Evolution of Powered Aircraft
Flight Controls
Tom Greetham
Evolution of Powered Flight Controls
February 10, 2012
1
Agenda
1. Introduction
2. Flight Control Basics
3. Un-powered Flight Controls
4. Powered Flight Controls
5. Stability Augmentation
6. Aircraft Control System Examples
7. Fly By Wire
8. Power By Wire
9. Aircraft Business Trends
10. Related SAE Publications
Evolution of Powered Flight Controls
February 10, 2012
2
Who Is This Guy?
Tom Greetham
• Ohio State University, B.S.M.E. 1981 & M.S.M.E
1982
• Moog Inc., Aircraft Group
Engineering Manager, Military Actuation
• Flight Control Actuation Experience
– B-2 Flight Control Actuation System (Simulation,
System Verification Testing)
– LCA (Light Combat Aircraft) – Indian Airforce
– V-22 Tilt Rotor Aircraft
– Bombardier Challenger 300 Mid-size Business Jet
Evolution of Powered Flight Controls
February 10, 2012
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Moog Inc.
• Moog Is a Worldwide Manufacture of
Motion Control Components and Systems
for Industrial, Medical and Aerospace
Applications
• Moog’s Aircraft Group Is the Company’s
Largest Business Segment and Is a
Leading Manufacture of Aircraft Control
Components and Systems, Mostly for
Flight Control Applications
Evolution of Powered Flight Controls
February 10, 2012
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What Are Aircraft Flight Controls?
•
Flight Controls (on Fixed Wing Aircraft) Are the Control Surfaces and the
Systems that Move Them to Control the Aircraft Attitude
–
–
–
Pitch (a.k.a. Longitudinal)
Roll (a.k.a. Lateral)
Yaw (a.k.a. Directional)
yaw axis
elevator
aileron
pitch
axis
flap
rudder
pedals
control
column
rudder
elevator
flap
aileron
roll
axis
Evolution of Powered Flight Controls
February 10, 2012
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What Are Powered Flight Controls?
• Powered Flight Controls Have Actuators that
Provide Significant Force Augmentation to the
Pilot to Move the Control Surfaces
• Increasingly Necessary for Larger Aircraft and/or
those Flying at Higher Airspeeds
Evolution of Powered Flight Controls
February 10, 2012
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Flight Control Technology Chronology
Entered Service
Technology
Un-Powered
Military
1910s
Commercial
1920s
Powered Boost
1940s
1940s
3000 psi Hydraulics
1940s
1950s
Auto Pilots
1950s
1950s
Fully Powered, with Reversion
1950s
1960s (Boeing 727)
Fully Powered, without Reversion
1950s (B-47)
1970 (Boeing 747)
Fly-By-Wire
1970s (F-16)
1980s (A-320)
Digital Fly-by-Wire
1970s
1980s (A-320)
5000 psi Hydraulics
1990s (V-22)
~2005 (A-380)
Power-By-Wire
~2006 (F-35)
~2005 (A-380)
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February 10, 2012
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Flight Control Design Drivers
• Safety, Safety, SAFETY
–
–
–
Design for <10-9 Critical Failures per Flight Hour
(That’s one failure per billion hours)
Redundancy
Conservative Design Philosophies
• Resist Deviating from What Is Known to Work
• More Prevalent in Commercial Aviation Than in Military
• Reliability
–
Minimize Complexity to Minimize Maintenance Actions (at Odds With Redundancy Above)
• Minimum Weight
–
Cost-to-Weight Trade-off:
• Commercial:
~$1000 per pound per aircraft (updated)
• Military:
~$2,000-$10,000 per pound per aircraft (8 Year Old Data)
• Cost
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Un-Powered Flight Controls
Pitch Up
Command
Control
Column
Manual
Trailing Edge
Up Result
Horizontal Tail
Pitch Up
Command
Elevator
Control
Column
Trailing Edge
Up Result
Servotabs
Elevator
Horizontal Tail
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February 10, 2012
