind-up clocks and helicopter batteries are both reversible processes because energy
can be restored by doing work on the clock or on the battery. A wound-up spring holds the
energy that drives a clock. A helicopter battery holds enough energy to start the engine. Work
done by winding the spring or charging the battery is turned into stored energy, most of which
can later be released to do work.
6-2
AUTOROTATION
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
TYPE OF ENERGY
Potential (PE)
Kinetic (KE)
Rotational (RE)
DEFINITION
Energy due to the height above a
surface (Helicopter Altitude)
Energy due to the motion with
respect to a point on the ground
(Helicopter Ground Speed)
Energy due to a rotating mass
(Rotor RPM)
CHAPTER SIX
BASIC FORMULA
mass × gravity × height
𝑚𝑔ℎ
1
mass × velocity2
2
1
𝑚𝑣 2
2
1
inertia of blades × rotor RPM2
2
1 2
𝐼
2
Table 6-1 Helicopter Energy
Changing a body’s position gives it potential energy, changing its motion gives it kinetic or
rotational energy or both. Under ideal conditions, if no work is being done on an object, its total
energy will remain constant. Such an object is in a closed system. In a closed system, the total
energy will remain constant, but energy is converted between types. If a weight is lifted against
gravity, it stores potential energy that can be released when the weight is lowered again.
Potential energy is proportional to the height through which the object is lifted. If a weight is
accelerated, the resultant force acting on it moves through a distance and does work which is
stored as kinetic energy. In aviation, the kinetic energy is based on the motion that the aircraft
has with respect to a point on the ground. In no-wind conditions, this motion is the aerodynamic
speed at which the helicopter is flying—practically the indicated airspeed. In a wind condition
the aircraft motion is the result of the indicated airspeed and the wind—practically the ground
speed. A helicopter flying with an indicated airspeed of 30 knots into a 30 knot headwind has
zero kinetic energy because its motion with respect to a point on the ground is zero. A helicopter
flying at 30 knots downwind with a 30 knot tailwind still has the same indicated airspeed, but its
ground speed is now 60 knots and it possesses more kinetic energy due to that ground speed.
The work done by an engine to accelerate and rotate the rotor is stored as rotational energy.
The application of the concept of energy becomes critical during autorotation because an
autorotation is basically landing a helicopter by managing and exchanging its total energy after
its engine flames out.
To fly a helicopter in powered flight, the pilot uses the engine to convert the energy stored in the
fuel to rotational energy stored in the main and tail rotors systems. The energy stored in the
rotors gives the helicopter the ability to generate work. By using the flight controls, the pilot
generates and directs forces that will accelerate the aircraft along its axes. The work done by
these forces are stored as kinetic or potential energy by respectively increasing the speed or the
height of the helicopter. Rotational energy is stored in the main rotor by keep it turning. The
faster the rotor turns, the higher the rotational energy stored in the rotor hub. If the engine does
not produce additional work on the helicopter, such as after an engine failure in flight, the pilot
flies the helicopter by managing energy (
Table 6-2).
AUTOROTATION
6-3
CHAPTER SIX
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
ENERGY MANAGEMENT DURING AUTOROTATION
Autorotation Phase
Potential (PE)
Kinetic (KE)
Rotational (RE)
Steady Descent
Transferred to KE
and RE until nearly
zero
Maintained by
converting PE
Maintained by
converting PE and KE
Flare
Nearly zero and kept
constant by KE
Transferred to RE
and PE until nearly
to zero
Maintained by KE
Touchdown
Zero
Zero or very low
Transfer to lift by
pulling collective to
cushion the touchdown
Table 6-2 Energy Management During Autorotation
604. AERODYNAMIC FORCES THAT AFFECT AUTOROTATION
Figure 6-2 illustrates the force vectors on the rotor disk regions (driven, driving, and stall) during
autorotation. Part A is the driven region, B and D are points of equilibrium, part C is the driving
region, and part E is the stall region. The blade element diagram is different in each region
because rotational relative wind is slower near the blade root and increases continually toward
the blade tip. Also, blade twist gives a more positive AOA in the driving region than in the
driven region. The combination of the upward airflow through the rotor and the rotational
relative wind produces different combinations of Total Aerodynamic Force (TAF) at every point
along the blade.
6-4
AUTOROTATION
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER SIX
Figure 6-2 Rotor Blade Regions in Autorotation
AUTOROTATION
6-5
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
The driven region, also called the propeller region, is nearest the blade tips. Normally, it consists
of about 30 percent of the radius. In the driven region, the TAF acts behind the axis of rotation,
resulting in an overall drag force. The driven region produces some lift, but that lift is offset by
drag. The overall result is a deceleration in the rotation of the blade. The size of this region
varies with the blade pitch, rate of descent, and rotor RPM (NR). When changing autorotative
RPM, blade pitch, or rate of descent, the size of the driven region in relation to the other regions
also changes.
There are two points of equilibrium on the blade, one between the driven region and the driving
region, and one between the driving region and the stall region. At points of equilibrium, TAF is
aligned with the axis of rotation. Lift and drag are produced, but the total effect produces neither
acceleration nor deceleration.
The driving region, or autorotative region, normally lies between 25 to 70 percent of the blade
radius. Part C of Figure 6-2 shows the driving region of the blade, which produces the forces
needed to turn the blades during autorotation. TAF in the driving region is inclined slightly
forward of the axis of rotation, producing a continual acceleration force. This inclination
supplies thrust, which tends to accelerate the rotation of the blade. Driving region size varies
with blade pitch setting, rate of descent, and NR.
By controlling the size of this region, a pilot can adjust autorotative RPM. For example, if the
collective pitch is raised, the pitch angle increases in all regions. This causes the point of
equilibrium between the driven and driving region to move inboard along the blade’s span, thus
increasing the size of the driven region. The stall region also becomes larger because the point
equilibrium between the driving region and the stalled region moves outboard along the blade’s
span, thus increasing the size of the stall region and decreasing the size of the driving region.
Reducing the size of the driving region causes the acceleration force of the driving region and
RPM to decrease. A constant rotor RPM is achieved by adjusting the collective pitch so blade
acceleration forces from the driving region are balanced with the deceleration forces from the
driven and stall regions.
The inner 25 percent of the rotor blade is referred to as the stall region and operates above its
maximum AOA (stall angle), causing drag, which tends to slow rotation of the blade. Part E of
Figure 6-2 depicts the stall region.
1.
Forward Flight Autorotation
Autorotative force in forward flight is produced in exactly the same manner as when the
helicopter is descending vertically in still air. However, because forward speed changes the
inflow of air up through the rotor disk, all three regions move outboard along the blade span on
the retreating side of the disk where AOA is larger, as depicted in Figure 6-3. With lower AOA
on the advancing side blade, more of the blade falls in the driven region. On the retreating side,
more of the blade is in the stall region. A small section near the root experiences a reversed flow
so the size of the driven region is reduced.
6-6
AUTOROTATION
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER SIX
Figure 6-3 Rotor Disk Regions in Autorotation During Forward Flight
Prior to landing from an autorotative descent, the pilot must flare the helicopter in order to
reduce the rate of descent and decelerate. The pilot initiates the flare by applying aft cyclic. As
the helicopter flares, the airflow patterns change around the blades causing the NR to increase.
Pilots must adjust the collective as necessary to keep the NR within operating limits.
2.
Autorotation Descent Variables
Performance in the descent depends upon the forces acting on the rotor. Several factors affect
the rate of descent in autorotation: airspeed, DA, gross weight, and rotor RPM.
The pilot primarily controls the rate of descent using aircraft attitude, which is directly correlated
to airspeed. Higher or lower airspeeds are obtained by adjusting the cyclic, just as in normal
flight. To minimize rate of descend, the helicopter should maintain the recommended minimum
rate of descend speed in autorotation. To extend the glide distance, the helicopter should
maintain the recommended maximum range airspeed in autorotation. Note that the minimum
rate of descent airspeed also corresponds with the best rate of climb airspeed (Vy), as this is the
airspeed where the coefficient of the lift to drag ratio is highest. The farther the pilot deviates
from Vy, the greater the descent rate.
Each type of helicopter has a specific airspeed at which a power-off glide is most efficient. The
best airspeed is the one that combines the greatest glide range with the slowest rate of descent.
The specific airspeed is somewhat different for each type of helicopter, yet certain factors affect
all helicopters in the same manner.
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
While in autorotation, helicopters should maintain the airspeed between the maximum and
minimum autorotational airspeeds. Conducting autorotations outside this range will negatively
affect aircraft performance in autorotation, including the inability to maintain minimum rotor
RPM.
Out of balance flight while in autorotation will also increase the rate of descent without
increasing airspeed. At a given rotor RPM, higher DA will increase the rate of descent due to
lower air density. With all other variables constant, a heavier helicopter will descend faster in an
autorotation than a lighter helicopter.
3.
Main Rotor RPM Stability and Control
In an autorotation, transient changes in aircraft attitude or wind shifts can change the airflow
through the rotor system, therefore, affecting RPM. However, the rotor system demonstrates
RPM stability in response to small changes.
Figure 6-4 graphically describes the blade region variations with RPM changes. In autorotation,
the blade is at (or near) flat pitch, which is designed to maintain a constant RPM at a given
descent rate. When an external force (winds or a change in airflow) causes a small transient
increase in the RPM, the regions shift inboard, enlarging the driven (prop) region, and associated
drag, while also reducing the moment arm for the driving (auto) region on the blades. This
reduction in the driving region causes the RPM to decrease back towards the original RPM. Just
the opposite happens with slight decreases in RPM. Thus, for minor RPM variations, the rotor
system has RPM stability.
6-8
AUTOROTATION
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER SIX
Figure 6-4 RPM Response to Small RPM Variations
However, with a large decrease in RPM, even though the driving region of the blade increases,
the stall region and the drag it produces also increase. The increased moment arm for the driving
region may not be sufficient to regain the lost RPM before the aircraft reaches the ground.
Since the amount of blade surface producing positive autorotative driving force varies according
to RPM and this driving force is synonymous with thrust produced, the pilot has additional
control over rate of descent by changing blade pitch using the collective. Excessively high rotor
RPM produces high rate of descent, and very low rotor RPM leads to low driving force in
proportion to high drag associated with a stalled profile. There is an optimum RPM range, which
produces the greatest net driving force and minimum descent rate. It is in the best interest of the
pilot to strive for this RPM range until reaching flare altitude. The pilot must monitor RPM
throughout the autorotation to ensure RPM stays within limits.
4.
RPM Tradeoffs
Rotor RPM is adjusted by varying collective. Adjusting RPM in an autorotation affects rate of
descent and energy stored in the rotor. Selection of a good autorotation rotor speed depends
upon desired performance. High RPM stores energy well, but involves a higher descent rate.

Increased centrifugal loads on hub
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS

Excessive driving region causes higher rate of descent

Increased rotational energy to trade off in a flare

Good for high-inertia systems, which would have difficulty building RPM rapidly in
a flare
Low RPM provides a slower descent and longer glide, but provides less stored power for use in
the flare. Taken to an extreme, low RPM can stall an excessive portion of the rotor and make
recovery extremely difficult.

Higher AOA, therefore, a slower rate of descent

Excessive stall region if RPM gets too low, resulting in an increase in rate of descent

Less rotational energy to trade off in a flare

Good for low-inertia systems, which can build RPM rapidly in a flare

Rotor blades lose centrifugal stiffness and cone upwards reducing the effective disk
area, increases material stresses, and increases the rate of descent
Individual types of helicopters have an optimum autorotation RPM that achieves the best
possible performance in a normal autorotation. This RPM is determined by the manufacturer
and is delineated in NATOPS.
5.
High- and Low-Inertia Rotor Systems
When discussing a rotating body such a blade or rotor, pilots should be familiar with the concept
of moment of inertia. The moment of inertia is the rotational equivalent of mass. In a rigid
body, such as the blade, the moment of inertia is the quantity that determines the torque needed
for the blade’s angular acceleration about its rotational axis. This is similar to how the size of a
mass determines the force needed for a desired linear acceleration. In a static rigid body, the
mass determines the correlation between force and acceleration, in a rotating rigid body, the
moment of inertia determines the correlation between torque and angular acceleration about a
rotational axis. The moment of inertia depends on the body’s mass distribution and the axis
chosen. Larger moments require more torque to change the body’s rotation rate. The moment of
inertia measures how difficult it would be to change an object’s current rotational speed.
Main rotors are often classified as either high-inertia or low-inertia systems. The type of rotor
system (low or high inertia) is a helicopter design characteristic that comes directly from the
helicopter manufacturing company, so pilots cannot modify this characteristic. However, pilots
must know the type of main rotor system they fly, and its characteristics. These characteristics
become critical during an autorotation.
6-10 AUTOROTATION
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
a.
CHAPTER SIX
Low-inertia Rotor Systems
Low-inertia rotor systems have a low moment of inertia because the rotor RPM and
its associated energy changes rapidly. Helicopters with low-inertia rotor systems lose
and gain rotor RPM faster than helicopters with high-inertia rotor systems, which
requires pilots to react quicker to maintain rotor RPM within operating range.
Low-inertia rotor systems tend to be more controllable than helicopters with highinertia rotor systems, but they are less stable. The rotor RPM rapidly responds to
flight control inputs because it requires a smaller aerodynamic forces to change the
rotor system rotational speed.
Low-inertia rotor systems also tend to be less forgiving to incorrect or late pilot inputs
because they change their rotational speed immediately, which does not allow the
pilot much time to react and correct flight control inputs before the RPM starts to
change. However, once the RPM and the rotational energy have changed, they
require small or short flight inputs to regain the desired RPM.
During an autorotation, all rotor systems are unforgiving of low rotor RPM, but
helicopters with low-inertia rotor systems are extremely unforgiving of low rotor
RPM because they lose rotor RPM faster, which requires the pilot to react more
quickly to prevent the rotor from reaching an unrecoverable condition. By reacting
quickly, the rotational energy stored in the main rotor is conserved and it can be used
by the pilot to land safely at the termination of the autorotation. In most rotor heads,
RPM is easier to control during a steady-state autorotation. It is the entry and
termination that demand pilot skill in controlling rotor RPM.
The rotor RPM slows down rapidly in a low-inertia rotor system after a total loss of
engine power, and builds up quickly during tight turns and in the flare at the
termination of an autorotation. Without effectively managing RPM, pilots run the
risk of using all the rotational energy stored in the rotor system and depleting the
RPM or over speeding the rotor.
The TH-73A has a low-inertia rotor head. It has good autorotation characteristics, but
pilots must ensure that they are using the collective to control rotor RPM throughout
the autorotation. Failure to control rotor RPM could result in exceeding aircraft limits
or putting the aircraft in a position of unrecoverable rotor RPM decay.
b.
High-inertia Rotor Systems
High-inertia rotor systems have a high moment of inertia because the rotor RPM and
its associated energy changes slowly. These systems give the pilot a few extra
seconds to react in order to keep rotor RPM within operating range. However,
because it is harder to change the rotational speed of high-inertia rotor systems, an NR
adjustment made by the pilot may take longer than in a low-inertia rotor system, and
it may require larger flight control inputs.
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
High-inertia rotor systems, tend to be more stable, but less controllable because it
requires a larger aerodynamic force to change its rotor system rotational speed. Rotor
RPM responds slowly to flight control inputs.
High-inertia rotor systems tend to be more forgiving to wrong or late pilot inputs
because they do not change their rotational speed immediately, allowing the pilot
extra time to react, and correct flight control inputs before RPM starts to change.
However, once the RPM and the rotational energy have changed, high-inertia systems
require large or long flight inputs to regain the desired RPM.
605. PHASES OF AUTOROTATION
While in level powered flight at high speed the lift and drag vectors acting on the main rotor
blades are large, and the TAF is inclined well to the rear of the axis of rotation. An engine
failure in this flight regime will cause a rapid rotor RPM decay. To stop the RPM decay the pilot
must immediately enter autorotation. Entering autorotation will quickly reduce blade drag and
incline the TAF vector forward, nearer the axis of rotation.
Autorotations are divided into three distinct phases: the entry phase, the steady-state descent
phase, and the landing phase. Each phase is aerodynamically different from the others.
1.
Entry
The entry phase occurs immediately after loss of engine power. The loss of engine power and
decrease in rotor RPM is more pronounced when the helicopter is at high gross weight, high
forward speed, or in high-DA conditions. Any of these conditions demand increased power and
a quicker reaction to the loss of that power. In most helicopters, it takes only a few seconds for
the RPM to fall into the minimum safe range, so it is imperative that pilots respond immediately
to an engine failure.
After an engine failure, the pilot enters an autorotation by lowering the collective and, depending
on the situation, by also adding aft cyclic. AOA lessens as airflow begins to shift to an upward
flow. The net result is that lower AOA and less blade pitch result in a smaller aerodynamic force
that is not tilted as far aft (see Figure 6-5). Vertical force is reduced, so a descent begins, but the
associated reduction in drag keeps the rotor from losing too much RPM.
6-12 AUTOROTATION
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER SIX
Figure 6-5 Force Vectors After Power Loss – Reduced Collective
Figure 6-5 shows the airflow and blade element diagram for a blade immediately after power
loss and the subsequent collective reduction, but before the aircraft has begun to descend. Lift
and drag are reduced and the TAF vector is inclined further forward than it was in powered
flight. As the helicopter begins to descend, the airflow begins to flow upward from under the
rotor system. This causes the TAF to incline further forward until it reaches an equilibrium that
maintains a safe operating RPM.
At the onset of an engine failure, the aircraft initial reactions are based upon sudden loss of
torque on the main rotor. The helicopter will yaw left due to a reduction in anti-torque required
(before pedals are adjusted), and may roll right due to residual tail rotor force. The primary
concern should be controlling rotor RPM. The rate of rotor RPM decay will determine how
quickly the collective must be lowered, and the rate of decay is determined by rotor inertia and
power required at the time of the engine failure. A high-inertia rotor head will tend to remain at
the same RPM longer after a loss of power than a low-inertia rotor head.
Power required is affected by DA and airspeed. In practical terms, this means the following
factors affect successful autorotation entry:

Rotor blade pitch at the time of the failure

Rotor inertia (high inertia—RPM decays slowly, low inertia—RPM decays rapidly)

Pilot reaction time

Entry altitude (time to establish a stabilized autorotation)

