Helicopter rotor dynamic inflow modeling for maneuvering flight

Finite-state dynamic inflow models for the wake of a helicopter rotor are necessary for the implementation of real-time flight simulation models. There has been a discrepancy between helicopter simulation model response and true vehicle response that has perplexed researchers for many years. This “off-axis problem” is believed to be caused by inaccurate representation of the inflow at the rotor disk during pitching or rolling maneuvers. Current simulation models predict an initial off-axis response to cyclic stick inputs that are opposite in sign to responses of the corresponding flight tests. This study addresses this problem by modifying existing inflow models to account for variations in the rotor wake during maneuvering flight. It is shown that only compact modifications to finite-state inflow models are needed to capture these effects in the inflow dynamics.; Vortex and momentum theories are used to model the effect of wake curvature expected in maneuvering flight. It is believed that the curvature of the wake results in inflow gradients that affect the flapping dynamics, mainly in the off-axis channel. While this effect is greatest in hover, where the inflow is largest, it is also significant for low speed conditions where the inflow is still plays a major role. The curvature and contraction of the wake vary greatly in the flight envelope, so it is necessary to construct a unified model (for the inflow coupling due to wake curvature) that is applicable in all flight conditions. The final result is a modified, Peters-He generalized dynamic wake model with curvature augmentation. Specifically, the L-matrix of the Peters-He model is modified by extending a general vortex tube result for arbitrary load distributions.; This extended wake model is then coupled with a rotor flapping model, and the flap and hub moment responses are studied in hover and forward flight. Comparisons are made between the simulation model and the Sikorsky Bearingless Main Rotor (SBMR) at 40 kts forward flight, and the deficiencies of the modeling in forward flight are addressed.