FLIGHT CONTROL SYSTEMS practical issues In design and implementation

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FLIGHT

CONTROL

SYSTE,,MS

practical"

issues In

"

design and

implementation

Edited by

Roger W. Pratt

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Copublished by:

The Institution of Electrical Engineers, Michael Faraday House,

Six Hills Way, Stevenage, Herts. SG1 2AY, United Kingdom and

The American Institute of Aeronautics and Astronautics 1801 Alexander Bell Drive

Suite 500 Reston VA 20191-4344 USA

This publication is copyright under the Berne Convention and the Universal Copyright Convention. All rights reserved. Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act, 1988, this publication may be reproduced, stored or transmitted, in any forms or by any means, only with the prior permission in writing of the Institution of Electrical Engineers (lEE) or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency or Copyright Clearance Centre Inc. Inquiries concerning reproduction outside those terms should be sent to the lEE at the address above. While the authors and the publishers believe that the information and guidance given in this work are correct, all parties must rely upon their own skill and judgment when making use of them. Neither the authors nor the publishers assume any liability to anyone for any loss or damage caused by any error or omission in the work, whether such error or omission is the result of negligence or any other cause. Any and all such liability is disclaimed.

The moral right of the authors to be identified as authors of this work has been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

British Library Cataloguing in Publication Data

A CIP catalogue record for this book is available from the British Library

ISBN 0 85296 766 7

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Contributors

B.D. Caldwell Aerodynamics (W310C) British Aerospace Warton Aerodrome Preston PR4 lAX UK M.V° Cook

Flight Test and Dynamics Group College of Aeronautics

Cranfield University

Cranfield, Bedfordshire MK43 0AL UK

L.F. Faleiro

Control Design Engineering Institute for Robotics and Mechatronics

German Aerospace Center DLR Oberpfaffenhofen Postfach 1116 82234 Wessling Germany R,D. Felton 14 Cromwell Court Eynesbury

St Neots, Cambs. PE19 2NZ UK

J. Fenton

Smiths Industries Aerospace Bishops Cleeve Cheltenham, Glos. GL52 4SF UK C. Fielding Aerodynamics (W427D) British Aerospace Warton Aerodrome Preston PR4 lAX UK J. Hodgkinson 7022 Starstone Drive

Rancho Palos Verdes, CA 90275 USA R.A. Hyde Cambridge Control Ltd Matrix House Cowley Park Cambridge CB4 0HH UK R. Luckner

DaimlerChrysler Aerospace Airbus GmbH

Flight Mechanics

Flight Guidance and Control PO Box 95 01 09

D-21111 Hamburg Germany

D.G. Mitchell

Hoh Aeronautics Inc. Vista Verde Center 217 2075 Palos Verdes Drive North Lomita, CA 90717

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xiv Contributors R.W. Pratt Formerly:

Department of Aeronautical and Automotive Engineering Loughborough University Loughborough UK Now with: Ricardo MTC Ltd.

Midlands Technical Centre Southam Road Radford Semele Leamington Spa Warwicks CV31 1FQ UK S.P. Ravenscroft Flight Systems (W354C) British Aerospace Warton Aerodrome Preston PR4 lAX UK T,D. Smith Flight Test (W27K) British Aerospace Warton Aerodrome Preston PR4 lAX UK R. Taylor Ricardo MTC Ltd.

Midlands Technical Centre Southam Road

Radford Semele Leamington Spa Warwicks CV31 1FQ UK

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Preface

I f you b e l o n g to the school of t h o u g h t that says 'give m e a m o d e l a n d I'll give you a c o n t r o l l e r ' then this b o o k is not for you. If, however, you believe that using linear-control design methodologies to develop flight control laws requires a fuller u n d e r s t a n d i n g o f the dynamics of the plant (aircraft), the p r o b l e m s associated with i m p l e m e n t a t i o n a n d the n e e d to satisfy the r e q u i r e m e n t s of a highly trained h u m a n o p e r a t o r (pilot) then the chapters in this b o o k should help you to develop that understanding. In essence, m u c h of this b o o k is a message to the academic researcher which says: ' I f y o u r work is to be useful to practising engineers in industry, then you n e e d to understand, or at least appreciate, the issues dealt with in this book'. Additionally, y o u n g engineers who are b e g i n n i n g their careers in the aerospace industry should find it useful to have a coverage o f the key aspects o f flight control in a single volume

T h e authors were chosen because of their d e p t h o f e x p e r i e n c e a n d mix of backgrounds, which I believe are reflected in their individual contributions. Additionally, in a n u m b e r o f cases the chapters were reviewed by senior m a n a g e r s who have spent their entire careers in the aerospace industry. Hopefully, the e x p e r i e n c e which lies b e h i n d the individual contributions will e n c o u r a g e a new generation o f engineers, mathematicians and scientists to b e c o m e involved in this exciting b r a n c h of e n g i n e e r i n g - - f l i g h t control systems.

In the late seventies a n d eighties very few texts were p r o d u c e d on flight control. T h e n in the nineties a n u m b e r of books appeared. For readers who are new to flight control it m i g h t be useful to a t t e m p t to assign a place for this text in the total grouping. F u n d a m e n t a l s o f the subject with varying degrees o f emphasis on aircraft dynamics a n d flight control are covered by a n u m b e r o f texts [1-7]. All of these texts should be of use to u n d e r g r a d u a t e students in the final year or years o f their courses, as well as to postgraduate students who are in the process o f strengthening their knowledge o f f u n d a m e n t a l concepts before i m m e r s i n g themselves in their specific topic. T h e texts by McLean [4] a n d Stevens a n d Lewis [7] will e x t e n d the r e a d e r ' s knowledge into the realms o f research work. T h e contribution edited by Tischler [8] is significantly different f r o m the o t h e r texts in that e x p e r i e n c e d practitioners, s o m e o f w h o m have c o n t r i b u t e d to this book, give a strong a c c o u n t of the state o f the art, for rotorcraft, c o m b a t aircraft a n d fixed-wing t r a n s p o r t

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xvi

Preface

aircraft. O u r text is seen as bridging the gap between the work on f u n d a m e n t a l principles and Tischler's excellent collection of research reviews.

