How Helicopters Fly ?
They have existed for only 70 years, yet they are without question one of the most versatile and
vital vehicles in the world. They transport world leaders and the critically wounded; they fight
forest fires and rescue people trapped in burning buildings; they can deliver huge payloads to areas
that no other vehicle can reach.
In the last 40 years, helicopters increased their speed from 150 Km/h to 400 and their lifting
capacity (payload) from 100 Kgs to 40.000
They were used extensively for the first time in the Korean war and today, they are used in all type
of rolls. Some of them are :
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Transports
Ambulance
Search and Rescue
Cranes
Fire suppression
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Mail Service
Observation
Arial photography
Traffic 's Control
Fertliser spreading
 Cattle 's Control
 Fence mending
 Militaries : Assault, Gunships,
Antisubmarine, Electronic Warfare, etc
 And more ...
Introduction
The helicopter is type of aircraft in which lift is obtained by means of one or more power-driven
horizontal propellers called rotors. When the rotor of a helicopter turns it produces reaction torque
which tends to make the craft spin also. On most helicopters a small rotor near the tail
compensates for this torque. On twin-rotor craft the rotors spin in opposite directions, so their
reactions cancel each other. The helicopter is propelled in a given direction by inclining the axis of
the main rotor in that direction. The helicopter's speed is limited by the fact that if the blades rotate
too fast they will produce compressibility effects on the blade moving forward and stall effects on
the rearward moving blade, at the same time.
Although the helicopter was only recently fully developed, its concept can date back to the late
1400's. Since then, helicopters have been put into use by society in many ways. One can find
helicopters in both civil and military areas. The early helicopters were mainly developed for
military use, but later became certified for civilian use. Since then helicopters have evolved
greatly, specifically with the design. Because a helicopter can perform more actions than a fixedwing aircraft can, it is more complicated to fly. The helicopter must compensate for a variety of
forces, like the spinning force induced by the main rotors. The engineering behind designing a
helicopter is complex with a variety of issues to be understood .
Principles:
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Principles of rotary flight
Flying a helicopter
Rotor 's configuration
The tail rotor
Skids or wheels ?
The servo flap controlled rotor
IGE and OGE / Recirculation
Blade Element - Momentum Theory
Subjects:
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What happens when the engine fails ? [Autorotation]
Why can't a helicopter fly faster than it does ?
Why were "compound helicopters" not successful ?
Blades and Lift (+ FAQ update)
How long can fly before it needs an overhaul ?
What is ground resonance ?
Specials:
 Rotopter A new type of winged aircraft
 BodyCopter
 Unicopter / SynchroLite
The Main Rotor
The lifting force is produced by the rotors. As they spin they cut into the air and produce lift. Each
blade produces an equal share of the lifting force.
Spinning the rotor against the air causes lift,
allowing the helicopter to rise vertically or
hover.
Tilting the spinning rotor will cause flight in
the direction of the tilt.
The Tail Rotor
The tail rotor is very important. If you spin a rotor
using an engine, the rotor will rotate, but the engine
and the helicopter will try to rotate in the opposite
direction.
This is called TORQUE
REACTION
The tail rotor is used like a small propeller, to pull
against torque reaction and hold the helicopter
straight.
By applying more or less pitch (angle) to the tail rotor blades it can be used to make the helicopter
turn left or right, becoming a rudder.
The tail rotor is connected to the main rotor through a gearbox.
When using the tail rotor trying to compensate the torque, the result is an excess of force in the
direction for which the tail rotor is meant to compensate, which will tend to make the helicopter
drift sideways.
Pilots tend to compensate by applying a little cyclic pitch, but designers also help the situation by
setting up the control rigging to compensate.
The result is that many helicopters tend to lean to one side in the hover and often touch down
consistently on one wheel first.
On the other hand if you observe a hovering helicopter head-on you will often note that the rotor is
slightly tilted. All this is a manifestation of the drift phenomenon.
Flying a helicopter
The helicopter is steered in any direction by inclining the axis of the main rotor in that direction.
Flying a helicopter requires great concentration.
You must use one hand on the control lever that is at your
side (the collective control stick) to raise or lower the
helicopter, while at the same time controlling the throttle
(not an easy task).
This is a control which is only found in helicopters and is
linked to the engine power. Moving this up and down
changes the pitch of the main rotors. As the pitch is
increased more power is required from the engines so that
the rotor speed is kept at the same level.
You must use your other hand on the control lever that is
just in front of you (the cyclic control stick) to move the
helicopter forward, backward and to either side, as if you
were in a conventional aircraft.
Moving it forward or back will point the nose of the
helicopter up or down. It does this by varying the angle of
the rotor blades as they go round, tilting the rotor back and
forth. When moved left or right the rotor tilts in that
direction and the helicopter banks and rolls.
And finally you must use the tail rotor pedals, on the floor,
to control the pitch of the tail-rotor. For straight flight, the
pitch of the tail rotor is set to prevent the helicopter from
turning to the right as the main rotor turns to the left. The
pilot pushes the left pedal to increase the pitch of the tail
rotor and turn to the left. Pushing the right pedal decreases
the pitch of the tail rotor and turns the helicopter to the
right.
Flying a helicopter requires entirely
different skills than flying
conventional aircraft. This is why it
is difficult to fly a vertical take-off
or landing (VTOL) aircraft since
both skills are required when making
the transition from vertical to
horizontal flight.
Notice the pilot 's hands
On tandem rotors helicopters, like
Boeing 's Chinook, that had not tail
rotor, the pedals are connected to the
swashplates and cyclicly change pitch
on both rotors in equal, but opposite
directions. For example, if the left pedal
is pressed at a hover, the front rotor disk
tips left and the rear rotor tips right so
that the helicopter yaws to the left.
Two types of panels:
Ok, now take a look how it is ...
How the Helicopter Flies ...
Contribution: Sikorsky Aircraft
Flight of a helicopter is governed by the pitch or angle of its rotor blades as they sweep through
the air. For climbing and descending, the pitch of all the blades is changed at the same time and in
the same degree.
To Climb, the angle ot pitch of the blades is increased. To descend, the pitch of the blades is
decreased. Because all blades are acting simultaneously, or collectively, this is known as
collective
pitch
.
For forward, backward and sideways flight an additional change of pitch is provided. By this
means the pitch of each blade increases at the same selected point in its circular pathway. This is
the cyclic pitch .
With these two controls in mind let us make an imaginary flight. With the engine warmed up and
the rotor blades whirling above us in flat pitch, that is, with no angle or bite in the air, we are
ready to start.
We increase the collective pitch. The rotor blades bite into the air, each to the same degree, and lift
the helicopter vertically.
Now we decide to fly forward. We still have collective pitch to hold us in the air and we adjust the
cyclic pitch so that as each blade passes over the tail of the helicopter, it has more bite on the air
than when it passes over the nose. Naturally the helicopter travels forward.
Now we decide to stop and hover motionless so we put the cyclic pitch in neutral, the rotor blades
now have the same pitch throughout their cycle, and the collective pitch holds the helicopter
suspended in space without moving in any direction.
In short, it is the cyclic and collective pitch which gives the helicopter its unique ability to fly
forward, backwards, sideways, rise and descend vertically and hover motionless in the air ,
making it one of the most versatile vehicles known by man .
