A Critical Examination of the Methods to Improve the Off-Axis Response Prediction of Helicopters

Recent advances in modeling the off-axis response of single-rotor helicopters have demonstrated the importance of correctly predicting the aerodynamic loads and induced velocity during maneuvering flight. This paper offers a critical examination of two correction methods used to improve the prediction of helicopter cross-coupling: extended momentum theory including angular rate effects and the aerodynamic phase lag correction. A generalized prescribed wake analysis of a rotor undergoing steady pitching and rolling motion is described, and numerical results are presented for a general forward flight condition. The influence of inflow and unsteady aerodynamic effects on an isolated rotor is examined, and comparisons with model rotor data, flight test identified parameters, and unsteady Euler calculations are made to provide bounds on the corrected model parameters. Results suggest current approaches cannot completely account for the observed discrepancies, and other physical mechanisms that are potentially important to the off-axis response prediction are briefly explored.