Flight Control Law Design: An Industry Perspective

1. 2003 European Control Conference Flight Control Law Design:An Industry Perspective Gary J. Balas balas@aem.umn.edu Aerospace Engineering and Mechanics University of Minnesota Minneapolis, MN 55105 USA September 4, 2003

2. Presentation Overview Survey of the control techniques being used by industry in Brazil, Europe, Russia and the United States of America to design flight control laws for fixed-wing aircraft.

3. Outline • 100 years of Controlled Flight • Introduction • Background • Countries • Brazil • Europe • France • Germany • Italy • Sweden • United Kingdom • Israel • Russia • United States of America • Boeing • Honeywell • Lockheed Martin • Summary

4. Wright Brothers: December 17, 1903 • First to develop a fully aerodynamic control system coupled to a powered aeroplane which was both flyable and maneuverable.

5. United Kingdom Most Influential Predecessor of the Wright’s • Sir George Cayley in 1799 at Brompton, near Scarborough in Yorkshire sketched a “conventional” configuration of an aeroplane. Indeed, in 1909 Wilbur Wright himself paid Cayley the following tribute: "About 100 years ago, an Englishman, Sir George Cayley, carried the science of flight to a point which it had never reached before and which it scarcely reached again during the last century.“ “The History of Flight from Around the World,” United Kingdom, Eur. Ing. Dr. J.A.D. Ackroyd

6. Cambridge University, England Lord Kelvin, President, Royal Society, undergraduate at Cambridge University, Senior Wrangler

7. Cambridge University, England Lord Kelvin, President, Royal Society, undergraduate at Cambridge University, Senior Wrangler “Heavier-than-air flying machines are impossible” (1895) "I have not the smallest molecule of faith in aerial navigation other than ballooning...I would not care to be a member of the Aeronautical Society," (1896)

8. Boeing 7E7 Dreamliner Airbus A380 Dassault Falcon 2000EX Embraer ERJ-170 Current/New Commercial Aircraft

9. F-35 JSF Rafale C F/A-22 Raptor Current/New Military Aircraft Saab Gripen

10. MIG 1.44 Indian Light Combat Aircraft Sukhoi-30MKK EurofighterTyphoon ChengduJ-10 Current/New Military Aircraft Sukhoi SU-37 F/A-18 E/F

11. Raven Neptune FPASS DragonEye Organic Air Vehicle Predator Global hawk Uninhabited Aerial Vehicles (UAVs)

12. Predator B UCAV Pointer Dragon Drone Uninhabited Aerial Vehicles (UAVs) Pioneer Dragon Drone FPASS

13. Uninhabited Aerial Vehicles (UAVs) • US Department of Defense (DoD) has 20 UAVs in service or under conceptual development: • DoD will have invested over $10 Billion in UAVs by 2007*. • DoD UAV systems will grow to 300 by the year 2010*. • 32 Nations are developing more than 250 models of UAVs*. • Over 60 small and Micro UAV programs are under way through out the world. * DoD Unmanned Aerial Vehicle Roadmap: 2002-2027

14. Background Flight control design research is a very active area: • In 2002 alone, 490 flight control design related papers were published. • Majority new theory or applying theory to aircraft simulations. • Basis from which the aircraft industry draws its “new” ideas. Working Group 23 of Advisory Group for Aerospace Research and development (AGARD now RTO) noted in 1996: • Skill required to design an advanced flight control system is not easily transferred and very little material exists in the open literature to be used as a reference handbook by designers. • RTO recommended better documentation of existing flight control system development process, lessons learned and best practices.

15. Background Survey of the current practices and control design approaches used by the aircraft industry. • Brazil , Europe (France, Germany, Italy, Sweden, United Kingdom), Israel, Russia, United States of America Caveats: • Limited information on techniques used in industry. • Some companies consider the control architecture, algorithms etc. to be IP. • Companies that publish more are better represented in this talk. • Almost all the references cited were published in English.

16. Brazil Embraer ERJ-170 aircraft Flight control design philosophy • Docile and benign as possible flying qualities behavior (behaviour) • Digital fly-by-wire (FBW) control system would allow complex flight controllers, cost constraints and accelerated time schedule led to selection of standard classical flight control system. • In-flight simulation using Veridian Variable Stability Learjet.

17. Europe

18. Europe Long tradition of aeronautical research. • Close links between universities and industry • To increase cooperation, the Association of European Research Establishments in Aeronautics (EREA) found in 1994. • CIRA (Italy), DERA (Great Britain), DLR (Germany), FFA (Sweden), INTA (Spain), NLR (Dutch) and ONERA (France, 1999). • Group for Aeronautical Research and Technology in Europe (GARTEUR) • GARTEUR Flight Mechanics Action Group 08 (1994-1997) • Robust control design methods • GARTEUR Flight Mechanics Action Group 11 (1999-2002) • New Analysis Techniques for Clearance of Flight Control Laws • Many European aircraft industries are multi-national and parts of the same aircraft flight control laws are designed in more than one country.

