.c- s p: - X I 3 - - - ADVISORY GROUP FOR AEROSPACE RESEARCH & DEVELOPMENT U 7 RUE ANCELLE 92200 NEUILLY SUR SEINE FRANCE m I I Stability in Aerospace Systems I (La Stabilitk dans les Systkmes Mrospatiaux) , Z', ADVISORY GROUP FOR AEROSPACE RESEARCH & DEVELOPMENT 7 RUE ANCELLE 92200 NEUILLY SUR SEINE FRANCE AGARD REPORT 789 Stability in Aerospace Systems (La Stabilitk dans les Systkmes Akrospatiaux) Papers presented at the Guidance and Control Panel Workshop on Stability in Aerospace Systems, held at ONERA in Mauzac (Toulouse), France from 23rd-25th June 1992. Note: Exceptionally, some authors have not submitted texts of their papers, only copies of the viewgraphs used during their presentations. AGARD apologisesf or the lack of continuity in these papers, and the poor quality of some, and hopes that readers will gain some understanding of the subject matter from them. - North Atlantic Treaty Organization Organisation du Traite de I’Atlantique Nord I The Mission of AGARD According to its Charter, the mission of AGARD is to bring together the Leading personalities of the NATO nations in the fields of science and technology relating to aerospace for the following purposes: - Recommending effective ways for the member nations to use their research and development capabilities for the common benefit of the NATO community; - Providing scientific and technical advice and assistance to the Military Committee in the field of aerospace research and development (with particular regard to its military application); - Continuously stimulating advances in the aerospace sciences relevant to strengthening the common defence posture; - Improving the co-operation among member nations in aerospace research and development; - Exchange of scientific and technical information; - Providing assistance to member nations for the purpose of increasing their scientific and technical potential; - Rendering scientific and technical assistance, as requested, to other NATO bodies and to member nations in connection with research and development problems in the aerospace field. The highest authority within AGAFCD is the National Delegates Board consisting of officially appointed senior representatives from each member nation. The mission of AGARD is carried out through the Panels which are composed of experts appointed by the National Delegates, the Consultant and Exchange Programme and the Aerospace Applications Studies Programme. The results of AGARD work are reported to the member nations and the NATO Authorities through the AGARD series of publications of which this is one. Participation in AGARD activities is by invitation only and is normally limited to citizens of the NATO nations. The content of this publication has been reproduced directly from material supplied by AGARD or the authors. Published February 1993 Copyright 0 AGARD 1993 All Rights Reserved ISBN 92-835-0702-9 Printed by Specialised Printing Services Limited 40 Chigwell Lane, Loughton, Essex IGlO 3lZ ii Theme Stability in guidance and control can be illustrated by the following examples: - Air vehicles subjected to atmospheric instabilities (particularly downbursts and windshears), including flight control in downbursts, detection of downbursts from the ground or by airborne systems, and aircraft guidance to avoid them. - Control of fighters at high angles of attack which may become unstable due to a bifurcation point beyond which their behaviour becomes unpredictable, though belonging to the deterministic domain. - Control of laminar-turbulent transition for high performance aircraft (subsonic and supersonic), including improvement of stability by control of transition, and the impact of fully laminar flow on aircraft control. The concept of stability also needs to be looked at from a theoretical point of view - for example, the impact of uncertainties in initial conditions on system behaviour. This has links with the concept of model robustness. The programme was as follows: 1st Session. Fundamental aspects of stability with examples: The dilemma of stability vs. manoeuvrability and its consequences on military and civilian aircraft; stability and controllability of non-linear systems; various types of stability of an equilibrium state. 