Feedback attenuation and adaptive cancellation of blade vortex interaction noise on a helicopter blade element

Blade vortex interaction (BVI) noise has been recognized as the primary determinant of the helicopter's far field acoustic signature. Given the limitations of design in eliminating this dynamic phenomenon, there exists a need for control. We present the application, first of feedback control strategies, and then of adaptive cancellation on Leishman and Hariharan's linear aerodynamic model of a trailing edge flap. Lift fluctuations caused by vortices are taken as output disturbance. The contribution of the vortices to lift is obtained from Leishman's indicial model for gusts. The use of an active structure for actuation is assumed, and the actuator is approximated as a lag element. To design an adaptive cancellation scheme that is applicable not only to BVI but also to general problems with periodic disturbances, we start with the classical sensitivity method, and arrive at an adaptive scheme whose stability we discuss via averaging. Sacks, Bodson, and Khosla (1996) arrived at the same result by introducing a phase advance into a pseudo-gradient scheme.