Rotor 's Configuration
When the engine develops enough power to lift the helicopter, the main problem is how to counter
the main rotor forcing the fuselage to rotate in the opposite direction of the rotor. This effect is
known as Torque.
The classic solution was a small tail rotor to push the fuselage in the opposite direction of the
torque force.
Another popular solution was tandem rotors. The primary advantage of this configuration is the
ability to lift heavy loads whose position relative to the helicopter's centre of gravity is less critical
than in the single rotor configuration.
Because there is no anti-torque rotor, full engine power can be applied to lifting the load.
Disadvantages of the tandem rotor system are a complex transmission and more drag due to its
shape and excessive weight.
Read about drag in our "Why can't a Helicopter fly faster than it does ?" article
And coaxial rotors, in which the goal is obtain a notably compact design.
A relative new solution is the NOTAR (NO TAil Rotor), that uses jet thrust rather than blades to
provide directional stability and reduce noise, providing the world's quietest helicopters.
NOTAR also utilizes Coanda Effect with the rotor downwash across the tailboom and an internal
airflow through the tailboom to produce a sideways "lift", or more correctly "thrust" to counter
main rotor torque.
The jet thrust from the nozzle at the end of the tailboom is primarily used for directional control,
with a very small contribution to anti-torque force.
After the torque effect, the other main problem was the tendency
of the helicopter to
roll laterally in the direction of the retreating rotor blades as the advancing
blades pass through denser air and generated greater lift than the retreating blades that pass
through less dense air.
This problem was eventually solved by the
introduction of a flapping hinge in the rotor head,
which allowed the advancing blade to climb
slightly, thereby reducing its angle of attack and
the amount of lift generated, while the retreating
blade fell slightly, thereby increasing is angle of
attack and the amount of lift generated.
The Tail Rotor
As we have already seen at Principles of Rotary Flight the tail rotor main function is to pull
against torque reaction and hold the helicopter straight.
But also, by applying more or less pitch (angle) to the
tail rotor blades, it can be used to make the helicopter
turn left or right, becoming a rudder.
The pilot use the tail rotor pedals, on the floor, to
control the pitch of the tail rotor. For straight flight, the
pitch of the tail rotor is set to prevent the helicopter
from turning to the right as the main rotor turns to the
left. The pilot pushes the left pedal to increase the pitch
of the tail rotor and turn to the left. Pushing the right
pedal decreases the pitch of the tail rotor and turns the
helicopter to the right.
The tailrotor in normally linked to the main rotor via a
system of driveshafts and gearboxes, so both are
usually connected to the same transmission, meaning
that if you turn the main rotor by hand, the tailrotor will also turn.
Most helicopters have between a 3:1 to 6:1 ratio. (In the first case, every time the main rotor turns
one rotation, the tail rotor makes three revolutions) For example: If the main rotor is turning at 324
RPM, then the tail rotor turns at 1944 RPM at 6:1.
In most helicopters the engine turns a shaft that connects to an input quill on the transmission; the
main rotor mast comes straight out of the top of the transmission and the tailrotor driveshaft
connects to an output quill 90 degrees out from the mast.
Skids or wheels ?
Skids are used mainly because they weigh less than wheels. On larger, more powerful helicopters,
wheels are used because the utility and convenience can be more important than the savings in
weight. In order to move a skid-equipped helicopter on the ground, one has to attach a set of
ground-handling wheels, jack up the helicopter, and roll it (into the hangar for maintenance, for
example). If your helicopter already has the wheels as a
permanent feature, it is more convenient to move around when the engine is shut down
or the pilot has wandered off.
The design decision between retractable or fixed wheels becomes a trade-off between the
complexity/weight but increased aerodynamic efficiency of retractable gear and the simplicity of
fixed gear (and increased drag/reduced efficiency).
It really depends on the primary use of the helicopter; if you are logging, skids make sense
because you can lift larger loads and are more concerned with hovering performance. If the
primary mission is medevac or air transport, retractable wheels allow greater speed and increased
fuel economy over long distances.
So the main factor is one of simplicity, a skid landing gear needs very little maintenance but there
is a drawback. Ground handling is a bit more difficult, so it follows that it is the smaller machines
that use them. Once you get above say 8,000lb or 4 tons, you really need some built in wheels to
move the thing around, a
Bell UH-1 must be somewhere near the practical limit, especially if your trying to get it
under cover in a hurry.
The Servo Flap Controlled Rotor
Helicopter design has come a long way from the days of Breguet, Pescara, Cierva and even since
Sikorsky 's first successful vertical lift machine. Along the way there have been attempts to design
the machine in order to :
 increase maximum forward speed
 improve the streamlining of the machine i.e. reduce drag and hence the power required
 reduce the vibrations in the helicopter
 improve the stability and control characteristics
These are some key issues associated with a conventional helicopter. The servo-flap is one method
that has been found to be of use in tackling some of these problems.
What is the Servo Flap and How Does it Work?
The servo flap is a small airfoil located at about 75 percent span of the rotor blade, situated on the
trailing edge of each rotor blade. These flaps are controlled by the pilot through push-pull control
rods and their function is similar to that of an elevator on fixed wing airplanes.
Moving the trailing edge of the flap upward moves the leading edge of the main rotor blade up.
This increases the rotor pitch or the lift in very much the same manner as the elevator, on a fixed
wing aircraft, changes the angle of attack on the wing. Thus the helicopter pilot can cause the
angle of attack of the flap to increase or decrease in pitch, causing the helicopter to alternately dive
or climb.
In the conventional rotor design the pitch of the rotor blade is varied by the introduction of a hinge
near the root of the blade, which can rotate the blade about the pitch change axis. As could be
imagined, the moment arm near the root being smaller than at the three-quarter radius of the blade
as it is for the servo-flap – the forces required to produce the pitching moment will be much larger.
The servo-flap does the work of more complex and heavy hydraulic control systems. Hence for
this system the total control forces would be much lower because the work to move the blade
happens right where lift is being generated. An accompanying advantage is the fact that this
dampens out the vibrations that are generated in the blade due to varying lift and eliminates the
transmission of these vibrations to the airframe. Vibrations being the cause for reduced life of the
hub and blades and accompanying parts, due to fatigue, is now no longer a factor to contend with.
This is what gives the helicopter blade and the hub in such a helicopter its well-touted infinite life,
which essentially implies that the life of the rotor blade is equivalent to the life of the airframe.
Consider this with the fact that for a conventional helicopter the rotor blade and hub has a far
shorter serviceable life than the airframe.
Furthermore since the servo flap uses energy drawn from the air-stream to pitch the blades up and
down, the control forces need only be high enough to deflect the small servo flaps, thereby
reducing the complexity of the control mechanism at the blade hub significantly. Also note the
additional stability effect that is factored into such a system where the servo flap by contributing
to additional rotational and flapping inertia, provides the system with angle of attack stability as
also acts as a gust alleviation device. Thereby justifying the analogy between the servo flap and
the elevator in a fixed wing airplane. So in the event of an engine failure, the servo flap responds
automatically to increased angle of attack caused by the change in airflow through the rotor and
decreasing rotor RPM. Although the pilot still has to lower collective to stabilize the autorotational
descent, the servo flap provides the pilot additional reaction time before rotor RPM decays too
low. An accompanying advantage is the ability of the system to provide for in-flight rotor blade
tracking. This is made possible by an electric actuator in each tab control, which allows tracking in
flight and on the ground.
