mae 331 lecture 16

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Aircraft Control Devicesand Systems

Robert Stengel, Aircraft Flight Dynamics, MAE 331,2012

Copyright 2012 by Robert Stengel. All rights reserved. For education al 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-anglelift 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

Eis an input for longitudinal motion

!!! ="

Mechanical, Power-Boosted System

Grumman A-6

McDonnell Douglas F-15

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Critical Issues for Control

Command and control of the control surfaces

Displacements, forces, and hinge moments of thecontrol mechanisms

Dynamics of control linkages included in model

Eis a state for mechanical dynamics

!!!E ="

Control Surface Dynamicsand Aerodynamics

Aerodynamic andMechanical Momentson Control Surfaces

Increasing size and speed of aircraft

leads to increased hinge moments This leads to need for mechanical or

aerodynamic reduction of hingemoments

Need for aerodynamically balancedsurfaces

Elevator hinge moment

Helevator

=CH

elevator

1

2!V

2Sc

Aerodynamic and MechanicalMoments on Control Surfaces

CHsurface =CH!!!!+CH

!

! +CH"

"+CHcommand + ...

CH!!

: aerodynamic/mechanical damping moment

CH!

: aerodynamic/mechanical spring moment

CH"

: floating tendency

CHcommand

:pilot or autopilot input

Hinge-moment coefficient, CH Linear model of dynamic effects

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Angle of Attack andControl Surface Deflection

Horizontal tail atpositive angle of attack

Horizontal tail withelevator controlsurface

Horizontal tail withpositive elevator

deflection

Floating and RestoringMoments on a Control Surface

Positive elevator deflection produces a negative (restoring)moment, H

, on elevator due to aerodynamic or mechanical spring

Positive angle of attack produces negative moment on the elevator

With stick free

, i.e., no opposing torques, elevator floats

up dueto negative H

Dynamic Model of a ControlSurface Mechanism

!!!" H!!

!!" H!! = H

##+ H

command + ...

mechanism dynamics

=

external forcing

Approximate controldynamics by a 2nd-

order LTI system

Bring all torques and inertias to right side

!!!E=H

elevator

Ielevator

=

CH

elevator

1

2"V2Sc

Ielevator

= CH!!E

!!E+ CH!E

!E+ CH##+ C

Hcommand+ ...$%

&'

1

2"V2Sc

Ielevator

( H !!E

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

Dynamic Model of a ControlSurface Mechanism

Ielevator

=effective inertia of surface, linkages, etc.

H!!E =" H

elevator I

elevator( )"!!

; H!E

=

" Helevator

Ielevator( )

"!

H# =

" Helevator

Ielevator( )

"#

Stability and control derivatives ofthe control mechanism

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Coupling of System Model and ControlMechanism Dynamics

2nd-order model of control-deflection dynamics Command input from cockpit

Forcing by aerodynamic effects Control surface deflection

Aircraft angle of attack and angular rates

Short period approximation

Coupling with mechanism dynamics

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

"E

! !q

!!#

$

%&&

'

())*

Mq M#

1 +L#VN

$

%

&&&

'

(

)))

!q

!#

$

%&&

'

())+

M"E 0

+L"EVN

0

$

%

&&&

'

(

)))

!"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

#

$%%

&

'((!"E

command+

0 0

Hq H

+

#

$%%

&

'((

!q

!+

#

$%%

&

'((

Short Period Model Augmented byControl Mechanism Dynamics

Augmented dynamic equation

Augmented stability and control matrices

FSP/!E

=

FSP

F!E

SP

FSP

!EF!E

"

#

$$

%

&

''=

Mq M

( M

!E 0

1 )L(VN

)L!EVN

0

0 0 0 1

Hq H

( H

!E H!!E

"

#

$$$$$$

%

&

''''''

!xSP ' =

!q

!"

!#E!!#E

$

%

&&

&&&

'

(

))

)))

!!xSP /"E

= FSP/"E

!xSP /"E

+GSP /"E

!"Ecommand

State Vector

GSP /!E

=

0

0

0

H!E

"

#

$$$$

%

&

''''

Roots of the Augmented ShortPeriod Model

Characteristic equation for short-period/elevator dynamics

!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$n"Es +$n"E2( )

Short Period Control Mechanism

Roots of the Augmented ShortPeriod Model

Coupling of the modes

depends on designparameters

M!E

,L

!E

VN

,Hq, andH

"

Desirable for mechanical naturalfrequency > short-period naturalfrequency

Coupling dynamics can beevaluated by root locus analysis

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Horn Balance

CH

!CH

"

"+ CH#E

#E+ CHpilot input

Stick-free case Control surface free to float

CH

!CH

"

" +CH

#E

#E

Normally

CH

!

