evolution of powered flight controls seminar
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Evolution of Powered Flight Controls February 10, 2012 1
Evolution of Powered Aircraft Flight Controls
Tom Greetham
Evolution of Powered Flight Controls February 10, 2012 2
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 3
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 4
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 5
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)
rudder
aileron
elevator
flap
rollaxis
pitchaxis
yaw axis
rudderpedals
controlcolumn
flapaileron elevator
Evolution of Powered Flight Controls February 10, 2012 6
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 7
Flight Control Technology Chronology Entered Service
Technology Military Commercial Un-Powered 1910s 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)
Evolution of Powered Flight Controls February 10, 2012 8
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
• Cost
(8 Year Old Data)
Evolution of Powered Flight Controls February 10, 2012 9
Un-Powered Flight Controls
Horizontal Tail Elevator
ControlColumn
Pitch UpCommand
Trailing EdgeUp Result
Horizontal TailElevator
ControlColumn
Pitch UpCommand
Trailing EdgeUp Result
Servotab
Manual
Servotabs
Evolution of Powered Flight Controls February 10, 2012 10
Simple Hydromechanical Servoactuator • Power Provided by High Pressure Hydraulic Fluid • Similar to an Automobile Power Steering System
Ps R
Control Surface
HydromechanicalServoactuator
Pilot Input
Evolution of Powered Flight Controls February 10, 2012 11
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
Horizontal TailElevator
Servotab
'Boost' Servoactuator(Moving-Body withPressureFeedback
and Tab Lock)
Evolution of Powered Flight Controls February 10, 2012 12
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
Horizontal TailElevator
Servotab
'Irreversible' Servoactuator(Dual-Tandem, Fixed-Body with
or without Tab Lock)
Feel/Trim
Evolution of Powered Flight Controls February 10, 2012 13
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
Ps R
Feel and Trim
Control Surface
HydromechanicalServoactuator
ControlColumn
Xs
Xp
Xs = Xp
Evolution of Powered Flight Controls February 10, 2012 14
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
Ps R
Feel and Trim
Control Surface
HydromechanicalServoactuator
ControlColumn 'FBW' Autopilot
Servoactuator
Xp Xa
Xs
Autopilot
Xs = Xp = Xa
Evolution of Powered Flight Controls February 10, 2012 15
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
Ps R
Feel and Trim
Control Surface
HydromechanicalServoactuator
ControlColumn
'FBW' Series DamperServoactuator
Xp
Xd
Xs
PFCS
AircraftSensors
Xs = Xp + Xd
Evolution of Powered Flight Controls February 10, 2012 16
Boeing 757 Elevator Control System
Evolution of Powered Flight Controls February 10, 2012 17
F-111 Pitch and Roll Control System
Evolution of Powered Flight Controls February 10, 2012 18
F-15 Pitch-Roll Control Assembly (A.K.A. Hydraulic Television Set)
16” 70 lbs
Evolution of Powered Flight Controls February 10, 2012 19
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
Autopilot
Airbus& Boeing
AircraftSensors
PFC
Loop ClosureCircuitry
Boeingonly
Feel and Trim
Xp
ControlColumn
Transducers
LVDT
ElectrohydraulicServovalve (EHV)
Ps
R
Xs
Control Surface
Autopilot Acts
Evolution of Powered Flight Controls February 10, 2012 20
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 In Case of Failure
Solenoid-OeratedPilot Valve (SOV)
Ps
R
EHV
BypassValve
LVDT
Servovalve
Servovalve Commands
Evolution of Powered Flight Controls February 10, 2012 21
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
Evolution of Powered Flight Controls February 10, 2012 22
Damping SOV(single coil)
Note:Solenoid Shown Enegized
InletFilter
LVDT
LVDT
Anti-Cavatition
Compensator / Indicator & Relief Valve
Mode Select Valve(Active, Damped)
EHSV
Aileron Balanced Actuator
Absolute Pressure Transducers
Manual Test Valve
Load Relief Valve
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
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)
Evolution of Powered Flight Controls February 10, 2012 23
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 up-float)
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 February 10, 2012 24
F-18 E/F Horizontal Tail 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
R2
Ps2
123
Quad, Rotary-Linear,Dual Tandem
Direct-Drive-Valve
QUAD LVDT
Partially-Balanced,Dual-Tandem Actuator
Ps1
R1
Inlet CheckValve
Inlet Screen
Return Line Compensator
InletScreen
InletCheckValve
Anti-Cavitation Valves
Quad, Direct-Drive,Dual TandemShutoff Valve
Two-PositionBypass Valve
Three-PositionMode Select Valve
Restrictor/CheckValves for Neutral
Lock Fail Safe Mode
4
QUADLVDT
LogicPiston
Evolution of Powered Flight Controls February 10, 2012 25
Relaxed Stability Aircraft
• 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
C.G.
Center of Lift
Trim Force
Evolution of Powered Flight Controls February 10, 2012 26
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 February 10, 2012 27
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
Evolution of Powered Flight Controls February 10, 2012 28
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 Fail-Safe Mode
Fixed Displacement Pump
M Variable Speed Motor
Motor Controller
Velocity Command
Evolution of Powered Flight Controls February 10, 2012 29
Tandem Electrohydrostatic Actuator and Power Control Electronics
Evolution of Powered Flight Controls February 10, 2012 30
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
Evolution of Powered Flight Controls February 10, 2012 31
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