The Conception of the Servo-Flap
The concept of trailing edge flap for active control essentially originated with Raul Pescara 's
helicopter of 1922, which featured plain flaps for 1/rev blade pitch control.
Corradino D'Ascanio 's conception of the servo flap as a control mechanism and its
application in his co-axial helicopter design later became an integral design feature of the
helicopters designed by Charles Kaman. However while D'Ascanio applied servo tabs for
collective and cyclic pitch control, Charles Kaman applied the servo tab for rotor pitch changes as
well, thereby making the blade twist rather than rotating the blade with the help of a hinge at the
root. This simplified the rotor hub significantly. The first helicopter that Charles Kaman designed
was the K-125 in 1947, and the servo-flap was a primary design feature in that. Since then servo
flaps have been the method of pitch control in helicopters designed by the Kaman Aerospace
Company. Interestingly the conception of pitch control took place at Sikorsky.
In 1940, Charles Kaman was working at Hamilton Standard, a division of United Aircraft in East
Hartford Connecticut, on propellers and later on the aerodynamic design of the Sikorsky
VS-300. Working on the problem of stability and control, he started analyzing ways to
overcome this. His initial attempts at providing a hinged surface similar to an aileron in fixed wing
aircraft had to be abandoned when he realized the inherent flaw in his design. The assumption that
the rotor blade was rigid was physically being violated by the fact that the blade was long and
flexible and was unable to contain the lift generated by the lowered flap. As a result the blade
twisted down each time the flap was deflected and remained that way until the flap was raised. He
abandoned his attempt to reproduce the ailerons on helicopters and started work on the servo flap.
While most helicopters control blade pitch by using mechanical force at the rotor hub, he found
that servo flaps could change pitch by utilizing aerodynamic forces acting on the blade itself. By
eliminating the pitch control mechanism at the hub, the hub could be simplified significantly since
the smaller surface area of the servo flap required lower operating forces. The two flaps he
designed were about the same size and looked much alike and Kaman bolted the servo to brackets,
which extended from the front and back of the blade. The servo flaps separation from the blade
was the key difference. After experimenting with different flap configurations he settled on
placing the device at the trailing edge of the blade, at the three-quarter-radius point. The angle of
the servo flap, controlled by the pilot twisted the flexible blade into the desired blade pitch,
eliminating the need to change pitch from the rotor hub and by compensating for each blades'
inconsistencies, the servos cut down on vibrations. When Kaman tried to get upper management at
Sikorsky interested in the servo flap, he was given an interesting reply. " Charlie, we have our own
inventor at United Aircraft. His name is Igor Sikorsky. We don't need another one. Soon after
Kaman left the company and with $2000 and his own invention, he started Kaman Aerospace
Corp.
The Configuration of a Helicopter Using Servo Flaps
A good way to understand the advantages of this system is to consider a helicopter that is designed
applying this technology. The
Kaman K-Max K1200 is a good study in this area.
Type: Single seater, external lift intermeshing rotor helicopter and military multi-mission
intermeshing
rotor
aircraft
(MMIRA).
Design Features: Kaman intermeshing rotor ensures all the engine power is produced for
lift, in addition rotor disc loading is very low – which provides greater lifting capability
per helicopter. It has Kaman intermeshing two bladed contra-rotation rotors with separate
inclined shafts emerging from a common transmission. Lifting power is increased
because induced drag and downwash of the intermeshing rotor system is reduced and
power drain of the tail rotor is eliminated. The blade centerline is offset from the hub and
there is a single drag bearing with drag damper. Small trailing edge tabs set the blade
pitch, light control loads and low feedback eliminate the need for powerful pitch change
rods and levers and hydraulic powered controls, all bending and twisting is caused by
pitch change accommodated by blade flexing. The engine is mounted horizontally behind
the transmission. Minimum overhaul life for all parts except the engine is 2400 hrs.
Flying Controls: Blade angle of attack is controlled by trailing edge tabs and light
control linkages, avoiding the need for hydraulic power. Normal powered flight turns at
or near hover are effected by applying differential torque to the rotors by means of
differential collective pitch commanded from the foot pedals. Intermeshing rotors cause
pronounced pitch attitude change in response to collective pitch change, The K-Max
tailplane is connected to the collective to alleviate this problem as well as to reduce blade
stresses and to produce touchdown and lift off in level attitude.
Structure: Light alloy airframe, composite main rotor blades and servo flaps. Tail
assembly weighs 36.3 kg and can be quickly removed by two people. Karon bearings,
Kaflex couplings are used which require no lubrication and zero maintenance.
Power Plant: One 1343 KW (1800 SHP) Textron Lycoming T53-17A turboshaft. Since
the K-Max only requires 1160SHP to operate at the maximum gross weight on a standard
day at SL, there is plenty of power for hot and high days.
The resulting helicopter can carry more payload for fuel used, maintained with minimum power
and fewer parts are required to maintain and track. It can move more weight reliably with less
support and lower operating cost. The intermeshing rotor configuration makes the K-Max one of
the quietest helicopters. (4dB lower than the FAA maximum dB level of 87 dB.)
Comparison of the Rotor System with a Conventional Rotor
Configuration
The servo flap mechanism in essence operates the flap on the trailing edge of the rotor blade in
order to change the pitch of the blade. Mechanical linkages from the rotor head run through the
blade to this small flap and changing its pitch in much the same manner as that for conventional
rotors. Flap and lag hinges are present as in the conventional helicopter blade. This mechanism is
different in that it does not require any hydraulics between the pilot and pitch change mechanism
because the moment arm is so large that suitable mechanisms can be designed such that pilot effort
is low. The collective and cyclic pitch system differs in manner of operation from the conventional
configuration due to the use of servo flaps.
Collective System: The primary components in this system are the collective stick, throttle and
push-pull control rods connected to the servo flaps through the azimuth assembly. The prime
function of the collective system is to control the pitch of all the rotors. Raising the collective
lever causes the servo flap trailing edge on each rotor blade to move upward, increasing the pitch
on all four blades, collectively and equally. This increases the lift causing the helicopter to rise.
Conversely, lowering the collective decreases lift and the helicopter descends. Engine power is
synchronized automatically with these pitch changes to hold the RPM constant. In a synchropter
like the K-Max, the collective is mechanically linked to the moving elevators on the tail boom. Up
collective results in the elevator leading edge to move up. This further reduces the pilot workload
by minimizing pitch attitude changes with collective lever movement.
Cyclic system: The cyclic control system consists of the cyclic control stick and push pull rods
connected through the azimuth assembly to the servo flaps. Movement of the cyclic stick in a
given direction causes one - or for the intermeshing system - both rotors to tilt and fly the
helicopter in the same direction and at speed relative to the amount of stick movement. When the
cyclic stick is moved forward one (or both) the rotors tilt forward equally. The same is true of the
aft cyclic stick movement. In the case of the intermeshing system the application of lateral cyclic
does not result in both the rotors tilting sideways the same amount as the fore and aft movement.