T h e aim of this text is to build on the fundamentals o f flight dynamics and flight control as described in References [ 1-7] and embellish these principles by assigning their relevance to the d e v e l o p m e n t o f flight control systems in the aircraft industry. T h e first seven chapters cover most o f the key areas within the discipline o f flight control systems with explicit reference to r e c e n t d e v e l o p m e n t programmes written by engineers who were closely involved in the work. T h e last two chapters look at just two of the multitude o f m o d e r n control methods which have been the subject of research studies. T h e text is largely restricted to military and civil fixed-wing aircraft. Only the constraint o f space has prevented equivalent material for rotorcraft and missiles from being included.

T h e book comprises nine chapters:

Chapter 1 'Industrial considerations for flight control', Chris Fielding and Robert Luckner: the authors set the scene for the whole b o o k by explaining the industry's perspective on flight control systems, giving a comprehensive overview of the subject with more detailed discussions of some particular topics being given in later chapters. T h e authors have carried t h r o u g h their c h a p t e r the parallel themes of military combat aircraft and civil aircraft, an interesting feature which strongly reflects their backgrounds.

T h e Chapter begins by examining the general objectives o f flight control and the role o f the flight control system (FCS). This is followed by the operational requirements for both types of aircraft and a discussion of the benefits o f fly-by-wire (FBW) in the pilot-vehicle system. T h e systems issues are explored, as are reliability and integrity, the twin versus--verification and validation. T h e Chapter is r o u n d e d off by a discussion o f the state-of-the-art and a look at some exciting future developments.

Chapter 2 'Aircraft modelling', Mike Cook: the a u t h o r summarises from his own text [3] the main elements of axis systems and the equations of motion for the longitudinal and lateral dynamics of fixed-wing aircraft. Aircraft- response transfer functions and state-space representations are then developed from the equations o f motion. Any reader who requires a fuller t r e a t m e n t than can be given within the confines o f this book is strongly r e c o m m e n d e d to refer to Mike's own text.

Chapter 3 'Actuation systems', Steven Ravenscroft: since the advent o f powered control surfaces without manual reversion, in the era o f the

Lightning, actuation systems have assumed great importance. T h e sig-

nificance o f actuation systems has been f u r t h e r e n h a n c e d by the drive to develop highly agile combat aircraft in which a safety-critical flight control system is required to stabilise the unstable open-loop dynamics o f the aircraft. This comprehensive chapter begins with an overview o f primary and secondary control surfaces and their operation and leads on to a discussion o f p e r f o r m a n c e criteria and modelling. T h e latter sections discuss m o r e

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Preface

xvii advanced topics: nonlinear frequency response, saturation analysis, j u m p resonance and failure transients.

Chapter 4 'Handling qualities', J o h n Hodgkinson and Dave Mitchell: uses the response transfer functions developed in Chapter 2 and examines the response of the aircraft from the pilot's viewpoint. T h e subjectivity which is i n h e r e n t in the assessment of handling qualities has, inevitably, given rise to a n u m b e r o f metrics and these are discussed in relation to the dynamic m o d e s for the longitudinal and the lateral motion. This leads on to stability and control augmentation systems and a discussion o f some control design concepts. Clearly, a chapter on handling qualities has to include a discussion o f pilot-induced oscillations (PIOs). This topic is given a t h o r o u g h and up-to- date treatment which reflects the very recent research carried out in the United States.

Chapter 5 'Automatic flight control system design considerations', J o h n Fenton: this chapter gives a clear and practical breakdown o f the tasks which are necessary in the m a n a g e m e n t of the d e v e l o p m e n t p r o g r a m m e for a complex flight control system. T h e conciseness of the chapter stems from the detailed breakdown of the main areas, the d e v e l o p m e n t p r o g r a m m e requirements definition and verification, system design considerations and AFCS architecture, into detailed subtasks.

Chapter 6 ' G r o u n d and flight testing a digital flight control system', T e r r y Smith: discusses the techniques which have been employed by the UK's major aircraft manufacturer, British Aerospace, as it has progressed with the d e v e l o p m e n t o f fly-by-wire combat aircraft. T h e chapter gives an excellent description o f the n e e d to progress a test p r o g r a m m e in a way which minimises both risk and cost, from the philosophy, tools and techniques o f flight testing t h r o u g h the elements o f simulator and rig testing, g r o u n d testing and, o f course, flight testing.

Chapter 7 'Aeroservoelasticity', Brian Caldwell, Roger Pratt, Richard Taylor and Richard Felton: discusses how a safety-critical flight control system can be affected by the elastic behaviour o f the aircraft structure, namely the p h e n o m e n o n o f aeroservoelasticity or structural coupling. As with the previous chapter, the material draws on the experience gained at British Aerospace with a series of aircraft in which the o p e n stability has b e e n r e d u c e d to the point of severe instability in o r d e r to e n h a n c e manoeuvrability. T h e contributions from Richard Taylor and Richard Felton are based on the results of research programmes which were carried out at the Universities o f L o u g h b o r o u g h and Lancaster, respectively.