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Control Surface Types

Elevator

Horizontal tail and elevatorin wing wake at selectedangles of attack

Effectiveness of lowmounting is unaffected bywing wake athigh angle ofattack

Effectiveness of high-mountedelevator is unaffected by wingwake atlow to moderate angleof 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 orbelow lower aerodynamic surface

Reduces hinge moment

Cross-coupling effects can be adverseorfavorable, e.g. yaw rate with roll

Up travel of one>down travel of other tocontrol yaw effect

Abzug & Larrabee, 2002

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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


Elevons

Combined pitch and roll controlusing symmetric andasymmetric surface deflection

Principally used on

Delta-wing configurations

Swing-wing aircraft

Grumman F-14

General Dynamics F-106

Canards Pitch control

Ahead of wing downwash

High angle of attackeffectiveness

Desirable flying qualitieseffect (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 dragflaps produce yawing moment

McDonnell Douglas X-36

Northrop Grumman B-2

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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

Rudder Has Mechanical As Well asAerodynamic Effects

! American Airlines 587 takeoff behind Japan Air 47, Nov. 12, 2001

! Excessive periodic commands to rudder caused vertical tail failure

Japan B-747American A-300

http://www.usatoday.com/story/travel/flights/2012/11/19/airbus-rudder/1707421/

NTSB Simulation of AmericanFlight 587

! Flight simulation derived from digital flight data recorder (DFDR) tape

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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

SupermarineSpitfire

Nonlinear Control Mechanism Effects

Friction

Deadzone

Control Mechanization Effects

Breakout force

Force threshold

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B-52 Control Compromises toMinimize Required Control Power

Limited-authority rudder, allowed by

Low maneuvering requirement

Reduced engine-out requirement (1 of

8 engines) Crosswind landing gear

Limited-authority elevator, allowed by

Low maneuvering requirement

Movable stabilator for trim

Fuel pumping to shift center of mass

Small manually controlled "feeler"ailerons with spring tabs

Primary roll control from poweredspoilers, minimizing wing twist

Internally BalancedControl Surface

! B-52 application!

Control-surface finwithflexible sealmoves within aninternal cavity inthe main surface

! Differentialpressuresreducecontrol hingemoment

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

Boeing B-52

B-52 Rudder Control Linkages

B-52 MechanicalYaw Damper

Combined stable rudder tab, low-friction bearings, smallbobweight, and eddy-current damper for B-52

Advantages Requires no power, sensors, actuators, or computers

May involve simple mechanical components

Problems Misalignment, need for high precision

Friction and wear over time

Jamming, galling, and fouling

High sensitivity to operating conditions, design difficulty

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Boeing B-47 Yaw Damper Yaw rate gyrodrives rudder to increase

Dutch roll damping

Comment: The plane wouldnt need thiscontraption if it had been designed rightin the first place.

However, mode characteristics --

especially damping -- vary greatly withaltitude, and most jet aircraft have yawdampers

Yaw rate washoutto reduce opposition tosteady turns

Northrop YB-49 Yaw Damper

Minimal directional stability due to small vertical surfacesand short moment arm

Clamshell rudders, like drag flapson the B-2 Spirit

The first stealth aircraft, though that was not intended

Edwards AFBnamed after test pilot, Glen Edwards,Princeton MSE, killed testing the aircraft

B-49swere chopped up after decision not to go intoproduction

Northrop had the last word: it built theB-2

Northrop YB-49

Northrop/Grumman B-2

Northrop N-9M

Instabilities Due ToControl Mechanization

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

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

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 with

Dutch roll mode

Douglas DC-2

Solutions Trailing-edge bevel

Flat-sided surfaces

Fully powered controls

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Roll/Spiral Limit CycleDue to Aileron Imbalance

Unstable nonlinearoscillation growsuntil it reaches asteady state

This is called alimit cycle

Lockheed P-38

Control Surface Buzz

North American FJ-4

At transonic speed, normal shocksmay occur on control surface

With deflection, shocks movedifferentially

Possibility of self-sustainednonlinear oscillation (limit cycle)

ARC R&M 3364

Solutions

Splitter-plate rudderfixes shock locationfor small deflections

Blunt trailing edge

Fully poweredcontrolswithactuators at thesurfaces

Rudder Lock Rudder deflected to stops at high

sideslip; aircraft trims at high#

3 necessary ingredients Low directional stability at high

sideslip 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 isunpoweredand requires highfoot-pedal force (rudder floatoflarge WWII aircraft)

Solutions Increase high-sideslip directional

stability 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

SAS = Stability Augmentation System

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Downsprings and Bobweights Adjustment of

Stick-free pitch trim moment

Stick-force sensitivity toairspeed*

Downspring

Mechanical spring with low springconstant

Exerts a ~constant trailing-edgedown moment on the elevator

Bobweight

Similar effect to that of thedownspring

Weight on control column thataffects feel or basic stability

Mechanical stability augmentation(weight is sensitive to aircraftsangular rotation)