Moving the cyclic to the left cause the left rotor to tilt to the left in proportion to the amount of left
control input and conversely for moving to the right. However in the synchropter the directional
control system analysis is a bit more involved and will not be included here, as it does not directly
allude to the topic at hand.
In comparison the conventional rotor-hub design is fairly complicated. The blade in such a design
rotates in pitch about a bearing, aligned in a radial direction, which can be a roller bearing stack or
a composite flexure. The pitch is applied via an arm projecting forwards from the pitch bearing
housing known as the pitch horn. The pitch horn is connected to its own individual track rod by a
swivel bearing and vertical movement of the track rod will cause the change in blade pitch angle.
The lower end of the track rod is connected to a spider or rotating star, which is constrained to
rotate with the rotor. A movement of the spider in a direction parallel to the rotor shaft will cause
all the blades to rotate in pitch by the same amount, have the same pitch angle, and hence effect a
change in collective pitch.
If the spider center maintains its location relative to the rotor shaft but its plane tilts, then it can be
seen that as the blade rotates around the shaft as the rotor turns, the spider arm moves up and
down once per rotor revolution. That is the blade pitch angle changes once per revolution and
cyclic pitch is achieved. The pilot controls must therefore be able to control the spider's location
and orientation with the added complication of the spider itself rotating both the shaft. The
majority of the helicopters achieve this using a swash plate. This is essentially a flat plate joined to
the spider such that they remain locked together in the same plane. The swash plate and spider
combination slides up and down the rotor shaft and tilts relative to its common center. The swash
plate is held stationary relative to the fuselage and its position and orientation is determined by
three actuators, or jacks connecting it to the top of the fuselage or main rotor gearbox casing. The
pilot's controls alter the strokes of the actuator which position the swash plate but rotates with the
rotor and its position determines the collective and cyclic pitch angles. If the actuators move in
unison the swash plate and spider maintain any tilt but slide along the rotor shaft and collective
pitch is adjusted. If the actuators move unequally then the rotation plane of the swash plate and
spider combination is altered and cyclic pitch is achieved.
As can be deduced from the description the conventional hub design is very complicated.
Compare this with the rotor hub for a servo-flap configuration. Evidently the use of the servo flap
simplifies matters far more than can be conceived. By far the most notable feature in this system is
in the simplicity it provides to the rotor hub. Consider the fact that the typical weight of a main
rotor blade would be around 200 lbs. while the weight of a servo-flap is around 6 lbs. Inevitably
the amount of force required to operate the servo flap is far less. The servo flap hence obviates the
need for the cumbersome design of conventional rotor hubs by getting rid of pitch change bearings
and heavy hydraulics required to operate those bearings at the root. In addition the reduction in
drag by the removal of many components in the hub is of tremendous advantage in streamlining
the helicopter.
In Conclusion
As can be gathered from this analysis, the servo flap provides the whole helicopter with more
stability with regard to angle of attack than other rotor control systems. It is easier to give wider
margins for rotor blade flutter and it reduces the requirements for an AFCS to give good handling
characteristics especially in gusts. It is lighter than other systems and needs no hydraulics, yet it
gives better rotational inertia, which is important for good autorotational characteristics.
The servo flap is extremely simple in concept and execution yet very effective in control. The
reasons for its application in helicopter design being limited to exactly one company are rather
hard to comprehend, nevertheless should not be attributed to flaws in the system.
IGE, OGE and Recirculation
In Ground Effect (IGE) is a condition where the downwash of air from the main rotor is
able to react with a hard surface (the ground), and give a useful reaction to the helicopter in the form of
more lift force available with less engine power required.
What is occuring is the air is impacting with the ground and causing a small build up of air pressure in the
region below the rotor disk.
The helicopter is then "floating" on a cushion of air. This means that less power is required to maintain a
constant altitude hover. IGE conditions are usually found within heights about 0.5 to 1.0 times the diameter
of the main rotor.
So if a helicopter has a rotor diameter of 48ft, the IGE region will be about 24 - 48ft above the ground. The
height will vary depending on the type of helicopter, the slope and nature of the ground, and any prevailing
winds
Out of Ground Effect (OGE) is the opposite to the above, where there are no hard
surfaces for the downwash to react against.
For example a helicopter hovering 150ft above the ocean surface will be in an OGE condition and will
require more power to maintain a constant altitude than if it was hovering at 15ft.
Therefore a helicopter will always have a lower OGE ceiling than IGE due to the amount of engine power
available.
Published performance figures for a given helicopter may state something like:
Hover Ceiling at Max Weight = 4000ft OGE and 6000ft IGE.
This means that the fully loaded helicopter can hover at 4000ft above the ocean (ie. no hard surfaces close
below), and can hover at 6000ft above a tall mountain top where there is the ground close below (within 0.5
- 1.0 rotor diameters).
Mountains this high are common in Papua New Guinea for example.
Recirculation is a condition which can occur during a low hover in ground effect.
Imagine the airflow which was directed to the ground to create the air cushion in a ground effect is now
rebounding off the ground and going back up into the top of the rotor system. When it passes back through
the rotor again it gets accelerated.
This process may continue with the air velocity increasing each time it passes through the rotor.
Eventually the velocity is so great that the air going into the rotor from above causes a loss of lift and the
helicopter will sink toward the ground unless the pilot increases power.
This means that if recirculation is occuring, the helicopter will need more power to hold a constant height.
Recirculation will not always happen but will be aggravated by the type of ground or nearby obstacles
causing the air which is trying to escape out to the sides of the helicopter to be directed back up toward the
rotor system.
The result is a "recirculation" of downwash air
Blade Element and Momentum Theory
Blade Element Theory (BET) is an analysis method that may be applied to a rotor, propeller, fan,
and even a lightly loaded compressor. BET is the foundation for almost all analyses of helicopter
aerodynamics because it deals with the detailed flow and loading of the blade. The theory gives
basic insights into the rotor performance as well as other characteristics. William Froude originally
conceived of BET in the 1870's. Stefan Drzewiecki however, was the first to rigorously examine
and apply BET. He performed his work between 1892 and 1920. BET is very similar to the Strip
Theory for fixed wing aerodynamics. The blade is assumed to be composed of numerous,
miniscule strips with width 'dr' that are connected from tip to tip.
The lift and drag are estimated at the strip using the 2-D airfoil characteristics of the section.
Also, the local flow characteristics are accounted for in terms of climb speed, inflow velocity, and
angular velocity. The section lift and drag may be calculated and integrated over the blade span.
BET is a very useful tool for the engineer. He or she may perform a fairly detailed local analysis
of the rotor in a short amount of time.
In contrast to BET, Momentum Theory is a global analysis which gives useful results but can not
be used as a stand-alone tool to design the rotor. It was originally intended to provide an analytical
means for evaluating ship propellers (Rankine 1865 & Froude 1885). Later Betz (1920) extended
Rankine and Froude's work to include the rotation of the slipstream. Momentum Theory is also
well known as Disk Actuator Theory. Momentum Theory assumes that the flow is inviscid and
steady, also the rotor is thought of as an acuator disk with an infinite number of blades, each with
an infinite aspect ratio. The useful results from momentum theory that are applied to BET are
listed below.
