Wake Unsteadiness and Tip Vortex System of Full–Scale Helicopters in Ground Effect

Abstract

The main rotor wake of free–flying helicopters in ground effect was investigated during a measurement campaign with the DLR test rotorcraft Airbus Bo105 and EC135. An advanced measurement system combined high–speed schlieren imaging with point–wise constant temperature anemometry for a comprehensive analysis of the flow. In hover conditions, the wake structure was similar to earlier data, in spite of the environmental conditions, which were found to have a significant influence. The schlieren system tracked the blade tip vortices convecting along the slipstream boundary. Depending on the rotorcraft, the vortices developed either instabilities of individual filaments or cooperative pairing instabilities. Both instability types occurred one to two revolutions below the rotor plane, and the decayed tip vortices fell below the schlieren sensitivity limit soon after. The velocity footprint of the vortices was detected by fiber–film sensors further downstream along the slipstream boundary, but harmonic oscillations play a minor role in comparison to broad–banded turbulence with a Kolmogorov–like spectrum. The wake was found to be hover–like for vertical take–off cases until breaking down into low–frequency oscillations when exceeding a hub height of approximately 1.4 rotor radii. In forward flight conditions, blade–vortex interactions were found in the frontal area of the main rotor plane, and between the main rotor tip vortices and the Bo105’s tail rotor.