19. Epsilon Rafale France Concorde

20. France • Mirage 2000, Rafale C • Airbus • Major partner in European consortium of French, German, Spain and U.K. companies • A300/A310, A320, A330/A340, A380 under design. • Developer of commercial fly-by-wire (FBW) system. • A320 was first commercial aircraft to enter service with a FBW flight control system (1988). • A340 2nd generation FBW certified in 1992. • All Airbus flight control surfaces are electronically controlled and hydraulically activated. • UAVs

21. France – Airbus A320 A320 Flight Control Laws • Improve the natural flying qualities, particularly the stability, control and flight envelope protection. Longitudinal Control • Load factor demands • Classical proportional plus integral control Lateral-directional Control • Roll rate, sideslip and bank angle commands • Classical proportional plus integral control with a gain matrix for stability and roll rate/sideslip decoupling.

22. France – Airbus A340 A340 Flight Control Laws • Reproduced architecture/principles for A320 • Increased size and flexibility, required addition of structural mode control to reduce structural mode vibration. • Structural Mode Suppression Controller • Manual flight controller/Autopilot modified to eliminate interaction. • Turbulence damping function added to attenuate fuselage response. • Sufficient bandwidth separation between two controllers minimized interaction. • Accelerometers were added to sense vibration.

23. France – Airbus A380 A380 Flight Control Laws • High capacity, long range • Flexibility increases interaction between control laws and structural dynamic modes. • Aeroservo-elastic coupling traditionally solved by filtering/decoupling, would require reducing control bandwidth.

24. France – Airbus A380 • Airbus A380 Approach • Airframe flexibility taken directly into account when designing the flight control laws. • Integrated flight control laws to achieve desired handling qualities and flexible mode damping requirements leading to extended control bandwidth. • Flight tested on A340 using A380 models • Robust to fuel, payload, etc. variations • Same concept applied to autopilot and manual control laws.

25. Germany Commercial Aviation • Airbus • DLR experimental aircraft (ATTAS) Advanced Technologies Testing Aircraft System. Military Aviation • Eurofighter • X-31 (US/Germany program)

26. Germany X-31A Post stall experimental aircraft (US/Germany program) • First X program with Int’l partner • Enhanced Fighter Maneuverability • EFM using thrust vectoring • Goal: tactical advantage of EFM in post stall up to 70 deg AoA • Rockwell and MBB • X-31A Flight Control Laws • Pilot cmd (p,q,r) • Sensed feedback (p,q,r,,) • Actuation cmd (SF ,DF,C,R,,) • K - LTI controller • Optimal LQ digital regulator • Scheduled with , M, h • Nonlinear feedforward blocks

27. Germany VECTOR X-31A program • Vectoring Extremely short-takeoff-and-landing Control Tailless Operation Research (VECTOR) • Boeing, US Navy, German BWB and EADS

28. Germany Flight control system for A380 • Size and flexibility • EU funded REAL (Robust and Efficient Autoland control Law design). • Industry and research institutions from France, Germany and the Netherlands • Benchmark was DLR ATTAS aircraft. • DLR REAL flight control design approach • Multi-Objective Parameter Synthesis (MOPS). • Robustness addressed via multi-model, optimization, Monte-Carlo analysis.

29. Germany - REAL • Stability/command augmentation, tracking, guidance • Inner loops designed using dynamic inversion. • Total Energy Control System (TECS) • Lateral tracking uses classical PI control with tuning parameters • Tuning based on multi-criteria/multi-model parameter opt using MOPS.

30. Germany - REAL DLR autoland controller flight tested in 2000 • DLR autoland control performed well. Nonlinear ATTAS simulations • Technical University of Deflt (Netherlands) developed a -controller to replace the dynamic inversion inner-loop controller. • ONERA (France) developed a fixed-order Hcontroller.

31. Germany - IRIS-T Infra-Red Imagining System-Tail (IRIS-T) Missile • Thrust-vectored control, next generation short-range missile. • Being developed with Greece, Italy, Norway and Sweden. • Extreme maneuverability. • -synthesis robust control technique used to design lateral and roll controllers. • Scheduled on dynamic pressure • Successful flight test in May 2000.

32. Italy Eurofighter • Germany, United Kingdom, Italy, Spain. • Quad redudant, full authority DFCS. Alenia responsible for basic autopilot • Longitudinal axis controls attitude or pitch angle, lateral axis controls bank angle and heading. • Autopilot designed using classical control tools. • Control structure defined, Nicholas/Bode plots, linear time responses. • Large amplitude, nonlinear closed-loop simulations. • Modified control structure with nonlinear elements. • Mode logic increased nonlinear elements to satisfy mode schedules.

33. SAAB SHARC Sweden Saab/BAE JAS 39 Gripen • Contract awarded in 1982 • Lightweight, multi-role combat aircraft • All moving, delta canard configuration • Hungary (2003) orders 14, 232 ordered

34. Sweden • Pilot-induced Oscillation (PIO) • First test aircraft crashed after 6th flight (1989), first operational aircraft crashed in 1993. • Partial cause: PIO related to control surface servo rate limits. • Reduced phase margin or extra delay in feedback loop.

