Adaptive flutter suppression of an unswept wing

The purpose of this study was to investigate the potential of an adaptive feedforward controller for active flutter suppression of a flexible wing. The flexible wing structure was modeled by a multi-degree-of-freedom finite element representation with beam elements for bending and rod elements for torsion. Control action was provided by a flap attached to the trailing edge of the wing, but extending for only a short length along the wingspan. The dynamics of the entire structure were simulated using only the first few flexible modes, thereby resulting in a reduced-order system for time integration. Both quasisteady and unsteady aerodynamics were used to generate the airforces acting on the wing. An adaptive feedforward controller was designed based on the filtered-X least mean squares (LMS) algorithm. The control configuration included an on-line system identification that provided the LMS controller with a reasonably accurate model of the plant. The linear wing model in closed loop exhibited highly damped responses at airspeeds where the open-loop responses were destructive. Simulations with the flexible wing model in a time varying airstream showed a 36% increase over the open-loop flutter airspeed, With 10% measurement noise introduced in the model, it demonstrated good robustness to the extraneous disturbances. Since the controller structure was adaptive, its effectiveness was not limited by the form of aerodynamics used in this investigation to model the airforces. In the example studied it was found that adaptation was rapid enough to successfully control flutter at accelerations in the airstream of up to 7 ft/s(2).