Investigation of Adverse Aeroelastic Rotorcraft-Pilot Coupling Using Real-Time Simulation

This paper presents the results of a flight simulator test campaign aimed at understanding the effect of high-frequency dynamics associated with helicopter aeroservoelasticity on the proneness to rotorcraftpilot couplings and specifically to pilot-assisted oscillations. Linearised aeroservoelastic models representative of helicopters in hover and in forward flight have been flown in a full motion flight simulator by trained test pilots, performing selected mission task elements. The handling qualities of the vehicles have been intentionally degraded by modifying the gearing ratios between the control inceptors and the flight controls and by introducing time delays representative of realistic fly-by-wire flight control systems. Clear evidence of pilot-induced oscillations has been found while performing the roll step manoeuvre, especially with the soft-inplane hingeless helicopter with a lightly damped main rotor first regressive lead-lag mode. Based on subjective pilot ratings and objective measures, the aeroservoelastic models require higher pilot workload than corresponding rigid-body models. The repeatable occurrence of an unstable pilot-assisted oscillation event with only one test pilot flying the aeroservoelastic model has been explained by the interaction of the pilot's involuntary biodynamic feedthrough, identified by specific experiments, and the above mentioned regressive lead-lag mode.