Entry airspeed
AUTOROTATION
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CHAPTER SIX
2.
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Steady-State Descent
Figure 6-6 shows the airflow and blade element diagram for a blade in a steady-state autorotative
descent. Airflow is now upward through the rotor disk because of the descent. This upward
flow of air creates a larger AOA, although blade pitch angle has not changed since the descent
began. TAF on the blade increases and is inclined further forward until equilibrium is
established. Rate of descent and rotor RPM stabilize, and the helicopter is descending at a
constant angle. Angle of descent depends on airspeed, DA, wind, and the type of helicopter.
Figure 6-6 Force Vectors in Autorotative Steady-State Descent
When autorotation is established, upflow tilts the relative wind downward, which moves the net
aerodynamic force forward. In a stabilized autorotation the component of lift in the horizontal
direction balances out the horizontal component of drag so that drag does not reduce the rotor
RPM.
3.
Rate of Descent and Glide Distance
Rather than using power required/power available charts for autorotation, many aircraft
NATOPS manuals contain autorotation charts specific for that aircraft (see Figure 6-7).
6-14 AUTOROTATION
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER SIX
Figure 6-7 Autorotational Glide Characteristics Chart – Rates of Descent Vs Airspeed
Airspeed is the most significant factor that affects rate of descent in autorotation. The rate of
descent is high at very low airspeeds, decreases to a minimum at some intermediate speed, and
increases again at faster speeds. Minimum rate of descent occurs at the bucket airspeed because
this is where the minimum power is required to remain airborne.
Maximum glide distance occurs when the ratio of power required to airspeed is at the minimum
so that the aircraft will fly the furthest horizontally. A line drawn from the point of origin
tangent to the total drag curve illustrates the airspeed for maximum glide distance, much the
same as for powered flight. However, in this case, higher speed and distance over the ground
reduces time aloft.
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
It can be assumed that the steady-state autorotation ends around 150 feet (depending on aircraft
and speed) and the landing procedure then begins. To execute a power-off landing for rotarywing aircraft, the aviator exchanges airspeed for lift by decelerating the aircraft during the last
150 feet. When executed correctly, deceleration is applied and timed so that the rate of descent
and forward airspeed are minimized just before touchdown. At about 15 feet, this energy
exchange is essentially complete. The primary remaining control input is application of
collective pitch to cushion the touchdown. Because all helicopter types are slightly different,
aviator experience in that particular aircraft is the most useful tool to safely land the helicopter.
4.
Flare and Touchdown
Figure 6-8 shows the airflow and blade element diagram for a blade in a steady-state autorotation
compared to the same blade during the deceleration flare. To make an autorotative landing, the
aviator must reduce airspeed and rate of descent just before touchdown. The aviator can partially
accomplish both actions by applying aft cyclic, which changes the attitude of the rotor disk in
relation to the relative wind. This nose-up cyclic flare tilts the rotor disk rearward, which
inclines the resultant thrust of the rotor system to the rear, slowing forward speed. It also
increases AOA on all blades by changing the direction of airflow through the rotor system. The
resulting increase in AOA creates more lift and causes the lift vector to become more vertical,
which decreases rate of descent. Moreover, the downward shift in relative wind tilts the lift
vector more forward, resulting in a larger pro-autorotative force which increases rotor RPM. The
increase in RPM can be used to cushion the landing but must be monitored to prevent over
speeding the rotor head. Additionally, the flare exposes more of the fuselage to the airstream,
thereby increasing fuselage parasitic drag, further slowing the aircraft down.
6-16 AUTOROTATION
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER SIX
Figure 6-8 Blade Element and Thrust During Steady-State Auto and Flare
If a pilot arrives at flare altitude at less than the minimum rate of descent airspeed, there is little
or no forward speed to trade off to increase rotor RPM and braking action. Forward speed is
already low, and if too much flare is combined with an improperly timed flare (too high),
forward speed may reduce to zero at a high altitude. This condition is known as becoming
vertical. Since the rotor system has little stored energy, when the pilot increases the collective,
there will not be enough thrust available to slow rate of descent to a safe touchdown speed.
The flare should be maintained to reach a point where forward speed is slow enough for a safe
and controlled power off landing. At this point, increasing collective increases thrust (trading
RPM for lift) and augments braking action, using part of the stored rotational energy. Due to the
aft-tilted thrust vector and the addition of collective, the pilot must put in forward cyclic to level
the aircraft and use that last rotational energy by pulling collective to cushion the landing.
Since there is no torque from the engine, drivetrain drag may cause the fuselage to follow the
rotor system when collective is pulled, causing the nose to yaw to the left and requiring some
right rudder, the opposite of powered flight. The key is to maintain heading control throughout
the autorotation using the rudder pedals as necessary.
The goal in any autorotation is to arrive at the optimum deceleration attitude just prior to the
collective pull. This allows the pilot to take the kinetic energy from the descent, translate it into
rotor RPM, and use the increased rotor RPM to cushion the touchdown. The deceleration flare
AUTOROTATION
6-17
CHAPTER SIX
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
attitude cannot be held for too long. If the flare is held for too long, it will result in an increased
descent rate, due to an increased stall region causing a loss of RPM.
5.
Windmill Brake State
The windmill brake state occurs when almost the entire rotor system is acting as the driving
region and accelerating the blades. The windmill brake state exists during the entry and the flare
of the autorotation.
During entry into autorotation, the sudden lowering of the collective and the possible aft cyclic
cause a large amount of upward inflow (low induced velocity) to occur. The greater the upward
inflow, the further forward the TAF moves, which increases the blade’s horizontal thrust and
increases rotor RPM. Once the helicopter is stabilized, it transits to the steady-state where the
rotor system divides itself into the three regions (driven – driving – stall).
At the bottom, the flare causes an increase in upward inflow again and the rotor system enters the
windmill brake state again. This state, combined with the Coriolis Effect due to coning, is what
helps build the rotor RPM during the flare. This combined effect is very pronounced in a lowinertia rotor system.
606. HEIGHT-VELOCITY DIAGRAM
No matter how well the pilot can execute an autorotation, there are some combinations of initial
altitudes and airspeeds from which a safe autorotational landing will be extremely difficult. In
fact, at some combinations of altitude and forward speed, it is almost impossible to perform an
autorotative landing without exceeding the landing force limits of the landing gear. The
boundaries of these combinations define the H-V diagram (see Figure 6-9).
Figure 6-9 Generic Height-Velocity Diagram
6-18 AUTOROTATION
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER SIX
The purpose of an H-V diagram is to identify the portions of the flight envelope from which a
safe landing can be made in the event of a sudden engine failure. The H-V diagram depicts the
areas to avoid.

Low-speed region. In the low-speed region, the helicopter does not have sufficient
altitude available to establish a steady-state glide and minimum flare airspeed. When
the engine fails in this sector a rapid rate of descent will occur, but with little or no
forward airspeed, a flare will not be capable of arresting the descent prior to landing.
Increasing the collective will cause the RPM to decay rapidly resulting in a hard
landing because of the limited rotor rotational energy available.

Low-altitude/high-speed region. At low altitude and high speed, a quick cyclic flare
can transfer kinetic energy to the rotor, provided time is sufficient to initiate the
maneuver before ground impact. In the low-altitude/high-speed region, velocity is
too great for a safe sliding landing, but altitude is too low for flare initiation. By the
time the pilot reacts (using typical reaction time) with a flare or zoom climb, the tail
sinks enough to impact the ground.

High hover height. At altitudes above the low airspeed avoid region, the pilot can
enter autorotation by making a diving transition to forward flight, reaching the
desired autorotation airspeed and then executing a normal flare.

Low hover height. Below the low hover height benchmark, a pilot can handle a
power failure by coming straight down and increasing the collective to cushion the
landing. Kinetic energy stored in the main rotor is traded in the cushion. The low
hover height benchmark is a function of the power required for hover, rotor inertia,
blade area, stall characteristics, and the capability of the landing gear to absorb the
landing forces without sustaining damage.
The size of the avoid region is affected by several variables.

Rotor inertia determines how quickly the rotor RPM decreases. A low-inertia
system would be more likely to lose valuable RPM before an autorotation is
established, so it will have a larger avoid area than a helicopter with high rotor inertia.
Low-inertia heads move the knee to the right.

High gross weight increases power required for flight, which in turn increases rate of
descent immediately after the engine fails and thus increases the size of the avoid
area. Higher gross weights will move the knee right.

DA decreases rotor efficiency and increases power required, so it has the same effect
as increased gross weight on size of the avoid area.

Average pilot reaction time is used to establish H-V diagrams. This is done by
specifying a definite delay time following the engine failure before initiating control
input. The military assumes that most pilots take about two seconds to respond to an
engine failure in normal flight conditions.
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
There are H-V diagrams for each type of helicopter. They are found in their respective NATOPS
manuals and all pilots should be familiar with the associated airspeed and altitude limitations
found in these diagrams. The H-V diagram for the TH-73A is pictured in Figure 6-10.
Figure 6-10 TH-73A Height-Velocity Diagram
6-20 AUTOROTATION
HELICOPTER PERFORMANCE PLANNING
700. INTRODUCTION
This chapter correlates helicopter aerodynamics principles with engine and aircraft performance.
All NATOPS publications include performance charts for all regimes of flight that can be used to
accomplish preflight planning and during flight as needed. A wide range of helicopter design
configurations and mission specific equipment exist in Naval Aviation, but the fundamentals of
performance relationships remain the same. An aviator flying the CH-53K at its 88,000 pounds
maximum gross weight will use the same performance planning considerations as an aviator in
the TH-73A.
701. LEARNING OBJECTIVES

Identify factors that affect helicopter performance.

Identify helicopter components.

Identify types of power.

State the difference between power requirements.

Recall the impact of speed, weight, and altitude on climb performance.

State the effect of tail rotor thrust power required and power available.
702. HELICOPTER PERFORMANCE FACTORS
1.
Weight
Weight is one of the most important factors affecting performance because the pilot can control
it. Most performance charts include weight as one of the variables. By reducing the weight of
the helicopter, a pilot may be able to take off or land safely at a location that otherwise would be
impossible. If ever in doubt about whether a takeoff or landing can be performed safely, delay
your takeoff until more favorable DA conditions exist. If airborne, try to land at a location that
has more favorable conditions, or one where a landing can be made that does not require a hover.
At higher gross weights, the increased power required to hover produces more torque, which
means more anti-torque thrust is required. During high-altitude operations in some helicopters,
the maximum anti-torque produced by the tail rotor during a hover may not be sufficient to
overcome torque, even if the gross weight is within limits.
HELICOPTER PERFORMANCE PLANNING 7-1
CHAPTER SEVEN HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
2.
Wind Direction and Velocity
Wind direction and velocity also affects hover, takeoff, and climb performance. Translational
lift occurs any time there is relative airflow over the rotor disk. This occurs whether the relative
airflow is caused by helicopter movement or by the wind. Assuming a headwind, as wind speed
increases, translational lift increases, resulting in less power required to fly.
The wind direction is also an important consideration. Headwinds are the most desirable as they
provide the greatest increase in performance. Strong crosswinds and tailwinds may require the
use of more tail rotor thrust to maintain directional control. This increased tail rotor thrust
requires power from the engine, which means there is less power available to the main rotor for
the production of lift. Some helicopters have a critical wind azimuth or maximum safe relative
wind chart. Operating the helicopter beyond these limits could cause Loss of Tail Rotor
Effectiveness (LTE).
Take off and climb performance is greatly affected by wind. When taking off into a headwind,
ETL is achieved earlier, resulting in more lift and a steeper climb angle. When taking off with a
tailwind, more distance is required to accelerate through translation lift.
3.
Density Altitude
DA contributes significantly to rotorcraft performance. An increase in DA degrades rotor
performance and all NATOPS manuals include DA as a factor to account for during performance
planning.
Humidity alone is not usually considered an important factor in calculating DA and helicopter
performance; however, it does contribute to DA. Some manufacturers include charts with 80%
RH columns as additional information. At 80% humidity there is an approximately 3–4 percent
reduction in performance compared to dry air at the same altitude and temperature, so expect a
decrease in hovering and takeoff performance in high humidity conditions. Although 3–4
percent seems insignificant, it can be the cause of a mishap when operating near the limits of the
helicopter.
It is critically important to conduct in-depth performance planning prior to flight, as well as
careful re-evaluation of the expected performance during flight should the mission require any
change to the planned flight. When the margin for error is minimal, unnecessary maneuvering
should be kept to a minimum, and increased vigilance is required to be best prepared for any
unanticipated situation. As with any other aspect of aviation, expect the unexpected.
703. POWER PERFORMANCE
Helicopter and engine performance require an understanding of the power required curves,
power available curves, and the relationship between them. Power is required to generate lift
and overcome the drag produced by the rotors and the fuselage. The power available to meet this
power requirement is produced by a turboshaft engine. A turboshaft engine is a jet engine
optimized to produce shaft power rather than jet thrust. In helicopters, the shaft power is used to
drive the main and tail rotor blades via a transmission gearbox.
7-2
HELICOPTER PERFORMANCE PLANNING
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER SEVEN
Turbojet engines, on the other hand, produce thrust directly from their engines and do not turn a
propeller or rotor. As such, jet aircraft performance charts look significantly different from
helicopter performance charts (Figure 7-1). For each pound of drag generated by a jet aircraft at
a specific airspeed, a pound of thrust must be generated by the jet in order to maintain level
flight. The amount of thrust produced by a jet engine is directly proportional to fuel flow;
therefore, endurance and range performance may be determined from an aircraft performance
curve.
Figure 7-1 Jet Thrust Vs Turboshaft Power
The differences between helicopter and jet performance curves can be attributed to the different
contributions of profile and induced drag in helicopters. The helicopter rotor also produces
thrust, but the production of thrust is not directly related to the fuel flow for turboshaft engines.
Turboshaft fuel flow is more closely related to how much power is being produced by the
engine. As a result, a total drag curve cannot be used as a performance chart for helicopters in
the same way it could in a jet aircraft.
Instead, power required curves (or more specifically, fuel flow curves) are presented in rotorcraft
NATOPS for use in mission planning (Figure 7-2). Power is simply the rate of doing work.
Turbine helicopters are equipped with a gauge for measuring torque, which the aviator uses in
the cockpit to measure power. Since power equals torque times RPM, if the RPM remains
constant, the torque is a direct representation of current engine power output. Further, a fuel
flow scale is usually provided opposite the torque scale of a cruise chart, thereby enabling the
aviator to convert torque directly to fuel flow.
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Figure 7-2 Power Required Curve
It is important to note that the lowest point on the power required curve is the point of minimum
power required (best lift to drag ratio) and not necessarily the point of minimum drag (as is the
lowest point on the total drag curve). The point of minimum power required results in the lowest
fuel flow and is, therefore, the airspeed for maximum endurance. The airspeed for minimum
power required is slower than the airspeed for minimum total drag because a decrease in velocity
to the minimum power required airspeed decreases the power required, even though flying at any
airspeed below minimum drag actually increases drag. Because the bottom of the total drag
curve is nearly flat, the slight increase in drag is more than offset by the decrease in velocity,
which slows the work rate resulting in an overall reduction in power required.
1.
Power Required
For a helicopter to remain in steady, level flight, forces and moments must balance. These forces
exist in the vertical plane, horizontal plane, and about the CG in the form of pitching, rolling, and
yawing moments.
In a hover, two types of power are necessary: induced and profile power. Induced power is
power associated with the production of rotor thrust. This value is at its highest during a hover
(60–85 percent of total main rotor power) and decreases rapidly as the helicopter accelerates into
forward flight. During forward flight, the increased mass of airflow introduced to the rotor
system reduces the amount of work the rotors must produce to maintain a constant thrust.
Therefore, induced power required continues to go down with increasing airspeed.
Profile power, which can be thought of as the power required to turn the main rotor, accounts for
15–40 percent of main rotor power in a hover and is used to overcome friction drag on the
blades. It increases slightly with increasing airspeed.
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In forward flight, parasite power is also required to overcome the parasite drag generated by all
the aircraft components, excluding the rotor blades. Parasite power can be thought of as the
power required to move the aircraft through the air. This power requirement increases in
proportion to forward airspeed cubed. Parasite power is inconsequential at low speed, but is
significant at high speed and is an important consideration for helicopter designers to minimize
drag.
In addition to the drag curves, which are the basis for the power required curves, there is a fourth
power requirement, labeled miscellaneous. This takes into account specific power required
curves for individual types of rotorcraft. This is the power required to run the tail rotor and
accessories such as generators and hydraulics. Accessory power requirements remain relatively
constant regardless of airspeed, with the exception of the tail rotor. Tail rotor power required
tends to decrease with increasing airspeed. Depending on the charts used, the miscellaneous
power requirement is sometimes combined with the profile power requirement, creating a total
rotor profile power required to maintain a given rotor RPM, taking into account the rotor profile
drag as well as the tail rotor and accessory requirements.
Different airspeeds optimize flight characteristics. The bowl shape of the power required curve
in Figure 7-3 graphically illustrates the reason why. Optimum speeds determined by this curve
are best rate of climb, maximum endurance, minimum rate of descent in autorotation, maximum
range, maximum glide in an autorotation, best angle of climb, and maximum airspeeds.
Figure 7-3 Optimum Airspeeds
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Best rate of climb airspeed is formed at the point where there is the largest difference between
maximum power required and power available. The bottom of the curve is called the bucket
airspeed. Since the goal of achieving maximum endurance is making the available fuel last as
long as possible, and since fuel flow is proportional to engine power, maximum endurance speed
is at this point. The minimum rate of descent in an autorotation is also found near this speed,
since the power required to keep the aircraft airborne is at a minimum.
The point on the power required curve corresponding to the point of minimum drag versus
airspeed is the maximum range airspeed. At the maximum range airspeed, the ratio of fuel flow
to velocity is at a minimum. This point is shown in Figure 7-3 where the tangent line meets the
power required curve and shows the best power (fuel flow) to airspeed (drag) ratio. Maximum
range airspeed will always be greater than bucket airspeed.
Maximum range speed can also be found on the fuel flow curve by drawing a line tangent to the
curve from the origin (Figure 7-4). This ratio of speed to fuel flow shows the distance the
helicopter can travel on a pound of fuel on a no-wind day. If there is a headwind, the line should
be originated at the headwind value, which derives a higher speed and lower range. For a
tailwind, the optimum airspeed decreases, but the range increases significantly. On generic
charts, the speed for maximum range and autorotation maximum glide distance sometimes
appear to be the same. However, when using aircraft specific charts, the two speeds are usually
different.
Figure 7-4 Maximum Range Airspeed Adjustment for Winds
A helicopter’s best angle of climb is a vertical climb at zero KIAS. The helicopter can only
achieve the performance if the power available at zero airspeed is greater than the power
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required at zero airspeed. If operating at high gross weights in high DA, the power available
may not be sufficient to achieve a zero KIAS climb. In this case, the pilot must pilot must utilize
a maximum gross weight takeoff and remain in ground effect until reaching the airspeed where
the curve of power available intersects the curve of power required. The lowest speed at which
this condition is achieved is the speed for the best angle of climb.
The maximum airspeed the helicopter can achieve in level flight is where the power available
and power required intersect, as shown in Figure 7-3. In general, helicopters do not have a
minimum airspeed. Minimum helicopter airspeed is a condition that arises when flying at high
gross weight in high DA or when a loss of power available occurs. In these cases, the power
available and power required curves intercept on the low end at an airspeed greater than zero.
2.
Power Available
Power available is the amount of power an engine is capable of producing based on current
conditions. Power available is affected most significantly by DA and engine performance. Less
dense air requires the engine to work harder to produce the same amount of mass flow (airflow
through the engine). In locations of extremely wide temperature variations, such as the desert
environment, high temperatures can also have an extra degrading effect on engine power
available. Operating conditions that affect fuel flow or airflow directly affect the ability of the
engine to generate power.

Fuel flow limitation (cold) – As temperature decreases, the density of air increases
so the fuel flow must increase in order to maintain the stoichiometric fuel/air ratio for
complete combustion. However, the amount of fuel flow through the fuel nozzles has
a limit. Therefore, at very cold temperatures, the fuel/air ratio will not be optimum,
and incomplete (lean) combustion will occur, resulting in less power available.

Turbine temperature limitation (hot) – The materials used to build turbines have
definite stress and temperature limits. To avoid unacceptable creep or component
failure, turbine temperature must be limited. Depending on the where the temperature
of the engine is taken, it can be called Inter-Turbine Temperature (ITT), Exhaust Gas
Temperature (EGT), Turbine Outlet Temperature (TOT), or Turbine Inlet
Temperature (TIT).

Gas generator limitation (hot) – As the OAT increases or the density of air
decreases, the gas turbine has to rotate faster in order to deliver the same air mass
flow rate. The increased rotational speed required at higher temperatures or higher
altitudes can approach limits that have been established to counter centrifugal loads
on the gas turbine blades.

Age of the engine – Compressor blades erode with time, and their degradation results
in a loss of blade area that will degrade engine performance.

