Active Tiltrotor Whirl-Flutter Stability Augmentation Using Wing-Flaperon and Swash-plate Actuation

Abstract

The effectiveness of active control employed via actuation of a wing trailing edge flap (wing-flaperon) and the rotor swashplate is examined for improving the whirl-flutter stability characteristics of the semi-span model of the XV-15 tiltrotor, over a wide range of conditions including variations in forward airspeed and operating RPM. Limits are enforced on the magnitude of control gains utilized based on assumptions on the maximum allowable flaperon/swashplate deflection and the expected magnitudes of disturbances experienced by the model. Full-state feedback, linear quadratic regulator (LQR) optimal controllers that are scheduled with respect to airspeed and RPM show significant improvement in the stability characteristics. However, constant-gain controllers and output-feedback controllers are appealing due to their simplicity; and the performance of such controllers (designed using a moving-point optimization scheme) are examined in terms of their robustness to variations in RPM and airspeed. Results show that for wing-flaperon actuation, constant gain (non-scheduled) full-state feedback controllers are robust over a fairly wide RPM range and increase critical whirl flutter speeds by as much as 90 knots at the cruise RPM, over baseline. Using output feedback of wing-states is as effective as fullstate feedback. For swashplate actuation constant-gain, full-state feedback controllers are robust over an even wider RPM range and increase the whirl flutter speed by as much as 75 knots at the cruise RPM. However, with swashplate actuation, feedback based on wingstates alone shows a reduced robustness to variation in RPM, suggesting that measurement or estimation of certain rotor states might be necessary. However, if the swashplate actuation limits are increased from a nominal value of ± 1 deg to ± 2 deg, control based on wing-state feedback is robust to large variations in RPM (over a range including both the cruise and the hover RPM of the XV-15).