 The down wash is twice as fast as the inflow
 The ideal power is a simple function of the thrust
 If the down wash is uniform, the ideal power is minimized
 The inflow is a simple function of the thrust
When the two theories are combined, it is possible to evaluate a field of induced velocity around
the rotor or propeller, and therefore correct the inflow conditions assumed in the basic blade
element theory. The induced velocities aren't known until the blade loads are computed. With the
loading available one can re-compute the field of induced velocities. This is an iterative method,
generally the quantity that is iterated for is the thrust coefficient. The combined Blade Element
Momentum Theory is a fairly accurate analytical tool (for lightly loaded rotors or propellers) that
can be used by the engineer early in the design of a rotor.
What happens when a helicopter's engine fails ?
Well, in a multi-engine helicopter, the remaining engine will still be able to power the rotors and
therefore normal flight will be able to be maintained. However this document will answer the
question you have probably have by now already asked yourself;
"What happens if I lose my only engine or god-forbid, I lose all my engines?"
The simple answer to this question is that the helicopter will enter autorotation.
What is autorotation?
Autorotation is a condition where the main rotor is allowed to spin faster than the engine driving
it. How is that achieved? It is actually quite simple.
All helicopters are fitted with a free wheeling unit between the engine and the main rotor, usually
in the transmission. This free wheeling unit can come in different forms but one of the most
popular is the sprag clutch. The free wheeling unit will allow the engine to drive the rotors but not
allow the rotors to turn the engine. When the engine/s fail the main rotor will still have a
considerable amount of inertia and will still want to turn under its own force and through the
aerodynamic force of the air through which it is flying. The free wheeling unit is designed in such
a way to allow the main rotor to now rotate of its own free will regardless of engine speed. This
principle is the same reason that if you are in your car and you push your clutch in, or put it into
neutral while the car is still moving, the car will coast along under it's own force. This occurs
regardless of what you do to the accelerator pedal.
Controlled Descent ?
The next question you are probably asking yourself is: "Does the pilot retain control of the
helicopter?" The answer is yes. The pilot will still have complete control of his descent and his
flight controls. The majority of helicopters are designed with a hydraulic pump mounted on the
main transmission. As the rotor will still be turning the transmission, the pilot will still have
hydraulically assisted flight controls. The pilot will be able to control his descent speed and main
rotor RPM with his collective control stick. He will be able to control his main rotor RPM by
increasing the collective pitch, which will increase drag on the rotor blades and thereby slow the
main rotor. If he needs to increase his rotor RPM, he can decrease his collective pitch therefore
decreasing drag.
The pilot will usually be able to find a suitable area for a safe landing by normal manipulation of
his cyclic control stick and his directional, or tail rotor pedals.
Larger helicopters will usually have a generator mounted on the transmission that will still provide
electrical power for flight and communication systems.
What happens to Torque Effect ?
Torque effect is the aircraft's tendency to rotate in the opposite direction to the main rotor due to
Newton's third law "Every action has an equal and opposite reaction". This is the reason why we
need a tail rotor or some other form of anti-torque control. The question at hand is what happens to
torque effect during autorotation? Well torque effect is directly proportional to the amount of force
driving the main rotor, so when when the engine fails the amount of force driving the main rotor
instantaneously decreases and therefore the torque effect decreases. This being the case the
fuselage of the helicopter will tend to rotate due to the sudden lack of torque effect. The pilot will
therefore have to immediately manipulate his directional pedals to overcome this problem and
retain control of his aircraft.
Conclusion
So in conclusion if your helicopter's engine/s should fail it is not just possible, but quite easy for
the pilot to retain control and land safely and gently. This is the reason I believe that helicopters
are far safer and more fun to fly in than fixed wing aircraft. A fixed wing aircraft will always need
forward speed to safely land, with or without an engine operating. A helicopter can be made to
land with zero forward speed whether the engine is operating or not.
Why can't a Helicopter fly faster than it does ?
In the following paragraphs, the reasons for this will be discussed in detail. For ease of explanation, all
descriptions will be based on a simple two bladed rotor system , which rotates counter-clockwise when
viewed from above. This makes the advancing blade on the right side of the aircraft swinging toward the
front of the helicopter.
The explanations will deliberately be kept fairly basic. For the more advanced out there, please don't send
e-mail saying that there is more to it than has been stated. However, do comment if you consider that any of
the explanations are fundamentally wrong.
There are a number of factors that govern the maximum speed of a helicopter. These are (in no particular
order):






1.- Drag
2.- Retreating Blade Stall
3.- Airflow Reversal
4.- Air Compressibility
5.- Cyclic Control Stick design
6.- Available engine power
 Summary
1. Drag
In aerodynamics, drag is the force opposing thrust . Drag is present in helicopters in two main types:
Parasite drag and Profile drag.
a. Parasite drag
Parasite drag is the drag forces created by the components that protrude into the airflow around the
helicopter. Because this drag is opposing thrust it is reducing the amount of thrust available to make the
helicopter fly faster. Parasite drag includes the landing gear, antennas, cowlings, doors, etc. The shape of
the fuselage will also produce parasite drag. On later helicopters where the manufacturer has attempted to
raise the speed of the helicopter, the landing gear is retractable to reduce the amount of parasite drag
produced. Generally, for a given structure, the amount of parasite drag is proportional to the speed that the
structure is passing through the air and therefore parasite drag is a limiting factor to airspeed.
b. Profile drag
Profile drag is the drag produced by the action of the rotor blades being forced into the oncoming airflow.
If a rotor blade was cut in half from the front of the blade (leading edge) to the rear of the blade (trailing
edge), the resulting shape when looking at the cross-section is considered to be the blade "profile". For a
rotor blade to produce lift, it must have an amount of thickness from the upper skin to the lower skin, which
is called the "camber" of the blade. In general terms the greater the camber, the greater the profile drag.
This is because the oncoming airflow has to separate further to pass over the surfaces of the rotor blade.
The blade profile for a given helicopter has been designed as a compromise between producing sufficient
lift for the helicopter to fulfill all of its roles, and minimising profile drag. To alter the amount of lift
produced by the rotor system, the angle of attack must be altered. As the angle of attack is increased then
the profile drag also increases. This is generally referred to as "induced drag", as the drag is induced by
increasing the angle of attack.
Have you ever stuck your hand out of the window while travelling in a car? If so, did you notice that if you
kept your hand flat with your thumb leading then you could keep you hand in that position fairly easily
with some effort. What happens if you turn your hand so that your palm is facing into the wind? It is not as
easy now to keep you hand still and it requires far greater effort to keep it there. This can be related to
profile drag and induced drag.
2. Retreating Blade Stall
To understand retreating blade stall it is first necessary to understand a condition known as "Dissymetry of
Lift" .
Consider a helicopter hovering in still air and at zero ground speed. The pilot is maintaining a constant
blade pitch angle with the collective pitch control lever and the aircraft is at a constant height from the
ground. The airflow velocity over the advancing blade and the retreating blade is equal.
If the tip of the advancing blade is travelling at 300mph then the tip of the retreating blade must also be
travelling at 300mph. The velocity of the airflow over the blade is progressively reduced as we look closer
toward the root end of the blade (toward the rotor hub) as the distance that the observed point has to travel
around the circle is reduced.