Component rating degradation – Transmission components have material
limitations, so their torque capacity must be considered. An engine may be
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CHAPTER SEVEN HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
artificially limited so it does not produce more power than the transmission can
handle.

Humidity/moisture effect – Increases in humidity decrease the air density, which
degrades engine performance. However, increased humidity or moisture may help
reduce the combustor inlet temperature, which improves engine performance. As a
result, humidity and moisture have a negligible effect on gas turbine engines.

Torque limits – Drive train limits, including drive shaft and transmission may have
torque limitations to prevent damage to components.

Airspeed effects (ram air) – Airspeed increases the flow rate into the engine, but at
the speed at which rotorcraft operate this effect is negligible.
The tail rotor does not affect the power available that the engine produces, but it does affect the
power required as a miscellaneous system. Since the engine drives both the main rotor and the
tail rotor, the power available from the engine must be shared between the two rotor systems.
When the tail rotor requires more power, less power is available for the main rotor. The tail rotor
uses 5–15 percent of the total power available, leaving only 85–95 percent for the main rotor.
Although other frictional losses of the drive train may be significant, the tail rotor robs the
greatest amount of power from the main rotor. The tail rotor makes its greatest demands on
engine power available when the greatest requirements are on the main rotor. For example, in a
climb, or at the termination of a steep approach, when power required to perform a maneuver is
closer, or equal to the power available. The main rotor system is creating the greatest amount of
torque when the power required exceeds the power available and rotor RPM starts drooping.
3.
Power Required Exceeds Power Available
When power required for a maneuver exceeds power available (PR>PA) under the ambient
conditions, an un-commanded descent or deceleration will result. Factors that may aggravate a
PR>PA situation include high G-loading, high gross weight, high DA, rapid maneuvering, loss of
wind effect, loss of wind direction, and loss of ground effect (transiting off the deck of a ship or
pinnacle takeoff).
Along with an un-commanded descent, a decrease in rotor RPM and un-commanded right yaw
may result. The right yaw is due to a Loss of Tail Rotor Authority (LTA) during which full left
anti-torque pedal does not stop the right yaw.
In addition to proper performance planning and situational awareness regarding the above
aggravating factors, the pilot should avoid excessive maneuvering, high descent rates, and
downwind takeoffs and landings, especially in environmental conditions where power available
may be marginal.
Power required exceeding power available is differentiated from Vortex Ring State (VRS) by
un-commanded descent being associated with maximum allowable torque and/or rotor droop and
possible LTA. VRS is not normally associated with either rotor droop or LTA.
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4.
Induced power
Induced power requirements are greatest in a hover and decrease as forward airspeed increases
(Figure 7-5). The requirement for a mass flow of air still exists, but forward velocity increases
the mass flow rate so the rotor does not need to apply as much work on the air, thus less power is
required. At speeds beyond that at which the tip vortices are outrun (speed for translational lift)
the rotor disk acts in a manner that is similar to a conventional wing.
Figure 7-5 Induced Power Requirements
704. PARASITE POWER
Just as high induced power requirements can cause a power required exceeding power available
situation at a hover or slow airspeeds, parasite power requirements can cause the situation at
higher speeds (Figure 7-6).
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Figure 7-6 Parasite Power Requirements
The point where power required exceeds engine power available at high speed is commonly
referred to as VH or the maximum speed in level flight at maximum power without developing
an un-commanded descent. VH should not be confused with VNE (Velocity Never Exceed) which
is a structural limitation. VNE is specified as a red line on many airspeed indicators. VH is
specific to the aircraft model and represents the edge of its performance envelope in terms of
speed. VH is achieved when the engine is performing at its best and the pilot is using all the
power available. High gross weight, high DA, and operating engines with marginal health are
the major factors affecting VH.
1.
Profile Power
Profile power requirements remain constant among different flight regimes. Profile power
requirements depend on the main rotor blade’s shape and the smoothness of their surface.
Blades with efficient aerodynamic profiles and smooth, clean surfaces have lower profile power
requirements. This power requirement alone will not exceed the power available, but its value is
added to the induced power at low airspeeds and parasite power at high speeds which might
generate a power required exceeds power available situation.
2.
Excess Power
Because power required exceeding power available is often associated with lower airspeeds, the
induced power requirement becomes the critical factor. As airspeed decreases during the
transition to a hover, induced power required increases due to a loss of translational lift and the
excess power available decreases (Figure 7-7). Each aircraft is inherently different, and
experiences a different amount of translational lift at different airspeeds. Typically, translational
lift is experienced during transition from hover into forward flight at airspeeds between 13–24
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knots, depending on disk size, blade area, and main rotor RPM.
Figure 7-7 Decrease in Excess Power as Airspeed Decreases
When a helicopter transitions from forward flight to a hover, it experiences decreased
performance because of increased induced power requirements that stem from the tip vortices
generated in a hover. As airspeed decreases to near 13–24 knots, the main rotor begins to
experience recirculation of vortices. Vortices also impact the fuselage and the tail. Power
required increases and if power available is not sufficient, an un-commanded descent, rotor
droop, and un-commanded right yaw may develop.
705. HOVER PERFORMANCE
Helicopter performance revolves around whether or not the helicopter can maintain altitude in a
hover. More power is required during the hover than in any other flight regime. Obstructions
aside, if a hover can be maintained, a takeoff from a hover can be executed, especially with the
additional benefit of translational lift. Aircraft specific NATOPS publications contain charts for
HIGE and HOGE under various conditions of gross weight, altitude, temperature, and power.
The highest altitude a helicopter can maintain HIGE is usually higher than HOGE because of the
added lift benefit produced by ground effect. A pilot should always plan HOGE when landing in
an area that is uncertain or unverified.
As DA increases, more power is required to hover. At some point, the power required is equal to
the power available. This establishes the hovering ceiling under the existing conditions. Any
adjustment to the gross weight by varying fuel, payload, or both, affects the hovering ceiling.
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CHAPTER SEVEN HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
The heavier the gross weight, the lower the hovering ceiling. As gross weight is decreased, the
hover ceiling increases.
706. CLIMB PERFORMANCE
Most of the factors affecting hover and takeoff performance also affect climb performance. In
addition, turbulent air, pilot techniques, and overall condition of the helicopter can cause climb
performance to vary.
A helicopter flown at the best rate-of-climb speed (Vy) obtains the greatest gain in altitude over a
given period of time. This speed is normally used during the climb after all obstacles have been
cleared and is usually maintained until reaching cruise altitude. Rate of climb must not be
confused with angle of climb. Angle of climb is a function of altitude gained over a given
distance. The Vy results in the highest climb rate, but not the steepest climb angle, and may not
be sufficient to clear obstructions. The best angle of climb speed (Vx) depends upon the power
available. If there is a surplus of power available, the helicopter can climb vertically, so Vx is
zero.
Wind direction and speed have an effect on climb performance, but it is often misunderstood.
Airspeed is the speed at which the helicopter is moving through the atmosphere and is unaffected
by wind. Atmospheric wind affects only the ground speed, or speed at which the helicopter is
moving over the Earth’s surface. This means that the only climb performance affected by
atmospheric wind is the angle of climb, not the rate of climb (Figure 7-8).
Figure 7-8 Rate of Climb Vs Best Angle of Climb
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707. CRUISE PERFORMANCE
Airspeeds for maximum speed, maximum range, and maximum endurance are distinct, and vary
with aircraft loading and environment. Choosing the appropriate level-flight airspeed is an
important part of obtaining the best performance for a mission. In a given set of conditions, one
performance parameter may be more crucial than others, so flight at the airspeed that would
maximize that potential makes sense. For example, if holding while awaiting deck-landing space
on a ship, the pilot should fly at the airspeed that gives the best endurance. In a long overwater
mission, best range may be appropriate. Maximum speed is required in a time critical situation,
such as a MEDEVAC. Fortunately, the required airspeed for any of these situations is easily
found on a power required versus airspeed chart (Figure 7-9).
Figure 7-9 Power Required Vs Airspeed
In some situations, a helicopter may consume less fuel at a rotor speed below 100 percent. This
benefit only occurs when profile power is a major contributor to power required, so it only
applies to a certain extent. When the rotor speed gets too slow the increase in AOA required to
generate lift at a slower rotational speed generates excessive drag forces.
In addition to possible drag increases, decreasing NR for fuel efficiency can present other
problems. Decreased main rotor speed produces lower tail rotor speed, so LTE can increase
power demands and, in the extreme case, make LTA more likely. Additionally, in the event of
an engine failure, rotor RPM will decay to dangerous levels more quickly.
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Nonetheless, it is true that in some cases a decrease in NR can yield a decreased fuel flow that
will increase range and endurance. Figure 7-10 shows that fuel flow increases at high gross
weights because the rotor system is attempting to lift a heavier aircraft at slower than optimal
rotor speed.
Figure 7-10 Example of RPM Vs Fuel Flow
Thus, fuel conservation benefits occur primarily at lower percentages of maximum gross weight.
Even at lower gross weights, use of this technique should be carefully considered for its
necessity, thoroughly planned, and not used routinely. One hundred percent NR (102 percent in
the TH-73A) is established by designers for good reasons, and is the best rotor speed for most
operations.
How does fuel consumption change with altitude? The specific fuel consumption of the gas
turbine varies with two primary operating parameters: temperature and power output. The
specific fuel consumption is defined as nautical miles per pound of fuel (Figure 7-11).
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Figure 7-11 Maximum Range Altitude Vs Gross Weight
As density is decreased, both the fuel flow and Shaft Horsepower (SHP) decrease proportionally,
so it can be said that density variations do not by themselves influence specific fuel consumption
(disregarding profile and parasitic drag). However, as altitude is increased, temperature
normally decreases. Because the turbine may deliver a given thrust output with less fuel at a
lower inlet temperature, the specific fuel consumption normally improves (decreases) with
altitude. If the atmosphere can be considered standard, the specific fuel consumption decreases
to the tropopause, and then remains constant until the efficiency of the compressor begins to
break down at sufficiently high altitudes. The standard atmosphere has a temperature decrease
up to the tropopause (approximately 36,000 feet).
Specific fuel consumption also varies with power output. The gas turbine is designed so that it
operates most efficiently at high power outputs. This means that the specific fuel consumption is
lowest at higher power settings, and that 100 percent Engine Gas Generator Speed (Ng) is the
optimum speed for greatest efficiency. The total fuel consumption does not go down at high
power settings; specific fuel consumption, or pound of fuel per hour per SHP does. For a given
amount of SHP output, the least amount of fuel is burned (highest efficiency) at high power
settings. This situation poses an interesting problem for helicopters. Because helicopters use
turboshaft engines, the fuel efficiency of the helicopter is determined by the efficiency of both
the engine and the rotor system. An increase in DA requires more work by the rotor system for
the same flight profile. Engine efficiency gains at altitude are balanced by rotor efficiency
losses. Actual fuel efficiency obtained at altitude thus depends upon rotor system efficiency,
installed aircraft engine characteristics, work output requirement, and total fuel load. In general,
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at low gross weights one gets better range at higher altitude while at high gross weights a better
range is achieved at sea level.
As might be expected, fuel flow increases at higher gross weights. As aircraft gross weight
increases, power required increases and hence fuel flow increases. In addition, the airspeed for
maximum endurance also increases due to the shift in the power-required curve. Maximum
range airspeeds increase with increasing gross weight due to the power-required curve shifting
up and to the right (see Figure 7-12). The increase in maximum endurance air speed due to an
increase in gross weight is universally true, but varying shapes of the power-required curve for
some helicopters make the trend of best range airspeed unpredictable. A survey of several fleet
helicopters reveals that maximum range (called long range in Figure 7-12) airspeed shifts depend
upon the aircraft and operating environment. For example, the MH-60R shows decreased
maximum range airspeed at higher gross weights for the conditions given.
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Figure 7-12 Maximum Range Airspeed Vs Gross Weight
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7-18 HELICOPTER PERFORMANCE PLANNING
FLIGHT PHENOMENA
800. INTRODUCTION
This chapter introduces various phenomena associated with helicopter flight. Knowing these
phenomena and the limitations they impose is critical.
801. LEARNING OBJECTIVES

Describe flight envelopes and V-n diagrams.

Describe helicopter CG concepts.

Describe dynamic rollover.

Describe rotor dynamics.
802. FLIGHT ENVELOPE/V-N DIAGRAM
Helicopter design is dictated by the expected use. Performance and load bearing requirements
for military aircraft are set by the Department of Defense (DoD). They are set by the FAA and
the engineering and sales departments for civil aircraft. Anticipated strength requirements to
meet design criteria at a range of speeds are consolidated in a V-n diagram.
The V-n diagram or V-G diagram (Figure 8-1) is a graph that summarizes an aircraft’s structural
and aerodynamic limitations at a particular weight, altitude, and configuration. The horizontal
axis is indicated airspeed. The vertical axis of the graph is load factor, or G’s. V-n diagrams
define the maneuvering envelope for fixed-wing aircraft and rotary-wing aircraft.
FLIGHT PHENOMENA
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CHAPTER EIGHT HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 8-1 V-n Diagram for Fixed-Wing Aircraft
Helicopter NATOPS manuals typically do not have V-n diagrams because most helicopters do
not have G-meters; therefore, pilots are unable to gauge loading. Rather, AOB limitations are
developed with consideration for associated load factors and general maneuver restrictions keep
the aircraft in the envelope.
Several critical factors are identified on the V-n diagram. Even though NATOPS typically does
not include a V-n diagram, consideration of the following factors goes into development of those
maneuver limits and they are worth knowing about before flying in critical situations.
8-2

Limit load factor is the greatest load factor an aircraft can sustain without any risk of
permanent deformation. The top and bottom of the V-n diagram in Figure 8-2 are
established by the structural limit line, or limit load factor. It is the maximum load
factor anticipated in normal daily operations. If the limit load factor is exceeded,
some structural damage or permanent deformation may occur. Aircraft will have
both positive and negative limit load factors.

Overstress/over-G is the condition of possible permanent deformation or damage
that results from exceeding the limit load factor. This type of damage will reduce the
service life of the aircraft because it weakens the aircraft’s basic structure.
Overstress/over-G may occur without visibly damaging the airframe. A variety of
components is inside the aircraft, such as hydraulic actuators and engine mounts,
which are not designed to withstand the same loads that the airframe can.
FLIGHT PHENOMENA
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER EIGHT
Figure 8-2 AH-64 Apache V-n Diagram
1.
Ultimate Load Factor
Ultimate load factor is the maximum load factor that the aircraft can withstand without structural
failure. There will be some permanent deformation at the ultimate load factor, but no actual
failure of the major load-carrying components should occur. If you exceed the ultimate load
factor, structural failure is imminent (something major on the aircraft will break). The ultimate
load factor is 150 percent of the limit load factor. Increases in gross weight and altitude require
increases in AOA and lifting forces, so the G envelope is reduced due to increased structural
bending and blade flapping limits (see Figure 8-2).
2.
Lift Limit
Lift limit is shown on the left-hand side of the V-n diagram. This is the maximum load factor
available at a given airspeed. Attempted hard aft cyclic maneuvering at low speeds will result in
further reduced speed and will bring the helicopter closer to its G limit. Many helicopters have
the capability of generating in excess of four transient G’s at high speed, but it is unusual for a
helicopter to be able to sustain more than two G’s.
3.
Limit Airspeed
The vertical line on the right side of the V-n diagram is called the redline airspeed (VNE) and is
the highest airspeed that an aircraft is allowed to fly. Flight at speeds above VNE can cause
FLIGHT PHENOMENA
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CHAPTER EIGHT HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
structural damage. VNE is determined primarily by excessive structural loads and power
available, but may also be affected by controllability limits, blade Mach limits, or airframe
temperature in jets designed to fly at supersonic speeds.
If an aircraft or component (advancing blade) reaches its Critical Mach Number (MCRIT), but it is
not designed to withstand supersonic airflow, the shock waves generated may damage the
structure of the blade and main rotor hub, drastically affecting the production of main rotor
lift/thrust.
VAFT is the maximum allowable rearward speed. This may be a structural limit, but
rotor/airframe configuration and rearward visibility from the cockpit are also factors, and V AFT
may be made as high as it is thought safe to test for.
Maneuvering speed occurs at the intersection of the lift limit and structural limit lines.
Maneuvering airspeed (Va), also known as the corner airspeed, is the maximum speed at which
full control deflection can be abruptly applied without overstressing the aircraft. Va varies with
aircraft weight, just as the size of the maneuvering envelope changes with weight. Above Va, the
rotor can generate high aerodynamic loads in excess of the limit load or can be pushed into
retreating blade stall. Below Va, the helicopter is aerodynamically limited by rotor thrust. At the
maneuver speed, the aircraft can achieve limit load at a low speed, so it offers maximum turn
rate and minimum turn radius.
Gust loading refers to the increase in the G load due to vertical wind gusts. The load imposed by
a gust is dependent upon the velocity of the gust. The higher the velocity, the greater the
increase in load. If an aircraft were generating the limit load factor during a maximum
performance turn and hit a vertical gust, the gust would instantaneously increase the AOA of the
airfoils and increase the lift on the rotor blades, enough to raise the G load above the limit load
factor. This is why intentional flight through severe or extreme turbulence and thunderstorms is
prohibited in many aircraft.
803. MOMENTS AND CENTER OF GRAVITY
It is often assumed that the CG of the helicopter is below the rotor shaft, but this it is not always
true. Loading passengers, fuel, cargo, and weapons can move the CG of the aircraft forward or
aft and left or right, potentially creating a longitudinal moment around the lateral axis and a
lateral moment around the longitudinal axis. Aerodynamic forces will create a moment around
the CG that will displace the helicopter if not corrected.
During flight, the position of the CG and the weight of the helicopter will change because the
aircraft will burn fuel. As a result, pilot control inputs required to maintain the same flight
attitude will change too.
For helicopters with a single main rotor system, their weight is supported at one point (the main
rotor system or mast) while in flight. If the helicopter were suspended by the rotor head, the
distribution of its total weight would determine how level the fuselage hangs. Since the fuselage
acts as a pendulum suspended from the rotor, changing the CG changes the angle at which the
aircraft hangs from the rotor.
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER EIGHT
In order to hover in no-wind conditions, the main rotor’s thrust/lift must be aligned with the CG.
If the thrust is not aligned with the CG, the helicopter will move laterally or longitudinally and
the pilot must displace the cyclic control in order to keep the thrust vector vertical. If the CG is
far away from the mast, a substantial amount of cyclic control will be required to compensate,
and there may not be enough cyclic authority left to control the helicopter.
CG limitations are important for aircraft operation. For example, if the helicopter is loaded with
too much weight up front, it will tilt more forward in flight. When slowing down, stopping, or
hovering, pilots might not have enough aft cyclic movement available to counteract this forward
tilt. The same would be true for lateral or aft CG situations. Every helicopter control input has a
mechanically limited range of movement and pilots should check the freedom of movement
within that range before each flight.
Failure to ensure the aircraft is within the allowable CG range specified in the NATOPS may
limit the aviator’s ability to control the aircraft in flight.
804. DYNAMIC ROLLOVER
A helicopter is susceptible to a lateral rolling tendency, called dynamic rollover, when it is in
contact with the ground during takeoffs or landings and a factor causes the helicopter to roll or
pivot around a skid or landing gear wheel until its critical rollover angle is reached (Figure 8-3).
The critical rollover angle is the angle at which main rotor thrust continues the rolling moment
and recovery using cyclic input alone is not possible. The critical rollover angle varies between
helicopter types and is usually delineated in NATOPS. If the critical rollover angle is exceeded,
the helicopter rolls on its side regardless of the cyclic corrections made.
Certain conditions reduce the critical rollover angle, thus increasing the possibility for dynamic
rollover and reducing the chance for recovery. The rate of rolling motion is also a consideration
because, as the roll rate increases, there is a reduction of the critical rollover angle at which
recovery is still possible. Other critical conditions include operating at high gross weights with
lift approximately equal to the weight.
FLIGHT PHENOMENA
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CHAPTER EIGHT HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 8-3 Dynamic Rollover
A dynamic rollover can occur for a variety of reasons, including the failure to remove a tie down
or skid-securing device, if the skid or wheel contacts a fixed object while hovering sideward, or
if the gear is stuck in ice, soft asphalt, or mud. It may also occur if you use an improper landing
or takeoff technique or while performing slope operations. In either case, the following three
factors must be present: rolling moment, pivot point, and thrust approximately equal to or
greater than weight.
The following conditions are most critical for helicopters with counterclockwise rotor rotation:

