Towards In-Flight Measurements of Helicopter Blade Tip Vortices

In the framework of the AIM project the near field of the blade tip vortex of a full-scale helicopter in simulated hover flight was investigated by combining three-component Particle Image Velocimetry and Background Oriented Schlieren measurements. The velocity field measurements in the range of wake ages of 1° to 30° in azimuth provided a reference for a quantitative analysis of the Schlieren results yielding vortex core density estimates. Ongoing vortex roll-up was observed at 1° while considerable aperiodicity was persistent thereafter. The vortex parameters for vortices older than 1° were consistent with the Scully vortex model. However, the particular challenges of full-scale, outdoor testing, especially the limited spatial resolution and aperiodicity effects, yielded a higher level of measurement uncertainty as compared to sub-scale experiments