In this condition the amount of lift being generated by each blade is the same because the amount of lift
produced is a function of velocity and angle of attack. However, if the helicopter started to move forward
then the airflow velocity over the advancing blade would be increased by the amount of the forward speed
as the blade is moving in the opposite direction to the flight. If the helicopter was then travelling forward at
100mph, then the airflow at the advancing blade tip would be:
Velocity induced by the blades turning:
Plus the velocity from forward flight:
Total effective velocity at the tip:
300mph
100mph
400mph
At the retreating blade the velocity is reduced by the amount of forward speed as the blade is travelling in
the same direction as the airflow created by forward flight. So the tip is now effectively travelling at
200mph, or half the speed of the advancing blade. From the Formula for Lift, it is known that the amount of
lift produced varies as the square of velocity. From the example above this means that the advancing blade
will produce four times more lift than the retreating blade. If this situation was not corrected, the helicopter
could not fly forward in a straight line when forward flight was attempted. (It would actually pitch nose-up,
but that's another story!)
To correct for this the rotor system is allowed to "flap" whereby one blade tip can rise above the other with
reference to the rotor plane of rotation. The effect this has is to reduce lift on the advancing blade and
increase lift on the retreating blade. The lift across both blades is then equalised.
Now that we understand "Dissymetry of Lift", we can look at retreating blade stall. You will recall that the
retreating blade has a lower airflow velocity than the advancing blade in forward flight. If we were to
accelerate our helicopter from the above example to 300mph, then the advancing blade would have an
airflow velocity of 600mph, and the retreating blade would be zero. For the blade to produce lift it must
have some airflow over it, so in this case the blade would "stall". Stall is a condition where there is a
breakdown of smooth laminar airflow over the surfaces of an aerofoil (rotor blade).
With each blade entering a stall condition as it passed down the left side of the helicopter, forward flight
could not be maintained at this speed. Before the blade actually stalled it would produce a series of harsh
vibrations known as "buffeting". When a manufacturer produces a new helicopter, the speed at which this
buffeting will occur is established during flight test trials and a lower figure is subsequently published
which is commonly known as VNE or Velocity - Never Exceed . This establishes a safety margin below
the speed where retreating blade stall may occur.
3. Airflow Reversal
Airflow Reversal will normally occur before retreating blade stall. You will recall that the airflow velocity
is progressively reduced along a blade from being highest at the tip, to lowest at the root end.
If the velocity is 300mph at the tip, it is feasible for the velocity to be as low as 100mph at the root.
Therefore when forward speeds as low as 100mph (approx. 60 Kts) are encountered, the root end of the
blade is effectively stalled. When higher speeds are attempted, the airflow across the root end of the blade
can actually reverse and travel from the trailing edge to the leading edge. This is because the airflow
velocity produced by the forward speed is greater than that being produced by the rotor blades turning.
Airflow reversal is counter-productive to producing lift and rotor thrust.
To reduce the effects of lift variations from the root to the tip of a blade the manufacturer will either twist
the blade along its length, or apply a taper to the blade. Twist is the reduction of angle of attack from the
root to the tip. Remember that lift increases with velocity and angle of attack? Because the tip is travelling
faster than the root, the angle of attack must be reduced toward the tip to maintain the same amount of lift
at the tip and the root ends. Taper is the gradual reduction of the width of a blade from the leading edge to
the trailing edge. A straight line drawn from the centre of the leading edge to the centre of the trailing edge
is called the "Chord Line". By reducing the chord line from the root to the tip, less surface area is available
for the airflow to act on to produce lift.
On higher speed helicopters (Westland Lynx), the root end of the blade is a blade spar and attachment area
only. The aerofoil shape does not start until several feet out from the centre of the rotor system. This is
done to reduce the effects of airflow reversal by placing the lift-producing surface further out where the
rotational velocity is higher.
4. Air Compressibility
Air is a gas and therefore conforms to the properties of a gas, namely the ability to be compressed. When
studying aerodynamics however, air must also be considered to have some of the properties of a fluid. A
fluid has far less compressibility than a gas.
When the airflow over a rotor blade strikes the leading edge, it is split into two streams, which then pass
above and below the blade. At lower speeds, this splitting action occurs relatively easily requiring little
energy. As speeds increase, the air striking the leading edge tends to be compressed before separating into
two streams. Think of this as slapping your hand onto a water surface. If you chop your hand into the water,
like a karate chop, you can separate the water fairly easily. If you slap your open hand onto the water
however, it takes considerably more force to submerge your hand. The airflow at the leading edge is very
similar. As the air at the leading edge is progressively compressed, it requires considerably more rotor
thrust for the blade to separate the airflow into two streams.
5. Cyclic Control Stick design
Helicopter designers are forever trying to fit more equipment into the cockpit of a helicopter to satisfy
market demands. At the same time, they are trying to minimise the weight of the aircraft so that it can carry
and lift more. When designing the pilot and copilots workstations the designers attempt to place the
controls in a position where the crew can easily and comfortably operate all controls without excessive
reaching or stretching. This places limitations on the amount of movement available at the cyclic control
stick.
The designers could feasibly arrange the controls such that very small amounts of stick movement were
required for normal flight, but this would make control in the hover very difficult as the controls would be
super sensitive to small inputs. For this reason, the controls are arranged so that a reasonable control
movement is available, generally 6-8 inches of stick movement depending on the particular aircraft model.
6. Available Engine Power
The engine system in a helicopter is required to provide power for a range of demands, not only the rotor
system. In the rotor system, thrust is required to overcome drag. As speed is increased, so does drag. If
more power is available to overcome drag, then potentially the helicopter can fly faster.
Summary
It can be seen that from these factors that it is very difficult for helicopter designers to increase the
maximum speed of a helicopter as many factors are beyond their control. Much research and development
has occurred in areas such as reducing drag, better rotor blade designs and increasing available engine
power.
The current World Helicopter Speed Record is held by the Westland Lynx at 217.5 Kts (402 km/h) using
specially designed high-speed rotor blades.
Why were compound helicopters not successful ?
This question is quite a debate.
First, Why compound? and the answer is of course to achieve higher speeds by providing extra
thrust.
But with the higher speeds come the problems of retreating blade stall and increased rotor stress.
Generally, the way around this has been to offload the rotor by adding a wing. However you
immediately pay a price in that you degrade the hover performance.
Even with compounding, you are unlikely to achieve cruising speeds higher than 450kph, which
can be achieved by fixed wing much more economically, and in any case conventional helicopters
can already fly at speeds of 300kph without difficulty.
The strength of the helicopter lies in its ability to hover efficiently, so to interfere with that and
add the complexity which compounding is bound to introduce, must call for a good reason.
To say that compounds have not been successful is not strictly true. The Fairey Rotodyne achieved
all its technical targets, but it must be admitted that it was expensive.
The Lockheed AH-56 Cheyenne demonstrated high manoeouverability and was capable of nearly
400kph. The
Kamov Ka-22 exceeded 365kph. And of course the V-22 Osprey is a compound which
can achieve very high speeds, and shows every sign of a bright operational future.