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Servotab
Simple Hydromechanical Servoactuator
• Power Provided by High Pressure Hydraulic Fluid
• Similar to an Automobile Power Steering System
Pilot Input
Ps
R
Hydromechanical
Servoactuator
Control Surface
Evolution of Powered Flight Controls
February 10, 2012
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Powered Boost
•
Hydraulic Servoactuated Surface Control (Servotab Locked)
•
Force Feedback to Pilot
–
•
Force Proportional to Actuator Load is Applied to Valve Input Link so that Pilot
Feels Surface Loads
Mechanical Reversion Mode
–
–
–
If Hydraulics Fail
Actuator Output and Surface Released to Move Freely
Pilot Input Moves Unlocked Servotab
'Boost' Servoactuator
(Moving-Body with
Pressure Feedback
and Tab Lock)
Elevator
Horizontal Tail
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Servotab
Fully Powered Flight Controls With
Reversion Mode
• Mechanical Reversion Mode
– Actuator Reverts to Bypass Mode
– Servotab Unlocked to “Fly” the Surface
• Pilot “Feel” Provided by Hydromechanical Feel
and Trim System
'Irreversible' Servoactuator
(Dual-Tandem, Fixed-Body with
or without Tab Lock)
Feel/Trim
Elevator
Horizontal Tail
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Servotab
Fully Powered Flight Controls, No Reversion Mode
• No Mechanical Reversion Mode
– Control Forces Too High For Pilot to Move Surface Sufficiently
to Control Flight
– Failures Covered by Redundant Actuators or Surfaces and
Redundant Hydraulic Systems (More On That Later)
• Pilot “Feel” Provided by Hydromechanical Feel and Trim
System
Xp
Control
Column
Ps
R
Feel and Trim
Xs = Xp
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Xs
Hydromechanical
Servoactuator
Control Surface
Autopilots
•
Autopilot Actuators “Fly” the Pilot Input Linkage and Control Column Via
Commands from an Autopilot Computer
•
Pilots Can Overpower Runaway (Failed) Autopilot
•
Autopilot and Feel Systems Provide Pilot Visual and Tactile Feedback,
Features Otherwise Lost by Powered Flight Controls
Xa
Xp
Autopilot
Control
Column
'FBW' Autopilot
Servoactuator
Ps
R
Feel and Trim
Xs = Xp = Xa
Xs
Hydromechanical
Servoactuator
Control Surface
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Stability and Control Augmentation
• Inputs from Sensors and a Fight Control Computer Are
Summed With Pilot or Autopilot Inputs to Improve the
Aircraft Stability and Handling Qualities
• Unlike Autopilot Inputs, Stability and Control Inputs Do Not
Move the Pilot’s Control Column
PFCS
Xp
Aircraft
Sensors
Xd
Control
Column
'FBW' Series Damper
Servoactuator
Ps
R
Feel and Trim
Xs = Xp + Xd
Evolution of Powered Flight Controls
February 10, 2012
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Xs
Hydromechanical
Servoactuator
Control Surface
Boeing 757 Elevator Control System
Evolution of Powered Flight Controls
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F-111 Pitch and Roll Control System
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F-15 Pitch-Roll Control Assembly
(A.K.A. Hydraulic Television Set)
70 lbs
16”
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Fly-By-Wire
• Mechanical Links Between the Pilot Controls and Surface
Actuators Are Replaced by Electronics
• This Offers a Significant Weight Savings Over
Hydromechanical Systems
• Requires Sophisticated Failure Management Techniques
• Early Fly-By-Wire Aircraft Used All Analog Electronics
Aircraft
Sensors
Autopilot
Airbus
& Boeing
Xp
Boeing
only
Control
Column
PFC
Loop Closure
Circuitry
Electrohydraulic
Servovalve (EHV)
Autopilot Acts
Ps
R
Feel and Trim
LVDT
Xs
Transducers
Control Surface
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Fly-By-Wire Actuators