Chapter 8 'Eigenstructure assignment', Lester Faleiro and Roger Pratt: represents o n e contribution to the work d o n e u n d e r the GARTEUR Action G r o u p on robust flight control in which a group o f universities, research establishments and aircraft companies contributed u n d e r Jan Terlouw's (NLR) excellent stewardship. Eigenstructure assignment was chosen in this case because it a p p e a r e d to offer a more visible methodology than o t h e r m o d e r n control techniques. T h e case study (RCAM) was based o n a flight

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xviii

Prefa~

profile for a civil aircraft which consisted o f a base leg and a two-stage final approach. T h e chapter is i n t e n d e d as an honest assessment o f eigenstructure assignment in this type of application.

Chapter 9 'An H0~ loop-shaping design for the VAAC Harrier', Rick Hyde: describes one o f the most exciting research programmes which has b e e n carried out in the field of m o d e r n control engineering applied to flight control. H0~ designs were evaluated extensively by piloted simulation and on the VAAC H a r r i e r at DERA Bedford where the controller was in competition with designs from British Aerospace and Smiths Industries. T h e early work benefited enormously from the r a p p o r t between Rick and the RAF's test pilot, Bj6rn Singer.

A step-by-step guide is given to the linear loop-shaping design process with a clear description of the use of the knowledge of the aircraft's dynamics. This is followed by the work on implementation and flight testing which explains the approach that was taken to gain-schedule controllers, deal with antiwind- up as well as describe the impressive results achieved during flight testing.

I would like to thank George Irwin, co-editor for the series, for inviting me 'to write or edit a text of flight control': certainly, there have been m o m e n t s when I have regretted yielding to George's Celtic persuasion. However, over twenty or so years I have benefited greatly from my association with the control community in the UK and, more recently, this has been equally true o f my association with the guidance, navigation and control activities within the AIAA in the United States and GARTEUR in Europe. My contribution to this book can be viewed as a partial repayment of a very large debt. Obviously, an edited text is the p r o d u c t o f a team of authors. I have been extremely fortunate to be able to assemble a very strong team, but more than that, they have been great people to work with. Although, inevitably, e x p e r i e n c e d p e o p l e have many calls on their time and from time to time this has caused the usual problems, everyone has come through and I have greatly appreciated their support and friendship t h r o u g h o u t the preparation o f the text. Additionally, I would like to thank the people who have volunteered to review individual chapters. Tony Lambregts (FAA) and Mike Walker (British Aerospace) are two people who are known to me, others have been acknowledged by individual authors.

T h e process o f publishing an edited text with several contributors is a d e m a n d i n g task. I have b e e n extremely fortunate to work with J o n a t h a n Simpson, then the IEE's commissioning editor for the project. J o n a t h a n ' s quietly efficient style impressed me greatly and on many, many occasions I have been extremely grateful for his support and guidance. I would also like to thank Robin Mellors-Bourne, Director o f Publishing, who m a n a g e d the project in addition to his normal duties during a very difficult period and Sarah Daniels, Book Production Editor, who j o i n e d the project at a late stage and injected some much n e e d e d energy and enthusiasm.

Finally, I would like to express my thanks to Penny Pilkington whose s u p p o r t and c o m m i t m e n t I have greatly appreciated t h r o u g h o u t this project.

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Preface

xix P e n n y has acted as the focal point for communications and retyped contributions and patiently, well mostly patiently, e n d u r e d the seemingly endless edits.

R e f e r e n c e s

[1] BABISTER, A.W.: 'Aircraft-dynamic stability and response' (Pergamon Press, 1980)

[2] BLAKELOCK, J.H.: 'Automatic-control of aircraft and missiles' (Wiley, 1991, 2nd edn.)

[3] COOK, M.V.: 'Flight dynamics: principles' (Arnold, 1997)

[4] ETKINS, B, and REID, L.D.: 'Dynamics of flight: stability and control' (Wiley, 1996, 3rd edn.)

[5] MCLEAN, D.: 'Automatic-flight control systems' (Prentice-Hall, 1990)

[6] NELSON, R.C.: 'Flight stability and automatic control' (McGraw-Hill, 1998, 2nd edn.)

[7] STEVENS, B.L., and LEWIS, EL.: 'Aircraft control and simulation' (Wiley, 1992) [8] TISCHLER, M.B. (Ed): 'Advances in aircraft flight control' (Taylor & Francis,

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N o m e n c l a t u r e

A B cg C

cL

D g G h

i,

I

I=

kq k~ kw ko kr L m M M N N o P q 1" $ t state matrix input matrix centre o f gravity o u t p u t matrix drag coefficient lift coefficient

direction cosine matrix; direct matrix acceleration due to gravity

transfer function matrix height

m o m e n t o f inertia in roll m o m e n t o f inertia in pitch m o m e n t o f inertia in yaw identity matrix

p r o d u c t o f inertia about ox or oz axes pitch-rate transfer function gain constant axial-velocity transfer function gain constant normal-velocity transfer function gain constant pitch-attitude transfer function gain constant turbojet engine gain constant

rolling m o m e n t mass pitching m o m e n t 'mass' matrix yawing m o m e n t n u m e r a t o r matrix origin o f axes roll-rate perturbation pitch-rate perturbation yaw-rate perturbation Laplace o p e r a t o r

time; m a x i m u m aerofoil section thickness roll-mode time constant

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xxviii Nomenclature U U U / ] V V W X X X Y Y Y Z Z R e % A 7/ 0 I"

spiral-mode time constant

numerator zero in axial-velocity transfer function numerator zero in normal-velocity transfer function

numerator zero in pitch-rate and attitude transfer functions turbojet engine time constant

axial-velocity perurbation input vector

total axial velocity

axial component of steady-equilibrium velocity lateral-velocity perturbation

eigenvector

perturbed total velocity; total lateral velocity lateral component of steady-equilibrium velocity steady-equilibrium velocity

normal-velocity perturbation total normal velocity

normal component of steady-equilibrium velocity longitudinal coordinate in axis system

state vector

axial-force component

lateral coordinate in axis system output vector

lateral-force component

normal coordinate in axis system normal-force component

angle-of-attack or incidence perturbation equilibrium incidence

sideslip angle perturbation equilibrium flight-path angle roll-control stick angle pitch-control stick angle rudder-pedal control angle transfer function denominator throttle-lever angle

rudder-angle perturbation; damping ratio dutch-roll damping ratio

phugoid damping ratio

short-period pitching-oscillation damping ratio elevator-angle perturbation

pitch-angle perturbation equilibrium pitch angle aileron-angle perturbation engine-thrust perturbation roll-angle perturbation

