Tiltrotor Whirl Flutter Alleviation Using Actively Controlled Wing Flaperons

Abstract

The present study is an evaluation of the effectiveness of wing flaperon active control for alleviation of tiltrotor whirl flutter. Control systems that increase whirl flutter speed and wing mode subcritical damping are designed while at the same time observing realistic limits on flaperon deflection. Both stiff- and soft in-plane tiltrotor configurations are examined, to develop an understanding of the influence of vehicle configuration on flaperon effectiveness. The importance of considering unsteady aerodynamic effects in stability analyses and in controller design is also discussed. 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. Feedback of wing vertical and chord wise bending modes are identified as the most powerful parameters, with wing vertical bending rate feedback being particularly beneficial, as it substantially increases wing vertical bending mode damping.