EXPERIMENTS ON THE ORIGIN OF TIP-VORTICES

Measurements of the structure and evolution of the tip vortex from a rotor in low-speed forward flight are presented. On the advancing blade spanwise flow on the pressure side of the blade is comparable in magnitude to the inflow. Core circumferential velocity profiles show multiple features during formation. Trajectories and peak core velocities of the tip-vortex during its formation on top of the blade are presented. The peak core circumferential velocity is constant during vortex formation. Peak core axial velocity tends towards tip-speed near the blade tip. In the near wake, the variation in core axial velocity exhibits a periodic behavior. Circulation measurements from different rotary-wing tests are correlated with an empirical model that was developed to relate the tip-vortex circulation to the blade parameters. The trailed circulation is seen to be only about 40 % of the bound circulation computed using the standard assumptions for calculation of blade loads