A fundamental study of vibration reduction in helicopters using higher harmonic blade pitch control was performed. The nonlinear equations of motion for a coupled rotor/flexible fuselage system have been derived using computer algebra on a special purpose symbolic computing facility. The trim state and vibratory response of the helicopter are obtained in a single pass by applying the harmonic balance technique and simultaneously satisfying the trim and the vibratory response of the helicopter for all rotor and fuselage degrees of freedom. The influence of the fuselage flexibility on the vibratory response is studied. New insight on vibration reduction in coupled rotor/fuselage is obtained from 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 conventional 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 reduction of hub loads and fuselage vibrations, a multiple frequency higher harmonic control input signal provided in the rotating frame is required. © 1993.