Three-dimensional Reconstruction of Blade Tip Vortices of a BO 105 using a Multi-camera BOS System

Noise and structural vibrations in rotorcraft are strongly influenced by interactions between blade tip vortices and the structural components of a helicopter. As a result, the three-dimensional localization of vortices is highly desirable, especially for the case of full-scale helicopters under realistic flight conditions. In the current study, we present results from a flight test with a full-scale BO 105 in an open pit mine. A background-oriented schlieren measurement system consisting of ten cameras was used with a natural background to visualize the vortices of the helicopter under maneuvering flight. Vortex filaments could be visualized and extracted up to a vortex age of 360°. Vortex instability effects were found for several flight conditions. Due to the large test dimensions, an iterative approach based on points on the helicopter fuselage was applied for the camera calibration. Point correspondence between vortex curves in the evaluated images was established by means of epipolar geometry. A three-dimensional reconstruction of the main part of the vortex system was carried out for the first time using triangulation-based stereo-photogrammetry. A quantitative evaluation of the 3D vortex system was carried out, demonstrating the potential of the multi-camera background-oriented schlieren measurement technique for the analysis of blade-vortex interaction effects in rotorcraft.