There are roles where compounding looks attractive:
 Aiborne Early Warning is an example where you only hover for take off and landing and
otherwise spend the whole flight in the criuse which is very efficient for a compound.
 The same applies for short haul transport, but city centre operation is a problem and once an
airfield is involved the helicopter loses its advantage.
 Search and Rescue, Attack, Anti-submarine, Police and Air Ambulance all call for prolonged
hover and low speed operation, where the compound is at a dis-advantage.
Personally, I do not think the age of the compound is with us yet, but as technology progresses
there may yet be a break through.
Blades and Lift
All rotor systems are subject to DISSYMETRY OF LIFT in forward flight. At a hover, the lift is
equal across the entire rotor disk. As the helicopter gains airspeed, the advancing blade develops
greater lift because of the increased airspeed (for example, if your blades at a hover move at 300
knots and you fly forward at 100 knots, your advancing blade is now moving at a relative speed of
400 knots and your retreating blade is moving at 200). This has to be compensated for in some
way, or the helicopter would corkscrew through the air doing faster and faster snap rolls as
airspeed increased.
( See also "Why can't a Helicopter fly faster than it does ?" article )
Dissymetry of lift is compensated for by BLADE FLAPPING . Because of the increased airspeed
(and corresponding lift increase) on the advancing blade, it flaps upward. Decreasing speed and
lift on the retreating blade causes it to flap downward. This INDUCED FLOW through the rotors
system changes the angle of attack on the blades and causes the upward-flapping advancing blade
to produce less lift, and the downward-flapping retreating blade to produce a corresponding lift
increase. Kinda spooky, huh? Anyway, it all balances out and the lift is equal across the disk.
In a two-bladed system (
AH-1, UH-1, TH-57 ) it is easy to visualize this because of the Bell/Textron insistance
on the semi-rigid, underslung, see/saw type rotor system. If you push down on one blade, the other
one goes up like a teeter-totter. Most three or four bladed systems ( TH-55, OH-6, MD500 ) are
"fully articulated" in that each blade can flap independently, without affecting the other blades.
The advancing blade still flaps upward and the retreating blade still flaps downwards, they just do
so without worrying about what the other blades are doing. A rigid rotor system (
BO-105, BK-117 ) have the blades fixed rigidly to the hub and compensate for
dissymetry of lift by BLADE FLEXING . The blades still flap up and down like all the other
helicopters, they just do so without hinges (really spooky). The advantage of the rigid hub is that
you don't have to worry about mast bumping and the helicopter is (theoretically) fully aerobatic.
The
OH-58D has a "four-bladed-soft-in-plane" rotor system, but you don't want me to go
there.
There are several trade-offs one has to make when designing or buying a helicopter. I would much
rather have a rigid or fully-articulated system, because they are more maneuverable and more
forgiving of abrupt (Panic!) control inputs. Most mechanics and owners/operators like the Bell
rotorhead because it is easier and cheaper to maintain. Things like max gross weight vs. empty
weight (cargo capacity) are much more important to determining the "quality" of lift than the
number of blades.
In any event, the four aerodynamic forces of lift, weight, thrust and drag all come into play, but
you have to be careful when defining your terms.
 Taylor Cox Web Site
 Butch Lottman Web Site
The weight of a helicopter is divided evenly between the rotor blades on the main rotor system. If
the helicopter weighs 5000 lbs and it has two blades, then each blade must be able to support 2500
lbs and so on. The more blades a helicopter has then the lower the weight that is carried on each
blade compared to the same helicopter with less blades.
In addition to the static weight of the helicopter, each blade must be able to accept enormous
aerodynamic loads as well. For example if a helicopter pulls up in a 2g manouvre (2 x the force of
gravity), then the effective weight of the helicopter doubles due to gravitational pull.
During a helicopter take off, the pilot causes the pitch angle on the rotor blades to increase slowly
until it reaches a point where the lift being developed by the main rotor system (not each blade) is
greater than the weight of the helicopter. At this point the helicopter will rise from the ground and
will continue to rise until the lift force is decreased to a point where it is equal to the weight. The
helicopter will then hover at a fixed height.
When the pilot wants to descend he will reduce the pitch angle so that the helicopter weight is
greater than the lift force, the helicopter will then come down due to gravity.
Contribution: Glenn Beare
FAQ Update
 What is the average flying hours before the rotorblades are due for repair ?
Most metal blades have a fixed life or TBO ( Time Before Overhaul ) , which is typically in the
range of 3000 to 6000 hours. The average time between repair varies greatly depending on the
type of operation.
 Where and who are the rotorblade repair centers in the world ?
All the airframe manufacturers have their own blade repair/overhaul facilities. In addition to this,
there are several manufacturer appointed service centres which are capable of doing blade repairs.
 Some claimed that to buy new rotorblade is cheaper than repairing it. To what extend this
statement is true ?
Rotor blades are expensive items and therefore any minor damage will be repaired. However,
certain damage, especially damage to the blade spar may be impossible to repair. As blades are
flight critical parts, no compromises can be accepted when carrying out repairs.
How long can a Helicopter fly before it needs an overhaul ?
Most phased maintenance programs have a minor inspection due every twenty-five flight hours
and a major inspection every 150. The military 150-hour is somewhat similar to the civilian
annual inspection.
As an example:
 If you have a civilian helicopter in the U.S. that you use to haul passengers under Part
91 of the Federal Aviation Regulations (FARs), you need an annual and the
recommendations of the manufacturer.
 If you are flying under FAR Part 135, the maintenance schedule is quite a bit more
extensive.
Of course this varies from country to country.
Check the Federal Aviation Administration FARs index for each case.
Other Aviation Regulations quick links
All components of an helicopter have a TBO ( Time Before Overhaul ) or an hour requirement for
replacement and the aircraft structure also has time related inspections.
As an example here are overhaul charts from the 23-1 maintenance manual of the AH-1F Cobra :
What is Ground Resonance ?
Before we can understand Ground resonance, we must first understand dissymetry of lift and how
it is compensated for in a fully-articulated system. This will lead to a discussion about blade
flapping, and why the blades must lead/lag (aka hunting).
Ground resonance happens in helicopters with lead-lag hinges. It occurs only on the ground. It
starts when the blades "bunch up" on one side of the rotor disc where they generate an unbalanced
centrifical force that gets in phase with the natural frequency of the aircraft rocking on it's landing
gear. Modern helicopters avoid this by using dampers on the blades and on the gear (shock
absorbed struts). It is less prominent on wheeled helicopters although can occur. The emergency
action is to lift the aircraft to a hover.
Ground resonance has not been "solved", and is still a big concern for anyone who flies a
helicopter with a fully-articulated rotor system. It basically is an out-of-balance condition in the
rotor system of a helicopter on the ground that rapidly increases in frequency until the helicopter
shakes itself apart. It is usually caused by a hard ground contact, and is much more likely in
aircraft with improperly maintained landing gear (deflated oleo struts, for example).
It cannot occur in a two bladed semi-rigid see-saw type rotor system, because the blades do not
lead and lag.
ROTOPTER
A new type of winged aircraft
A new type of winged aircraft was proposed recently by Dr. Vladimir Savov from
Bulgarian Air Force Academy. The new type of aircraft was derived from a classification of the
winged aircraft by the type of wing movements in horizontal and vertical plane.
