SUMMARY

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-to-aft gradient in induced flow that can reverse the predicted sign of the roll coupling. Other research as shown that this result can also be obtained with momentum and vortex theory. There are many issues still under debate regarding the magnitude of wake distortion and its effectiveness in predicting off-axis dynamics. In the present work, a generalized ynamic wake model is augmented to include wake distortions. This model is then coupled with a flap model for simulation in low speed forward flight. Frequency responses from the simulation are collected with and without wake distortion, and these are compared with wind tunnel test data. LIST OF SYMBOLS thrust coefficient aerodynamic rolling moment coefficient aerodynamic pitching moment coefficient coefficients of pressure potential function hinge flap spring wake distortion parameter (curvature) matrix of inflow gains apparent mass matrix for dynamic inflow normalized Legendre polynomials, 1 ’ and 2ti kind number of rotor blades rotor radius wake radius of curvature rotor climb speed lift curve slope blade chord blade hinge offset blade root cutout longitudinal inflow gradient distribution bound circulation rotor azimuth rotor angular velocity coefficients of inflow distribution tip path plane angular ate blade Lock number normalized ellipsoidal coordinates flap mode shap