2nd Session. Basic theoretical aspects and chaos: Newtonian mechanics and thermodynamics; transition from stability to chaos; boundary layer control on a wing or fuselage, stabilisation of an unstable aircraft (decoupling of the controls of aircraft, helicopters, or convertibles). 3rd Session. Applications of aerospace techniques: a) External parameters: the atmosphere, turbulence, windshears and downbursts; their detection and modelling (methods of detection - whether ground or air based - the information processing of the data collected); control laws when flying through atmospheric disturbances; criteria. b) Control of the flow around wings and fuselage. C, and C, variations with boundary layer effects. Stability of a spin. Prediction of the instability and consequences for vehicle guidance. ... 111 I I I 1 Thkme Les problkmes de stabiliti dans l’optique du guidage et du pilotage peuvent &trei Llustris par les exemples suivants: - Vihicules airiens soumis aux instabilitis atmosphiriques (particulierement les cisaillements de vents) incluant le contrBle du vol dans un “downburst”a insi que la detection de ceux-ci, soit du sol, soit bord de l‘avion, et le guidage de l‘avion pour kiter ces phinomknes. - ContrBle des chasseurs a des trks grandes incidences qui peuvent devenir instables a cause de “bifurcations” demkre lesquelles le comportement des vehicules devient imprivisible, bien qu’appartenant au domaine “determint”. - Contrble de la transition laminaire-turbulent pour les avions a hautes performances (subsoniques ou supersoniques), incluant l’amtlioration de la stabiliti grlce au contrBle de la transition et l’impact d‘un icoulement totalement laminaire sur le comportement de l’avion. Le concept de stabiliti doit &re tgalement considiri d’un point de vue thiorique, par exemple, en examinant les constquences des incertitudes sur les conditions initiales sur le comportement du systeme. Ce problkme est lie, d‘ailleurs, au concept plus giniral de la “robustesse” d’un modkle. Le programme de l’atelier a 6t6 le suivant: Itre Session. ‘Aspects fondamentaux de la stabiliti et exemples” Le dilemme stabilitUmanoeuvrabilit6 et ses consiquences sur la stabiliti des aironefs civils et militaires; la stabiliti et la pilotabiliti des systkmes non-lintaires; diffirents types de stabiliti d’un “point”d ‘iquilibre (Ctat). 22me Session. “Aspects thionques fondamentaux et chaos” Micanique newtonienne et thermodynamique; transition stabilitUchaos; contrble de la couche limite sur une aile ou un fuselage, stabilisation d’un avion instable (dicouplage des commandes dans un avion, un hklicoptkre ou un convertible). I 32me Session. ‘Applications aux techniques airospatiales” a) Les paramktres extemes: l‘atmosphkre, la turbulence, les cisaillements de vents et les “downbursts”;l eurs dttecteurs et leur i modilisation (mithode de detection - sol ou airoportie - et traitement de l’information collectie); lois de contrBle en vol en atmosphkres trks turbulentes; critkres. I b) ContrBle de l’icoulement autour des ailes et du fuselage; C, et Cx lorsqu’on prend en compte la couche limite; stabiliti d’un avion en vrille; prkdiction de l’instabilitt et constquences pour le guidage du vihicule. I I I I I ! iv I I Guidance and Control Panel Chairman: Mr S. Leek Deputy Chairman: Mr J.K. Ramage British Aerospace Chief, Flight Control Defence Dynamics Ltd Advanced Development Branch PO Box 19 Wright Laboratory (WL/FIGX) Six Hills Way, Stevenage Wright-Patterson AFB, Herts SG12DA OH 45433 United Kingdom United States TECHNICAL PROGRAMME COMMITTEE Chairman: Dr M.J. Pelegrin P) Members: Dr A. Benoit (BE) Prof. R.C. Onken FE) Mr J. Bardal (NO) Mr J.K. Ramage (US) HOST NATION COORDINATOR Dr M.J. Pelegrin Haut Conseiller a I'ONERA ONEWC E R T BP 4025 2 avenue Edouard Belin F-31055 Toulouse Cedex France PANEL EXECUTIVE Commandant M. Mouhamad, FAF Mail from Europe: Mail from US and Canada: AGARD-OTAN AGARD-NATO Attn: GCP Executive Attn: GCP Executive 