Wing movement
In vertical
plane
In horizontal plane
translational
rotational
no movement
airplane
helicopter
reciprocal
ornithopter
?
The new rotorcraft concept was named rotopter ( from 'rotor' and 'ornithopter').
The essence of the rotopter concept is to substitute progressive motion of the flapping wing with
rotation: flap ergo turn ( fig.1 ). But maybe the real significance of the rotopter concept is that it
allows welldeveloped helicopter rotor technology to be applied to the flapping flight. That's why
the rotopter rotor to great extent looks like a helicopter rotor
( fig.2 ).
The rotopter operates as follows. The crank mechanism drives the slider into reciprocal motion,
which is being converted by the link mechanism into flapping of the blades. The mass
characteristics of the blade and rigidity of the elastic element are selected so as to ensure changing
of the blade angle of incidence lagging by 90o from the angle of flapping. The flapping blade of
the rotopter creates thrust and lift similarly to the flapping wing of a bird or an insect. Due to the
thrust the blade begins to rotate. Magnitude and direction of the lift force is controlled via the
swash plate mechanism analogously to the way it is carried out on the helicopter rotor. When in
progressive flight, velocity head on the advanced blade is greater than the head on the retreating
one, respectively the lift on the advanced blade is greater than on the retreating. Increased lift
creates greater nose-down moment around the axial hinge, the angle of incidence decreases, so
does the lift and the banking moment is being reduced.
It is expected that the rotopter has some advantages in comparison with the helicopter rotor:
 there is no torque reaction;
 the induced power losses are possibly lower due to the unsteady flow.;
The rotopter has certain advantages in comparison with the ornithopter:
 velocity triangles in different crossections along the rotopter blade are geometrically
similar, hence no twisting is necessary;
 the forward speed of the blade is not directly connected with the speed of the aircraft,
hence vertical flight or maintaining optimal speed of rotation when in level flight is
possible;
 the centrifugal force reduces compressive stress on the upper surface of the blade.
The unfavorable load distribution along the blade, the cyclic variation of the lift and increased due
to the centrifugal forces bending moment at the root of the blade probably confine the use of the
rotopter concept for small aircraft only (micro air vehicles).
At the current moment the development of the rotopter concept is in the following state:
 mathematical model of the rotopter aerodynamics based on the element-impulse theory has been
created and used for optimizing rotopter kinematics.
 laboratory model was built, allowing measuring of the lift and consumed power (fig.3). The
experiments with the laboratory model demonstrated that the concept of the rotopter works i.e. the
blades rotate and create lift force. It was also ascertained that the efficiency of the rotopter is
ultimately sensitive to the law of variation of the blade angle of incidence. At the current stage,
achieved coefficient of efficiency of the rotopter is considerably less than the usual for a
conventional helicopter rotor. There is hope that if more sophisticated numerical model of the flow
around rotopter blade is developed and the motion of the blade is optimized, the efficiency
coefficient of the rotopter will exceed the one of a conventional rotor.
 a small (20 cm) flying model is under construction.
It is early to predict what the rotopter could be used for besides completing the list of the winged
aircraft. Let's hope that the fresh main idea and existing rotor technology will ensure fast
developing of the new hatched out rotopteryx.
The BodyCopter
US Patent Pending
The BodyCopter is the aircraft you will be able to store in your closet, fly to work, put in your
trunk, and take off and land in any 15' open space.
The BodyCopter,
designed by Charles
Medlock, is unique
in several ways, but
the most interesting
is the use of vertical
airfoils to control
the reverse torque
created by spinning
the horizontal rotor
blades.
The BodyCopter
uses the down draft
created by the
horizontal rotor to
achieve reverse
torque and
rotational control.
Basically, free
floating variable and
fixed vertical
airfoils create
"horizontal lift" in the same direction but on opposite sides of the axis of the horizontal rotor. By
changing the angle of attack of the vertical airfoils while maintaining balanced "horizontal lift",
rotational control is achieved thus eliminating the need for a tail rotor blade.
Used a 20 hp engine to turn a 10 foot blade at around 900 rpms and looking for total lift of about
400 pounds. The horizontal lift, vertical airfoils are fixed and variable pitch and are free floating
so they balance any angle of attack. The unit currently weighs around 120 pounds but the
production model will have two engines for safety sake and weigh only about 60 pounds. There
are also plans for a folding unit with three blades. It will stand on its own on the three blades and
conceivably, you could store it in the space of a closet.
As of September 19, 2002 the BodyCopter is being ground tested and has not flown.
Unicopter & SynchroLite
The intent of the UniCopter/SynchroLite project is to assimilate
and disseminae technical information, both positive and negative,
about the intermeshing configuration.
The worlds only unsymmetrical vehicle is the single rotor
helicopter, with its tail-rotor. I believe that this adherence to the
tail-rotor has inhibited progress in VTOL craft. Symmetry in both
animals and vehicles is beautiful, and this beauty is due to its
natural functionality. Germany, during WW II, was the strongest
proponent of symmetrical helicopters. Its side-by-side Focke Fa-61
and intermeshing Flettner FL-282 are without equal. After the war,
the Germans were in no position to develop and promote their
laterally symmetrical helicopters.
The SynchroLite project is directed at the US FAA Part 103
ultralight category, where the maximum dry weight is 254 lbs. The
intermeshing rotors and their symmetrical handling result in a craft
that has a low power to weight ratio and more importantly is easier to fly then 'conventional'
helicopters. This is critical, since the ultralight category does not demand a pilots license.
The UniCopter project is a move toward optimal VTOL, by overcome many of the problems
that have beset rotorcraft since their inception. Many of its features have be used before, but never
unified in one craft.
The following is a listing of its
primary features, and the reason for
them;
 Symmetrical Rotor
Configuration: Two counter-rotating
intermeshing rotors eliminate the need
for a tail rotor. This symmetry reduces
the demands on the pilot in
coordinating the cyclic, collective and
pedals. In addition, it results in a
power to weight ration that is
approximately 15% better than the
single rotor.
 Absolutely Rigid Rotors [ARR]:
Consisting of: Totally rigid blades. No
flapping, no lead-lag and no cone or
pre-cone angle. Rigid coupling
between rotor hubs and semi rigid coupling to fuselage.This minimizes the pilot's off-axis
compensation, caused by cross-coupling. As well, it results in rapid and positive responses to the
pilot's control inputs.
 Safety: The addition of a rotor governor will reduce the pilot's workload and eliminate low
rotor speed. A rotor governor (not an engine governor) provides automatic entry into autorotation.
 Less Rotor Induced Vibration: The intent is to incorporate leading and trailing edge sealed
tabs on the blades to provide high rate pitch change.
 High Speed Flight: The Advancing Blade Concept [ABC] with its low tip speed and rotor
rigidity will reduce retreating blade stall and advancing blade compression during high-speed
flight. In addition, the pusher prop contributes to the higher speed.
 Quiet: The elimination of the tail rotor and the slower rotational speed of the main rotors will
result in less noise.
 Aerobatics: The extreme rigidity of the rotor system should, theoretically, provide the ability
for prolonged inverted flight, should any exhibitionist desired it.