Aeroelasticity

1. Investigations of Nonlinear Pathologiesin Aeroelastic SystemsThomas W. Strganac(and many others)Department of Aerospace EngineeringTexas A&M UniversityCollege Station, Texas

2. Aeroelasticity RIGID BODY Thermal Control

4. time domain simulations + V < Vflutter V > Vflutter frequency domainsolutions Vf , wf

6. USAF SEEK EAGLE OFFICEEglin AFB, Florida

7. Limit Cycle Oscillations > Nonlinear behavior leads to “Wing-with-Store Flutter” > Found in high performance aircraft > Flutter is a linear case of aeroelastic instability > LCOs are bounded amplitude oscillatory responses Placards are required … restricting mission performance.

8. Characteristics ( Flight Test & Lab Observations ) o LCOs below linear flutter predictions o LCOs as low as M ~ 0.6 o configuration dependent o spring-hardening stiffness evident o onset sensitive to AOA and maneuvers o hysteresis exists in recovery o performance limiting – pilot and aircraft

9. Flight Operation Placards Altitude kft downloading case configuration case Velocity, KCAS

11. continuous nonlinearities (seen in flight vehicles) NATA - Nonlinear Aeroelastic Test Apparatus Simulation & Validation Tools Ko and Thompson Large amplitude LCOs

12. Nonlinear Example: Pendulum w/ Extension Motion

13. Nonlinear system response to gust input “detuned” system c.m. Small shift in store center of mass (within mil. std.) tuned to a 2:1 resonance Shift in c.m. Duangsungnaen

14. 3:1 w2 2:1 w flutter w1 V Autoparametric (internal) resonances 2 DOF nonlinear aeroelastic system Cubic nonlinearity in aero Frequencies depend on V Commensurate frequencies occur at 3:1 and 2:1 (below flutter V) Large response at 3:1 only Gilliatt

15. Related findings of interest : + Transient Response External Forcing o A stiffening (continuous) structural nonlinearity is present o if modified frequencies are commensurate, then large amplitude LCO response is found at sub-flutter conditions. o linear theory fails to predict this response Thompson

16. Kim, Nichkawde Large wing deformations + Aerodynamic stall (subsonic) + Rigid store kinematics

17. Dz << Dx rCG = 0 Store terms : ( )s , ( )m, ( )* O(3) terms retained +/- xEA locations

18. { W - large beam deformations A - aerodynamic stall S - store rigid-body kinematics Treatment of all nonlinearities is required decay to 0, 0 unstable LCO LCO

19. A subcritical bifurcation occurs for specific system nonlinearities. o full system nonlinearities are required. o mimics flight test observations … - LCO depends on magnitude of input, > pilot control input > gust load or turbulence level > maneuver loads - hysteresis exists in onset/recovery speed bifurcation depends on system parameters - store mass and inertia - store chordwise and spanwise location - pylon length

20. @ AFRL w/Beran et al. • Streamwise position placed to achieve LCO • Underwing store CM located on elastic axis at midspan 1 ft below midplane • Store mass = wing mass / 10

21. LCOs and Subcritical Bifurcations

22. Subcritical Bifurcationsanalysis via AUTO Helios

23. TAMU 2’x3’ Low Speed Wind Tunnel top view leading edge trailing edge Barnett, O’Neil, Block, Kajula side view

24. Active Control – Theory and Experiments • Linear multivariable control - LQG ( Block ) • Feedback Linearization ( Ko, Kurdila* ) • Adaptive feedback linearization ( Ko, Kurdila* ) • Model reference adaptive control ( Junkins*, Kurdila*, Akella* ) • Adaptive control of a multi-control surface wing ( Platanitis )

25. Active Aeroelastic Wing

26. 1.0 0.5 0.0 r = dLE/dTE -0.5 0.0 0.5 1.0 1.5 Suppression of Roll Reversal measured ∆r = -2 ○ r = -0.7 □ r = 0 rrigid wing = -11 rrev  ∞ = - 6.7 Lb g = 20 ; r = -2 Insufficient loads g = 0 ; r = 0 dLE g = 10 ; r = -0.7 V dTE l Platanitis

27. Partial Feedback Control note: animation of measured data (via Working Model)

28. Structured Model Reference Adaptive Control note: animation of measured data (via Working Model)

29. 0.02 plunge (m) 0 0.02 -0.02 20 plunge (m) 0 pitch (deg) 0 -0.02 20 -20 30 pitch (deg) 0 TE ctrl. defl. (deg) 0 -20 30 -30 control 0 defl. (deg) -30 5 6 7 8 9 10 11 12 13 14 15 time (s) Closed-loop responses: LCO control (wing w/ leading & trailing edge control) Free Response Closed Loop Response Closed Loop Response Free Response b 30 g meas. LE ctrl. cmd. 0 defl. (deg) -30 10 11 12 13 14 15 16 17 18 19 20 time (s) Simulated response Measured response Platanitis

30. Intelligent Technologies in a UAV Demonstrator Demo Features/Lessons • Wing Warping Control • Highly Deformable Wings • Fluid-Structure Interaction • Composite wing spar • Autonomous control • AUVSI UAV Student Competition (Summer 2004) • Indoor Flight Capabilities   w/o skin wing w/ skin  The Albatross CRCD Project – Fall 2003 Specifications • Total Vehicle Weight = 4.5 lb • Available Payload Weight = 1.5 lb • Wing Span = 14 ft; Airfoil: SA7038 • AR = 15, W/S = .35 lb/ft2, L/D = 20 • Electric engine (lithium polymer batt.) • variable speed, thrust = 1.4 lb • VMAX = 31 mph, VSTALL = 10 mph • Roll control via active wing warping conventional pitch & yaw control Future • Semi-autonomous • Micro-autopilot: onboard 3-axis accels, 3-axis rate gyro, and GPS • position and altitude sensors programmable for waypoints and control laws • Distributed Control for Flexible Wings • Piezoelectric • SMA wires • Micro-servos