•
Actuator Position Control Loop Implemented With Electronics, Rather than
Linkages
–
–
•
Actuator Motion Determined by an Electronically Controlled Servovalve
Actuator Position Feedback Provided Electronically by a LVDT Position
Transducer
Bypass Valve Allows Surface to Be Controlled Freely by Another Actuator
Servovalve
In Case of Failure
Commands
EHV
Servovalve
Solenoid-Oerated
Pilot Valve (SOV)
Ps
R
Bypass
Valve
LVDT
Evolution of Powered Flight Controls
February 10, 2012
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Digital Fly-By-Wire
• First Generation Fly-By-Wire Electronics Were Analog
– Uncertain Reliability and Failure Modes of Digital Processors
• Industry Has Transitioned to Digital Control Electronics
– System Complexity Growing Exponentially
– Reliability of a Single DSP (Digital Signal Processor) Predicted
to Be Better than that of Accumulated Analog Components
– Maturing DSP Failure Management
– Many of the Typical Early System Changes Are More Easily
Made In Software Without Requiring Hardware Changes
• However, Software Changes Still Come at a High Price
• Commercial Aircraft Use Digital Flight Control
Electronics, but Often Use Analog Reversion Modes
– Example of Conservative Design Philosophy
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FBW Primary Surface Actuator Schematic
(With Damped Fail-Safe Mode)
Active Mode
• Actuator motion responds to electrical
commands to servovalve (EHSV)
Damped Mode
• Cylinder chambers connected together
through an orifice
• Actuator moves with external forces
• Damping suppresses flutter
• Compensator provides emergency fluid
Note:
Solenoid Shown Enegized
Manual Test
Valve
Inlet
Filter
Compensator /
Indicator & Relief
Valve
Damping SOV
(single coil)
EHSV
LVDT
Typical Components
• Inlet Filter (Screen)
• Inlet check valve
• Servovalve with LVDT
• Mode Select Valve
• Damping Solenoid Valve
• Piston & Cylinder with LVDT
• Compensator with Manual Release
• Load Relief Valves
• Pressure Transducers (Optional)
Mode Select Valve
(Active, Damped)
AntiCavatition
Absolute Pressure
Transducers
Load Relief
Valve
LVDT
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Aileron Balanced Actuator
FBW Spoiler Surface Actuator Schematic
Active Mode
• Actuator motion responds to electrical
commands to servovalve (EHSV)
Fail-Safe Mode
• EHSV biased to drive actuator to
retract (surface moves down)
• Loss of hydraulic Power
 Hold-down check valve prevents
actuator from extending (surface upfloat)
 Hold-down check valve allows
surface to freely retract (surface
down)
Typical Components
• Inlet Filter (Screen)
• Inlet check valve
• Servovalve with LVDT
• Anti-Extend Valve With Manual
Release
• Piston & Cylinder with LVDT
• Load Relief Valve
Evolution of Powered Flight Controls
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F-18 E/F Horizontal Tail Dual Tandem Actuator
Quad, Direct-Drive,
Dual Tandem
Shutoff Valve
Quad, Rotary-Linear,
Dual Tandem
Direct-Drive-Valve
QUAD
LVDT
1
2
3
4
Inlet Check
Valve
Ps2
Ps1
Inlet
Screen
Inlet Screen
Inlet
Check
Valve
Restrictor/Check
Valves for Neutral
Lock Fail Safe Mode
R2
R1
Return Line Compensator
Two-Position
Bypass Valve
Logic
Piston
Three-Position
Mode Select Valve
Anti-Cavitation Valves
QUAD LVDT
Partially-Balanced,
Dual-Tandem Actuator
•
Dual Hydraulic Supplies Feed Separate but Connected Pistons
•
Quad Redundant Electrical Channels
– Quad Servovalve and Shutoff Valve Coils
– Quad Servovalve and Ram Position Transducers (LVDTs)
•
Direct Drive Servovalve
Evolution of Powered Flight Controls
February 10, 2012
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Relaxed Stability Aircraft
Trim Force
C.G.
