Numerical Simulation of Autorotation in Forward Flight

Abstract

Autorotation in forward flight is an important subject for gyroplanes or compound helicopters. By integrating the flapping and rotational equations of motion, we investigate in this work the steady-state autorotation of a three-bladed rotor for a given set of three independent variables: forward velocity, collective pitch angle, and shaft angle. Two-dimensional aerodynamic coefficients as functions of the angle of attack and the Reynolds number from the Navier-Stokes simulation and Pitt/Peters inflow model to determine induced flowfield are adopted to implement the simulation. Transient behavior of the solution from arbitrary initial conditions to a steady-state periodic solution is described. Variations of rotor speed and flapping angle of autorotation with the shaft angle and the collective pitch angle for a given forward velocity are presented and discussed in detail. Drastic changes in the region of autorotation near a specific shaft angle is found to exist, below which the range of collective pitch angle for stable autorotation becomes very limited and the flapping angle changes abruptly with the pitch angle.