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Nomenclature xxix

¢

~0 d f-O n

%

Ws yaw-angle perturbation

dutch-roll u n d a m p e d natural frequency d a m p e d natural frequency

p h u g o i d u n d a m p e d natural frequency

short-period pitching-oscillation u n d a m p e d natural frequency

SUBSCRIPTS

0 free-stream flow conditions

b aeroplane body axes

d dutch roll

e equilibrium, steady or initial condition

E datum-path earth axes

p roll rate; phugoid

q pitch rate

r yaw rate; roll m o d e

s short-period pitching oscillation; spiral m o d e

u axial velocity

v lateral velocity

w aeroplane wind or stability axes; normal velocity

( r u d d e r elevator 0 pitch ailerons ~- thrust EXAMPLES OF O T H E R SYMBOLS A N D N O T A T I O N

x u a s h o r t h a n d notation to d e n o t e a concise derivative, a dimensional derivative divided by the appropriate mass or inertia parameters

OX

. ~ a shorthand notation to d e n o t e the dimensional Ou

N { (s) a shorthand notation to d e n o t e a transfer function n u m e r a t o r

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Glossary of terms

Accident (aircraft): an unintended event that causes death, injury, environ-

mental or material damage

Active control technology: the use of feedback control to enhance the performance or controllability and handling of a vehicle

Actuator: physical device for producing motion a n d / o r force

Adaptive control: real-time parameter identification and controller update Aerodynamic derivative: partial derivative defining changes in vehicle force or moment due to changes in control or motion parameters

Air data system: provides flight-condition and velocity vector information from external aircraft measurements

Airworthiness: an all-embracing term to describe an aircraft's ability to fulfil its role safely

Aliasing: phenomenon in digital systems in which input signal frequencies above half the sampling frequency appear at lower frequencies on the output signal, owing to the sampling process

Analogue (computer): using electrical signals that are directly proportional (i.e. analogous) to a continuous physical parameter

Angle of attack (AoA): the angle formed by the vector addition of the aircraft body-axis normal and longitudinal velocity components

Anti-aliasing filter: function for reducing aliasing by restricting the bandwidth of the signal to be sampled--usually an analogue filter with a natural frequency set to less than half the sampling frequency

Authority limit" permissible maximum amplitude of a signal or physical parameter

Autopilot- outer-loop automatic control system for reducing pilot workload a n d / o r augmenting weapon-system performance

Autostabiliser: simple stability-augmentation system, usually to provide increased damping and often with limited authority

Averaging (rolling average): digital process used to provide a smoothing and anti-aliasing function

Backlash: a form of hysteresis found in mechanical systems Bandstop filter: see notch filter

Bandwidth: range of frequencies over which the amplitude of the frequency response of a device remains essentially constant (numerical definitions vary)

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Glossary of terms

xxi

Bode diagram: frequency-response plots covering gain (usually in decibels, dB) against frequency and phase against frequency

Break point: frequency at which attenuation (or amplification) appears to occur, for the frequency response of a real pole or zero term

Built-in test: checks that are carried out automatically on the system or part of the system by failure-detection algorithms within the flight control system. These checks may be carried out continuously or at specific instances, for example, on start-up

Carefree handling: protection of aircraft from both departure and exceed- ance of loading limits, regardless of pilot-input demands, through the functionality of the flight control system

Certification: process for demonstrating that system safety is satisfactory for flight operation

Characteristic equation: polynomial defining the linear-stability characteristics of the system (defined by setting the denominator of a transfer function equal to zero)

Classical control: range of design and analysis techniques developed early in the 20th century, principally the methods referred to as Bode, Nyquist, Nichols, R o o t - L o c u s . . .

Clearance: see certification

Closed-loop control: outputs from the aircraft (or system) are measured and fed back to provide corrective action

C o m m a n d path: part of control system between physical input (e.g. pilot's stick) and the point where feedback is applied

Conditionally stable: a system that is stable only for a range of values of a particular gain; the system can be made unstable by either increasing or decreasing the nominal gain value by a sufficient a m o u n t

Control-configured vehicle (CCV): one which incorporates the control system capabilities and limitations at the onset of the project design

Control law: architecture containing controller(s), feedback filtering, nonlinear compensation and scheduling

Controller: algorithm or filter to provide desired control behaviour, usually acting on an error signal

Cooper-Harper rating: a m e t h o d for quantifying pilot opinion of an aircraft handling task, in terms of perceived controllability and operational effectiveness

Crossover frequency: gain crossover occurs when the gain of a system equals unity (0 dB); phase crossover occurs when the phase equal - 1 8 0 degrees. These are the frequencies at which the stability margins are measured Damping: attribute which determines the nature of a response, in terms of the rate of decay of oscillatory behaviour

DC block: see high-pass filter

Dead-beat (response): exhibiting no overshoot when tracking a step input signal

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xxii

Glossary of terms

Dead-zone: nonlinearity in which no output is achieved until the input exceeds some threshold

