The influence of time-varying ground effect (e.g., induced by ship deck motion) or even of static, inclined ground planes (e.g., hillsides) on the flow field and on the rotor inflow in hover is not yet understood. Therefore, experiments and CFD simulations were performed to study the flow field below a two-bladed 0.8 m-diameter rotor in hover over a parallel and a 15 degree inclined ground plane at a height of one rotor radius above the ground plane pivot point. Particle image velocimetry measurements were used to measure the rotor wake, and CFD simulations were correlated to the experimental results. To investigate the flow field, instantaneous, phase-averaged, and time-averaged data were used. The flow field was found to be sensitive to the ground plane inclination angle. It was found that the inclined ground plane reduced the unsteadiness in the flow field. The phase-averagcd experimental results were predicted well by the numerical simulation. The computations captured the flow phenomenology well, but underestimated the influence of the inclined ground plane on the rotor inflow.
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