Right side skid or landing wheel down landing, since translating tendency adds to the
rollover force

Right lateral CG

Crosswinds from the left

Left yaw inputs
For helicopters with clockwise rotor rotation, the opposite conditions would be true.
Dynamic rollover is possible even during normal takeoffs and landings on relatively level
ground, if one wheel or skid is on the ground and thrust (lift) is approximately equal to the
weight of the helicopter. If the takeoff or landing is not performed properly, a roll rate could
develop around the wheel or skid that is on the ground. When taking off or landing, perform the
maneuver smoothly, and carefully adjust the cyclic so that no pitch or roll rates build up.
8-6
FLIGHT PHENOMENA
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER EIGHT
During slope operations, excessive application of cyclic control into the slope, together with
excessive collective pitch control, can result in the downslope skid or landing wheel rising
sufficiently to exceed lateral cyclic control limits, and an upslope rolling motion can occur
(Figure 8-4).
Figure 8-4 Slope Takeoff or Landing Upslope Roll
When performing slope takeoff and landing maneuvers, follow the published procedures and
keep the roll rates small. Slowly raise the downslope skid or wheel to bring the helicopter level,
and then lift off. During landing, first touchdown on the upslope skid or wheel, then slowly
lower the downslope skid or wheel using combined movements of cyclic and collective.
The collective is more effective in controlling the rolling motion than lateral cyclic, because it
reduces the main rotor thrust (lift). A smooth, moderate collective reduction, at a rate of less
than approximately full up to full down in two seconds, may be adequate to stop the rolling
motion. Additionally, if the helicopter is on a slope and the roll starts toward the upslope side,
reducing collective too fast may create a high roll rate in the opposite direction. When the
upslope skid or wheel hits the ground, the dynamics of the motion can cause the helicopter to
bounce off the upslope skid or wheel, and the inertia can cause the helicopter to roll about the
downslope ground contact point and over on its side (Figure 8-5).
FLIGHT PHENOMENA
8-7
CHAPTER EIGHT HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 8-5 Slope Takeoff or Landing Downslope Roll
Under normal conditions on a slope, the collective should not be pulled suddenly to get airborne
because a large and abrupt rolling moment could occur. This moment might be uncontrollable
and the helicopter will roll over onto its side on the upslope side of the slope.
805. ROTOR DYNAMICS
1.
Ground Resonance
Helicopters with articulated rotor heads (usually designs with three or more main rotor blades)
are subject to ground resonance, a destructive vibration phenomenon that occurs at certain rotor
speeds when the helicopter is on the ground. Ground resonance is a mechanical design issue that
results from the helicopter’s airframe having a natural frequency that can be intensified by an
out-of-balance main rotor. The unbalanced main rotor disk vibrates at the same frequency (or a
multiple thereof) of the airframe’s resonant frequency. The harmonic oscillation increases
because the engine is adding power to the system, increasing the magnitude (amplitude) of the
vibrations until the aircraft structure fails. This condition can cause a helicopter to self-destruct
in a matter of seconds.
Ground resonance does not occur in rigid or semi-rigid rotor disks because they do not use lead
and lag hinges. In addition, skid-type landing gear are not as prone to ground resonance as
wheel-type landing gear, since the rubber tires’ resonant frequency can match that of the
spinning rotor, unlike the condition of a rigid landing gear.
If ground resonance occurs, the pilot has two options: take off or shut down. If the pilot can
safely take off, the vibrations will no longer resonate and the harmonic oscillation will dissipate.
Shutting the aircraft down removes the rotor disk vibrations. Regardless of which action the
pilot decides to take, he or she must act quickly to avoid severe damage to the aircraft.
8-8
FLIGHT PHENOMENA
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER EIGHT
2.
Ground Vortex
In a hover, the rotor downwash travels outward from the aircraft after impacting the ground. The
height of this outward traveling airflow is approximately equal to 1/3 the rotor diameter and has
a curling tendency. This is called the ground vortex (Figure 8-6). The speed of the vortex as it
moves further from the aircraft slows due to friction from the ground. As the helicopter moves
forward, it catches up with the ground vortex, and the rotor downwash mixes with increased
relative wind to create a rotating vortex, which eventually causes an increased downwash
through the rotor system. This simulates a climbing situation, thus increasing power required.
Eventually this vortex is overrun at a higher speed.
Figure 8-6 Ground Vortex
3.
Vibration Analysis
All objects have a natural frequency, including aircraft. This natural frequency is determined by
the aircraft’s mass and stiffness. The main source of vibrations for helicopters is the main rotor
system. Main rotor vibrations are equal to the rotor rotating frequency or multiples of that
frequency (vibrations per revolution). The frequency of the main rotor is a function of the speed
at which it rotates in revolutions per minute
Table 8-1).
FLIGHT PHENOMENA
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CHAPTER EIGHT HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CATEGORY
Low Frequency
(Most Common)
INDICATIONS
1:1 Lateral
1:1 Vertical
2:1
Medium
Frequency
4:1 TO 6:1
High Frequency
Too fast to count or
buzz in pedals.
CAUSE
Main Rotor Blade out of balance.
Main Rotor Blade out of track.
Inherent in two-bladed helicopter. Increase
indicates worn rotating control part of rotor hub
part.
Change in ability of Aerodynamic Center (AC)
to absorb normal vibrations. Loose component
(landing gear most common), loose cargo, etc.
Anything that rotates or vibrates at the speed of
the tail rotor (transmission, engine, driveshaft).
Table 8-1 Vibration Analysis Indications and Causes
Vibration analysis provides a quick reference for basic analysis of vibrations typically felt in the
cockpit while flying. The number or beats of the main rotor vibrations can vary depending on
the number of rotor blades installed. For example, on a two-bladed helicopter, one blade out of
track would create a one per revolution vibration. On a larger aircraft, such as an H-53E with
seven blades, several blades could be out of track. If three blades are out of track, there will be a
three per revolution vibration.
Tail shake is a problem that is usually worse in autorotation than in other flight conditions. It is
usually traced to turbulent airflow generated at the main-rotor hub that reaches the tail rotor or
empennage (vertical and/or horizontal stabilizers) surfaces with high turbulence. If the
frequency of the turbulence happens to match one of the empennage’s (vertical and/or horizontal
stabilizers) natural frequencies, the resulting resonance causes vibrations that can be felt
throughout the entire helicopter.
4.
Compressibility
Because the forward speed of the helicopter is added to the rotational velocity of the advancing
blade, the highest relative wind velocities occur at the tip of the advancing blade. When the
Mach number of the tip section of the advancing blade exceeds the MCRIT for the rotor blade
section, it causes compressibility effects. The principal effect of compressibility is a large
increase in drag and a rearward shift of the airfoil AC.
MCRIT is the flow speed at which the local velocity at some point on an airfoil first reaches sonic
speed. Because airfoils speed up flow on the upper surface to generate lift, the flow over the top
is faster than the free stream. When the free stream past a section of the rotor blade is
approximately Mach .72, the local speed over an airfoil’s surface may reach Mach 1.0, or the
speed of sound (Figure 8-7). That flow Mach number, in this case .72, is known as the MCRIT.
The actual MCRIT depends on the shape of the airfoil.
8-10 FLIGHT PHENOMENA
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER EIGHT
Figure 8-7 Critical Mach Number
Compressibility effects on the helicopter increase the power required to maintain rotor RPM and
cause rotor roughness, vibration, cyclic shake, and an undesirable structural twisting of the rotor
blade. Compressibility effects become more severe at higher lift coefficients (higher blade
AOAs) and higher Mach numbers.
The following circumstances represent the conditions that contribute to compressibility:

High airspeed

High rotor RPM

High gross weight

High DA

High-G maneuvers

Low temperature—the speed of sound is proportional to the square root of the
absolute temperature; therefore, the speed of sound decreases as temperature
decreases.

Turbulent air—sharp gusts momentarily increase the blade AOA and thus lower the
MCRIT to the point where compressibility effects may be encountered on the blade.
Corrective actions are any actions decrease the AOA or velocity of the airflow. Techniques to
recover from compressibility conditions include decreasing the blade pitch by lowering the
collective, decreasing the airspeed, decreasing the severity of maneuver, and decreasing the rotor
RPM.
FLIGHT PHENOMENA
8-11
CHAPTER EIGHT HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
5.
Retreating Blade Stall
In forward flight, the relative airflow through the main rotor disk is different on the advancing
and retreating side. The relative airflow over the advancing side is higher due to the forward
speed of the helicopter, while the relative airflow on the retreating side is lower. This
dissymmetry of airspeed increases as forward speed increases.
To generate the same amount of lift across the rotor disk, the advancing blade flaps up while the
retreating blade flaps down. This causes the AOA to decrease on the advancing blade, which
reduces lift, and to increase on the retreating blade, which increases lift (Figure 8-8). At some
point, as the forward speed increases, the low relative speed of the retreating blade combined
with its high AOA cause a stall and loss of lift.
Figure 8-8 Stall Region of the Retreating Blade
Retreating blade stall is a limiting factor for a helicopter’s never-exceed speed (VNE).
Indications of an impending retreating blade stall include a low-frequency vibration, pitching up
of the nose, and a roll in the direction of the retreating blade. High weight, low rotor RPM, high
DA, turbulence, and steep, abrupt turns are all conducive to retreating blade stall at high forward
airspeeds. As altitude increases, higher blade angles are required to maintain lift at a given
airspeed. Thus, retreating blade stall is encountered at a lower forward airspeed at altitude.
Most manufacturers publish charts and graphs showing a VNE decrease with altitude.
Corrective actions for a retreating blade stall include any action that will decrease the AOA of
the retreating blade or increase the relative wind over the retreating blade. There are similarities
in the critical conditions for compressibility and retreating blade stall, with one notable exception
compressibility occurs at high rotor RPM, and retreating blade stall occurs at low rotor RPM.
8-12 FLIGHT PHENOMENA
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER EIGHT
Retreating blade stall recovery techniques include decreasing the severity of the maneuver,
decreasing the blade pitch by lower the collective, decreasing the airspeed, descending, and
increasing the rotor RPM.
6.
Vortex Ring State (VRS)
VRS occurs during powered flight descents when a helicopter descends into its own downwash.
In normal HOGE, the helicopter is able to remain stationary by propelling a large mass of air
down through the main rotor. Some of the air is recirculated near the tips of the blades, curling
up from the bottom of the rotor system and rejoining the air entering the rotor from the top. This
phenomenon is common to all airfoils and is known as tip vortices. Tip vortices (Figure 8-9)
generate drag and degrade airfoil efficiency. As long as the tip vortices are small, their only
effect is a small loss in rotor efficiency. However, when the helicopter begins to descend
vertically, it settles into its own downwash, which greatly enlarges the tip vortices. As a result,
most of the power developed by the engine is wasted in circulating the air in a donut pattern
around the rotor, not producing lift.
Figure 8-9 Rotor Tip Vortex
As VRS develops, the helicopter may descend at a rate that exceeds the normal downward
induced flow near the blade root. As a result, the airflow of the inner blade section is upward
relative to the disk. This produces a secondary vortex ring in addition to the normal tip vortices.
The secondary vortex ring is generated about the point on the blade where the airflow changes
from up to down. The result is an unsteady, turbulent flow over a large area of the disk. Rotor
efficiency decreases, even though power is still being supplied from the engine, and the
helicopter will begin to descend faster and faster.
VRS is particularly dangerous for helicopters because it typically occurs in the landing profiles
near the ground. Indications of VRS include the following:

Increased noise and vibrations

Aircraft shuttering
FLIGHT PHENOMENA
8-13
CHAPTER EIGHT HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS

Increased descent with the addition of power
VRS can only occur in very specific flight profiles. In order to enter VRS, the following must be
true:

Airspeed at or below ETL (approximately 30 KIAS or less)

Excessive rate of descent (generally greater than 800 fpm)

Power must be applied to the aircraft (high blade pitch)
Certain ambient and aircraft conditions increase the likelihood of entering VRS. Any regime
that requires more power from the aircraft or a particularly steep approach increase the likelihood
of VRS or the speed at which VRS will occur.
These specific conditions include the following:
7.

High DA

High gross weight

Steep approaches with airspeed near zero

Confined area landings that require a near vertical descent

Approaches and landings with a tailwind
Vortex Ring State Recovery
In order to recover from VRS, the aircraft must fly out of its own downwash either by gaining
airspeed or sliding laterally. Simply raising the collective and increasing power will not affect a
recovery; it will only exacerbate the situation and increase the descent rate. There are two
techniques to recover from VRS state. The first traditional recovery requires the pilot to freeze
or lower the collective and apply forward cyclic and gain airspeed until the helicopter outruns its
own downwash. Once the helicopter has passed through translational lift, the pilot can adjust the
flight controls and return to normal flight.
The second method is called the Vuichard recovery. It is a maneuver that enables the pilot to
recover more quickly and without losing as much altitude.
Vuichard Recovery Procedure
a.
Raise the collective to maximum continuous power. (Note that this differs from the
traditional method where power is either maintained or reduced.)
b.
Simultaneously enter a sideslip by applying left pedal and right cyclic. (To utilize the
8-14 FLIGHT PHENOMENA
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER EIGHT
Vuichard recovery most effectively, sideslip by applying the pedal that increases tail
rotor pitch. In counterclockwise rotating aircraft, such as the TH-73A, this is the left
pedal.)
8.
c.
Keep the nose straight using pedal and increase the AOB to approximately 15–20
degrees.
d.
As soon as the helicopter slides out of its rotor wash, it will begin to climb. At that
point, the pilot can adjust the flight controls to transition to forward flight.
Vortex Ring State Prevention
VRS is a serious condition that can result in the loss of life and aircraft if not properly corrected.
The best way to handle potential VRS situations is to avoid entering into VRS in the first place.
By maintaining positive control of the helicopter and following procedures, pilots can avoid
VRS.
In situations that are more likely to cause VRS, the pilot should pay particular attention to the
following parameters:
9.

Maintain a controlled rate of descent. The steeper an approach, the slower the rate of
descent. A good rule of thumb is to never exceed 800 fpm during a steep approach.
If the landing zone cannot be reached without exceeding 800 fpm, the pilot should
elect to wave off and set up for another approach.

Avoid landing with any amount of tailwind. Whenever possible, land with a direct
headwind, particularly in confined areas.

Know whether or not the ambient conditions will allow the helicopter to maintain
HOGE. Attempting HOGE without sufficient power may cause the helicopter to
begin a descent into its own downwash.
Low-G Conditions
Helicopters rely on positive G to provide much or all of the response to pilot control inputs. The
pilot uses the cyclic to tilt the rotor disk, and, at one G, the rotor is producing thrust equal to
aircraft weight. Tilting the thrust vector provides a moment about the CG to pitch or roll the
fuselage. In a low-G condition, the thrust and consequently the control authority are greatly
reduced.
Although their control ability is reduced, rigid and fully articulated main rotor heads can
generate some moment about the fuselage independent of thrust due to the rotor hub design.
However, helicopters with two-bladed, semi-rigid rotor heads rely entirely on the tilt of the thrust
vector for control. Therefore, low-G conditions can be catastrophic for two-bladed helicopters.
FLIGHT PHENOMENA
8-15
CHAPTER EIGHT HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
An abrupt forward cyclic input or pushover in a two-bladed helicopter can be dangerous and
must be avoided, particularly at higher speeds. During a pushover from moderate or high
airspeed, as the helicopter noses over, it enters a low-G condition. Thrust is reduced, and the
pilot has lost control of fuselage attitude but may not immediately realize it. Tail rotor thrust or
other aerodynamic factors will often induce a roll. The pilot still has control of the rotor disk,
and may instinctively try to correct the roll, but the fuselage does not respond due to the lack of
thrust. If the fuselage is rolling right, and the pilot puts in left cyclic to correct, the combination
of fuselage angle to the right and rotor disk angle to the left becomes quite large and may exceed
the clearances built into the rotor hub. This results in the hub contacting the rotor mast, which is
known as mast bumping (Figure 8-10). Low-G mast bumping has been the cause of numerous
fatal accidents. The accident sequence may be extremely rapid, and the energy and inertia in the
rotor system can sever the mast or allow rotor blades to strike the tail or other portions of the
helicopter.
Figure 8-10 Mast Bumping
Turbulence, especially severe downdrafts, can also cause a low-G condition and, when combined
with high airspeed, may lead to mast bumping. Typically, helicopters handle turbulence better
than a light airplane due to smaller surface area of the rotor blades. During flight in turbulence,
momentary excursions in airspeed, altitude, and attitude are to be expected. Pilots should
respond with smooth, gentle control inputs and avoid over controlling. Most importantly, pilots
should slow down, as mast bumping is less likely at lower airspeeds.
Multi-bladed rotors may experience a phenomenon similar to mast bumping known as droop
stop pounding if flapping clearances are exceeded, but because they retain some control authority
at low G, occurrences are less common than for two-bladed rotors.
8-16 FLIGHT PHENOMENA
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS CHAPTER EIGHT
10.
Rotor Stall
Rotor stall is very similar to the stall of an airplane wing at low airspeeds. The airplane wing
relies on airspeed to produce the required airflow over the wing, whereas the helicopter relies on
rotor RPM. As the airspeed of the airplane decreases or the speed of the helicopter rotor slows
down, the AOA of the wing/rotor blade must be increased to support the weight of the aircraft.
At a critical angle (about 15 degrees), the airflow over the wing or the rotor blade will separate
and stall, causing a sudden loss of lift and increase in drag. An airplane pilot recovers from a
stall by lowering the nose to reduce the AOA and adding power to restore normal airflow over
the wing. However, the falling helicopter is experiencing upward airflow through the rotor disk,
and the resulting AOA is so high that even full down collective will not restore normal airflow.
When the helicopter rotor stalls, it does not do so symmetrically because any forward airspeed
will produce a higher airflow on the advancing side than on the retreating side. This causes the
retreating blade to stall first. During the stall, the weight of the retreating blade makes it descend
as it moves aft while the advancing blade is climbing as it goes forward. The resulting low aft
blade and high forward blade become a rapid aft tilting of the rotor disk. As the helicopter
begins to descend, the upward flow of air acting on the bottom surfaces of the tail boom and any
horizontal stabilizers tend to pitch the aircraft nose down. These two effects, combined with any
aft cyclic by the pilot attempting to keep the aircraft level, allow the rotor blades to tilt aft and
contact the tail boom, in some cases actually severing the tail boom.
Rotor stall should not be confused with retreating blade stall, which occurs at high forward
speeds and over a small portion of the retreating blade tip. Retreating blade stall causes vibration
and control problems, but the rotor is still very capable of providing sufficient lift to support the
weight of the helicopter. Rotor stall, however, can occur at any airspeed when the blade AOA
exceeds the blade’s stall AOA, and the rotor quickly stops producing enough lift to support the
helicopter, causing it to lose lift and descend rapidly.
11.
Low Rotor RPM
Low rotor RPM can occur during power-off and power-on operations. During power-off flight, a
low RPM situation can be caused by the failure to quickly lower the collective after an engine
failure or by raising the collective too high during the autorotation.
However, low rotor RPM incidents are more common during power-on operations. Power-on
incidents occur when the engine is operating normally, but the pilot demands more power than is
available by pulling up too much on the collective. Sometimes called overpitching, this can
easily occur at high DA, where the engine is already producing its maximum horsepower (hp)
and the pilot raises the collective. The corresponding increased AOA of the blades requires more
engine hp to maintain the speed of the blades; however, the engine cannot produce any additional
hp, so the speed of the blades decreases.
As the RPM decreases, the amount of thrust/lift produced by the main rotor also decreases. With
less power from the engine and less thrust/lift from the decaying rotor RPM, the helicopter will
start to settle. If the pilot raises the collective to stop the settling, the situation will feed upon
itself rapidly leading to rotor stall.
FLIGHT PHENOMENA
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CHAPTER EIGHT HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
There are a number of ways the pilot can recognize the low rotor RPM situation. The pilot can
see the NR indication decrease and will experience a yaw excursion due to a change in torque.
There will also be a noticeable decrease in engine noise, and at higher airspeeds or in turns, an
increase in vibration. Many helicopters have a low-RPM warning system that alerts the pilot to
the low rotor RPM condition.
To recover from the low rotor RPM condition the pilot must simultaneously lower the collective,
increase throttle if available, and adjust the cyclic to achieve bucket airspeed. At higher
airspeeds, additional aft cyclic may be used to help recover lost RPM. Recovery should be
accomplished immediately before investigating the problem and must be practiced to become a
conditioned reflex.
8-18 FLIGHT PHENOMENA
TAIL ROTOR CONSIDERATIONS
900. INTRODUCTION
This chapter describes the relationship between the tail rotor and main rotor as well as
considerations inherent to a conventional tail rotor system.
901. LEARNING OBJECTIVES

Describe tail rotor aerodynamic considerations.