Decade: frequency interval in which the frequency changes by a factor of ten

Decibel (riB): defined at each frequency as 101ogl0(g) where g is ratio of powers, or 201ogl0(g ) if g is a ratio of voltages or signal amplitudes

Defect: the nonconformance of an item to one or more of its required parameters, within the limits defined in the specification

Derivative control/action: a function proportional to the rate of change o f the applied signal (i.e. differentiation with respect to time)

Describing function: approximation of nonlinear behaviour (amplitude dependence) of a system element, by modelling the gain and phase characteristics of the fundamental components of its Fourier transform Digital: described by a function of regularly sampled values

Dissimilar redundancy: multiplex arrangement where different lanes use different software a n d / o r hardware to perform the same function

Disturbance: an unwanted signal or force which can impair the quality of control

Drop back: a reduction in attained angle, following the removal of an angular rate d e m a n d

Duplex: having two hardware lanes operating in parallel, with cross- monitoring for detection of a single failure

Error: a state, resulting from a fault or h u m a n mistake, which is liable to lead to incorrect operation

Error signal: a control system signal equal to the calculated value between the parameter value c o m m a n d e d and that achieved

Failure: an occurrence in which a previously acceptable item is no longer able to perform its required function within the limits defined in the specification

Fault: see defect

Feedback: signal generated by sensor and applied with the aim of corrective action

Feedforward: signal from the c o m m a n d path which bypasses the controller to boost the downstream c o m m a n d to an a c t u a t o r - - i m p r o v i n g transient response without affecting stability

Flight control laws: control laws (or algorithms) within the flight control system which have broarder capabilities, for example, the monitoring of i n d e p e n d e n t signal channels for possible failures

Flight envelope: boundaries which define the limitations imposed on the operation of the aircraft defined in terms of altitude, airspeed/Mach n u m b e r and load factor

Flight management systems: system designed to assist the flight crew in managing the aircraft's systems, for example, fuel and navigational

Fly-by-wire (light): connection between pilot's control column, yoke or inceptor made electrically (or by fibre optics) rather than by mechanical

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Glossary of terms

xxiii

system consisting of rods, cables and levers Hying qualities: see handling qualities

Frequency response: variation of an output signal magnitude and phase characteristics, relative to a sinusoidal input signal, as frequency varies Full authority: allowing the maximum useable range

Full-state feedback: all the system states are used as feedback signals

Functional requirements document: specification of function requirements (e.g. control laws)

Gain: control law parameter for providing a signal-scaling capability

Gain margin: the factor by which the gain may be increased or decreased before system instability results

Gain schedule: variation of a gain or gains within a control law with respect to some measured scheduling variable (s)

Governor: a mechanical system for regulating a controlled parameter Handling (or flying) qualities: piloting characteristics with respect to how easy or safe the aircraft is to fly (for a particular task)

Hang-off (also hang-on): transient response characteristics whereby the c o m m a n d e d response fails to achieve its steady-state value within an acceptable time; is associated with undershoot, and with overshoot

Hard-over: a failure that causes a control surface to rapidly drive its output to the authority limit

Hazard: a state of the system, often following some initiating event, that can lead to an accident

High-pass filter: attenuates low frequency signals, allowing high frequencies to pass

Hysteresis: nonlinear function in which the i n p u t / o u t p u t relationship for increasing an input is different from that for decreasing the input

Inceptor: physical device with variable force a n d / o r motion, for enabling pilot input to the flight control system. Examples might be a centre-stick control column, a side stick or a throttle lever

Incidence: see angle-of-attack

Incident: an event which results in equipment or property sustaining damage or any person receiving any injury, or which might have resulted in an accident

Integrating filter: function for performing integral action on a signal Integrity: freedom from flaw or corruption (within acceptable limits) J u m p resonance: undesirable nonlinear saturation p h e n o m e n o n with a

sudden j u m p in its frequency-response characteristics

l a n e : a signal path containing all the hardware and functional elements of the control system within a multiplex arrangement

Limited authority: having access to only part of the full range available Limit cycle: b o u n d e d amplitude and fixed-frequency oscillation of a system, involving nonlinear beahviour

Line-replaceable unit or item: equipment fitted into an aircraft

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xxiv

Glossary of terms

outputs by the same factor; the principle of superposition applies

Linear quadratic Gaussian (LQG): linear design method which uses a quadratic cost performance and Gaussian noise to determine optimum feedback gains

Low-pass filter: function which attenuates high frequency signals but allows low frequency signals to pass

Minimum phase: a system which has no zeros in the right half of the complex plane

Mission-critical: loss of capability leading to possible reduction in mission effectiveness; compare with safety-critical

Mode (FCS): a selectable function of the FCS, e.g. terrain following

Modern control: a range of design and analysis techniques developed, generally considered as post 1960

Multi-input multi-output (MIMO) system: a system which has at least two inputs and at least two outputs. Often it is understood that the system possessses significant interaction or cross-coupling

Multiplex: having several hardware lanes to enable detection and isolation of equipment failures

Multivariable control: theory and techniques for addressing multi-input multi-output systems

Natural frequency (damped): the frequency at which a system will tend to respond when excited by a sudden input

Nichols chart: frequency response rectangular plot with gain in decibels (dB) plotted against phase in degrees and with frequency varying as a parameter. The chart contains contours of closed-loop gain and phase characteristics superimposed, assuming unity negative feedback

Noise: usually, an unwanted signal corrupting the desired signal

Nonlinearity: characteristic which introduces amplitude dependency into a system; linear behaviour is not preserved, in that the output magnitude no longer scales with the input

