Mitigation of transverse gusts via open- and closed-loop pitching maneuvers

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Unsteady flow conditions present significant challenges to stable flight, and gust rejectionremains a concern for flight control in many modern flight environments. Examples of gustdominated flight conditions include flight in stormy conditions, aircraft takeoff and landing in strong crosswinds or ship air wakes, and micro air vehicles in strong shear flow engendered by urban settings and complex terrain. Improving flight stability during gust encounters relies on an improved understanding of the flow physics and the development of effective mitigation control strategies. To this end, the present work seeks to (1) improve our understanding of the unsteady flow physics of a pitching wing encountering a transverse gust and (2) develop and characterize successful open- and closed-loop control strategies to mitigate aerodynamic lift transients induced by the gust using wing pitching input. Classic unsteady aerodynamic theory was used to construct the open-loop pitch maneuvers and tune the closed-loop controller for closed-loop control. The dynamical systems treatment of the problem during control design revealed several important physical features important to vehicle control. Two sets of wing-gust encounter experiments were conducted using a flat-plate wing model in a water towing tank. The transverse gust was generated in the center of the towing tank using a recirculating water jet. Data was acquired using a combination of Particle Image Velocimetry (PIV), force, and torque measurements. In the first set of experiments, the constructed openloop pitch maneuvers were implemented as open-loop kinematics in the water towing tank. This study revealed several findings regarding the change in the flow topology due to pitch actuation, the necessity of modeling added mass for open-loop pitch maneuver construction, and the increase in the pitching moment transients due pitch control. This study also demonstrated how lift-mitigating pitching maneuvers minimized the disturbance to the gust’s flow field, thereby reducing the momentum exchange between the gust and the wing. The second set of experiments implemented a proportional control strategy based on classic unsteady aerodynamic theory using a pitch acceleration input and real-time force measurements. The closed-loop control experiments spanned upwards and downwards gusts of various strengths and lift tracking at pre- and post-stall angles of attack. The controller yielded an average rejection performance of 80% without a priori knowledge of gust strength or onset time and for various aerodynamic conditions. Reasons for the controller’s success include using lift measurements directly in control feedback, aerodynamic models that capture the salient physics in the control design process, and wing pitching as input. Simultaneous time-resolved PIV and force measurements were used to discover and understand the flow physics underlying the lift transients and how applying closed-loop control mitigated those transients.