UNDERSTANDING THE PECULIARITIES OF ROTORCRAFT - PILOT - COUPLINGS

Rotorcraft pilots are familiar with potential instabilities or with annoying limit cycle oscillations that arise from the effort to control aircraft with high response bandwidth actuation systems. The destabilization of a vehicle due to active participation of the pilot in the control loop corresponds to the so-called ‘rotorcraftpilot coupling’ phenomenon (RPC). RPCs, in the past frequently called ‘pilot induced/assisted oscillations’ (PIO/PAO), can be problematic for the safety of the aircraft. Generally, it is accepted that RPCs are much more difficult to predict and suppress than aircraft-pilot-couplings (APCs); APCs have been mainly associated with the lower frequency spectrum of the flight modes, while for modern helicopters RPCs can also be associated with the higher frequency spectrum of structural dynamic and rotor aeroelastic modes. The goal of the present paper is to present results of an analytic investigation to provide an improved understanding of the peculiar physical mechanisms through which the pilot excites the rotor regressive flap and lag modes in an RPC event, and how these modes can couple through the flight control system (FCS) to the airframe body roll mode. It will be demonstrated that for a hovering helicopter the FCS is primary responsible for transferring energy from the roll to the flapping motion but usually no energy is transferred back from the flapping to the roll motion. In the case of an RPC induced by a time delay between the pilot input and the aircraft response it appears that the time delay has not much influence on the limits of the attitude controller, however, there is energy transferred back from the flapping to the roll motion. The roll mode tends to couple primarily with the flapping motion which in turn couples with the lag motion and can contribute to the destabilizing flap-roll coupling in an RPC event. For the FCS or rotor system designer, the paper will derive stability criteria and boundaries for the roll attitude feedback/roll rate feedback gains for a hovering helicopter as a function of aircraft parameters.