Nouminimum phase: having zeros in the right-half complex plane

Notch filter: function which produces attenuation over a specified frequency range, normally with minimal attenuation either below or above that range Nyquist diagram: a polar plot of a system's frequency response in the complex plane, with frequency varying as a parameter

Open-loop: without the use of any feedback

Order: the number of poles in the characteristic polynomial, remembering that a complex pair consists of two poles

Overgearing: where the control system gains have been increased beyond the point of optimum performance

Overshoot: transient response characteristic whereby the commanded response exceeds its steady-state; usually measured as a percentage

Pad6 approximation: a transfer function technique for establishing a low- order approximation to exponential functions (e.g. to model pure time delays)

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Glossary of terms

xxv

Phase: the relative angle between a sinusoidal input signal and the corresponding output signal

Phase advance filter: function for providing a low frequency phase lead, at the expense of increasing high frequency gain

Phase margin: the a m o u n t of phase lag (or advance) a system can tolerate before instability is reached

Phase-plane analysis: rectangular plot of two system states, usually position and velocity, for analysing system behaviour, particularly when nonlinear characteristics are present

Phase-retard filter: function for providing high frequency attenuation, with the associated phase loss being recovered at higher frequencies

Pilot-induced oscillation (PIO): p h e n o m e n o n whereby the pilot inadver- tently triggers and sustains an oscillation of the aircraft through a control input, owing to adverse coupling with the system dynamics

Plant: a device which is to be controlled, for example, an aircraft

Pole: real or complex root of transfer function denominator polynomial, sometimes referred to as an eigenvalue of the system

Power spectrum: plot of power against frequency where power is defined as the square of the signal magnitude

Primary controls: those controls which are fundamental for the safe operation of the system, for example, elevators, ailerons and r u d d e r

Proportional, integral and derivative: three-term controller with inherent phase advance and tracking capability

Quadruplex: having four hardware lanes for detection and isolation of up to two identical failures

Qualification: process for demonstrating that the system meets the custom- er's requirements

R a n d o m failure: a failure which results from a variety of degradation mechanisms in the hardware

Rate limit: physical or functional limit on rate of change of a parameter, of particular significance in actuation systems

Reconfigurable control: redistribution of system functions or hardware following a failure, to maintain satisfactory operation

Redundancy: duplication of components or software, to improve system integrity

Regulator: a control system in which the design driver is satisfactory disturbance rejection, in order to hold some desired parameter value constant; c o m m a n d tracking is usually of secondary importance

Reliability: the probability that a system will be free from faults

Resonant frequency: frequency at which the ratio of the magnitudes of a system's output to input is a maximum

Rise time: the time taken for the system response to a step input to change from ten per cent to ninety per cent of its steady-state value

Risk: the combination of the frequency, or probability, and the consequence of an accident

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xxvi Glossary of terms

Robustness: the ability of a system to tolerate variations in system parameters without u n d u e degradation in p e r f o r m a n c e

Roll-off: rate o f gain reduction at extremes of frequency (usually specified as d B / d e c a d e or dB/octave)

R o o t locus: parametric plot showing variation o f closed-loop poles, as a function o f a particular system parameter, almost invariably b u t not essentially, to the controller gain

Safe: the state in which the perceived risk is lower than the m a x i m u m acceptable risk

Safety: the expectation that a system does not, u n d e r defined conditions, lead to a state in which h u m a n life is e n d a n g e r e d

Safety-critical: failure or design e r r o r could cause a risk to h u m a n life Sample and hold: device for producing an analogue signal from a series o f discrete digital pulses

Saturation: a state where authority limits, rate limits or acceleration limits are r e a c h e d

S e c o n d a r y controls: those controls which are not essential for safe operation o f the system, but are likely to result in degraded p e r f o r m a n c e if they are not available (for example, flaps)

Self-monitoring: capability of a lane o f computing to detect its own failures Sensor: physical device for detection of inceptor positions, feedback meas- u r e m e n t s or scheduling information

Servomechanism: control system, literally slave mechanism, in which the design driver is accurate tracking o f a varying input signal and where disturbance rejection is usually o f secondary importance

Servovalve: a hydraulic device applied to a control valve or ram for switching the pressure and controlling the direction and magnitude of flow o f hydraulic fluid

Settling time: time taken for the c o m m a n d e d response to remain within a specified percentage, often five per cent, o f its steady-state value

Sideslip: the angle f o r m e d by the vector addition of the aircraft body-axis lateral and longitudinal plane velocity c o m p o n e n t s

Similar redundancy: multiplex a r r a n g e m e n t where different lanes have identical software and hardware to p e r f o r m the same function

Single-input single-output (SISO): system which has only o n e input with o n e associated controlled o u t p u t

Stability margin: a measure o f system stability--see gain margin and phase margin

Validation: process of d e t e r m i n i n g that the requirements are c o r r e c t and complete

Verification): evaluation o f results of a process to ensure correctness and consistency with respect to the inputs and standards provided to that process

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3.3.2 Maximum rate capability 3.3.3 Frequency response 3.3.4 Dynamic stiffness 3.3.5 Failure transients 3.4 Actuation system modelling 3.5 Nonlinear frequency response 3.6 Saturation analysis 3.7 J u m p resonance 3.8 Failure transients 3.9 Conclusions 3.10 Acknowledgements 9O 90 90 90 91 96 98 99 100 104 105 107 109 110 112 112 116 118

