Experimental investigation of tip anhedral effects on the aerodynamics of a model helicopter rotor in hover

This study experimentally investigates the effects of tip anhedral on the rotor aerodynamic performance and the tip vortex characteristics in hovering flight. A five-bladed scaled helicopter rotor with blades that have either rectangular (baseline) or anhedral tip geometries was used as the experimental model. Thrust and torque measurements were performed at the tip Mach numbers (Mtip) of 0.3 and 0.4 at five different pitch angles. In addition, flow field measurements via phase-locked particle image velocimetry technique were conducted at Mtip = 0.4 for a selected pitch angle. Thrust and torque measurements reveal that the anhedral tip configuration reaches a maximum Figure of Merit (FOM) value of 0.67 yielding approximately 5% FOM improvement when compared to the rectangular blade at Mtip = 0.3. It is observed that the tip anhedral changes the tip vortex trajectory by increasing the radial contraction of the wake and decreasing the average axial convection speed in the range of the wake age considered in this study. The meandering amplitude of the tip vortex increases abruptly after the passage of the proceeding blade in both cases yet a greater increase is observed in the anhedral tip configuration. Moreover, the axial convection of the previously shed tip vortices in the downstream direction is significantly slowed down during the course of the blade passage such that they reside close to the rotor disc in the anhedral case. The resultant proximity of the tip vortices to the blade with anhedral tip configuration promotes the induced flow in the radially inwards direction, which probably attenuates the strength of the newly forming tip vortex. Accordingly, the maximum tangential velocity and circulation of the tip vortex are reduced by 20% and 13%, respectively, when compared to the rectangular blade case. The reduced tip-vortex circulation is consistent with the increased FOM value. A good agreement was found with the Vatistas vortex model using n=3 for the rectangular blade and n=2.2 for the blade with tip anhedral