Describe tail rotor malfunctions and degraded performance considerations.
902. TAIL ROTOR AERODYNAMICS
A single conventional main rotor system that rotates counterclockwise when viewed from above
imparts a moment on the fuselage and, if left unbalanced, causes the fuselage to rotate clockwise
around the vertical axis. This moment is compensated for by placing an anti-torque tail rotor a
certain distance from the CG of the aircraft. The thrust of the anti-torque tail rotor multiplied by
the distance to the CG results in a moment in the opposite direction to that generated by the main
rotor.
1.
Torque Effect
Any change in power setting will change the torque and, therefore, the anti-torque requirement.
The effects of wind on the tail rotor’s effectiveness must also be considered if the helicopter is to
be usable in a wide range of operating conditions. See Figure 9-1. Pilots of helicopters with a
clockwise rotation (and thus a left main rotor moment), such as the MH-65 Dolphin, simply need
to consider the effects in the opposite direction.
Figure 9-1 Tail Rotor Unbalanced Force
TAIL ROTOR CONSIDERATIONS
9-1
CHAPTER NINE
2.
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Translating Tendency and Hover Attitude
While the tail rotor system produces anti-torque effect, it also produces thrust in the horizontal
plane. This causes the aircraft to drift right laterally in a hover (Figure 9-2), for a
counterclockwise rotating, single-rotor helicopter. The aviator must compensate for this right
translating tendency of the helicopter by tilting the main rotor disk to the left. This lateral tilt
creates an equal but opposite main rotor force to the left that compensates for the tail rotor thrust
to the right. These two horizontal forces are often offset from each other vertically. The main
rotor force to the left coupled with the tail rotor force to the right commonly causes a left skid
low hover attitude.
Figure 9-2 Translating Tendency
3.
Vertical Stabilizer
The vertical stabilizer helps in reducing the amount of tail rotor thrust required in forward flight
(Figure 9-3). Shaped like a wing, a vertical stabilizer provides lift (thrust) in the direction of
anti-torque. The vertical stabilizer can be either a cambered airfoil or a symmetrical airfoil
mounted on an offset angle. The higher the aircraft’s velocity, the more the vertical stabilizer
will contribute to the anti-torque effort. At higher speeds, the vertical stabilizer significantly
reduces tail rotor power requirements; therefore, more engine power is now available to drive the
main rotor system.
Design tradeoffs have precluded the production of any military helicopters that can actually fly
in level, balanced flight with a complete tail rotor failure because of the power interactions
between the tail rotor and vertical stabilizer. Making a vertical stabilizer large enough to
compensate for a lost tail rotor would compromise sideward flight capability.
9-2
TAIL ROTOR CONSIDERATIONS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER NINE
Figure 9-3 Vertical Stabilizer
4.
Tail Boom Strakes
Some helicopters, including the TH-73A, incorporate a tail strake along the tail boom to aid in
maintaining direction control during a hover. The strake looks like a shelf running along the left
side of the tail boom. In a hover, downwash from the main rotor passes over tail boom. The
strake disrupts the downward airflow on the left side of the tail boom while the airflow on the
right side of the boom continues unimpeded. The difference in airflow on either side of the
boom creates lift pushing tail rotor and boom to the right. Rotating the tail rotor to the right
causes the nose to yaw left, aiding the tail rotor in counteracting the main rotor torque.
The process that strakes use to make the tail rotor more effective is sometimes called the Coanda
effect. Other helicopters that take advantage of the Coanda effect, such as NOTAR tail systems,
use the Coanda effect both in a hover and in forward flight. The strake, however, is most
effective at counteracting torque in a hover and provides less benefit in forward flight.
5.
Weathervaning
In a no-wind hover, the tail rotor provides all of the anti-torque compensation. As the helicopter
moves into forward flight, the tail rotor is assisted by the weathervaning effect in addition to the
vertical stabilizer (Figure 9-4). The weathervaning effect is caused by the increased parasitic
drag produced on the longitudinal surface of the helicopter as the relative wind increases. This
causes the helicopter to steer into the relative wind. The weathervaning effect increases
proportionally with airspeed and provide minor assistance to the anti-torque effect.
TAIL ROTOR CONSIDERATIONS
9-3
CHAPTER NINE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Figure 9-4 Weathervaning Effect
903. TAIL ROTOR MALFUNCTIONS
Tail rotor malfunctions can be grouped into four major categories: a complete loss of tail rotor
thrust, fixed pedal settings, Loss of Tail Rotor Authority (LTA), or Loss of Tail Rotor
Effectiveness (LTE). In order to distinguish one tail rotor malfunction from another, it is
important to understand the cause of each malfunction (mechanical or aerodynamic), aircraft
response to each type of malfunction, and the correct pilot reaction. Each type of malfunction
will require a combination of collective and airspeed adjustments to maintain controlled flight
and land safely.
1.
Loss of Tail Rotor Thrust
A complete loss of tail rotor thrust occurs whenever the tail rotor is no longer being driven. This
mechanical failure can occur due to a tail rotor drive shaft failure, a tail rotor gearbox failure or
seizure, or if the tail rotor departs the aircraft. The result of a complete loss of tail rotor thrust is
a rapid right yaw. High power settings, such as at high airspeed or in a hover, will cause the
aircraft to rotate even faster.
A complete loss of tail rotor thrust requires attention to airspeed. With increased airspeed the
main rotor operates more efficiently so it generates less torque. As velocity increases, both the
power required and anti-torque required decrease until the aircraft reaches its minimum power
required or bucket airspeed. After the bucket airspeed, the power and anti-torque required again
increases up to VNE. The best airspeed to fly after a complete loss of tail rotor thrust is the
airspeed that requires the least amount of anti-torque.
When considering what airspeed requires the least amount of torque, the impact of the vertical
stabilizer must be taken into account. A vertical stabilizer can help reduce the amount of tail
rotor thrust required in forward flight. Since it is shaped like a wing, the vertical stabilizer
provides lift (thrust) in the direction of anti-torque. The higher the aircraft’s velocity the more
anti-torque the vertical stabilizer will contribute. Individual aircraft have specified airspeeds that
minimize anti-torque requirements while maximizing vertical stabilizer effectiveness.
In some instances of complete loss of tail rotor thrust, it may not be possible to control yaw
through airspeed. For example, if a complete loss of tail rotor thrust occurs in a hover, the pilot
9-4
TAIL ROTOR CONSIDERATIONS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER NINE
will not have time to build airspeed before yaw begins and the aircraft becomes uncontrollable.
In these cases, it is necessary to remove the anti-torque requirement completely by entering an
autorotation. Pilots experiencing a complete loss of tail rotor thrust may control yaw using
airspeed to fly to a suitable landing site, and then plan for a sliding/rolling landing or initiate an
autorotation in order to safely land the helicopter. If yaw cannot be controlled by airspeed the
pilot should not hesitate to enter in autorotation. Specific recovery procedures are delineated in
individual NATOPS.
2.
Fixed Pedal Conditions
Fixed pedal conditions (or stuck pedals) occur when something is preventing the pilot from
making pedal control inputs. This can be caused by Foreign Object Damage (FOD) or a control
linkage failure.
The helicopter reaction to a fixed pedal condition depends on where the pedals are stuck and the
current flight regime of the helicopter. If the pedals are stuck with a significant amount of right
pedal (low power setting condition), the yaw rate will be lower at low power settings. If a high
power setting is applied to the helicopter, a yaw rate to the right will develop. If the pedals are
stuck with a significant amount of left pedal (high power setting), the yaw rate will be lower at
high power settings. If a low power setting is applied to the helicopter, a yaw to the left will
develop.
Recovery from a fixed pedal condition varies depending on the situation and the aircraft.
However, for any fixed pedal situation, the pilot will use a combination of airspeed, collective, or
twist grip adjustment to control the helicopter and land safely. Specific recovery procedures are
delineated in individual NATOPS.
3.
Loss of Tail Rotor Authority (LTA)
LTA is related to power available to the main and tail rotor. It is a mechanical phenomenon.
LTA occurs when power required exceeds power available. Power supplied to the main rotor is
delivered as torque at a certain RPM.
Power = Torque × RPM
If the engines are operating at the maximum amount of power they are capable of providing, they
will be unable to provide additional power if the pilot demands it. If the pilot demands more
performance by continuing to raise the collective, the AOA on the main rotor blades will
increase. Lift will increase, but so will drag. Because power is a constant (at the engine
maximum), the main rotor response to the increased drag will be a decrease in RPM because the
engine cannot produce enough power to overcome the additional drag and maintain main rotor
RPM. Increasing collective will only worsen rotor RPM decay.
Tail rotor thrust required for flight is a function of main rotor torque. Tail rotor thrust available
is a function of RPM squared. When the main rotor slows down it, also slows down the tail
rotor, providing less tail rotor thrust. Thus, as main rotor RPM slows, the increased tail rotor
TAIL ROTOR CONSIDERATIONS
9-5
CHAPTER NINE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
thrust required to counteract increasing main rotor torque is not available. The pilot can call for
more tail rotor thrust by increasing tail rotor torque with increased left pedal, but at some point,
the pilot will reach full pedal input and the ability to increase tail rotor AOA runs out. When tail
rotor thrust required exceeds tail rotor thrust available, LTA occurs and the nose of the aircraft
yaws to the right.
In order to recover from an LTA event, the pilot must decrease power required and anti-torque
demand on the tail rotor. To decrease power required, the pilot can decrease the anti-torque
requirement by lowering the collective, thereby decreasing the main rotor blade pitch or increase
forward airspeed, thereby increasing translational lift. If possible, the pilot may elect to jettison
external cargo or dump fuel to decrease the gross weight of the helicopter.
4.
Loss of Tail Rotor Effectiveness (LTE)
LTE is caused by a change in the direction from which the wind strikes the tail rotor, resulting in
an unexpected loss of tail rotor thrust. It results in an un-commanded yaw (usually to the right)
that does not subside of its own accord. It is important for pilots to understand LTE is caused by
an aerodynamic interaction between the wind relative to the helicopter and tail rotor and NOT a
mechanical failure.
LTE occurs when the flow of air through a tail rotor is altered in some way. A change in either
the angle or the speed at which the air passes through the rotating blades of the tail rotor disk
may cause LTE. An effective tail rotor relies on stable and relatively undisturbed airflow in
order to provide steady and constant anti-torque. A change in airflow over the tail rotor alters the
amount of thrust delivered for the same yaw input, creating a discrepancy between the pilot input
and the helicopter reaction. Taking this discrepancy to the extreme will result in the loss of
effective control in the yaw axis, and LTE will occur.
Tail rotor thrust can be affected by numerous external factors. The main factors contributing to
LTE include the following:
9-6

Airflow and downdraft generated by the main rotor blades that may interfere with the
airflow entering the tail rotor system.

Main rotor blade vortices developed at the blade tips that may enter the tail rotor disk.

Turbulence and other natural phenomena that may affect the airflow surrounding the
tail rotor.

A high-power setting, associated with large main rotor pitch angle, may induce
considerable main rotor blade downwash and more turbulence than a low-power
condition.

The loss of translational lift results in an increased power demand and additional antitorque requirements. If the loss of translational lift causes a right yaw, an increase in
power will accelerate the yaw unless a corrective action is taken by the pilot.
TAIL ROTOR CONSIDERATIONS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS

CHAPTER NINE
The airflow relative to the helicopter, which plays a significant role in LTE.
Certain flight activities, such as low, slow flight in undetermined winds, lend themselves to a
higher risk of LTE. Unfortunately, the flight conditions and contributing factors that might cause
LTE are numerous. To maximize the chances of avoiding LTE, the pilot should do the
following:

Maintain normal power-on rotor RPM. If the main rotor RPM is allowed to decrease,
the anti-torque thrust available decreases proportionally.

Avoid tailwinds below airspeeds of 30 knots. If loss of translational lift occurs, it
results in an increased power demand and additional anti-torque requirements.

Avoid OGE operations and high-power demand situations below airspeeds of 30
knots at low altitudes.

Be especially aware of wind direction and velocity when hovering in winds of about
8–12 knots.

Be aware that if a considerable amount of left pedal is being maintained, a sufficient
amount of left pedal may not be available to counteract an unanticipated right yaw.

Be alert to changing wind conditions, which may be experienced when flying along
ridgelines and around buildings.