Center of Lift
• C.G. Always Located Forward of Center of Lift for Positive Stability
• Modern Fly-By-wire Aircraft Are Designed With Reduced Distance Between the
Center of Gravity (C.G.) and Center of Lift
–
–
–
–
Requires Smaller Surfaces and Forces
• Reduced Weight and Cost
Requires Lower Trim Loads (Less Drag)
Reduces Aerodynamic Airframe Stability (Less Tendency to Fly Straight)
Requires More Control Loop Augmentation
• Active Damping for Example
• Higher Dynamic Response
Evolution of Powered Flight Controls
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System Pressure
• When Hydraulics Were Introduced to Aircraft in the 1930s
and 40s They Operated at ~1,500 psi
• In the 1950s 3000 psi Became the Standard
• Increasing System Pressure Enables Higher Actuator
Forces and/or Smaller Sizes
– Smaller Actuators Demand Lower Flow Rates
– Lower Flow Rates Enable Smaller Tubing and Pumps; Thus
Reduced Weight
• But Higher Pressures:
– Increase Hydraulic Component Fatigue Stresses
– Decreased Actuator Dynamic Stiffness, Because Actuators Are
Smaller, Increasing Control Surface "Flutter" Vulnerability
• Flutter Is an Aeroelastic Phenomenon In Which a Control Surface Becomes
Violently Unstable If Not Restrained Adequately
Evolution of Powered Flight Controls
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Power-By-Wire
• Traditionally, Hydraulics Has Been the Technology of Choice for
Powered Flight Controls
–
–
–
–
–
High Power Capability
High Reliability
Compact Components
Distribution (Long Hydraulic Lines) Provides Natural Cooling
Reliable Fail-Safe Modes
• Advances In Electronics and Magnetics Has Made Electric
Actuators Become More Attractive
– Magnets and Magnetic Materials
– Electronics Reliability
– Computer Power
• Electric Actuation Offers Some Advantages Over Centralized
Hydraulics
– Fewer Leaks
– Can Remove Components Without Breaking Into Hydraulic Lines
– Easier To Physically Separate Redundant Electrical Systems than
Hydraulic Systems
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Electrohydrostatic Actuators (EHAs)
•
Actuator is an Electrically Powered Self Contained Hydraulic System
•
No External Hydraulic Connections
•
Actuator Motion Proportional to Motor/Pump Rotation
•
Adding a Bypass Valve Across the Ram Piston Provides a Reliable FailSafe Mode
Velocity
Command
Motor
Controller
M
Variable Speed
Motor
Fixed Displacement
Pump
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Tandem Electrohydrostatic Actuator and
Power Control Electronics
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Business Trends
• Aircraft Builders Increasingly Are Subcontracting Larger Systems
to Other Companies
– Shifts Risks and Costs from Airframers to Suppliers
• Fun and Headaches, Too
– Examples:
•
•
•
•
B-2 Flight Control Actuation System (Moog)
Boeing 777 Flight Control Actuation System (Teijin Seiki)
F-35 Flight Control Actuation System (Moog/Parker Hannifin)
Boeing 787 Flight Control Actuation System
• In Commercial Aviation Pressure to Reduce Costs has Become
Brutal
• Consolidation of Industry
– More Teaming Arrangements On New Aircraft
• Cost and Risk Sharing
– Acquisitions
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Related SAE Publications
•
Books
– Raymond, E.T., C.C. Chenoweth, Aircraft Flight Control Actuation System Design
•
Documents
– ARP1281D: General Specification For Power Operated Hydraulic Flight Control
Actuators
– ARP490F: Electrohydraulic Servovalves
– ARP4493A: Direct Drive Servovalves
– AS94900 : Aerospace - General Specification for Flight Control Systems Design, Installation and Test of Piloted Military Aircraft
– ARP4386C : Terminology and Definitions for Aerospace Fluid Power, Actuation
and Control Technologies
– ARP5007 : Development Process - Aerospace Fly-By-Wire Actuation System
– AIR4253A: Description of Actuation Systems for Aircraft With Fly-By-Wire Flight
Control Systems
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