Beechcraft B-18

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

Effect of Scalar Feedback Controlon 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

H(s)=kn(s)

d(s)

=KH(s) !yc(s)"!y(s)[ ]

!y(s)=KH(s)!yc(s)"KH(s)!y(s)

K

Closed-Loop Transfer Function

1+KH(s)[ ]!y(s)= KH(s)!yc(s)

!y(s)

!yc(s)

=

KH(s)

1+KH(s)[ ]

Roots of the Closed-Loop System

Closed-loop roots are solutions to

!closedloop

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

orKkn(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

( )

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

KH s( ) = K !q(s)

!"E(s)= K

kq

s #zq( )

s2+2$SP%nSP s+%nSP

2 = #1

! # of roots = 2

! # of zeros = 1

! Destinations of roots (for k=#):

! 1 rootgoes to zero ofn(s)

! 1 rootgoes toinfinite radius

! Angles of asymptotes, $, forthe roots going to#

! K-> +#: 180 deg

! K-> #: 0 deg

Root Locus Analysis of PitchRate Feedback to Elevator

(2nd-OrderApproximation)

Center of gravity: doesnt

matter

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

the zero

K< 0: Segment to the right ofthe zero

Feedback effect is analogousto changing Mq

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

Flight Path Angle

Pitch Rate

Pitch Angle Angle of Attack

* p. 524, Flight Dynamics

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

Flight Path Angle

Pitch Rate

Pitch Angle Angle of Attack

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Direct Lift andPropulsion Control

Direct-Lift Control-ApproachPower Compensation

F-8 Crusader Variable-incidence wing,

better pilot visibility

Flight path controlat low

approach speeds requires throttle use

could not be accomplishedwith pitch control alone

Engine response time is slow

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

Approach powercompensationfor A-7 CorsairIIand direct lift controlstudiedusing Princetons Variable-Response Research Aircraft

Princeton VRA

Vought A-7

Vought F-8

Direct-Lift/DragControl

Direct-lift control on S-3AViking

Implemented with spoilers

Rigged up

during landingto allow lift.

Speed brakes on T-45AGoshawkmake up for slowspool-up time of jet engine BAE Hawk's speed brake

moved to sides for carrierlanding

Idle speed increased from55% to 78% to allow moreeffective modulation viaspeed brakes

Lockheed S-3A

Boeing T-45

Next Time:

Flight Testing forStability and Control

ReadingFlight Dynamics, 419-428

Aircraft Stability and Control, Ch. 3Virtual Textbook, Part 17

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upplementary

aterial

Trailing-EdgeBevel Balance

Bevel has strong

effect onaerodynamic hingemoments

See discussion inAbzug and Larrabee

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

Control Tabs

Balancing or geared tabs Tab is linked to the main surface

in opposition to control motion,reducing the hinge moment with

little change in control effect

Flying tabs

Pilot's controls affect only thetab, whose hinge momentmoves the control surface

Linked tabs

divide pilot's input between taband main surface

Spring tabs put a spring in the link to the

main surface

Control Flap Carryover Effect onLift Produced By Total Surface

from Schlichting & Truckenbrodt

CL!E

CL"

vs. c

f

xf +

cf

c f x f +c f( )

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Aft Flap vs. All-MovingControl 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 ofmechanical instability

Aerodynamic hinge moment is lower

Can be mounted on structurally rigid mainsurface

Mechanical and AugmentedControl Systems

Mechanical systemPush rods, bellcranks, cables, pulleys

Power boostPilot's input augmented by hydraulic servo that

lowers manual force

Fully powered (irreversible) systemNo direct mechanical path from pilot to

controls

Mechanical linkages from cockpit controls toservo actuators

Boeing 767 Elevator Control System


Boeing 777 Fly-By-Wire Control System

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Classical

Lateral Control Logic fora Fighter Aircraft (c.1970)

MIL-DTL-9490E , Flight Control Systems - Design, Installation and Test ofPiloted Aircraft, General Specification for, 22 April 2008

Superseded for new designs on same date bySAE-AS94900

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

The Unpowered F4DRudder 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 forceto neutralize the rudder

Fortunately, the test aircraft had a spin chute

Powered Flight Control Systems

Early powered systems had a singlepowered channel, with mechanicalbackup

Pilot-initiated reversion to

"conventional" manual controls

Flying qualities with manual controloften unacceptable

Reversion typically could not beundone

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

Bobweightresponds 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

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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

United Flight 232, DC-10Sioux City, IA, 1989

Uncontained engine failure damaged all three flight controlhydraulic systems (http://en.wikipedia.org/wiki/United_Airlines_Flight_232)

United Flight 232, DC-10Sioux City, IA, 1989

Pilot maneuvered on differential control of engines to make a runway approach

101 people died

185 survived

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

mae 331 lecture 16

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