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

The main rotor wakes of the free-flying DLR test helicopters Airbus Bo105 and EC135 were investigated in ground effect during hover, vertical takeoff, and forward flight. A high-speed schlieren system tracked the blade tip vortices at about 60 images per revolution. In addition, a constant temperature anemometry system utilized arrays of fiber film sensors, providing velocity statistics and spectra in the rotor flow. The overall wake structure agreed to preceding studies, but the velocity profiles and tip vortex trajectories were sensitive towards the environmental wind conditions. The tip vortices were observed in the schlieren images up to an age corresponding to about two revolutions below the rotor plane, before developing instabilities and falling below the detection limit. Systematic vortex pairing was found for the Bo105 but not for the EC135. The remnants of the tip vortices were identified further downstream in the wake by means of rotor-harmonic velocity signals, but they play a minor role in comparison to broad-banded turbulent fluctuations with a Kolmogorov-like spectrum. For vertical takeoff cases, the rotor wake had a hover-like structure until breaking down into low-frequency oscillations when exceeding a hub height of approximately 1.4 rotor radii. In forward flight, different types of wake velocity footprints were categorized on the basis of the normalized advance ratio. Blade–vortex interactions were found in the frontal area of the main rotor planes and between the main rotor tip vortices and the Bo105's tail rotor. The interactions prevent a further evolution of the tip vortices.