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Nonlinear and Time-Dependent Aerodynamics: Implications for Testing and Flight Mechanics Analysis

Nonlinear and Time-Dependent Aerodynamics: Implications for Testing and Flight Mechanics Analysis. Jerry E. Jenkins Voluntary Emeritus Corps AFRL Wright-Patterson AFB, OH. The Delta Wing Model. Free-to-Roll Tests. Instantaneous motion state insufficient

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Nonlinear and Time-Dependent Aerodynamics: Implications for Testing and Flight Mechanics Analysis

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  1. Nonlinear and Time-Dependent Aerodynamics:Implications for Testing and Flight Mechanics Analysis Jerry E. Jenkins Voluntary Emeritus Corps AFRL Wright-Patterson AFB, OH

  2. The Delta Wing Model

  3. Free-to-Roll Tests • Instantaneous motion state insufficient • Bypasses stable trim points

  4. Nonlinear & Unsteady Aero Characteristics: 65° Delta Wing AIAA-97-0742 • Free-to-Roll tests perplexing results • Aerodynamic responses at moderate angles of attack • Not determined by instantaneous motion state • Highly dependent on motion history

  5. Nonlinear & Unsteady Aero Characteristics: 65° Delta Wing AIAA-97-0742 • Free-to-Roll tests perplexing results • Aerodynamic responses at moderate angles of attack • Often not determined by instantaneous motion state • Highly dependent on motion history • Viscous effects superimposed on potential flow • L. E. Vortex system structure • Vortex breakdown dynamics

  6. Nonlinear & Unsteady Aero Characteristics: 65° Delta Wing AIAA-97-0742Flow Structure • The steady-state flow-field can become unstable • At some flight conditions (Critical States) • Bifurcations in static force and moment characteristics

  7. Critical States

  8. Flow Structure & Bifurcations Left Wing

  9. Flow Structure & Bifurcations Right Wing

  10. Nonlinear & Unsteady Aero Characteristics: 65° Delta Wing AIAA-97-0742Flow Structure • The steady-state flow-field can become unstable • At some flight conditions (Critical States) • Bifurcations in static force and moment characteristics • Must transition to a new stable state when perturbed • Can require a considerable amount of time • Static tests give us no clue as to how long

  11. Nonlinear & Unsteady Aero Characteristics: 65° Delta Wing AIAA-97-0742Flow Dynamics • Flow processes acting on at least three time scales • Transitions between equilibrium states • Potential flow phenomena • Vortex breakdown movement in response to the motion

  12. Ramp Across a Critical State65o Delta Wing

  13. Attached and Vortical Flow Contributions

  14. Can isolate CS transients including motion history

  15. Harmonic Motion With and Without Critical State Encounters Rolling Moment Pitching Moment k = 0.02 & 0.14

  16. Harmonic Motion With and Without Critical State Encounters Rolling Moment Pitching Moment k = 0.02 & 0.14

  17. Static Nonlinearities - AIAA 2004-5275 3 Region 1 2 4 Region 5

  18. Multiple Time Scales in Linear RegionAIAA 2004-5275 • Slow responses cannot keep up with rapid motions

  19. Broadband Input • Allwine, et. al., “Nonlinear Modeling of Unsteady Aerodynamics at High Angle of Attack,” AIAA 2004-5275

  20. Multiple Time Scales (F-16XL)AIAA 2001-4016 • Variation of in-phase & out-of-phase components Lift-Curve slope Reducedfreq. Lift due to pitch rate

  21. Schroeder sweep • Murphy, P.C., and Klein, V., “Estimation of Aircraft Unsteady Aerodynamic Parameters from Dynamic Wind Tunnel Testing,” AIAA 2001-4016

  22. Langley Fighter Model

  23. Roll-Damping “Derivative”AIAA-2004-5273

  24. Nonlinear & Unsteady Aero Characteristics: Summary • Free-to-Roll tests difficult to explain results • Aerodynamic responses at moderate angles of attack • Often not determined by instantaneous motion state • Highly dependent on motion history • Traced to leading edge vortex system dynamics • Vortex system structure • Vortex breakdown phenomenon • Response characteristics not unique to delta wings • Static discontinuities, i.e. flow-field instabilities • Multiple time scales