Execute right turns slowly. This limits the effects of rotating inertia, and decreases
loading on the tail rotor to control excessive or un-commanded descent and right yaw.
If a suspected LTE event occurs, the pilot should apply pedal to counteract the rate of rotation.
Simultaneously, the pilot should apply forward cyclic to increase airspeed and lower the
collective, if altitude permits. As the recovery continues, adjust the flight controls for normal
forward flight.
Lowering the collective during an LTE event may cause an excessive rate of descent. Any large,
rapid increase in collective to prevent ground or obstacle contact may decrease the rotor RPM
and increase the yaw rate. If the pilot elects to lower the collective in order to recover from LTE,
he or she must be sure that there is a suitable landing site below the helicopter.
5.
Weathercock Stability (120–240 Degrees)
In the 120 to 240 degree region, the helicopter attempts to weathervane, or weathercock, to
position its nose into the relative wind. Unless an opposing pedal input is made, the helicopter
will start an un-commanded turn either to the right or left, depending upon the wind direction. If
the pilot allows a right yaw rate to develop and the tail of the helicopter moves into this region,
the yaw rate can accelerate rapidly. In order to avoid the onset of LTE in this downwind
condition, it is imperative to maintain positive control of the yaw rate.
TAIL ROTOR CONSIDERATIONS
9-7
CHAPTER NINE
6.
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Tail Rotor Vortex Ring State (210–330 Degrees)
Winds within the 210 to 330 degree region cause a tail rotor VRS to develop. The result is a
non-uniform, unsteady flow into the tail rotor. The VRS causes tail rotor thrust variations, which
result in yaw deviations (Figure 9-5). The net effect of the unsteady flow is an oscillation of tail
rotor thrust. Rapid and continuous pedal movements are necessary to compensate for the rapid
changes in tail rotor thrust when hovering in a left crosswind. Maintaining a precise heading in
this region is difficult, but this characteristic presents no significant problem unless corrective
action is delayed. However, high pedal workload, lack of concentration, and over controlling
can lead to LTE.
Figure 9-5 Tail Rotor Vortex Ring State
When the tail rotor thrust being generated is less than the thrust required, the helicopter yaws to
the right. When hovering in left crosswinds, concentrate on smooth pedal coordination and do
not allow an un-commanded right yaw to develop. If a right yaw rate is allowed to build, the
helicopter can rotate into the weathercock stability region, which will accelerate the right turn
rate. Pilot workload during a tail rotor VRS is high, but pilots should be very careful not allow a
right yaw rate to increase.
7.
Main Rotor Disk Vortex (285–315 Degrees)
Winds at velocities of 10–30 knots from the left front cause the main rotor vortex to be blown
into the tail rotor by the relative wind. This main rotor disk vortex causes the tail rotor to operate
in an extremely turbulent environment. During a right turn, the tail rotor experiences a reduction
of thrust as it comes into the area of the main rotor disk vortex. The reduction in tail rotor thrust
comes from the airflow changes experienced at the tail rotor as the main rotor disk vortex moves
across the tail rotor disk.
9-8
TAIL ROTOR CONSIDERATIONS
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
CHAPTER NINE
The effect of the main rotor disk vortex initially increases the AOA of the tail rotor blades, thus
increasing tail rotor thrust. The increase in the AOA requires that right pedal pressure be added
to reduce tail rotor thrust in order to maintain the same rate of turn. As the main rotor vortex
passes the tail rotor, the tail rotor AOA is reduced. The reduction in the AOA causes a reduction
in thrust and right yaw acceleration begins. This acceleration can be surprising, since previously
adding right pedal maintained the right turn rate. The thrust reduction occurs suddenly, and if
uncorrected, develops into an uncontrollable rapid rotation about the mast. The pilot must react
quickly to counter this reduction with additional left pedal input.
8.
AOA Reduction (060–120 Degrees)
In a right crosswind, the relative wind shifts toward a tail rotor blades chord line because of
effectively increased induced velocity. The shifted relative wind impacts the tail rotor blades at a
lower AOA, which develops lower lift and results in less thrust (Figure 9-6). The pilot should
compensate by adding more left pedal, but in some cases can reach pedal travel limits before
adequate thrust can be generated.
Figure 9-6 Tail Rotor AOA Reduction in a Right Crosswind
9.
Tail Rotor Control During an Engine Failure
Should the engine fail in a single engine helicopter, the aircraft will tend to yaw left. The yaw
inputs made prior to the engine failure compensate for a much greater torque than what is
instantly now required after the engine failure. Because of that left yaw, a pilot’s initial response
to an engine failure must always include a right pedal input.
After an engine failure, there is no longer a requirement for anti-torque. However, the tail rotor
must still produce thrust to provide yaw control during the autorotation and landing. The upward
TAIL ROTOR CONSIDERATIONS
9-9
CHAPTER NINE
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
airflow during an autorotation drives the main rotor, which mechanically drives the tail rotor. As
the pilot adjusts the collective to maintain main rotor RPM, the tail rotor RPM will change as
well. Following an engine failure in multi-engine helicopters, the remaining operating engines
will provide tail rotor drive.
9-10 TAIL ROTOR CONSIDERATIONS
APPENDIX A
GLOSSARY
Acceleration (a) – The rate of change of velocity per unit of time. It is a vector quantity.
Advance angle – The number of degrees the manufacturer sets the control input in advance of
the desired reaction point in order to compensate for the phase lag and ensure the rotor disk
reacts in the same sense as the cyclic movement.
Advancing blade – The blade moving in the same direction as the helicopter. In forward flight,
the advancing blade is on the right side of the helicopter.
Aerodynamic Center (AC) – The point along the chord line about which changes in AOA do
not result in a change of moment.
Aerodynamic force – The resultant vector of lift and drag vectors. Sometimes called the Total
Aerodynamic Force (TAF) is the combination of all of the lift and drag forces on the blade. The
vector sum of lift and profile drag.
Aerodynamic twist – The twist of an airfoil having different absolute angles of incidence at
different span wise stations.
Aerodynamics – A branch of dynamics that deals with the motion of air and other gaseous fluids
and with the forces acting on bodies in motion relative to such fluids.
Air density () – The total mass of air particles per unit of volume. The distance between
individual air particles increases with altitude resulting in fewer particles per unit volume. Air
density decreases with an increase in altitude.
Air density/Atmospheric density – The mass of air per unit volume ( = m/V). It is the most
important atmospheric variable concerning aircraft performance.
Airfoil – A device designed to produce more lift (or thrust) than drag when air flows over it at an
appropriate speed and angle.
Airfoil section – The cross section taken at right angles to the span axis or some other specified
axis of the airfoil. It shows the form or shape of an airfoil profile and the area defined by it.
Airspeed – The speed of an aircraft in relation to the air through which it is passing.
Altimeter setting – The value of the atmospheric pressure used to adjust the sub-scale of a
pressure altimeter so that it indicates the height of an aircraft above a known reference surface.
The current reported altimeter setting of an airport along the route and within 100 NM of the
aircraft.
GLOSSARY
A-1
APPENDIX A
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Angle of Attack (AOA) – The angle between the chord line of the blade and the relative
airflow/wind. This angle is independent of the pitch angle. Depending on the flight regime and
the position of the blade in relation to the direction of flight, it may be less than, the same as, or
greater than the pitch angle.
Angle of climb – The angle between a horizontal plane and the flight path of a climbing aircraft.
Angle of Incidence (AOI) – The acute angle between the chord line of the airfoil and a selected
reference plane, usually the longitudinal axis of the aircraft. For rotating airfoils such as
propellers and helicopter main and tail rotors, it is the acute angle between the chord line of the
airfoil and the tip path plane. The AOI is normally called pitch angle for helicopter’s main and
tail rotors, and for propeller blades.
Angular acceleration – A simultaneous change in both speed and direction of movement.
Anti-autorotative force – In autorotational flight, the decelerating horizontal component of the
aerodynamic force along the driven and no-lift regions.
Anti-torque device – A method used to counteract torque reaction of the helicopter fuselage in
response to the rotation of the main rotor. Typical and most popular anti-torque devices are tail
rotor, Fenestron, and NOTAR.
Articulated rotor system – A rotor system in which the hub is mounted rigidly to the mast and
the individual blades are mounted on hinge pins, allowing them to flap up and down and move
forward and backward (lead and lag). Individual blades are allowed to feather by rotating about
the blade grip retainer bearing.
Aspect Ratio (AR or) – Design quality of an airfoil. It is defined as the span or length (b) of a
blade divided by the chord or width (c) [AR = b/c]. A more practical and useful method to
determine the AR is to divide the square span of the airfoil by its area (S). [AR = b/c = b × b/c ×
b = b2/S]
Attitude – The position of a body as determined by the inclination of the axes to some frame of
reference. If not otherwise specified, this frame of reference is fixed to the Earth (horizon).
Most common helicopter attitudes are pitch and roll attitude.
Autorotation – The descending flight of a helicopter without engine power where the air
approaching from below the rotor disk (upward induced flow) keeps the rotor blades turning at
an operational speed.
Axis – A line passing through a body about which the body rotates or may be assumed to rotate,
any arbitrary line of reference such as a line about which the parts of a body or system are
symmetrically distributed, or a line along which a force is directed; for example, an axis of
thrust.
A-2 GLOSSARY
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
APPENDIX A
Axis of rotation – The axis around which the helicopter blades rotate. A line through the rotor
head at right angles (perpendicular) to the plane of rotation (tip path plane). It is not always in
line with the rotor mast.
Balancing tab – A moveable tab linked to the trailing edge of a control surface. When the
control surface is deflected, the tab is deflected in an opposite direction, creating a force, which
aids in moving the larger surface. Sometimes called a servo tab.
Blade element theory – Uses graphically depicted representation of the airflow and
aerodynamic forces applied to a selected airfoil section. Gives a more accurate representation of
rotor performance than the Momentum Theory. It also details the movement of individual blades
around the disk.
Blade pitch angle – The angle between the tip path plane and the chord line. The pilot controls
the Blade Pitch Angle using the collective and cyclic.
Blade span – The length of the rotor blade from center of rotation to tip of the blade.
Blade taper – The progressive reduction of the chord from the root to the tip. This design
reduced the amount of lift produced by the blade progressively from the root to the tip.
Blowback – The pitch-up tendency as the aircraft accelerates due to the flapping, which
compensates for dissymmetry of lift. The separation of the virtual axis from the control axis
causes this tendency.
Boundary-layer control – The control of the flow in the boundary layer about a body, or of the
region of flow near the surface of the body, to reduce or eliminate undesirable aerodynamic
effects and hence to improve performance.
Calibrated Airspeed (CAS) – The Indicated Airspeed (IAS) corrected for instrument and
position error. When flying at sea level under ISA conditions (15 °C, 1013 hPa/29.92 inHg, 0
percent humidity), calibrated airspeed is the same as equivalent airspeed and true airspeed. If
there is no wind, it is also the same as ground speed. Under any other conditions, CAS may
differ from true airspeed and ground speed.
Camber – The curvature of the surfaces of an airfoil or airfoil section from leading edge to
trailing edge. The distance between the chord line and the mean camber line.
Camber line – Line equidistant from the upper and lower surface of the airfoil; same as chord
line for a symmetrical airfoil.
Center of Gravity (CG) – A point within an object through which all the forces of gravity are
considered to act. If an object were suspended from this point, it would be in balance. It is
generally expressed along the longitudinal or lateral axis. For a rotor blade, the CG should
ideally be on the feathering axis.
GLOSSARY
A-3
APPENDIX A
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Center-of-pressure travel – The movement of the center of pressure of an airfoil along the
chord with changing AOA; the amount of this movement is expressed in percentages of the
chord length from the leading edge.
Centre of Pressure (CP) – The point on the chord line where the sum of all the aerodynamic
forces are considered to act. This point moves toward the leading edge of an airfoil when the
AOA is increased. The movement of the CP is less with a symmetrical airfoil than it is with a
nonsymmetrical one. For a rotor blade, the CP should ideally be on the feathering axis.
Centrifugal force – The outward force created by the rotation of the main rotor and opposed by
centripetal force. The large centrifugal force is what allows the weight of the helicopter to be
distributed across otherwise flexible rotor blades. Centrifugal force is proportional to the square
of Rotor RPM (NR) and increases dynamic blade rigidity. Rotation/centrifugal force gives
rigidity to the rotor blades.
Centripetal force – The accelerative force acting on a body moving in a curved path. It is the
component of force that is directed toward the center of curvature or axis of rotation. Centripetal
force causes a change in the direction of the linear velocity vector of a body in motion, resulting
in an acceleration of the body. Centripetal force is the out-of-balance force that causes an
aircraft to turn. During a turn, it is the horizontal component of lift that is directed toward the
center of the turn.
Chord – The length of the chord line between the leading and trailing edge of an airfoil.
Chord Line – A an infinitely long, straight line which passes through the leading and trailing
edges of the airfoil.
Coanda effect – The tendency of a jet of fluid emerging from an orifice to follow an adjacent
flat or curved surface and to entrain fluid from the surroundings so that a region of lower
pressure develops.
Coefficient of Drag (CD) – A dimensionless number indicating the inefficiency of an airfoil
which is determined by AOA and airfoil design. It is derived from wind tunnel testing.
Coefficient of Lift (CL) – A dimensionless number indicating the efficiency of the airfoil which
is determined by AOA and airfoil design. It is derived from wind tunnel testing.
Collective feathering – The equal and simultaneous mechanical change of blade pitch (the AOI)
of all rotor blades in a rotor system.
Compressibility – At high forward airspeeds, the advancing rotor blade creates large pressure
changes, which result in significant air density changes. As the blade’s velocity approaches the
speed of sound, the blade becomes less efficient because of a nose-down pitching moment and a
significant increase in drag.
Compressible flow – The flow at speeds high enough that density changes in the fluid can no
A-4 GLOSSARY
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
APPENDIX A
longer be neglected.
Coning – The upward displacement of the main rotor blades due to increased lift and balanced
somewhat by centrifugal force.
Coning angle – The angle between the feathering axis of a blade and its plane of rotation. This
varies with Rotor Revolution per Minute – R/RPM (centrifugal force) and rotor thrust. It can be
visualized as the angle between the tip path plane and the main rotor blades.
Control axis – See Axis of Rotation.
Control surface – A movable airfoil designed to be rotated or otherwise moved to change the
speed or direction of an aircraft.
Conventional helicopter – A single-engine, single-rotor helicopter with the rotor turning anticlockwise (as viewed from above), and a conventional tail rotor as anti-torque system.
Coriolis Effect – The tendency for a rotating body to increase its RPM when its CG moves
closer to its axis of rotation. A rotor blade moves forward on its drag hinge when it flaps up (the
CG moves inboard) and the R/ RPM increases when the coning angle increases (all the blades
flap up). Technically, it should be referred to as the conservation of angular momentum.
Critical Mach number (MCRIT) – The free-stream Mach number at which a local Mach number
of 1.0 is attained at any point on the body under consideration.
Cyclic feathering – The mechanical change of blade pitch (the AOI), of individual rotor blades
independently of the other blades in the system.
Density ( or rho) – The mass per unit volume.
Density Altitude (DA) – The altitude relative to standard atmospheric conditions at which the
air density would be equal to the indicated air density at the place of observation. The denser an
air mass (cold, dry air), the lower the corresponding value corrected to a standard atmosphere
(High density = Low DA). The opposite is also true. DA increases as temperature and/or RH
increases, thus it is inversely proportional to atmospheric density and directly proportional to
temperature and RH.
Density ratio (σ) – The ratio of the density of air at a specific altitude to that of the standard
altitude (sea level).
Disk area – The area contained within the tip path plane. The size of this area is affected by the
coning angle and therefore varies in flight.
Disk loading – The gross weight of the helicopter divided by the disk area. As the disk area
varies in flight with changes in Total Rotor Thrust (TRT), R/ RPM, etc., so does the disk loading,
even at a constant weight.
GLOSSARY
A-5
APPENDIX A
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Dissymmetry of lift – The unequal lift produced by each rotor blade as its relative airspeed is
increased as it becomes the advancing blade and decreased as it becomes the retreating blade (lift
varies at the square of the speed). It is automatically corrected by blade flapping.
Downwash – The induced downward flow of air resulting from the passage of an airfoil
(induced flow).
Downwash angle – The angle, measured in a plane parallel to the plane of symmetry of an
aircraft, between the direction of downwash and the direction of the undisturbed airstream. This
angle is positive when the deflected stream is downward. (See Upwash angle.)
Drag – The retarding force acting in line with the relative airflow. It is caused by the resistance
to movement brought to bear on an aircraft by the atmosphere through which it passes. Drag =
CD ½  v2 S; Where – CD = Coefficient of drag (the efficiency of the airfoil in minimizing
drag) it changes with the shape of the airfoil, surface friction (smoothness and cleanliness) and
AOA.  = Air density (rho), V2 = Velocity squared, S = Surface area of the airfoil.
Dragging – See Lead-lag.
Droop snoot airfoil – A nonsymmetrical airfoil design used on many helicopters included the
TH-73A. The droop snoot design incorporates a symmetrical blade design with a
nonsymmetrical nose. A droop snoot design provides good stall characteristics at high angles of
attack by maximizing the lift coefficient, and produces very low pitching moments by precluding
large movements in the CP along the chord line during changes in AOA.
Dynamic pressure – The pressure of a fluid resulting from its motion is equal to one-half the
fluid density times the fluid velocity squared (q = ½  v2). In incompressible flow, dynamic
pressure is the difference between total pressure and static pressure.
Dynamic rollover – The lateral rolling of the helicopter onto its side due to exceeding the
critical rollover angle regardless of cyclic corrections. For dynamic rollover to occur the
helicopter must have (a) a ground pivot point, (b) collective applied, (c) rolling moment.
Dynamic stability – The property that causes a body to dampen the oscillations setup by
restoring moments and returning gradually to its original state when disturbed from the original
state of steady flight or motion.
Effective Translational Lift (ETL) – The pronounced increased in translational lift during
transition to forward flight (approximately 13–24 knots) due to the rotor disk experiencing a
significantly decreased induced airflow.
Empennage – The assembly of stabilizing and control surfaces at the tail of an aircraft.
Endurance – The time an aircraft can continue flying under given conditions without refueling.
A-6 GLOSSARY
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
APPENDIX A
Energy – A scalar measure of a body’s capacity to do work. There are two types of energy:
potential energy and kinetic energy. Energy cannot be created or destroyed but may be
transformed from one form to another. This principle is called the law of conservation of energy.
Equivalent Airspeed (EAS) – CAS corrected for the compressibility of air. When flying at sea
level under ISA conditions (15 °C, 1013 hPa/29.92 inHg, 0 percent humidity), EAS is the same
as CAS and True Airspeed (TAS). If there is no wind, it is also the same as Ground Speed (GS).
Under any other conditions, EAS may differ from the aircraft’s CAS, TAS, and GS.
Feathering – A mechanical change in the pitch angle of an airfoil. The rotation of the blade
around the feathering axis (changes in pitch angle) due to cyclic and collective control inputs.
Feathering axis – A line between the root end and the tip of a blade around which the blade
rotates to alter its pitch angle.
Fin – A fixed vertical airfoil that aids directional stability. It is part of the empennage.
Flapping – The movement of the blade in a vertical plane.
Flapping to equality – Whenever dissymmetry of lift occurs, the blades automatically flap to
eliminate this dissymmetry and equalize the lift; the advancing blade decreases its AOA as it
flaps up, whilst the retreating blade increases its AOA as it flaps down. The blades continue to
flap until their lift is equal. When this occurs, the blades stop flapping, hence the term flapping
to equality.
Flight path – The line connecting the continuous positions occupied or to be occupied by an
aircraft as it moves with reference to the vertical or horizontal planes.
Flow separation/boundary layer separation – The breakaway of flow from a surface. When
the flow separated from the surface of a body, it no longer follows surface contours or airfoil
profile.
Force (F) – A vector quantity equal to a mass (m) times an acceleration (a). 𝐹 = 𝑚 × 𝑎
Form drag – The drag due to the shape and size of the object.
Fully articulated rotor – The rotor head has hinges and bearings that allow the blades to feather
flap and drag independently of each other.
Fuselage – The body to which most of helicopter’s components are attached. It is the supporting
structure of the aircraft. Engine, transmission, tail boom, skids/landing gear, fuel cells are
attached or are part of the fuselage.
Geometric imbalance – Occurs when the radius of the center of mass for a single rotor blade
changes due to excessive flapping and is no longer equidistant from the center of rotation relative
to the individual centers of mass for the other rotor blades. This phenomenon may lead to
GLOSSARY
A-7
APPENDIX A
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
excessive hunting.
Geometric twist – An engineered design of the rotor blade span wise that incorporates a twist
beginning with an increased AOI at the root of the rotor blade, which decreases from the root to
the tip. Geometric twist helps to distribute lift more equally across the rotor blade.
Gravity – An attraction of two objects for each other that depends on their mass and the distance
between them.
Gross weight – The total weight of an aircraft and its contents.
Ground effect – The name given to the positive influence on the lifting characteristics of the
horizontal surfaces of an aircraft wing when it is close to the ground. At approximately one rotor
