A Finite-State Aeroelastic Model For Rotorcraft Pilot-Assisted-Oscillations Analysis

Rotorcraft-pilot couplings denote interactions between pilot and helicopter (tiltrotors) that may become adverse. They are usually divided into two main classes of phenomena: that including Pilot Induced Oscillations (PIO) phenomena driven by flight dynamics and behavioural processes, and the one conventionally named Pilot Assisted Oscillations (PAO) which is caused by unintentional actions of pilot on controls, due to his involuntary reaction to seat vibrations. The aim of this paper is the development of mathematical helicopter models suited for analysis of PAO phenomena. PAO are strictly related to the structural dynamics of the fuselage and to aervoelasticity, but a crucial role is played by main rotor aeroelasticity. This paper presents a finite-state model of main rotor aeroelastic behavior that may conveniently be applied for PAO stability and response analysis, as well as for control applications aimed at PAO alleviation. It is validated, and its sensitivity to the aerodynamic modeling used within the aeroelastic operator is examined. Further, it is coupled with fuselage dynamics, servo-elastic and pilot models in order to carry out a numerical investigation concerning the stability analysis of vertical bouncing, which is a type of PAO instability which might be caused by the coupling, through the pilot, of vertical acceleration of pilot seat with collective control stick.