  25. Unsteady and Nonlinear Aerodynamics:A Flight Mechanics Viewpoint • Unsteady Aero prescribed motion • Flight Mechanics motion is unknown a priori • Stability and Control • Flight Control System Design

  26. Unsteady and Nonlinear Aerodynamics:A Flight Mechanics Viewpoint • Unsteady Aero prescribed motion • Flight Mechanics motion is unknown a priori • Stability and Control • Flight Control System Design • Small-amplitude dynamic data inadequate • Stability “derivatives” • Exhibit frequency and amplitude dependence • Powerless to describe the aerodynamics

  27. Unsteady and Nonlinear Aerodynamics:A Flight Mechanics Viewpoint • Unsteady Aero prescribed motion • Flight Mechanics motion is unknown a priori • Stability and Control • Flight Control System Design • Small-amplitude dynamic data inadequate • Stability “derivatives” • Exhibit frequency and amplitude dependence • Powerless to describe the aerodynamics • Need math models for aerodynamics • Applicable to arbitrary motions • Functions of the translational and rotational DOF

  28. Nonlinear & Unsteady Aero Characteristics:AIAA-97-0742AIAA-2001-4016AIAA-2004-5273 • Results were for single DOF motions in wind tunnel • Understanding requires that we • acknowledge the existence of multiple time scales • Consider the individual effects of translation and rotation • Include lags present in both responses

  29. Stability Derivatives – Reduced Frequency RangeWhat happens as MAV scales are approached? • Assumptions: • Square-Cube Law holds

  30. Vehicle Inertia Variation with Mass

  31. Vehicle Weight Variation with Wing Area

  32. Stability Derivatives – Reduced Frequency RangeWhat happens as MAV scales are approached? • Assumptions: • Square-Cube Law holds • Want to fly in similar CL range • Conclusions:

  33. Stability Derivatives – Reduced Frequency RangeWhat happens as MAV scales are approached? • Assumptions: • Square-Cube Law holds • Want to fly in similar CL range • Hold non-dimensional derivatives constant • i.e. ignore Re effects • Conclusions:

  34. Stability Derivatives – Reduced Frequency RangeWhat happens as MAV scales are approached? • Consequences: • Magnitude of atmospheric disturbances do not scale • Relative angular disturbances, • Responses to disturbances up to not attenuated • Control system rates must increase • Sensor sampling rates • Servo response times • Aerodynamic effects • Convective time lags unaltered • Separated, vortex dominated flows ( low )

  35. Static Test Recommendations • Closely spaced static data • Critical state detection • Examine all components of the force and moment • Critical States are flow field events • Make sweeps should in both directions • Hysteresis detection • Another indication of critical states

  36. Dynamic Test Recommendations • Structure dynamic tests based on static test results

  37. Dynamic Test Recommendations • Structure dynamic tests based on static test results • Filtering (except anti-aliasing) should not be used • Ensemble averaging recommended

  38. Dynamic Test Recommendations • Structure dynamic tests based on static test results • Filtering (except anti-aliasing) should not be used • Ensemble averaging recommended • Record the complete response • potential nonlinear effects -- Linearize off line

  39. Dynamic Test Recommendations • Structure dynamic tests based on static test results • Filtering (except anti-aliasing) should not be used • Ensemble averaging recommended • Record the complete response • potential nonlinear effects -- Linearize off line • Cover wide range reduced frequencies • Try to saturate the viscous effects • Extract both "static" and dynamic stability derivatives • Frequency dependence multiple time scales

  40. Linear Aero Model from Broadband DataAIAA 2004-5275 • Linear system ID works quite well

  41. Dynamic Test Recommendations • Structure dynamic tests based on static test results • Filtering (except anti-aliasing) should not be used • Ensemble averaging recommended • Record the complete response • potential nonlinear effects -- Linearize off line • Cover wide range reduced frequencies • Try to saturate the viscous effects • Extract both "static" and dynamic stability derivatives • Frequency dependence multiple time scales • Ramp and hold motions invaluable • Isolate critical state transients • Provide quantitative measures for response times • Examine history effects. • Consider other types of "motion and hold" experiments

  42. Ramp-Between-Harmonics

  43. Nonlinear Model Constructed from SSM’sAIAA 2004-5275

  44. Nonlinear Model Constructed from SSM’sAIAA 2004-5275

  45. Nonlinear Model Constructed from SSM’sAIAA 2004-5275

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