Adverse Rotorcraft-Pilot Couplings - Prediction and Suppression of Rigid Body RPC

Abstract

Rotorcraft-pilot-couplings (RPCs) of today helicopters seem to be highly more problematic for safety than aircraft-pilot couplings (APCs). GARTEUR organisation (Group for Aeronautical Research and Technology in Europe) has recently engaged the industry, research laboratories and academia in the group GARTEUR HC-AG16 with the goal of assessing the problems associated with undesirable RPC’s. The goal of the present paper is to give an overview of the GARTEUR work performed on rigid body RPC. Rigid body RPC involve adverse coupling phenomena dominated by helicopter lower frequency dynamics (range at frequencies of approximately 1 Hz and below) characteristic to flight mechanics modeling and an active pilot “keen“ to fulfil the mission task by actively controlling the rotorcraft. Using a state-space identified model enhanced by a full authority fly-by-wire RCAH-type flight control system, the paper will demonstrate that: 1) the bandwidth/phase delay criterion and Gibson average phase rate criterion used for Cat. I PIO prediction of fixed wing aircraft can be applied also for helicopters. Low dynamics and high time delays result in general in a PIO prone system. 2) the OLOP criterion used for Cat. II PIO analysis shows that the boundaries for helicopters lie between the fixed wing aircraft original OLOP boundary and the modified OLOP2 boundary. The paper investigates the concept of “boundary avoidance” for RPC analysis. In this respect, it will be shown that the condition for a RPC onset depends on the value of the minimum time to boundary which has to be proportional to the system time delay. The closer are the boundaries, the higher the tendency to develop RPC.