A stochastic model of unsteady ship airwake disturbances on rotorcraft

Shipboard operations present challenging piloting tasks due to unsteady ship motion and airwake behind the ship. Researchers have been working to accurately simulate the ship airwake through computational fluid dynamics (CFD). Although CFD can create high fidelity solutions, they often require significant computing power to implement in real time simulation applications. In an effort to reduce computing power and to more directly tie the airwake model to experimental flight data, this study presents a stochastic airwake model. The airwake model relies on stochastic filters that are derived from the vehicle response and pilot control activity when hovering at a particular location relative to the ship deck. When these filters are excited with a white noise input, the filters will recreate similar disturbances to the aircraft as the original unsteady airwake model. Only the stochastic filter coefficients have to be stored into memory, instead of 3-axes of wind velocities at every CFD grid point for the length of the solution. In this study, the stochastic filters are derived from flight simulations using a CFD database. However, these filters could also be derived from flight test data, thereby providing an independent experimental verification of the CFD based simulations. An initial feasibility study developed a stochastic model that was tested in two simulation environments. Results show similar pilot control activity when the power spectral density of the stick inputs are examined. The study suggests the stochastic airwake model could be used in real time simulation applications as an alternative to current CFD simulations.opyright