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Contents ix

4 Handling qualities

J. Hodgkinson and D. Mitchell

4.1 Introduction

4.2 Longitudinal flying qualities

4.2.1 Control-input transfer functions 4.2.2 Modal criteria

4.2.3 Phugoid flying qualities 4.2.4 Short-period flying qualities

4.2.5 Criteria for the longitudinal short-period dynamics 4.2.6 Model criteria for the short period

4.2.7 Other short-period criteria 4.2.8 Equivalent systems 4.2.9 Equivalent time delay 4.2.10 The bandwidth method 4.2.11 The Neal-Smith m e t h o d 4.2.12 Gibson's dropback criterion 4.2.13 Time-history criteria 4.2.14 Flight-path stability 4.3 Lateral-directional flying qualities

4.3.1 Roll mode 4.3.2 Spiral mode 4.3.3 Coupled-roll spiral 4.3.4 Dutch-roll mode 4.3.5 The parameter t%/~a

4.3.6 Phi-to-beta ratio, ~b//3

4.4 Stability and control-augmentation systems 4.4.1 The influence of feedback

4.4.2 The influence of actuators, sensors and processors 4.4.3 Multiple-input, multiple-output flying quality possibilities 4.4.4 Response types

4.5 Notes on some control design concepts 4.5.1 Integration in the forward path 4.5.2 Notch filters 4.5.3 Stick prefilters 4.5.4 Model prefilters 4.6 Pilot-induced oscillations ( P i t s ) 4.6.1 P I t categories 4.6.2 P I t and APC

4.6.3 Criteria for category I P i t s 4.7 Modal P I t criteria

4.7.1 STI high-gain asymptote parameter 4.7.2 A'Harrah-Siewert criteria

4.7.3 Dynamic stick force per g 4.8 Non-modal P I t criteria

4.8.1 Some current criteria 4.8.2 Effectiveness of the criteria 4.9 Effects of rate limiting on P I t

4.9.1 Criteria for category II P i t s 4.9.2 The consequences of rate limiting 4.10 Concluding remarks 4.11 References 119 119 121 121 121 122 123 124 126 126 127 131 132 132 134 134 136 136 136 139 139 139 140 141 142 142 144 146 147 147 147 149 150 150 150 150 151 151 152 152 155 155 155 156 161 164 164 165 167 167

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x Contents

5 Automatic flight control system design considerations

J. Fenton

5.1 AFCS development programme 5.1.1 Study phase/vendor selection 5.1.2 Interface definition

5.1.3 System definition 5.1.4 Software design and code

5.1.5 Hardware design and development 5.1.6 System integration and test 5.1.7 Qualification testing

5.1.8 Preliminary (final) declaration of design and performance (PDDP/FDDP)

5.1.9 Flight testing 5.1.10 Certification 5.1.11 Design reviews

5.2 Requirements definition and verification 5.2.1 Introduction

5.2.2 Design and test methodology 5.2.3 Safety considerations 5.3 System design considerations

5.3.1 Primary considerations 5.4 AFCS architecture

5.4.1 Introduction

5.4.2 AFCS flying control interfaces 5.4.3 AFCS system interfaces 5.4.4 AFCS configurations

5.4.5 Flight control computer data processing 6 Ground and flight testing of digital flight control systems

7: Smith

6.1 Introduction

6.2 Philosophy of flight testing 6.2.1 Ground testing

6.2.2 Simulator and rig testing 6.3 Aircraft ground testing

6.3.1 FCS build tests

6.3.2 Ground-resonance tests 6.3.3 Structural-coupling tests

6.3.4 Electromagnetic-compatibility testing 6.3.5 Engine-running tests

6.4 Flight test tools and techniques 6.5 Flight testing

6.5.1 FBWJaguar demonstrator flight test programme 6.5.2 The EAP demonstrator flight test programme 6.6 Conclusion

6.7 Acknowledgements 6.8 References

7 Aeroservoelasticity

B.D. CaldweU, R. W. Pratt, tL Taylor and R.D. Felton

7.1 Introduction

7.2 Elements of structural coupling 7.2.1 Flexible-aircraft modal dynamics

170 170 170 172 172 172 172 172 173 173 173 173 173 174 174 175 176 178 178 184 184 184 184 188 189 197 197 2O0 201 202 209 210 210 210 211 213 213 214 214 217 223 223 223 225 225 226 226

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Contents xi 7.2.2 Inertial excitation of the flexible-aircraft control surface 226 7.2.3 Actuators, flight control computers and the aircraft-motion sensor

unit 228

7.2.4 Aerodynamic excitation of the flexible-aircraft's control surface 230 7.2.5 Flexible-aircraft modal aerodynamics 230 7.2.6 Formulation for solution and design trade-offs 230 7.3 FCS-SC structural coupling: design examples 234

7.3.1 Jaguar-first flight 1968 235

7.3.2 Tornado-first flight 1974 236

7.3.3 Experimental aircraft programme (EAP)--first flight 1986 237 7.3.4 Eurofighter 2000 (EF2000)--first flight 1994 248

7.4 Future developments 260

7.4.1 Limit-cycle prediction and specification of alternative clearance

requirements 260

7.4.2 Active control for rigid body and structural-mode stabilisation 284

7.4.3 Flexible aircraft modelling 297

7.5 Conclusions 298

7.6 References 299

8 Eigens~ucture assignment applied to the design o f an autopilot function

for a civil aircraft 301

L.F. Faleiro and R. W. Pratt

8.1 Introduction 301

8.2 The RCAM control problem 302

8.2.1 A landing-approach simulation 304 8.2.2 Performance specifications 305 8.2.3 Robustness specifications 306 8.2.4 Ride-quality specifications 306 8.2.5 Safety specifications 306 8.2.6 Control-activity specifications 307

