Flutter suppression control design for a small flexible flying-wing aircraft

Several modern aircraft designs involve high aspect ratio lightweight wings for efficiency, leading to a flexible airframe. As aircraft become more flexible, the frequency gap between rigid-body and structural dynamics is reduced, leading to dynamic coupling that poses challenges in aircraft design, modeling, and control. To maintain a desired operational flight envelope, active flutter suppression is required for these aircraft that achieves targeted damping of specific aeroelastic modes in isolation. This paper describes three different active modal suppression controllers that were designed for a small testbed UAS featuring flexible wings. The objective for all three controllers is to significantly increase the damping of the body-freedom-flutter mode at a selected sub-critical airspeed, and to successfully demonstrate such targeted modal damping via flight test. In addition, these controllers are not to significantly alter the vehicle dynamics at lower frequencies, thus not affecting vehicle handling qualities or the primary flight-control system. The three controllers are designed using three different synthesis approaches – a classical design, a modal isolation and suppression design, and a robust H_∞ design. A linear, flight-test-updated aeroelastic model is used for control design. A detailed comparison of controllers is presented, and it is shown that all controllers meet the objective of isolated modal damping and additionally, two of them succeed in expanding the flight envelope beyond the open-loop flutter boundary.