Aircraft Control Devicesand Systems

Robert Stengel, Aircraft Flight Dynamics, MAE 331,

2010

Copyright 2010 by Robert Stengel. All rights reserved. For educational use only.http://www.princeton.edu/~stengel/MAE331.html

http://www.princeton.edu/~stengel/FlightDynamics.html

Control surfaces

Control mechanisms

Flight control systems

Design for Control

Elevator/stabilator: pitch control

Rudder: yaw control

Ailerons: roll control

Trailing-edge flaps: low-angle lift control

Leading-edge flaps/slats: High-angle

lift control

Spoilers: Roll, lift, and drag control

Thrust: speed/altitude control

Critical Issues for Control Effect of control surface deflections on aircraft motions

Generation of control forces and rigid-body moments on the aircraft

Rigid-body dynamics of the aircraft

!E is an input

Command and control of the control surfaces

Displacements, forces, and hinge moments of the control mechanisms

Dynamics of control linkages

!E is a state

!!! ="

!!!E ="

Control Surface Dynamicsand Aerodynamics

Aerodynamic and

Mechanical Moments

on Control Surfaces Increasing size and speed of aircraft leads to

increased hinge moments

This leads to need for mechanical or aerodynamic

reduction of hinge moments

Need for aerodynamically balanced surfaces

Control surface hinge moment

Hsurface = CHsurface1

2!V 2Sc

CHsurface = CH !!

!! + CH! ! + CH"" + CHcommand + ...

CH !!E

: aerodynamic/mechanical damping moment

CH!E

: aerodynamic/mechanical spring moment

CH"

: floating tendency

CHcommand

: pilot or autopilot input

Hinge-moment coefficient, CH Linear model of dynamic effects

Angle of Attack andControl Surface Deflection

Horizontal tail at positiveangle of attack

Horizontal tail withelevator control surface

Horizontal tail with positiveelevator deflection

Floating and RestoringMoments on a Control Surface

Positive elevator deflection typically produces a negative (restoring) moment, H!" , on

the elevator due to aerodynamic or mechanical spring

Positive angle of attack typically produces a negative moment on the elevator

With stick free, i.e., no opposing torques, elevator floats up due to negative H#

Dynamic Model of a ControlSurface Mechanism

!!!E =H

elevator

Ielevator

=

CHelevator

1

2"V 2Sc

Ielevator

= CH !!E

!!E + CH!E

!E + CH## + C

Hcommand+ ...$%

&'

1

2"V 2Sc

Ielevator

( H!!E

!!E + H!E!E + H## + Hcommand + ...

Ielevator

= effective inertia of surface, linkages, etc.

H!!E=" H

elevatorIelevator( )

" !!; H

!E=" H

elevatorIelevator( )

"!

H#=" H

elevatorIelevator( )

"#

!!! " H!!

!! " H!! = H

## + H

command+ ...

mechanism dynamics = external forcing

Approximate control dynamics

by a 2nd-order LTI system

Bring all torques and

inertias to right side

Stability and control

derivatives of the controlmechanism

Coupling of System

Model and Control

Mechanism Dynamics

Second-order model of control-deflection dynamics

Short period approximation

!!xSP = FSP!xSP +GSP!uSP = FSP!xSP + F"ESP!x"E

! !q

! !#

$

%&&

'

())*

Mq M#

1 +L#VN

$

%

&&&

'

(

)))

!q

!#

$

%&&

'

())+

M"E 0

+L"E

VN0

$

%

&&&

'

(

)))

!"E

! !"E

$

%&&

'

())

!!x"E = F"E!x"E +G"E!u"E + FSP"E!xSP

! !"E

! !!"E

#

$%%

&

'(()

0 1

H"E H !"E

#

$%%

&

'((

!"E

! !"E

#

$%%

&

'((+

0

*H"E

#

$%%

&

'((!"Ecommand +

0 0

Hq H+

#

$%%

&

'((

!q

!+

#

$%%

&

'((

Augmented Short Period Model

Augmented dynamic equation

Augmented stability and control matrices

FSP /!E =FSP F!E

SP

FSP!E

F!E

"

#

$$

%

&

''=

Mq M( M!E 0

1 )L(VN

)L!E

VN0

0 0 0 1

Hq H( H!E H !!E

"

#

$$$$$$

%

&

''''''

!xSP ' =

!q

!"

