Blade tip vortex detection in maneuvering flight using the Background Oriented Schlieren (BOS) technique

Abstract

The background oriented Schlieren (BOS) technique was used to visualize the blade tip vortices of a Eurocopter AS532UL Cougar helicopter in maneuvering flight. The test program included a wide range of maneuvers covering a large part of the flight envelope such as hover, fast forward flight, flare maneuvers and high speed turns. For selected flight conditions, the aerodynamic results are presented here. It is shown that with the reference-free BOS method, the detection of vortex filaments up to vortex ages of 540° is possible. The visualization of the vortex system is used to detect aerodynamic phenomena like blade vortex interactions (BVI), vortex airframe interactions and the occurrence of smooth sinuous disturbances. A detailed description of the applied reference-free BOS setup is given and the suitability of different natural backgrounds for the BOS method is analyzed.