Airborne Application of the Background Oriented Schlieren Technique to a Helicopter in Forward Flight

Blade-tip vortices and their interaction with the helicopter blades play an important role in the generation of noise on rotorcrafts. Full-scale vortex visualization is essential for the understanding of these effects and the validation of sub-scale experiments and numerical simulations. In the present work, the reference-free Background Oriented Schlieren (BOS) method was used to visualize blade-tip vortices of a full-scale BO 105 helicopter. Two flight tests were conducted with a dual-camera BOS system. In the first experiment, the ground-based camera system was tested on the helicopter in hovering flight with an artificial background pattern. The main and tail rotor tip vortices and the exhaust gases from the helicopter engines were visualized during take-off and hovering flight. Main rotor tip vortices up to a vortex age of ψv = 450°, vortex instability effects and interactions with the tail boom were detected. In the second flight test, a modified version of the dual-camera BOS system was used to visualize the main rotor blade-tip vortices of the BO 105 during forward, curve and accelerating forward flight with velocities over the ground of 60 − 80 knots. The camera system was deployed aboard a microlight airplane flying above and to the side of the helicopter with fields and meadows serving as natural backgrounds. It was shown that, during curve flight, vortices up to a maximum vortex age of ψv = 630° at distances of up to one rotor diameter behind the rotor plane could be visualized in some parts of the rotor wake. For accelerating forward flight, blade-vortex interaction effects and deformations of the vortex system were observed. In general, vortex visibility varied greatly between different parts of the rotor wake and different measurement images. This mainly resulted from the background noise level, which can vary based on the suitability of the natural background for the BOS method. A variety of natural backgrounds was analyzed and compared to the artificial dot pattern used for the hover test. Image contrast, homogeneity and structure size were found to vary between different natural backgrounds and to be significantly lower than for the artificial background pattern, but still within a useable range. The visualization results obtained during the inflight measurements show that the reference-free BOS method is highly suitable for the detection of helicopter blade-tip vortices during full-speed forward and maneuvering flight, therefore removing the restrictions imposed by ground-based measurements.