The effect of wake distortion on rotor inflow gradients and off-axis coupling

Abstract

For many years, analysts have been puzzled by the fact that the off-axis coupling of a helicopter exhibits the opposite sign in flight tests as compared to simulations. Recently, researchers have shown that the effect may be attributable to the bending of the wake during a pitching maneuver, which introduces a fore-toaft gradient in induced flow that can reverse the predicted sign of the roll coupling. Other research has shown that this result can also be obtained with momentum and vortex theory. In the present work, the vortex lattice is used to compute the inflow that is responsible for off-axis coupling. The results are then compared with momentum theory, vortex tube theory, and potential flow theory.