A Fundamental Investigation of Ship Airwake Influence on Rotorcraft

Rotorcraft responses to idealized disturbances are examined to gain insights into model fidelity requirements for flight simulations of the ship-rotorcraft dynamic interface. Two disturbance fields are considered: an isolated straight vortex that represents the canonical vortex that results from the corners of flat top ships in oblique wind-over-deck conditions and a horseshoe vortex derived from a nondimensional characterization of the time-averaged flow observed aft of a simplified ship superstructure. Rotorcraft models considered include: an analytical blade element theory-based rotor model, where the disturbance velocities are integrated over the rotor, and a coupled blade element / free wake flight dynamic model of the full UH-60 aircraft, which is used to perform time-marching simulations with the disturbances modeled as a frozen field that is fixed in space and not interacting with the aircraft (one-way coupling), and as a distorting field (two-way coupling). Analytical thrust responses of four idealized rotors to the disturbance associated with the horseshoe vortex suggest the existence of a scaling relationship between nondimensional blade loading and the product of nondimensional vortex width and strength. Time-averaged rotor thrust responses to the disturbance induced by the isolated straight vortex indicate significant distortion of the rotor wake and disturbance field at low forward speeds, with the frozen field assumption indicating greater disk tilt responses. Modeling the horseshoe vortex disturbance as a frozen field generates a greater rotor response than in the distorting case. Inclusion of full aircraft dynamics attenuates rotor responses to the disturbances when compared to isolated rotor responses.