Wake Characterization of a Free-Flying Model Helicopter in Ground Effect

Measurements of the flow field around a free-flying model helicopter in ground effect were performed for both quasi-steady and unsteady maneuvering flights using stereoscopic particle image velocimetry. Changing flow patterns like recirculation and ground vortex flow were observed for forward flight at low advance ratios and found to be in good agreement with existing wind tunnel data. Parameters describing both general wake structures and single blade tip vortices showed significant dependence on the forward flight velocity. Landing approaches were performed, and large vortical structures were observed close to the rotor disk, which contained distinctly higher velocities and momentum fluxes than expected from quasi-steady conditions at the same advance ratios. For a vertical takeoff maneuver with a rapid increase of collective pitch, the bundling of blade tip vortices into a "starting vortex" was observed, with circulation values up to six times higher than for a single blade tip vortex.