Effects of anhedral on S-76 hover aerodynamics

A systematic study of the effects of tip shape on hover performance of a baseline S-76 rotor has been conducted by a number of researchers worldwide. Rectangular planforms, and swept-tapered planforms with and without anhedral have been systematically studied. The simulations and the test data all indicate that rotors with an anhedral tip achieve the best performance in hover. In this work, the underlying physical mechanisms behind this improvement are explored. For a given thrust level, the figure of merit does improve as the anhedral angle is progressively increased. Our results indicate that rotors with anhedral tips experience a slightly greater radial contraction of the tip vortices, causing the tip vortices from the preceding blades to move farther inboard compared to other planforms. Our results also indicate that anhedral tip vortices are somewhat weaker, and more diffused. This was traceable to a smoother radial variation of the blade loading near the tip. Finally, rotors with anhedral tips had a more uniform inflow at the rotor disk. Additional parametric studies are in progress.