The Role of Rotor Coning in Helicopter Proneness to Collective Bounce

Collective bounce is a rotorcraft-pilot coupling phenomenon caused by vertical vibrations in the aircraft cockpit that are transmitted to the collective lever through the torso, the left arm and the hand of the pilot, and fed back to the rotor through the collective pitch control. This paper shows that collective bounce is rooted in the coupling of the pilot biodynamics with the rotor coning mode. The damping of the coning mode, usually large, introduces significant phase delay in the response to collective pitch. When coupled in feedback with the pilot biodynamics, such delay may lead to marginal stability conditions or even to instability. The basic mechanism of this coupling is shown using simple analytical models, and confirmed using detailed helicopter models. The influence of several design parameters is investigated, and possible means of prevention are briefly discussed.