The paper investigates the basic mechanism of aeroservoelastic Pilot Assisted Oscillation phenomenon (PAO) about the roll axis due to the interaction with the pilot’s arm biomechanics. The motivation stems from the observation that a rotor imbalance may occur as a consequence of rotor cyclic lead-lag modes excitation. The instability mechanism is analogous to the ‘air resonance’ phenomenon, in which the pilot’s involuntary action plays the role of the AFCS. Using robust stability analysis, the paper demonstrates that, in particular, the introduction of a gain and a time-delay between the stick motion and the servoactuator displacements may reduce the gain and phase margins of the pilotvehicle system. The mechanism of instability proves that the pilot biodynamics is participating to the destabilization of the system by firstly inputting energy directly into the flapping mode. This destabilizes the airframe roll motion which, in turn, causes lag motion imbalance.

The paper reassesses the mechanism of biodynamical feedthrough coupling to helicopter body motion in lateral-roll helicopter tasks. An analytical bio-aeroelastic pilot–vehicle model is first developed and tested for various pilot’s neuromuscular adaptions in the lateral/roll axis helicopter tasks. The results demonstrate that pilot can destabilize the low-frequency regressing lead-lag rotor mode; however he/she is destabilizing also the high-frequency advancing lag rotor mode. The mechanism of pilot destabilization involves three vicious energy circles, i.e. lateral-roll, flap-roll and flap-lag motions, in a very similar manner as in the air resonance phenomenon. For both modes, the destabilization is very sensitive to an increase of the steady state rotor coning angle that increases the energy transfers from flap to lag motion through Coriolis forces. The analytical linear time-invariant model developed in this paper can be also used to investigate designs proneness to lateral/roll aeroelastic rotorcraft–pilot couplings.