diameter above the surface the rotor system of the helicopter increases in efficiency, which
decrease the power require, and allows the aircraft to require less power to land. This effect
increase as the helicopter approaches the ground.
Ground resonance – A destructive force created by an imbalance of the main rotor causing the
helicopter to rock from side to side on its landing gear. Normally associated with the fully
articulated rotor system and an inoperative blade dampener, ground resonance is a destructive
oscillation caused when the helicopter is in contact with the ground and one or more rotor blades
are displaced due to a gust of wind, sudden control movement, or a hard landing. When this
occurs, the CG of the rotor system spirals violently outward. (See Geometric imbalance.)
Ground Speed (GS) – The horizontal speed of an aircraft relative to the ground. In no-wind
condition, it is equal to the TAS.
Ground vortex – During a normal transition to forward flight, the helicopter’s downwash
creates a vortex in front of the path of flight. As the helicopter accelerates, the aircraft flies
through the vortex. This serves to increase the induced flow causing an increase in the power
required.
Gyroscopic precession – The characteristic of a rotating body to delay its reaction by 90 degrees
after an external force is applied perpendicular to the plane of rotation (see Phase lag).
Harmonic vibration – See Sympathetic Resonance.
Hinge offset – The distance between the flapping hinge (effective flapping hinge in a rigid rotor)
and the center of the rotor hub.
Hooke’s joint effect – The movement of the blades in the lead lag plane as they accelerate and
decelerate due to the flapping motion required to eliminate dissymmetry of lift.
Horsepower – Horsepower (hp) is the common unit of power, or the rate at which work is done,
usually in reference to the output of engines or motors. A unit of power equal to 550 footpounds per second. It represents the power necessary to raise 550 pounds one foot in one
A-8 GLOSSARY
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
APPENDIX A
second. Thus, a 1,000-hp engine develops 1,000 times 550 foot-pounds of work per second.
Humidity – The amount of water vapor in the air. As humidity increases, water molecules
displace an equal number of air molecules. Since water molecules have less mass (H2O, MW
18) than air (N2, MW 28; and O2, MW 32) and occupy approximately the same volume, the
overall mass in a given volume decreases. Therefore, as humidity increases, air density
decreases. Compared to dry air, the density of air at 100 percent humidity is 4 percent less.
Hunting – See Lead-lag.
Indicated Airspeed (IAS) – The airspeed read directly from the airspeed indicator (ASI) on an
aircraft, driven by the pitot-static system. It either uses the difference between total pressure and
static pressure, provided by the system, to mechanically or electronically measure dynamic
pressure.
Induced drag – The drag created by the production of lift. It is caused by the downward
induced velocity/flow and it is parallel to the plane of rotation. Induced drag decreases with
speed.
Induced flow– In relation to a rotor blade, it is the component of the relative airflow that is at
right angles to the plane of rotation. In relation to the rotor disk, it is the total mass of air
induced to flow by the action of the rotor, most of the induced flow passes through the rotor but
some may miss it altogether. Generally, in powered flight, the induced velocity is downward and
in non-powered flight, the induced velocity is upward through the rotor disk.
Induced Velocity – The vertical component of relative wind. It is always perpendicular to the
tip path plane.
Inertia (I) – The property of matter to retain its state of rest or its velocity along a straight line
until it is acted upon by an external force.
In-plane drag – The summation of all decelerating forces in the plane of rotation (induced drag
plus the horizontal component of profile drag).
International Standard Atmosphere (ISA) – A static atmospheric model of how the pressure,
temperature, density, speed of sound, and viscosity of the Earth’s atmosphere change over a wide
range of altitudes or elevations.
Kinetic Energy (K.E.) – The energy of a system because of motion. Kinetic energy is part of
helicopters total energy. It is due to the motion of the helicopter with respect to a point on the
ground; helicopter ground speed It is defined as KE = 1/2 mv2 (1/2 mass × velocity squared).
Lag – In a rotating system, this is the occurrence of a momentary decrease in the rotational
velocity, normally about a vertical hinge pin in an articulated system.
Laminar flow – A smooth flow in which no cross flow of fluid particles occurs, hence a flow
GLOSSARY
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HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
conceived as made up of layers.
Laminar separation – The separation of a laminar-flow boundary layer from a body.
Lateral axis – An axis going from side to side of an aircraft passing through the CG and
perpendicular to the longitudinal and vertical axes. The axis about which pitching action occurs.
Sometimes called a Transverse axis.
Lateral stability – The tendency of a body, such as an aircraft, to resist rolling or lateral
displacement; the tendency of an aircraft to remain wings-level, either in flight or at rest.
Lead – Opposite of lag or, a momentary increase in the rotational velocity in a rotating system.
Leading edge – The forward edge of an airfoil/blade. The edge, which normally meets the air or
fluid first.
Lead-lag – The horizontal movement of the blade. Also called hunting.
L/DMAX – The point where the lift-to-drag ratio is greatest.
Lift – The force derived from an airfoil that acts at right angles to the relative airflow. The
component of the TAF (thrust on a blade element), which is perpendicular to the relative wind.
The amount of lift an airfoil produces is determined by the formula Lift = CL ½ V2 S Where –
CL = Coefficient of lift (the efficiency of the airfoil in producing lift).
Lift component – A force acting on an airfoil perpendicular to the direction of its motion
through the air.
Lift limit – The left-hand side of the V-n diagram. It is the maximum load factor available at a
given airspeed.
Lift/ drag ratio – Whenever an airfoil is producing lift, it is also producing drag. The amount of
lift produced compared to the amount of drag produced is the lift/ drag ratio and is an indication
of the efficiency of the airfoil. It is determined by dividing the lift coefficient by the drag
coefficient, CL/CD.
Limit airspeed – The vertical line on the right side of the V-n diagram. It is the highest airspeed
that an aircraft is allowed to fly. It is represent by a red line on the airspeed indicator. It is
commonly known as redline airspeed, or VNE (Velocity never-to-exceed).
Limit load factor – The greatest load factor an aircraft can sustain without any risk of
permanent deformation. If the limit load factor is exceeded, some structural damage or
permanent deformation may occur. The top and bottom of the V-n diagram.
Linear flow – The horizontal/lateral component of resultant relative wind in a rotating system,
the V-rotational flow +/– the V-translational, adjusted for any existing wind condition.
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APPENDIX A
Linear Velocity – The horizontal component of relative wind. It is always parallel to the tip
path plane.
Load – The forces acting on a structure and. These may be static (as with gravity), dynamic (as
with centrifugal force), or a combination of static and dynamic. It is also frequently used to
describe an aircraft’s cargo. Aircrafts cargo is commonly called payload, which is the carrying
capacity of an aircraft usually measured in terms of weight. Depending on the nature of the
flight or mission, the payload of a vehicle may include cargo, passengers, flight crew, munitions,
scientific instruments or experiments, or other equipment. Extra fuel, when optionally carried, is
also considered part of the payload. In a commercial context (i.e., an airline or air freight
carrier), payload may refer only to revenue-generating cargo or paying passengers.
Load factor – The sum of the loads on a structure, including the static and dynamic loads;
expressed in units of G. Frequently known as the G factor it is ratio of load being support by the
lifting surface of the helicopter, the rotating blades, compared to the actual weight of the aircraft
and its contents. The G factor is determined by the sized of the lateral force (AOB) and not the
airspeed or rate of turn. The load factor increases in any positive G maneuver, either in banking
(rolling) or pulling out of a dive (positive pitching).
Longitudinal acceleration – Acceleration substantially along the longitudinal axis of an aircraft.
Longitudinal axis – A straight line through the CG running from the nose to the tail of an
aircraft and perpendicular to the lateral and vertical axes. It is the axis about which rolling action
occurs.
Loss of Tail Rotor Effectiveness (LTE) – A sudden reduction in tail rotor efficiency that occurs
when the wind strikes the aircraft from certain sectors. Due to the wind, the tail rotor will lose
its ability to provide anti-torque thrust and the helicopter will enter in an uncontrolled yaw (right
yaw for conventional helicopters). Any low-airspeed high-power environment provides an
opportunity for LTE.
Mach number – The ratio of the velocity of a body to that of sound in the surrounding medium.
Thus, a Mach number of 1.0 indicates a speed equal to the speed of sound; 0.5, a speed one-half
the speed of sound; 5.0, a speed five times the speed of sound, and so on.
Mach wave – A pressure wave traveling with the speed of sound caused by a slight change of
pressure added to a compressible flow.
Maneuver – Any planned motion of an aircraft in the air or on the ground.
Maneuverability – The ease with which an aircraft will move out of its equilibrium position.
Maneuverability and stability are opposites.
Maneuvering speed (Va) – The maximum speed at which full control deflection can be abruptly
applied without overstressing the aircraft. Above maneuver speed, the rotor can generate high
aerodynamic loads in excess of the limit load or can be pushed into a retreating blade stall. The
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APPENDIX A
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
intersection of lift limit and structural limit lines occurs at the maneuvering airspeed.
Maneuvering airspeed (Va), also known as the corner airspeed.
Mass (m) – The matter contained by a body.
Maximum endurance cruise airspeed – The airspeed required to achieve the maximum flight
time. It is identified by the lowest point on the power required curve where the ratio of lift
versus drag is maximized (also called bucket airspeed). It provides maximum excess power and
maximize airtime.
Maximum range cruise airspeed – The airspeed required to achieve the maximum range. It is
identified by the point where a line drawn from the origin (corrected for winds) is tangent to the
power required curve. It provides maximum flying distance or maximum range.
Maximum rate of climb airspeed – The lowest point on the power required curve. Ratio of lift
versus the drag is maximized thereby allowing for the greatest power excess. (Also referred to
as best rate of climb.)
Mean aerodynamic chord – The chord of an imaginary rectangular airfoil that would have
pitching moments throughout the flight range the same as those of an actual airfoil or
combination of airfoils under consideration, calculated to make equations of aerodynamic forces
applicable.
Mean camber line – A curved line that passes mid-way between the upper and lower surface of
an airfoil. The curvature of the mean camber line in relation to the chord line is very important
in determining the aerodynamic characteristics of an airfoil section. On symmetrical airfoils, the
mean camber line and the chord line are the same.
Mechanical axis – The extension of the centerline of the rotor mast (the actual axis of the rotor
head).
Moment (M) – created when a force is applied at some distance from an axis and tends to
produce rotation about that point. A moment is a vector quantity equal to a force (F) times the
distance (d) from the point of rotation on a line that is perpendicular to the applied force vector.
This perpendicular distance is called the moment arm. Torque (Q) is another word for a moment
created by a force. M = F × d
Momentum theory – Theory that helps explain rotary-wing lift production, primarily based on
Isaac Newton’s three laws of motion. The action of accelerating a mass of air downward
produces a reaction that lifts the helicopter. Momentum theory is most applicable in hovering
and forward flight.
Negative G – Any G-force in the opposite direction of positive G-force, caused by your body
accelerating towards the Earth faster than the forces of gravity. This causes the liquids in your
body to move away from your feet and towards your head.
Neutral stability – The stability of a body such that after it is disturbed, it tends neither to return
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APPENDIX A
to its original state nor to move further from it. Its motions or oscillations neither increase nor
decrease in magnitude.
Never-to-exceed airspeed (VNE) – See Limit airspeed.
Newton’s Laws of Motion – Newton’s third law (the law of interaction), “For every action,
there is an equal and opposite reaction.”
Nonsymmetrical airfoil – An airfoil with a different shape or size above and below the chord
line.
Over-G – see Overstress.
Overstress – The condition of possible permanent deformation or damage that results from
exceeding the limit load factor.
Parasite drag – The drag of non-lift producing elements of the aircraft (i.e., fuselage, rotor head,
and skids).
Pendulum effect – The uncommand nose-up tendency during deceleration that occurs in
response to an increase in collective pitch before mechanical and virtual axes are realigned.
Compensated for by pilot-induced feathering through forward cyclic.
Phase lag – The angular difference between the control input and the subsequent reaction
(gyroscopic precession). It is normally 90 degrees, but on helicopters with a large diameter rotor
head, it may be one or two degrees less due to the offset of the flapping hinges, bearing friction,
etc.
Pitch angle – The angle between the chord line of the blade and the reference plane of the rotor
hub (the plane of rotation of the rotor disk). It is a mechanical angle, not an aerodynamic angle,
which is set by the amount of collective and/or cyclic pitch that is applied. (See also AOI.)
Pitching – The movement of the aircraft around its lateral axis.
Pitching moment – A moment about a lateral axis of an aircraft, rocket, airfoil, and so on. This
moment is positive when it tends to increase the AOA or to nose the body upward.
Positive G – The footwear inertial force produced by a headword acceleration. The force occurs
in a gravitational field or during an acceleration when the human body is so positioned that the
force of inertia acts on it in a head-to-foot direction. Positive G-forces are anything that exceeds
one G. This force is commonly experienced during heaving acceleration and causes the liquids
in your body to move away from your head.
Positive lift – Lift acting in an upward direction.
Potential Energy (P.E.) – The energy of a system derived from position. Potential energy is
part of helicopter’s total energy. It is due to the height above a surface, which is the helicopter
GLOSSARY
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APPENDIX A
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
altitude. It is defined as PE = mgh (mass × gravity × height).
Power (P) – The rate of doing work; often expressed in units of hp.
Power Available (PA or Pavail) – The amount of power an engine is capable of producing for
given conditions. As DA increases, engine power available decreases.
Power excess/excess power – The ratio of power available to the power required. If the ratio is
less than one then power required exceeds the power available.
Power Required (PR or Preq) – The amount of power necessary to turn the rotor system at a
constant speed. As the DA increases, the pitch angle of the rotor blades must increase to
generate the same amount of lift. This creates more drag forces on the rotor system and therefore
more power is required to maintain a constant rotor speed.
Preconing – The engineered design used to reduce stress associated with flexing on the root of
the rotor blades, the yoke, and the blade grips. All helicopters have some pre-cone in the rotor
head, because the blades must angle upward from the hub while they are generating lift. Only an
infinitely stiff rotor blade can produce lift without bending upward. Since the blades must angle
upward, the rotor head is shaped that way with a pre-cone, so there is less bending on the blade
and hub.
Pressure Altitude (PA) – The altitude of a given pressure in the standard atmosphere. See
Standard atmosphere. As pressure increases, density increases and density altitude DA
decreases. PA can be easily determined by setting the altimeter setting to 29.92 inHg and read
PA directly from the altimetry.
Pressure gradient – A change in the pressure of a gas or fluid per unit of distance.
Pro-autorotative force – In unpowered flight, the accelerating horizontal component of the TAF
vector in the driving region. In this region the horizontal component of the TAF is tilted forward
of vertical/axial and drives the blade forward.
Profile drag – A combination of Form Drag and Skin Friction. In the case of a rotor blade, it is
the drag created by the movement of the rotor blade through the air without creating lift. In a
hover, profile drag accounts for 15–45 percent of the total power consumption.
Rate of climb – The rate at which an aircraft gains altitude. It is the vertical component of the
aircraft speed in climbing.
Rate of descent – The rate at which an aircraft descends. It is the vertical component of the
aircraft speed in descending.
Redline airspeed – See Limit airspeed.
Relative airspeed – The sum of the velocity of the air and the velocity of the object it is
A-14 GLOSSARY
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
APPENDIX A
approaching.
Relative Airflow (RAF) – The direction of the airflow in relation to the airfoil. It is created by
the movement of the airfoil through the air, the movement of air over the airfoil or a combination
of both. It is the resultant of the rotational relative wind/airflow, the induced airflow, helicopter
velocity, and blade is flapping. It can also refer to the airflow in relation to the rotor disk.
Relative Humidity (RH) – The ratio of the partial pressure of water vapor (amount of water
vapor in the air measured in g/m3) to the saturation vapor pressure (the maximum amount of
water vapor the air can hold in g/m3) at the gas temperature. The resulting number is multiplied
by 100 to get a percentage, which is the RH. In a more practical way, it is how much water
vapor is in this air versus how much could possibly get in the air at this temperature.
Relative wind – Represents all the wind the blade experiences. It is the sum of the Induced
Velocity and Linear Velocity.
Relative wind resultant – see Relative airflow.
Retreating blade – The blade moving in the opposite direction to the helicopter. The rotor
blade experiences a decreased in relative wind. In the USN fleet and in most helicopter designs
the retreating blade is on the left side of the helicopter.
Retreating blade stall – Aggravated case of dissymmetry of lift, which results in the aircraft
pitching up and rolling left. As airspeed increases the retreating blades linear flow is reduced,
the blade flaps down, decreasing induced flow and increasing AOA. Eventually, as airspeed
increases further, the blade will exceed its airfoil critical AOA and will stall. With current blade
designs, a helicopter’s forward airspeed is primarily limited by (a) retreating blade stall, (b) sonic
and supersonic speed at rotor blade’s tip, and (c) noise production of the blades.
Reynolds number (Re) – The Reynolds number (Re) helps predict flow patterns in different
fluid flow situations. At low Reynolds numbers, flows tend to be dominated by laminar (sheetlike) flow, while at high Reynolds numbers flows tend to be turbulent. The turbulence results
from differences in the fluid’s speed and direction, which may sometimes intersect or even move
counter to the overall direction of the flow (eddy currents). These eddy currents begin to churn
the flow, using up energy in the process. Reynold numbers are used to describe the relationship
between dimensions and profile drag and are mainly used in model testing. During the model,
testing of an airfoil a speed is chosen to produce the same Reynolds number in the model as in
the full-sized airfoil and then the drag of the model will be representative.
Rigid rotor – The blades rotate in the feathering plane, but movement in the flapping and
dragging plane is achieved by bending of the blades. Sometimes referred to as hingeless since
the rotor blades are fixed rigidly to the hub without mechanical hinges for flapping, lead and lag
(hunting), and on some systems pitch change (feathering).
Rigidity of the rotor blades – The rotation of the rotor system creates centrifugal force (inertia),
which tends to pull the blades straight outward from the main rotor hub. The faster the rotation,
GLOSSARY
A-15
APPENDIX A
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
the greater the centrifugal force. This force gives the rotor blades their rigidity and, in turn, the
strength to support the weight of the helicopter.
Rolling – The movement of the aircraft around its longitudinal axis.
Rotational Energy (R.E.) – The energy of a system derived from a mass in rotation. Rotational
energy is part of the helicopter’s total energy and it is due the main rotor rotating mass; Rotors
RPM. It is defined as RE = 1/2 I2 (1/2 inertia of blades × rotor RPM squared).
Rotational relative wind – The component of the relative wind produced by rotation of the rotor
blades. It is the airflow opposite to the linear velocity of the blade. The term rotational refers to
the method of producing relative wind. Rotational Relative Wind moves opposite to the physical
flightpath of the airfoil, striking the blade at 90 degrees to the leading edge and parallel to the
plane of rotation. It is the velocity of airflow across the airfoil due to its rotation about the
mechanical axis.
Rotor disk – The area of the circle inscribed in the tip path plane.
Rotor drag – The portion of drag on a rotor blade that is in line with the plane of rotation of the
rotor disk. Rotor drag must be overcome by engine torque.
Rotor driving force – The force that causes the rotor blade to move in the direction of rotation.
It can come from engine power, aerodynamic forces, or from a combination of both. If the rotor
driving force is not equal to the rotor drag, the R/ RPM must be either increasing or decreasing.
Rotor system – A general term referring primarily to the design that holds the rotor blades to the
mast. The three general types of rotor systems are – fully articulated, semi-rigid, and rigid.
Rotor thrust – The thrust produced by the rotor.
Semi-rigid rotor – The blades are free to feather individually, but the rotor head teeters to allow
the rotor to flap as an assembly. The blades are connected to the mast by a trunnion that allows
blades to flap. Pitch change (feathering) is allowed at the rotor hub about the blade grip retainer
bearing.
Separated flow – The flow over or about a body that has broken away from the surface of the
body and no longer follows its contours.
Settling with power – A hazardous helicopter flight condition in which the power required for a
given maneuver or flight regime is greater than the power available under the current ambient
conditions. The terms “settling with power” and “power settling” are commonly miss
understood and improperly used. Therefore, the term “power required exceeds power available”
is preferred. “Power Required Exceeds Power Available” must not be confused with “Power
Settling” which is more correctly called “Vortex Ring State.” Power Required Exceeds Power
Available is a power issue, VRS is an aerodynamic issue.
Sideslip – A movement of an aircraft such that the relative wind has a velocity component along
A-16 GLOSSARY
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
APPENDIX A
the lateral axis.
Skid – Rate of turn is greater than normal for a degree of bank established. During a skid, the
aircraft is turning too fast for the bank angle, and yaws into the turn. (Most likely, you are
pushing too much pedal in the direction of turn and causing the skid.) That causes the outside of
the helicopter to move faster. Skidding the helicopter can be used to increase the rate of descent