!#E

! !#E

$

%

&&&&&

'

(

)))))

!!x

SP /"E= F

SP /"E!x

SP /"E+G

SP /"E!"E

command

State Vector

GSP /!E =

0

0

0

H!E

"

#

$$$$

%

&

''''

Roots of the AugmentedShort Period Model

Characteristic equation

!SP /"E s( ) = sIn # FSP /"E =

s # Mq( ) #M$ #M"E 0

#1 s +L$

VN( )L"E

VN0

0 0 s #1

#Hq #H$ #H"E s # H !"E( )

= 0

!SP /"E s( ) = s

2+ 2#

SP$

nSPs +$

nSP

2( ) s2 + 2#"E$n"E s +$n"E2( )

Coupling of the modes

depends on values of

M!E

,L!E

VN

,Hq, and H

"

Root locus evaluation of design parameters

Assuming high aero/mechanicaldamping of the control mechanism

Horn Balance

Inertial and aerodynamic effects

Control surface in front of hinge line

Increasing improves pitch stability,

to a point

Too much horn area

Degrades restoring moment

Increases possibility of mechanicalinstability

Increases possibility of destabilizing

coupling to short-period mode

CH ! CH"" + CH#E#E + CHpilot input

Stick-free case Control surface free to float

CH! C

H"" + C

H#E#E

Normally

CH!

< 0 : reduces short-period stability

CH"E

< 0 : required for mechanical stability

CH!

NACA TR-927, 1948

Overhang orLeading-EdgeBalance

Effect is similar tothat of horn balance

Varying gap andprotrusion intoairstream withdeflection angle

CH ! CH"" + CH# # + CHpilot input

NACA TR-927, 1948

Control Flap Carryover Effect on

Lift Produced By Total Surface

from Schlichting & Truckenbrodt

CL!E

CL"

vs.cf

x f + cf

c f x f + c f( )

All-Moving Control Surfaces

Particularly effective at supersonic speed (BoeingBomarc wing tips, North American X-15 horizontal

and vertical tails, Grumman F-14 horizontal tail)

SB.4!s aero-isoclinic wing

Sometimes used for trim only (e.g., Lockheed L-1011horizontal tail)

Hinge moment variations with flight condition

Shorts SB.4

Boeing

Bomarc

North American X-15

Grumman F-14

Lockheed L-1011

Control Surface Types

Elevator Effectiveness of high

mounting is unaffected by

wing downwash at low to

moderate angle of attack

Effectiveness of low mounting is

unaffected by wing downwash at

high angle of attack

Ailerons When one aileron goes up, the other goes down

Average hinge moment affects stick force

Compensating Ailerons Frise aileron

Asymmetric contour, with hinge line at or below loweraerodynamic surface

Reduces hinge moment

Cross-coupling effects can be adverse or favorable, e.g.

yaw rate with roll

Up travel of one > down travel of other to control yaw

effect

Abzug & Larrabee, 2002

Spoilers Spoiler reduces lift, increases drag

Speed control

Differential spoilers

Roll control

Avoid twist produced by outboardailerons on long, slender wings

free trailing edge for larger high-liftflaps

Plug-slot spoiler on P-61 BlackWidow: low control force

Hinged flap has high hinge moment

North American P-61

Abzug & Larrabee, 2002

Elevons Combined pitch and yaw control using

symmetric and asymmetric surfacedeflection

Principally used on Delta-wing configurations

Swing-wing aircraft

Grumman F-14

General Dynamics F-106Canards

Pitch control

Ahead of wing downwash

High angle of attack effectiveness

Desirable flying qualities effect(TBD)

Dassault Rafale

SAAB Gripen

Yaw Control of Tailless

Configurations Typically unstable in pitch and yaw

Dependent on flight control systemfor stability

Split ailerons or differential drag flapsproduce yawing moment

McDonnell Douglas X-36

Northrop Grumman B-2

Rudder Rudder provides yaw control

Turn coordination

Countering adverse yaw

Crosswind correction

Countering yaw due to engine loss

Strong rolling effect, particularly at high #

Only control surface whose nominalaerodynamic angle is zero

Possible nonlinear effect at low deflectionangle

Insensitivity at high supersonic speed Wedge shape, all-moving surface on North

American X-15

Martin B-57

Bell X-2

Control Tabs Balancing or geared tabs

Tab is linked to the main surface inopposition to control motion, reducingthe hinge moment with little change incontrol effect

Flying tabs

Pilot's controls affect only the tab,whose hinge moment moves thecontrol surface [BAC 1-11 deep stallflight test accident]

Linked tabs

divide pilot's input between tab andmain surface

Spring tabs

put a spring in the link to the mainsurface

BAC 1-11

Control MechanizationEffects

Control Mechanization Effects

Fabric-covered controlsurfaces (e.g., DC-3, Spitfire)subject to distortion under airloads, changing stability andcontrol characteristics

Control cable stretching

Elasticity of the airframechanges cable/pushrodgeometry

Nonlinear control effects friction

breakout forces

backlash

Douglas DC-3

Supermarine

Spitfire

Nonlinear Control Mechanism Effects

Friction

Deadzone

Control Mechanization Effects

Breakout force

Force threshold

Instabilities Due ToControl Mechanization

Aileron buzz (aero-mechanical instability; P-80)