7, rue Ancelle Unit 21551 F-92200 Neuilly-sur-Seine APO AE 09777 France Tel: 33(1)47 38 57 80 Telex: 610176 (France) Telefax: 33 (1) 47 38 57 99 ACKNOWLEDGEMENTS/REMERCIEMENTS The Panel wishes to express its thanks to the French National Delegates to AGARD for the invitation to hold this meeting in 1 Mauzac and for the facilities and personnel which made the meeting possible. I Le Panel tient a remercier les Dtlkgues Nationaux de la France prks I'AGARD de leur invitation a tenir cette rtunion a Mauzac I et de la mise a disposition de personnel et des installations ntcessaires. V 1 Contents Page Theme iii Theme iv Guidance and Control Panel V Introduction viii by M.J. Pelegrin Reference Expos6 d’ouverture (Opening Address) OA par M. Btnichou - SESSION I FUNDAMENTAL ASPECTS OF STABILITY WITH EXAMPLES Chairman: Dr A. Benoit (BE) StabilitC Hydrodynamique des Ecoulements CisaillCs 1 (Hydrodynamic Stability of Free Shear Flows) par P. Huerre et L.G. Redekopp Chaotic Time Series Analysis 2 by J. Stark Exploring Chaos: A Toolkit and Some Ways to Use It 3 by M. Samuelides Stability Analysis and Aerospace Vehicle Dynamics 4 by P.Y. Willems Chaos Mechanisms in Some Turbulent Shear Flows* 5t by B. Nicolaenko On Non-Linear Longitudinal Stability of an Aircraft in a Dive in the 6 Presence of Atmospheric Disturbances** by L.M.B.C. Campos and A.A. Fonseca Discussion: Questions and Answers (Papers 1,2,6) D1 - SESSION I1 BASIC THEORETICAL ASPECTS AND CHAOS Chairman: Mr S. Leek (UK) Stability of Viscoelastic Flow: Physical and Numerical Considerations 7 by M.J. Crochet and C. Bodart - Dynamics and Control of Coherent Structures in the Turbulent Wall Layer An Overview* 8 by G. Berkooz, P. Holmes and J. Lumley Chaos, Entropy and Reversibility: A Simple Example 9 by C. Marchal Analyse Non LinCaire et Dynamique du Vol 10 par P. Guicheteau * FDP contribution. ** FMP contribution. t Not available at time of printing. vi Reference Decoupling of Aircraft Responses 11 by D.J. Moorhouse Discussion: Questions and Answers (Papers 8,10,11) D2 - SESSION 111 APPLICATIONS TO AEROSPACE TECHNIQUES Chairman: Dr C.H. Houpis (US) Boundary Layer Transition: Prediction and Wind Tunnel Simulation' 12 by D. Arnal Modeling Nonlinear Aerodynamic Loads for Aircraft Stability and Control Analysis* 13 by J.E. Jenkins and J.H. Myatt Stability Model of the Atmosphere** 14 by A. Knuppel, D. Martens and A.H. Siemer Utilizing Quantitative Feedback Theory Design Technique for Flight Control System 15 by C.H. Houpis Adaptive Reconfigurable Flight Controls for High Angle of Attack Aircraft Agility 16 by T. Sadeghi, M. Tascillo, A. Simons and K. Lai Toward Intelligent Flight Control 17 by R.F.S tengel X-29: Longitudinal Instability at High Angle-of-Attack 18 by L.A. Walchli Discussion: Questions and Answers (Papers 13,15,16,17,18) D3 * FDP contribution. ** FMP contribution. vii INTRODUCTION by Dr M.J. PELEGRIN, Programme Committee Chairman Is stability a measurable quantity - like mass or an Recently. new vocabulary has been introduced: strange identifiable quantity - like temperature? There are many attractor instead of equilibrium point or limit cycle. definitions of stability, sometimes contradictory: in fact, it is a subjective quantity which should be defined in the All chapters of physics are affected by the concepts of context of the theme considered The reference system in stability. Mechanical systems were the first to be affected which the system evolves should be defined stability by Poincark's approach. Theories and studies concern may exist in a given reference system, but no longer non linear systems. All that can be said about linear exists in other reference systems. Stability seems to be a systems has been said at the present time. No global dominant factor for aircraft or missile control - or any solution is expected The robustness concept of a control type of vehicle. However, stability and manoeuvrability system is an extension of linear systems studies. This are two opposing factors which intervene in aircraft concept is important for applications in industry or control: for