while at the same time maintain the same forward airspeed.
Skin friction – The amount of drag created by the retardation of the boundary layer of air in
contact with the surface of the airfoil.
Slip – The rate of turn is less than normal for the degree of bank established. During a slip, the
aircraft is turning too slow for the bank angle, and yaws outside the turn. (Most likely, you are
pushing too much pedal in the opposite direction of turn and causing the slip.) The nose of the
helicopter will be pointing in the opposite direction to the bank. Slipping the helicopter can be
used to increase the rate of descent while at the same time maintain the same forward airspeed.
Solidity ratio – The ratio of the combined area of the plan form of all the blades to the disk area.
Total blade area divided by the disk area.
Span – The dimension of an airfoil from end to end or tip to tip. In a rotor system, the span of a
blade is the distance from blade root to blade tip. Sometimes it can be also used to indicate the
distance from the rotor centerline to the blade tip when the rotor blade is horizontal. Span is
always measured in a straight line.
Speed – The rate at which an object moves in relation to time and distance.
Speed of sound – The distance travelled per unit time by a sound wave as it propagates through
an elastic medium. At 20 °C (68 °F), the speed of sound in air is about 343 meters per second
(1,235 km/h; 1,125 ft/s; 767 mph; 667 kts), or a kilometer in 2.9 s or a mile in 4.7 s. It depends
strongly on temperature as well as the medium through which a sound wave is propagating. The
speed of sound decreases with a decrease in temperature. The speed of sound in air between
36,000 feet and 65,000 feet (cruising altitude for commercial jets) at −57 °C (−70 °F) is 295
meters per second (1,062 km/h; 660 mph; 573 kts). At higher altitudes, the speed of sound is
reached at a lower True Airspeed. For a given Indicated Airspeed, True Airspeed increase with
altitude. Airplanes flying at higher altitudes may have to fly at airspeeds that are closed to the
speed of sound and at the same time close to stall speed. This particular situation is known as the
coffin corner. Higher the airplane flies, the more narrow this corner is going to be. At high
altitudes the supersonic and stall, True Airspeed will start to converge. If the airplane will fly
faster, it will enter the sonic and supersonic regime if it will fly slower it will stall. The solution
to this problem is to descend at a lower altitude to have a larger operational speed range.
Stabilator – A horizontal surface that pivots as a whole. It is a fully movable aircraft horizontal
stabilizer and it is different from the usual combination of fixed and movable surfaces.
Stability – The property of an aircraft to maintain its attitude or to resist displacement and, if
GLOSSARY
A-17
APPENDIX A
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
displaced, to develop forces and moments tending to restore the original condition.
Stabilizer – A fixed or adjustable airfoil or vane that provides stability for an aircraft. An
aircraft stabilizer is an aerodynamic surface, typically including one or more movable control
surfaces, that provides longitudinal (pitch) and/or directional (yaw) stability and control. A
stabilizer can feature a fixed or adjustable structure on which any movable control surfaces are
hinged, or it can itself be a fully movable surface such as a stabilator. Single main rotor
conventional helicopters are designed with a fixed horizontal and vertical stabilizer. Horizontal
stabilizer provides negative lift to augment longitudinal stabilization at high speeds. Some
horizontal stabilizers are allowed to move as speed changes. Vertical stabilizer known also as
vertical fins provide directional stabilization at high speed. By providing more directional
stabilization less tail rotor trust is required during forward flight. Less rotor trust requires the
pilot to use less power pedal and provides more power to the main rotor.
Stall – The condition in which a wing or other dynamically lifting body flies at an AOA greater
than that for maximum lift, resulting in a loss of lift and an increase of drag. In a sonic or
supersonic regime, a loss of lift and an increase of drag brought on by a shock wave. The
condition under which the airflow separates from the airfoil, resulting in a massive increase in
drag and an almost total loss of lift.
Stall speed – The airspeed at which, under a given set of conditions, an aircraft will stall.
Airplanes stall at a relative low speed because in order to maintain lift the AOA exceed the
critical or stall AOA. Helicopters stall at a relative high speed because the retreating blade
reaches the critical or stall AOA at high speed. It is important to notice that when we think an
aircraft will stall because of the airspeed, the real reason why they stall is that the aircraft airfoil
(wing or blade) exceed the stall AOA. We use stall speeds because are easy to represent in terms
of flight parameters. However, some airplanes are equipped with AOA indicators that provide
stall information based on the value of AOA instead of airspeed.
Stalling AOA – The minimum AOA of an airfoil or airfoil section or other dynamic lifting body
at which a stall occurs it is frequently know as critical AOA.
Standard atmosphere – A model of atmospheric conditions (pressure, temperature, and
density), that vary with altitude above the model was derived from global averages and is used in
performance.
Standard lapse rate – In a thermodynamic system, the rate of heat loss of two °C per every
1,000 feet due to an expansion of the atmosphere corresponding to an increase in altitude. Also
referred to as average or adiabatic lapse rate.
Static pressure – The atmospheric pressure of the air through which an aircraft is flying. The
force each air particle exerts on those around it. On a more macroscopic scale, ambient static
pressure (14.7 psi at sea level on standard day) is equal to the weight of a column of air over a
given area. The force of static pressure acts perpendicularly to any surface with which the air
particles collide. As altitude increases, less air is above you, so the weight of the column of air
decreases. Thus, atmospheric static pressure decreases with an increase in altitude at a rate of
A-18 GLOSSARY
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
APPENDIX A
approximately 1.0 inHg per 1,000 feet, near the Earth’s surface. Standard pressure lapse rate is
1,000 feet of PA for each one inch of Hg.
Strake – See Tail strake.
Sweepback – The backward slant from root to tip (or inboard end to outboard end) of an airfoil
or of the leading edge or other reference line of an airfoil. Sweepback usually refers to a design
in which both the leading and trailing edges of the airfoil have a backward slant. Sweepback
airfoils perform better at high airspeeds.
Symmetrical airfoil – An airfoil with the same size and shape above and below the chord line.
Sympathetic resonance – A resonance or vibration set up when two components are rotating at
an RPM where they react with one another to create a vibration that is usually destructive to one
or both components.
Tab – A small auxiliary airfoil set into the trailing edge of an aircraft control surface and used
for trim or to move or assist in moving the larger surface.
Tail rotor – The anti-torque device of a single-rotor helicopter. Control of this rotor is through
the foot pedals.
Tail strake – This L shape aerodynamic component usually protrudes about an inch on the side
of the tail boom and runs for the entire length of the tail boom. A Tail strake acts like a spoiler.
It aerodynamically augments the authority of the tail rotor in hover and at slow speeds.
Tandem rotor system – A main lifting rotor is used at each end of the helicopter. The rotor
systems rotate in opposite directions to counteract torque.
Taper – See Blade taper.
Taxi – The operation of an airplane or helicopter under its own power on the ground, except that
movement incident to actual takeoff and landing. The forward movement of a helicopter at a
hover is referred to as a hover taxi or air taxi depending on speed and altitude.
Temperature (T) – A measure of the average kinetic energy of air particles. As temperature
increases, particles begin to move and vibrate faster, increasing their kinetic energy. Air
temperature decreases linearly with an increase in altitude at a rate of approximately 2 °C
(3.57 F) per 1,000 feet up through 36,000 feet MSL. This is called the standard or adiabatic
lapse rate. Above 36,000 feet lies the isothermal layer where air is at a constant temperature of
-56.5 °C.
Thrust – In an airplane, thrust is the force from the propeller or the jet engine that drives the
aircraft through the air. In a helicopter, the horizontal component of total rotor thrust that is
providing the force to pull the helicopter through the air is often referred to as thrust. In order to
avoid confusion with other uses of the terms lift and thrust, we will refer to the horizontal
GLOSSARY
A-19
APPENDIX A
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
component of TRT as Htrt and the vertical component of TRT as Vtrt throughout this book.
Thrust axis – The line or axis through an aircraft along which the thrust acts. For helicopters,
the total rotor thrust acts perpendicular to the tip path plane through the rotor head and is called
virtual axis.
Tip path – The path described by the tips of the rotor blades.
Tip path plane – The plane (disk) within the tip path. It is parallel to the plane of rotation. The
tip path plane contains the rotor disk, and rotor thrust is perpendicular to the Tip Path Plane.
Tip vortex – A vortex springing from the tip of a wing because of the flow of air around the tip
from the high-pressure region below the surface to the low-pressure region above it.
Torque – Force times a distance. It causes the fuselage to react in yaw because the drive train
turns the rotor.
Torque effect – In a counterclockwise single main rotating rotor system, due to the momentum
of the advancing rotor blade on the right side of the aircraft, there is an equal and opposite
reaction (torque) which causes the helicopter to rotate to the right. The anti-torque system
counteracts torque effect. Remember Newton’s third law of motion, which states, every action
has an equal, and opposite reaction.
Total drag – The sum of the form, parasite, and induced drags.
Total energy – The energy stored in the helicopter and is the result of adding – (a) Potential
Energy (P.E.), due to the height above a surface; Helicopter Altitude. (b) Kinetic Energy (K.E.),
due to the motion with respect to a point on the ground; Helicopter Ground Speed. (c) Rotational
Energy (R.E.), due to the main rotor rotating mass; Rotors RPM.
Total reaction – A single vector representing the sum of all the aerodynamic forces acting on an
airfoil.
Total Rotor Thrust (TRT) – The force created by a rotor at right angles to the plane of rotation
of the rotor disk. This force acts through the rotor head and is broken up into a vertical
component that opposes the weight of the helicopter and a horizontal component that pulls the
helicopter through the air.
Trailing edge – The rearmost edge of an airfoil.
Trailing vortex – A vortex that is shed from a wing or other lifting body and trails behind it is
sometimes referred to as wake turbulence. Wake turbulence must be avoided. Heavy, slow, and
clean (flaps, spoilers, slats, and gear not deployed) airplanes generate larger and stronger wake
turbulence then light, fast, and dirty (flaps, spoilers, slats, and gear deployed) airplanes. In nowind condition wake vortex moved behind and below the aircraft that produced them. Always
avoid flying in trail with heavy airplanes.
Translating tendency – The tendency for a helicopter to translate laterally due to tail rotor
A-20 GLOSSARY
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
APPENDIX A
thrust.
Translational flight – Any horizontal movement of a helicopter with respect to the air.
Translational lift – The increased efficiency of the rotor system in the production of lift by
increasing the horizontal mass flow of air through the rotor disk, reducing the induced flow and
vortices. (See also Effective translational lift.)
Translational velocity (V-trans) – Airflow through a rotor system or across a blade element due
to movement of the aircraft. Added geometrically to v-rotational on the advancing blade and
subtracted on the retreating blade.
Transverse flow effect – A non-uniform induced velocity flow pattern across the rotor disk that
produces a pronounced rolling tendency and lateral vibrations during transition through
approximately 10–20 knots.
Trim – The condition of a heavier-than-air aircraft in which it maintains a fixed attitude with
respect to the wind axes, with the moments about the aircraft axes being in equilibrium. The
word trim is often used with special reference to the balance of control forces.
Trim tab – A tab that is deflected to a position where it remains to keep the aircraft in the
desired trim. Adjustment of a trim tab on a rotor blade causes the blade to maintain a given track
or plane of motion.
True Airspeed (TAS) – EAS corrected for error that is due to air density (altitude and
temperature). It is the speed of the aircraft relative to the air mass in which it is flying. For a
give Indicated Airspeed, True Airspeed increase with altitude. When flying at sea level under
ISA conditions (15 °C, 1013 hPa/29.92 inHg, 0 percent humidity), TAS is the same as EAS and
CAS. If there is no wind, it is also the same as GS.
Turbulence – An agitated condition of the air or other fluids. A body in motion through the air
can create a disordered, irregular, mixing motion of air, called air turbulence.
Turbulent boundary layer – A boundary layer characterized by random fluctuations of a
velocity and by pronounced layer mixing of the fluid.
Turbulent flow – A flow characterized by turbulence. Turbulent flow is an irregular, eddying,
fluctuating flow in which the velocity of a given point varies erratically in magnitude and
direction with time.
Twist (washout) – The decrease in pitch angle from the root of the blade to the tip.
Ultimate load factor – The maximum load factor an aircraft can withstand without structural
failure. At the ultimate load factor, there will be some permanent deformation, but no actual
failure of the major load-carrying components should occur.
Underslinging – Attachment of the rotor head occurs with a pivot point above the blade grips
GLOSSARY
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APPENDIX A
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
and centered midway between the opposing blade centers of gravity. Semi-rigid rotor head
design which compensates for geometric imbalance by keeping the individual centers of mass for
each rotor blade equidistant from the center of rotation. Underslinging geometrical designs
allows for flapping but minimizes hunting.
Uniform flow – An idealized flow in which the streamlines are parallel and the velocity is
constant throughout.
Unsteady flow – A flow whose velocity components vary with time at any point in the fluid.
Unsteady flow is of fixed pattern if the velocity at any point changes in magnitude but not
direction and of variable pattern if the velocity at any point changes in direction.
Upwash – A flow deflected upward by a wing, rotor, or tail blade.
Upwash angle – An acute negative downwash angle measured in a plane parallel to the plane of
symmetry of an aircraft, between the direction of upwash and the direction of the undisturbed
airstream.
Useful load – The difference, in pounds or kilograms, between the empty weight and maximum
authorized gross weight of an aircraft.
Vector – A quantity having both magnitude and direction. A graphic illustration of a quantity
having both magnitude and direction.
Velocity – The time rate of motion in a given direction. It is a combination of speed and
direction. It is represented by a vector quantity that includes both magnitude (speed) and
direction relative to a given frame of reference.
Venturi – A converging-diverging passage for fluid that increases the fluid velocity and lowers
its static pressure. A Venturi tube is a converging-diverging passage where the venturi effect
(increase fluid velocity – decrease fluid static pressure) takes place.
Vertical axis – A straight line through the CG running from top to bottom and perpendicular to
the longitudinal and lateral axis. It is the axis about which yaw occurs. It is also called a Normal
axis.
Vertical stabilizer – A vertical aerodynamic surface mounted approximately perpendicular to
the longitudinal axis of an aircraft to which a rudder may be attached. The vertical stabilizer aids
in directional stability. Single main rotor conventional helicopters are designed with vertical
stabilizer attached at the end of the tail boom and generally, some degrees offset from the
longitudinal axis. A helicopter’s vertical stabilizer is commonly known as a vertical fin. It
provides directional stabilization at high speed. By providing more directional stabilization less
tail rotor trust is required during forward flight. Less rotor trust requires the pilot to use less
power pedal and provides more power to the main rotor.
V-G diagram – See V-n diagram.
Virtual axis – The axis of rotation perpendicular to the tip path plane, as opposed to the
A-22 GLOSSARY
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
APPENDIX A
mechanical axis. As the rotor disk tilts with control inputs, the virtual axis tilts and remains
perpendicular to the plane of rotation. Total rotor thrust acts through the virtual axis.
Viscosity () – A measure of the air’s resistance to flow and shearing. Air viscosity can
determine its tendency to either stick to a surface or how easily it flows past it. For liquids, as
temperature increases, viscosity decreases. Recall that the oil in your car flows better or gets
thinner when the engine gets hot. Just the opposite happens with air—air viscosity increases
with an increase in temperature.
V-n diagram – A graph that summarizes an aircraft’s structural and aerodynamic limitations at a
particular weight, altitude, and configuration. The horizontal axis of the graph is indicated
airspeed. The vertical axis of the graph is load factor, or G’s. V-n diagrams define the
maneuvering envelope for fixed-wing aircraft and rotary-wing aircraft.
VNE – Velocity never-to-exceed. See Limit airspeed.
Volume (v) – The amount of space occupied by an object.
Vortex Ring State (VRS) – The settling of the helicopter into its own downwash. During VRS,
airflow is downward over the outer portion of the rotor disk and upward in both an area
expanding outward from the hub as well as the area outside the tip path plane. This rapidly
decaying phenomenon may result in zero net lift, high vibrations, and high sink rates/rates of
descent. VRS has also been called power settling, a term commonly confused with the term
settling with power which defines a power required exceeding power available situation.
V-rotational – See Rotational velocity.
V-translational – See Translational velocity.
Vtrt – The vertical component of TRT (Total Rotor Thrust).
Wake turbulence – See Trailing vortex.
Weathervane effect – The tendency of a helicopter to face into the wind while in a hover, air
taxing, or slow flying.
Weight (W) – A measure of the mass of an object under the acceleration of gravity.
Work (W) – A force exerted over a given distance. Work is done when a force acts on a body
and moves it. It is a scalar quantity equal to the force (F) times the distance of displacement (s).
𝑊 =𝐹×𝑠
Yaw/yawing – The movement of the aircraft around its vertical axis.
Zero AOA – The position of an airfoil, fuselage, when no AOA exists between two specified or
understood reference lines. Depending of the airfoil design (symmetric or asymmetric) lift may
be created even at zero AOA.
Zero-lift AOA – The geometric AOA when no lift is created. Often called the angle of zero lift
GLOSSARY
A-23
APPENDIX A
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
or the zero-lift angle. Zero lift AOA may not coincide with zero AOA. Depending on the airfoil
design, zero lift could be created even at positive AOA.
A-24 GLOSSARY
APPENDIX B
RECOMMENDED READING LIST
Anderson, D.F., & Eberhardt, S. (2001). Understanding flight. (Vol 2). McGraw-Hill.
http://ae.sharif.edu/~iae/Download/Anderson%20D.F.,%20Eberhardt%20S.%20Understanding%
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Anderson, J. D. (1997). A history of aerodynamics and its impact on flying machines.
Cambridge University Press.
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u_Z2EAo&sig=iLFTAF7NJa4lzmHRqnCFYJkTys8#v=onepage&q&f=false
Anderson, J. D. (2010). Fundamentals of aerodynamics (5th ed.). McGraw-Hill.
https://cds.cern.ch/record/1392990/files/9780071289085_TOC.pdf
Chiles, J. R. (2007). The god machine: From boomerangs to black hawks, the story of the
helicopter. Bantam Dell.
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cC1ef&sig=uK50pYSwLHQE_XKD_RI1bQDCuNQ#v=onepage&q&f=false
Coyle, S. (2013), The little book of autorotations. Eagle Eye Solutions, LCC.
Dole, C. E., Lewis, J. E., Badick, J. R., & Johnson, B. A. (2016). Flight theory and
aerodynamics a practical guide for operational safety. John Wiley & Sons.
https://books.google.com/books?hl=en&lr=&id=RAdLDQAAQBAJ&oi=fnd&pg=PR11&ots=u
Vw_Mu8qJZ&sig=8jGpma8TDExqJmGA6yFS9ieKJZI#v=onepage&q&f=false
Electronic Code of Federal Regulations (e-CFR) (2021, May 25). Electronic Code of Federal
Regulations. https://www.ecfr.gov/cgi-bin/textidx?c=ecfr&tpl=/ecfrbrowse/Title14/14tab_02.tpl
Federal Aviation Administration. (2019). Helicopter flying handbook. (FAA-H-8083-21B) U.S.
Department of Transportation.
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handb
ook/media/helicopter_flying_handbook.pdf
Hurt, H.H. (1965). Aerodynamics for naval aviators (NAVAIR 00-80T-80). U.S. Government
Printing Office.
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Isaacson, S. (2021, May 25). FAR AIM.org. FAR/AIM. http://www.faraim.org/
Johnson, W. (1980). Helicopter theory. Dover Publications, Inc.
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Helicopter+theory&ots=NKZiY0VZyt&sig=SHBrSu7jLkQvTqG4AiqMfvvFGjo#v=onepage&q
&f=false
RECOMMENDED READING LIST
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APPENDIX B
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
Leishman, G. J. (2000). Principles of helicopter aerodynamics. Cambridge University Press.
https://books.google.com/books?hl=en&lr=&id=nMV-TkaX9cC&oi=fnd&pg=PR19&dq=2000).+Principles+of+helicopter+aerodynamics&ots=Cs9Qg91Ge
z&sig=HWIdDQmeCTzdDzxw9poyfduNJc#v=onepage&q=2000).%20Principles%20of%20helicopter%20aerodynamics&f=false
Lengel, R. (2019). Everything explained for the professional pilot. (13th ed.). Aviation-Press.
McKenzie, F. (2019). 71 Lessons From The Sky Civilian Helicopters. Squabbling Sparrows
Press.
Montgomery, J. R. (1964). Sikorsky helicopter flight theory for pilots and mechanics. Sikorsky
Aircraft Division of United Aircraft Corporation.
http://manosparnai.lt/e107_files/public/1292454390_636_FT7882_sikorsky-helicopter-flighttheory-300bw.pdf
Newman, R. (2015). The technical, aerodynamic & performance aspects of a helicopter.
BookBaby.
Padfield, R. (2014). Learning to fly helicopters. McGraw-Hill Education.
Prouty, R. W. (1985). Even more helicopter aerodynamics. PJS Publications Inc.
Prouty, R. W. (1985). Helicopter aerodynamics. PJS Publications Inc.
Prouty, R. W. (1995). Helicopter performance, stability, and control. Krieger Publishing
Company, Inc.
Prouty, R. W. (2016). Helicopter aerodynamics volume III. Eagle Eye Solution LLC.
Saunders, G. H. (1975). Dynamics of helicopter flight. John Wiley & Son Inc.
Seddon, J. (1990). Basic helicopter aerodynamics. AIAA.
Stepniewski, W. Z. (1981). Rotary-wing aerodynamics. Dover Publication Inc.
U.S. Army Headquarters. (1979). Rotary wing flight, FM 1-51. Department of the Army.
U.S. Army Headquarters. (1982). Meteorology for army aviators, FM 1-230. Department of
the Army.
U.S. Army Headquarters. (1988). Fundamentals of flight, FM 1-203. Department of the Army.
U.S. Department of Transportation Federal Aviation Administration. (2016). Pilot’s handbook
of aeronautical knowledge. United Sates Department of Transportation, Federal Aviation
Administration, Airman Testing Standard Branch.
B-2 RECOMMENDED READING LIST
HELICOPTER FUNDAMENTALS AND BASIC AERODYNAMICS
APPENDIX B
U.S. Department of Transportation. (2020). 2021 FAR/AIM. Aviation Supplies & Academics,
Inc.
U.S. Navy. (2013). Introduction to helicopter aerodynamics TH-57 (CNATRA P-401 Rev 02
13). Naval Air Training Command.
Wagtendonk, W. J. (1996). Principles of helicopter flight. Aviation Supplies & Academics Inc.
Watkinson, J. (2004). Art of the helicopter. Elsevier Butterworth-Heinemann.
RECOMMENDED READING LIST
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