Rudder snaking (Dutch roll/mechanical coupling; Meteor, He-162, X-1)

Aeroelastic coupling (B-47, Boeing 707 yaw dampers)

Rudder Snaking Control-free dynamics

Nominally symmetric control position

Internal friction

Aerodynamic imbalance

Coupling of mechanical motion withDutch roll mode (see Flight Dynamics)

Solutions Trailing-edge bevel

Flat-sided surfaces

Fully powered controls

Douglas DC-2

Roll/Spiral Limit Cycle

Due to Aileron

Imbalance

Control Surface

Buzz

North American FJ-4

At transonic speed,normal shocks may

occur on control surface

With deflection, shocks

move differentially

Possibility of self-

sustained nonlinear

oscillation (limit cycle)

ARC R&M 3364

Solutions

Splitter-plate rudder

fixes shock location for

small deflections

Blunt trailing edge

Fully powered controls

with actuators at the

surfaces

Rudder Lock Rudder deflected to stops at high

sideslip; aircraft trims at high $

3 necessary ingredients

Low directional stability at highsideslip due to stalling of fin

High (positive) hinge moment-due-to-sideslip at high sideslip(e.g., B-26)

Negative rudder yawing moment

Problematical if rudder isunpowered and requires highfoot-pedal force (rudder floatof large WWII aircraft)

Solutions

Increase high-sideslip directionalstability by adding a dorsal fin(e.g., B-737-100 (before), B-737-400 (after))

Hydraulically powered rudder

Martin B-26

Boeing 737-100

Boeing 737-400

Control Systems

Downsprings and Bobweights Adjustment of trim and stick-

force sensitivity to airspeed*

Downspring

Long mechanical spring withlow spring constant

Exerts a trailing-edge down

moment on the elevator

Bobweight

Weight on control column thataffects feel or basic stability

Mechanical stabilityaugmentation

Beechcraft B-18

* See pp. 541-545, Section 5.5, Flight Dynamics

Mechanical and Augmented

Control Systems

Mechanical system Push rods, bellcranks, cables, pulleys

Power boost Pilot's input augmented by hydraulic servo that

lowers manual force

Fully powered (irreversible) system No direct mechanical path from pilot to

controls

Mechanical linkages from cockpit controls toservo actuators

Mechanical, Power-Boosted Systems

Grumman A-6

McDonnell Douglas F-15

Boeing 767 Elevator Control System

Abzug & Larrabee, 2002

Classical Lateral Control Logic for

a Fighter Aircraft (c.1970)

MIL-DTL-9490E, Flight Control Systems - Design, Installation and Test

of Piloted Aircraft, General Specification for, 22 April 2008

Superseded for new designs on same date by

SAE-AS94900http://www.sae.org/servlets/works/documentHome.do?comtID=TEAA6A3&docID=AS94900&inputPage=dOcDeTaIlS

Effect of Scalar Feedback Control

on Roots of the System

!y(s) = H (s)!u(s) =kn(s)

d(s)!u(s) =

kn(s)

d(s)K!"(s)

Block diagram algebra

Aircraft Transfer Function (open-loop): H (s) =kn(s)

d(s)

Control System Gain :K

= Kkn(s)

d(s)!yc (s) " !y(s)[ ] = K

kn(s)

d(s)!yc (s) " K

kn(s)

d(s)!y(s)

Closed-Loop Transfer Function

!y(s) = Kkn(s)

d(s)!yc (s) " K

kn(s)

d(s)!y(s)

!y(s) + Kkn(s)

d(s)!y(s) = K

kn(s)

d(s)!yc (s)

!y(s)

!yc (s)=

Kkn(s)

d(s)

1+ Kkn(s)

d(s)

"

#$

%

&'

Roots of the Closed-Loop System

Closed-loop roots are solutions to

!closedloop

(s) = d(s) + Kkn(s) = 0

or Kkn(s)

d(s)= !1

!y(s)

!yc (s)=

Kkn(s)

d(s)

1+ Kkn(s)

d(s)

"

#$

%

&'

=Kkn(s)

d(s) + Kkn(s)[ ]=

Kkn(s)

!closedloop

s( )

Root Locus Analysis of Pitch Rate Feedback toElevator (2nd-Order Approximation)

Kkn(s)

d(s)= K

!q(s)

!"E(s)= K

kq s # zq( )s2+ 2$SP%nSP s +%nSP

2= #1

Number of roots = 2

Number of zeros = 1

Destinations of roots (for k = "):

1 root goes to the zero of n(s)

1 root goes to infinite radius from the origin

Angles of asymptotes, %, for theroots going to "

