Evolution of Powered Flight Controls Seminar

Evolution of Powered Flight Controls February 10, 2012 1

Evolution of Powered Aircraft Flight Controls

Tom Greetham


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


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


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


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


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


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)


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)


Un-Powered Flight Controls

Horizontal Tail Elevator

ControlColumn

Pitch UpCommand

Trailing EdgeUp Result

Horizontal TailElevator

ControlColumn

Pitch UpCommand

Trailing EdgeUp Result

Servotab

Manual

Servotabs


Simple Hydromechanical Servoactuator • Power Provided by High Pressure Hydraulic Fluid • Similar to an Automobile Power Steering System

Ps R

Control Surface

HydromechanicalServoactuator

Pilot Input


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


Servotab

'Boost' Servoactuator(Moving-Body withPressureFeedback

and Tab Lock)


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


Servotab

'Irreversible' Servoactuator(Dual-Tandem, Fixed-Body with

or without Tab Lock)

Feel/Trim


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


ControlColumn

Xs

Xp

Xs = Xp


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


ControlColumn 'FBW' Autopilot

Servoactuator

Xp Xa

Xs

Autopilot

Xs = Xp = Xa


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


ControlColumn

'FBW' Series DamperServoactuator

Xp

Xd

Xs

PFCS

AircraftSensors

Xs = Xp + Xd


Boeing 757 Elevator Control System


F-111 Pitch and Roll Control System


F-15 Pitch-Roll Control Assembly (A.K.A. Hydraulic Television Set)

16” 70 lbs


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


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


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


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)


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


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


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


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


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


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


Tandem Electrohydrostatic Actuator and Power Control Electronics


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


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