Comparison of Different Approaches for Modeling Vortex - Rotor Wake Interference on Rotor Trim

The flight of a helicopter within the wake of a preceding fixed-wing aircraft, for example during air-to-air refueling, is accompanied by vortex-rotor interactions. Prior studies made use of many simplifying assumptions such as ignoring any interactions of the rotor wake with the fixed-wing tip vortex, or assuming a vortex deflection into the rotor slipstream once the vortex reaches the rotor disk. The full mutuality of vortex and rotor wake interactions requires a free-wake solution to the problem. In the paper free-wake solutions are presented for different advance ratios and compared to existing simplified solutions. The results indicate that the simplified solutions are valid for higher advance ratios whereas very small advance ratios require the use of a free-wake approach.