COUPLED ROTOR-FUSELAGE VIBRATION REDUCTION USING OPEN-LOOP BLADE PITCH CONTROL

A fundamental study of vibration reduction in helicopters using higher harmonic blade pitch control was performed. The nonlinear equations o f motion for a coupled rotor/flexible fuselage system have been derive d using computer algebra on a special purpose symbolic computing facil ity. The trim state and vibratory response of the helicopter are obtai ned in a single pass by applying the harmonic balance technique and si multaneously satisfying the trim and the vibratory response of the hel icopter for all rotor and fuselage degrees of freedom. The influence o f the fuselage flexibility on the vibratory response is studied. New i nsight on vibration reduction in coupled rotor/fuselage is obtained fr om the sensitivity of the hub shears to the frequency and amplitude of the open loop HHC signal in the rotating frame. It is shown that conv entional higher harmonic control through the swashplate, is capable of suppressing either the hub loads or only the fuselage vibrations, but not both simultaneously. It is demonstrated that for simultaneous red uction of hub loads and fuselage vibrations, a multiple frequency high er harmonic control input signal provided in the rotating frame is req uired.