35. Sweden Controller modification based on PIOs • Feedback phase compensation based on anti-windup methods. • Increased or advanced phase around nonlinearity. • Low pass filters used to eliminate biases and high frequency roll off issues • Flight tested and verified. • Phase compensation technique used in place of rate limiters in Gripen production flight control system.

36. United Kingdom Commercial • Airbus • Military • Harrier • Saab/BAE JAS 39 Gripen • Eurofighter • Lockheed Martin/BAE F-35 JSF

37. UK - Jaguar and EAP Programs • FBW Jaguar program (1980s) precursor to VAAC • Prove principles of active control technology, establish design and flight clearance techniques for DFCS. • First UK aeroplane equipped with full authority DFCS.

38. UK - Jaguar and EAP Programs • Experimental Aircraft Programme (EAP, 1983-1995) • Follow on from Jaguar programme. • Control design process: • Linear low frequency: PI scheduled as function control • Nonlinear: trim distribution, nonlinear control power, nonlinear variations of stability • Linear high frequency: avoid structural coupling • Lessons learned: separate regulator and command path designs. • FBW Jaguar and EAP shaped Eurofighter flight controller

39. United Kingdom Vectored thrust Aircraft Advanced Control (VAAC) program • Inception in 1984. • Handling, control and display requirements for future short takeoff/vertical landing (STOVL) aircraft. • Experimental FBW VAAC Harrier. • Development and testing of advanced aircraft flight control algorithms. • Longitudinal axis, integrated management of thrust vectoring and aerodynamic forces for decoupled control.

40. UK - VAAC Control Strategies • Classical control, loop-at-a-time • Frequency shaping, gain-scheduling, significant nonlinearities linearized with inverse functions, iterative design. • Anti-windup scheme and control allocation. • Nonlinear static inverse • Nonlinear inverse of the aircraft to determine control effectors to trim. aircraft at a given maneuvering state. • Constrainted design process used to define unique solution to non-linear inverse problem (trim map). • Nonlinear inverse feed-forward combined with low gain, classical feedback design for stability.

41. UK - VAAC Control Strategies • Nonlinear Dynamic Inversion (NDI) • Nonlinear dynamic model of aircraft used to invert nonlinearities and a classical PI controller designed to track desired pitch rate command. • Pilot commands filtered prior to input to NDI controller.

42. UK - VAAC Control Strategies • H loop shaping • Multivariable linear controllers at 4 points: hover-to-forward flight. • Inner-loop pitch rate feedback used to reduce effect of pitch moment due to thrust changes. • Outer-loop 3-input/3-output, H loop shaping to control normal and forward acceleration and incidence. • Weight selection similar to classical loop-shaping. • Four linear point designs gain-scheduled throughout flight envelope. • Controller implemented in observer form. • Interpolated controller gains and interpolated controller outputs. • H loop shaping techniques also used to synthesize an integrated longitudinal/lateral flight and propulsions control system for VAAC.

43. UK - VAAC Control Strategies • Linear, parameter-varying (LPV) controller • System dynamics written as LTI models whose state-space coefficients are a function of scheduling variable(s). • LPV H loop shaping uses LPV model of nonlinear aircraft dynamics to directly synthesize a scheduled LPV controller. • Successfully implemented at tested between 1995 and 1998.

44. Israel • Light, multi-mission fighter “Lavi” • Initial flight test: 31 Dec 86, program terminated: 30 Aug 87 • Flight control laws • Classical technique with optimal control methods used in preliminary design process. • Lessons learned: Relationship between control design parameters and flying qualities. • UAVs

45. SU-27 SU-30MK SU-35 SuperFlanker Russia

46. SU-37 Terminator SU-27 TU-22 MIG-29 Russia

47. SU-30MK TU-160 SU-37 Russia

48. Russia Extensive history of military/commercial aircraft development. • English language literature on Russian industry flight control design techniques is limited. Sukhoi 37 FBW flight controller • Quad redundant DFCS • Design requirements • Good handling qualities. • Optimal trimming. • Reconfigurable under flight control system failures to maximize control moments and trim configuration. • Adaptive controller designed to eliminate small amplitude self-induced oscillations due to actuator nonlinearities.

49. Russia - SU-37 Aircraft • Canards and thrust vectoring (TV loop not shown.). • Longitudinal controller synthesized with classical control methods.

50. United States of America (USA) Commerical • Boeing (McDonnell Douglas) • B-717, B-737, B-747, B-757, B-767, B-777 • Honeywell Military • Lockheed Martin (General Dynamics) • F-16, F-22, F-35 (JSF) • Northrup Grumann • F-14, F-20, B-2 • Boeing (McDonnell Douglas, North American Rockwell) • B-52, B-1B, C-17 C-40A, F/A-18, KC-10 • Honeywell