K -> +": 180 deg

K -> ": 0 deg

Root Locus Analysis of PitchRate Feedback to Elevator(2nd-Order Approximation)

Kkn(s)

d(s)= K

!q(s)

!"E(s)= K

kq s # zq( )s2+ 2$SP%nSP s +%nSP

2= #1

Center of gravity : doesn!t matter

Locus on real axis K > 0: Segment to the left of the zero

K < 0: Segment to the right of the zero

The feedback effect is analogousto changing Mq

Root Locus Analysis of AngularFeedback to Elevator (4th-Order Model)*

Flight Path Angle Pitch Rate

Pitch Angle Angle of Attack

* p. 524, Flight Dynamics

Direct Lift andPropulsion Control

Direct-Lift Control-ApproachPower Compensation

F-8 Crusader

Variable-incidence wing toprovide better pilot visibility

Flight path control at lowapproach speeds requires throttleuse, could not be made with pitchcontrol alone

Engine response time is slow

Flight test of direct lift control(DLC), using the ailerons as flaps

Approach power compensationfor A-7 Corsair II and direct liftcontrol were studied at Princetonusing our Variable-ResponseResearch Aircraft

Princeton VRA

Vought A-7

Vought F-8

Direct-Lift/Drag Control

Direct-lift control on S-3A Viking

Implemented with spoilers

Rigged up during landing toallow lift.

Speed brakes on T-45A Goshawkmake up for slow spool-up timeof jet engine

BAE Hawk's speed brake movedto sides for carrier landing

Idle speed increased from 55% to78% to allow more effectivemodulation via speed brakes

Lockheed S-3A

Boeing T-45

Next Time:Flight Testing

Supplementary

Material

Trailing-EdgeBevel Balance

See discussion in

Abzug and Larrabee

CH ! CH"" + CH# # + CHpilot input

Aft Flap vs. All-Moving

Control Surface

Carryover effect

Aft-flap deflection can be almost as effective as

full surface deflection at subsonic speeds

Negligible at supersonic speed

Aft flap

Mass and inertia lower, reducing likelihood of

mechanical instability

Aerodynamic hinge moment is lower

Can be mounted on structurally rigid main

surface

The Unpowered F4D Rudder Rudder not a problem under normal flight conditions

Single-engine, delta-wing aircraft requiring small rudder inputs

Not a factor for upright spin Rudder was ineffectual, shielded from flow by the large delta wing

However, in an inverted spin rudder effectiveness was high

floating tendency deflected rudder in a pro-spin direction

300 lb of pedal force to neutralize the rudder

Fortunately, the test aircraft had a spin chute

Powered Flight Control Systems

Early powered systems had a single

powered channel, with mechanical

backup

Pilot-initiated reversion to"conventional" manual controls

Flying qualities with manual controloften unacceptable

Reversion typically could not be

undone

Gearing change between control stickand control to produce acceptable pilot

load

Flying qualities changed during a high-stress event

Hydraulic system failure was common

Redundancy was needed

Alternative to eject in military aircraft

A4D

A3D

B-47

Advanced Control Systems

Artificial-feel system

Restores control forces to those of an

"honest" airplane

"q-feel" modifies force gradient

Variation with trim stabilizer angle

Bobweight responds to gravity and tonormal acceleration

Fly-by-wire/light system

Minimal mechanical runs

Command input and feedback signalsdrive servo actuators

Fully powered systems

Move from hydraulic to electric power

Boeing 777 Fly-By-Wire Control System Control-Configured Vehicles Command/stability augmentation

Lateral-directional response

Bank without turn

Turn without bank

Yaw without lateral translation

Lateral translation without yaw

Velocity-axis roll (i.e., bank)

Longitudinal response

Pitch without heave

Heave without pitch

Normal load factor

Pitch-command/attitude-hold

Flight path angle

USAF F-15 IFCS

Princeton Variable-Response Research AircraftUSAF AFTI/F-16

Root Locus Analysis of AngularFeedback to Thrust (4th-Order Model)

Flight Path Angle Pitch Rate

Pitch Angle Angle of Attack

United Flight 232, DC-10

Sioux City, IA, 1989 Uncontained engine failure damaged all three flight control

hydraulic systems (http://en.wikipedia.org/wiki/United_Airlines_Flight_232)

296 people onboard

185 survived due to pilot!sdifferential control of engines

http://www.youtube.com/watch?v=vTXr1QR3rbQ

Propulsion Controlled Aircraft Proposed backup attitude control in event of flight control system failure

Differential throttling of engines to produce control moments

Requires feedback control for satisfactory flying qualities

NASA MD-11 PCA Flight Test

NASA F-15 PCA Flight Test

Proposed retrofit to McDonnell-Douglas (Boeing) C-17