Rotor-Wake Modeling for Simulation of Helicopter Flight Mechanics in Autorotation

Rotorcraft flight mechanics simulations typically use finite-state induced velocity models to mimic the effect of the rotor wake on blade loads. However, with recent developments in computational methods coupled flight mechanics and comprehensive rotor-wake analyses have become more tractable. This paper compares results from a finitestate induced velocity model with a vorticity transport wake model that computes the flowfield in a domain around the aircraft. The study focuses on flight mechanics in autorotation. Simple parameter estimation using three- and five-state induced velocity model structures facilitates interpretation of the vorticity transport induced velocity field as well as providing a direct comparison with the finite-state model. Comparisons of performance, trim, and control response indicate that there is little difference between the two models where the autorotative descent is shallow. However, as airspeed reduces the descent angle steepens, and clear discrepancies become apparent. These are directly related to a single parameter in the finite-state model. It is concluded that finite-state models are adequate for much of the autorotation regime, but for low speeds in steep descents the improved resolution of the rotor inflow provided by more comprehensive rotor-wake models might be essential.