civilian aircraft stability is the dominant vehicle control. Robustness can be defined as the factor; for military aircraft or missiles manoeuvrability is capacity (capability) of coping with the specified the dominant factor. The above are some of the reasons performances in spite of some unknown concerning the which led to the organization of a Workshop on parameters which define the system to be controlled - or "Stability" for the AGARD community. sometimes, the controller parameters themselves. Obviously, a linear differential equation with such Basically, stability is related to irreversibility which uncertainties on coefficients is no longer a linear means energy dissipation for linear systems, but linear equation. In fact, even in the beginning of linear system systems are very rare though they also often represent a studies "phase margin" and "gain margin" were used to suitable approximation of non linear systems. Stability is compensate for some errors in the system description. also a matter of accuracy. Let's take the earth's rotation: Nowadays more elaborate techniques such as H, is it stable or unstable? This question has no meaning optimization enables us to deal with multi-input, multi- until the range of accuracy we are looking for, and in output systems. fact, the whole context is specified. Due to the accuracy of existing atomic clocks, it is demonstrated that daily The stability concept is widely used in theimodynamics, variations are of the order of lms yearly or pluri-annual at least before statistical thermodynamics came into variations of the order of tens of ms, occur in a pseudo being. Irreversibility - at least from a practical point of periodic manner. However, the angular velocity is view - leads to the entropy concept, which simply says necessarily decreasing on a long-range basis: this is that "in an autonomous system entropy can only mainly due to the water/earth friction of tides. In the pre- increase". Irreversibility is no longer accepted as a Cambrian period (400 M years) the day was 15 hours! universal law in statistical thermodynamics, namely those What has been said about the angular velocity of the who study "chaos". Maxwell's devil could operate ... if earth could also be said about the direction of the earth's we wait an appropriate very long time. momentum. At the pole the Face of the rotational vector moves continuously in a circle of about 2m in diameter. We can probably say that the dilemma !'stability- However, for all human activities the earth's rotation is instability" has mainly progressed in fluid mechanics, considered (except by some astronomers) as stable. aero and hydro dynamics and spectacular (in both meanings of the word) results arose from the Bemard's Poincark in the 1870s studied stability for non- curls; they directly derive from a non-organized autonomous and autonomous problems. Ljapounov in the structure; order came up from disorder! If the heating of 1900s introduced a way of proving whether or not the fluid is adjusted, it can be stable as long as we do stability was sufficient, but not the necessary conditions. not modify the heat transfer (it is a non-autonomous Thereafter the behaviour of a system in the vicinity of an system). "equilibrium point" was studied in detail (Poincark) and equilibrium points or "singularities" were classified as Nonetheless. in this domain, the aerodynamic flow nodes-summits-focus-saddle. around a wing can be stable though instability may locally appear in the boundary layer: during a "normal A variety of possible behaviour in the vicinity of a point, flight" the boundary layer becomes turbulent (i.e., locally the limit cycles. which can be stable or unstable. were unstable) somewhere between one-half or two-thirds of introduced; they generalized the stability point by letting the wing chord. Buffet phenomena is due to the escape a periodic motion, noimally of a small amplitude. around of curls from the boundary layer, a phenomena which the stability point (in the phase plane or space). should be avoided for aircraft performance and passenger viii