Vortical lift on retreating rotor blades at high advance ratios

Abstract

A retreating helicopter rotor blade encounters reverse flow over large portions of the rotor disk in high advance ratio flights. Measurements of flow velocity and loads are described for a two-bladed teetering rotor for advance ratios ranging from 0.6 to 1.2 in a low speed wind tunnel. The tip vortex trajectory is separately mapped. A pressure extraction technique is used to map the static pressure field on the lower surface of the rotor blade near sharp edge. The reverse flow region shows a proof of an attached, helical Sharp Edge Vortex (SEV). The proximity of the SEV to the rotor blade creates sharp excursions in the static pressures at the sharp edge. The negative lift generated by the vortex is comparable to the sectional bound circulation. The presence of the SEV explains the shift in the center of pressure from 75 to 70% of chord. Stereo particle image velocimetry, a pressure extraction technique, and load measurements provide closure in estimating aerodynamic loads. As the blade approaches 270 $$^\circ$$ azimuth vortex strength stops growing and its core detaches from the blade surface but the vortex stays close to the blade. Flow on blunt edge of the blade in reverse flow is found to be largely attached, unlike expectations of vortex shedding.