The present work reports the results of a research investigation into the performance effects of hovering over sloping terrain as characterized in both laboratory-scale testing of an isolated rotor as well as flight testing of a UH-72A helicopter. The design of the experiment included variation in the slope magnitude, rotor hub height, thrust/power setting, and aircraft orientation with respect to the slope (for the flight test data). In addition to performance measurements, flow field measurements of the laboratory flows were computed via particle image velocimetry, and rotor wake visualizations from flight testing were created via background-oriented Schlieren post-processing techniques. The results show that hovering over sloped terrain results in performance effects that are both nonintuitive and operationally significant. Additionally, the hover performance data collected over level terrain show that in-ground-effect hover performance is truly a complicated, interdependent relationship that includes multiple specifying conditions; it is not simply a one-dimensional function of hub height above the ground plane. Lastly, a comparison of the flow field measurements from laboratory testing and the rotor wake visualization from flight testing is presented to speculate on the fundamental flow field mechanisms at play and propose an approach for a more rigorous reconciliation of these two data sets in future studies.
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