Correlation of helicopter rotor tip vortex measurements

The roll-up and development of the tip vortices generated by subscale helicopter rotors were measured and analyzed. The measurements were performed on one- and two-bladed hovering rotors using three-component laser Doppler velocimetry. A trip strip was used to simulate boundary-layer transition at the higher Reynolds numbers typical of the full-scale rotor. The detailed velocity field of the tip vortex was investigated, a strong self-similarity of the swirl velocity profile being found. The measurements suggest a general logarithmic growth of the viscous core, which for older vortex ages is proportional to the square root of time (vortex age). Measurements from other helicopter rotor and fixed-wing experiments are used to support the observations.