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Meeting	ACGS Committee Meeting 101 - Salt Lake City - March 2008
Agenda Location	6 SUBCOMMITTEE D - DYNAMICS, COMPUTATIONS, AND ANALYSIS 6.2 Modeling Airwake Disturbances on Rotorcraft in Shipboard Operations
Title	Modeling Airwake Disturbances on Rotorcraft in Shipboard Operations
Presenter	Joe Horn
Affiliation	Penn State
Available Downloads*	presentation
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Abstract	Several methods for modeling and simulation of ship airwake disturbances on rotorcraft were presented. In one-way coupled simulations, the ship flowfield is calculated off-line using time-accurate computational fluid dynamics (CFD). The time history of the airwake velocities are then injected into a blade element simulation model of the helicopter by adjusting the local velocities at various rotorcraft components based on the location of the component within the domain. The solutions are one-way coupled since the helicopter and its rotor downwash do not influence the flowfield. Simulations were performed for the UH-60A operating off of the LHA ship with the aircraft controlled by a pilot model. Comparisons with flight test data show reasonable prediction of pilot control activity with a frequency range of 0.2 to 2 Hz. Fully coupled simulations were also investigated, in which the flight dynamics model is run in parallel with the CFD calculation and the effect of the rotor loads are modeled in CFD using source terms. The CFD velocity field then replaces the rotorcraft inflow model. Preliminary results showed that the coupling effects had relatively little impact on pilot control activity for the LHA approaches. A method for modeling airwake disturbances as a stochastic process (similar to traditional atmospheric turbulence models) was also investigated. The spectral properties of the airwake disturbances are identified through simulation and linear filters were derived to fit the spectrum of these disturbances. Simulation results show that the stochastic model results in similar pilot control activity, when the filters are driven by unity white noise and the disturbances are injected into the non-linear simulation of the UH-60A with pilot model. Finally, a flight control design methodology for reducing pilot workload during shipboard operation of helicopters is presented. The objective of the control synthesis is to improve the gust rejection properties of the aircraft when operating in an unsteady ship airwake. An airwake compensator is designed to integrate with a standard model following control scheme. The spectral properties of the ship airwake are used in the design of the airwake compensator. The model following controller and airwake compensator are implemented in a simulation model of a UH-60A Black Hawk operating near an LHA-class ship. Simulations were performed for approaches and hovering over the deck. The results show that the airwake compensator results in significantly lower angular motion of the aircraft when compared to a basic model following controller without the airwake compensator.