8.3 Eigenstructure analysis and assignment 307

8.3.1 Eigenstructure analysis 307

8.3.2 Eigenstructure assignment 310

8.4 The eigenstructure assignment design cycle 316

8.4.1 Controller structure 316

8.4.2 Construction of a desired eigenstructure 320

8.4.3 Initial synthesis 324

8.4.4 Methods of controller analysis 324

8.4.5 Analysis of the longitudinal controller 327 8.4.6 Analysis of the lateral controller 329

8.4.70ptimisation of the controllers 330

8.5 Nonlinear simulation of the controlled aircraft 331

8.5.1 Performance specifications 333

8.5.2 Robustness specifications 337

8.5.3 Ride-quality specifications 337

8.5.4 Safety specifications 338

8.5.5 Control-activity specifications 338

8.5.6 Evaluation using a landing-approach simulation 339

8.6 Conclusions 343

8.7 References 346

9 An H ® loop-shaping design for the VAAC Harrier

R.A. Hyde

9.1 Introduction

348 348

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xii Contents

9.2 The VAAC Harrier 9.3 Ha Loop shaping

9.4 Linear design for the VAC 9.5 Implementation and flight testing

9.5.1 Gain scheduling 9.5.2 Anti-windup 9.5.3 Flight modes 9.5.4 Flight testing 9.6 Flight clearance 9.7 The way ahead 9.8 References 350 350 354 359 359 360 362 364 366 371 372 Index 375

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Chapter 3 Actuation systems

S. Ravenscroft

3.1 Introduction

Actuation systems are a vital link in the flight control system, providing the motive force necessary to move flight control surfaces. W h e t h e r it is a primary flight control, such as an elevator, rudder, taileron, spoiler or foreplane, or a secondary flight control, such as a leading edge slat, trailing edge flap, air intake or airbrake, some means o f moving the surface is necessary. P e r f o r m a n c e o f the actuator can have a significant influence on overall aircraft p e r f o r m a n c e and the implications o f actuator p e r f o r m a n c e on aircraft control at all operating conditions must be considered during flight- control system design and d e v e l o p m e n t programmes. Overall aircraft p e r f o r m a n c e requirements will dictate actuator p e r f o r m a n c e requirements, which can lead to difficult design, control and manufacturing problems in their own right.

In this chapter an overview o f c u r r e n t actuation system technologies as applied to m o d e r n combat aircraft is presented, and their p e r f o r m a n c e and control requirements are discussed. T h e implications for aircraft control are considered and an overview o f selected modelling and analysis m e t h o d s is presented.

3.2 Actuation system technologyman overview

3.2.1 Control-surface types

Aircraft have a n u m b e r o f different flying control surfaces. Some are for primary flight control (control o f roll, pitch and yaw manoeuvring a n d stabilisation) and others are secondary controls (high-lift or lift-dump devices, for example). T h e type and use o f a control surface has a significant impact on the requirements for the actuation system for that surface, in particular the actuator post-failure operation.

Primary flight control capability is critical to safety and loss o f control in o n e or m o r e o f the primary flight control axes will, in most cases, hazard the

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Actuation systems

91

aircraft. T h e advent o f concepts such as active control technology, control- c o n f i g u r e d vehicles and relaxed static stability, resulting in highly unstable c o m b a t aircraft to improve p e r f o r m a n c e and agility, have led to an even greater reliance o n primary flight control surface availability to the e x t e n t that many m o d e r n combat aircraft could not be controlled without the c o n t i n u e d operation of the primary flight control surfaces. Accepting that failures within an actuator are inevitable at some time in the life o f a fleet o f aircraft, the actuation systems for primary flight control o f such aircraft are designed to comply with a fail-op-fail-op philosophy; that is the actuator will c o n t i n u e to operate at, or very close to, full p e r f o r m a n c e following o n e or two failures to m e e t the safety and integrity requirements.

For many secondary control surfaces, it is not necessary to ensure full operation following failures. Although the loss of operation o f a secondary surface may introduce flight restrictions, such as requiring a flapless landing or limiting the m a x i m u m incidence angle the aircraft can achieve, these will n o t directly lead to the loss o f an aircraft. However, the nature o f the failure may, in itself, p r o d u c e a hazardous situation, such as possible engine flame- o u t if air-intake cowls fail in a closed position, or handling and speed restrictions if an airbrake fails in the o p e n position. In such cases a fail-op-fail- safe or simply fail-safe philosophy is used, where one design feature is to ensure that the secondary control surface can be moved to a safe position or simply frozen following a failure. In the examples given above, the actuators may be required to o p e n the intake cowl or close the airbrake following a failure, albeit at a lower rate than would normally be achieved. A similar philosophy exists for landing gear, where the safe state is gear down; the actuators have an e x t e n d only capability following loss of n o r m a l e x t e n d and retract functions.

Although secondary flight controls are, o f course, very i m p o r t a n t components o f an aircraft and the provision of e m e r g e n c y operation m o d e s can p r o d u c e interesting engineering challenges, it is the primary flight control actuators that have most influence on basic aircraft stabilisation and handling qualities. T h r o u g h o u t the r e m a i n d e r o f this chapter we will c o n c e n t r a t e primarily on actuators for primary flight control surfaces.

3.2.2 Actuator operation

Most flight control actuation systems on c u r r e n t aircraft are electrically or mechanically signalled and hydraulically powered. Until the early 1970s most actuators were mechanically signalled, but the advent o f fly-by-wire technology has led to many actuators now having electrical signalling as the primary, if n o t only, form o f demand. T h e d e m a n d signal is used to drive a spool valve, o p e n i n g ports t h r o u g h which high-pressure hydraulic fluid flows. T h e fluid is m e t e r e d to the actuator ram, causing the piston rod to e x t e n d or retract and providing the force to move the control surface. Movement o f the spool valve could be achieved by mechanical input, using mechanical feedback o f