ACTIVE AND PASSIVE TECHNIQUES FOR TILTROTOR AEROELASTIC STABILITY AUGMENTATION

Open Access
Author:
Hathaway, Eric L
Graduate Program:
Aerospace Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 27, 2005
Committee Members:
  • Farhan Gandhi, Committee Chair
  • Edward C Smith, Committee Member
  • Joseph Francis Horn, Committee Member
  • Christopher Rahn, Committee Member
Keywords:
  • aeroelastic stability
  • whirl flutter
  • tiltrotor
  • active control
  • optimization
Abstract:
Tiltrotors are susceptible to whirl flutter, an aeroelastic instability characterized by a coupling of rotor-generated aerodynamic forces and elastic wing modes in high speed airplane-mode flight. The conventional approach to ensuring adequate whirl flutter stability will not scale easily to larger tiltrotor designs. This study constitutes an investigation of several alternatives for improving tiltrotor aerolastic stability. A whirl flutter stability analysis is developed that does not rely on more complex models to determine the variations in crucial input parameters with flight condition. Variation of blade flap and lag frequency, and pitch-flap, pitch-lag, and flap-lag couplings, are calculated from physical parameters, such as blade structural flap and lag stiffness distribution (inboard or outboard of pitch bearing), collective pitch, and precone. The analysis is used to perform a study of the influence of various design parameters on whirl flutter stability. While previous studies have investigated the individual influence of various design parameters, the present investigation uses formal optimization techniques to determine a unique combination of parameters that maximizes whirl flutter stability. The optimal designs require only modest changes in the key rotor and wing design parameters to significantly increase flutter speed. When constraints on design parameters are relaxed, optimized configurations are obtained that allow large values of kinematic pitch-flap (delta-3) coupling without degrading aeroelastic stability. Larger values of delta-3 may be desirable for advanced tiltrotor configurations. An investigation of active control of wing flaperons for stability augmentation is also conducted. Both stiff and soft-inplane tiltrotor configurations are examined. Control systems that increase flutter speed and wing mode sub-critical damping are designed while observing realistic limits on flaperon deflection. The flaperon is shown to be particularly effective for increasing wing vertical bending mode damping. Controller designs considered include gain scheduled full-state feedback optimal control, constant gain full-state controllers derived from the optimal controllers, and single-state feedback systems. The dominant feedback parameters in the optimal control systems are identified and examined to gain insight into the most important feedback paths that could be exploited by simpler reduced-order controllers.