TWENTY FIFTH EUROPEAN ROTORCRAFT FORUM Paper no. C13 ACTIVE SUPPRESSION OF STALL ON HELICOPTER ROTORS BY

This paper describes the numerical analysis of a stall suppression system for helicopter rotors. The analysis employs a finite element method and includes advanced dynamic stall and vortex wake models. The stall suppression system is based on a transfer function matrix approach and uses blade root actuation to suppress stall directly. The rotor model used in this investigation is the UH-60A rotor. At a severe stalled condition, the analysis predicts three distinct stall events spreading over the retreating side of the rotor disk. Open loop results show that 2P input can reduce stall only moderately, while the other input harmonics are less effective. The responses of the stall index, a measure of stall, to individual input harmonics are highly nonlinear. Such nonlinear stall behavior makes the closed-loop controller ineffective in suppressing stall and the combined effects of individual harmonics nonadditive. Also, stall reduction does not guarantee gains in rotor performance.