Unsteady Aerodynamics of Rotorcraft at Low Advance Ratios in Ground Effect
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The aerodynamic characteristics of rotorcraft flying at low speed close to the ground are investigated. This will help better understand and quantify the flowfield structures and unsteadiness associated with various in ground effect flight conditions. This study aims to separate out the various phenomena according to their causal factors. Experimental investigations first involved flow visualization, which helped in identifying the various flight regimes and in getting an approximate estimate of the unsteadiness. The problem was divided into its unsteady and quasi-steady aspects. Hotwire measurements were performed and the unsteadiness in the flow structure was quantified. It was found that there were long time scale and fluctuations in the upwind side of the rotor disk, with significant changes in the in and flow. On the quasi-steady side, the fuselage loads for two fuselage cross-sectional shapes were investigated both in and out of ground effect. The fuselage cross-section shape had a significant effect on the loads felt by the fuselage in ground effect. The power required for the experimental configuration was measured to provide a basis for comparison. Finally, the flowfield around the ground vortex was quantified, and the structure of the ground vortex was investigated using Particle Image Velocimetry. It was found that tip vorticity was ingested by the ground vortex and that the strength of the ground vortex was considerably more than the tip vortex.