Blade Tip Vortex System of a Rotor with Cyclic Pitch

To characterize the vortex system of a helicopter rotor with cyclic pitch, an experimental investigation is carried out at the rotor test facility of the DLR, German Aerospace Center in Gottingen. A test case with attached flow conditions is chosen to study the blade tip vortex aerodynamics on a four-bladed rotor. The resulting azimuth-dependent vortex system of the rotor is analyzed. In addition to velocity fields acquired by a particle image velocimetry system, the current investigation also uses a background-oriented schlieren setup to capture the density gradients caused by the blade tip vortices. These optical measurement techniques enable the reconstruction of the azimuth-dependent vortex system for vortex ages up to 70 deg. The pitch variation, and thus the change in lift, is found to be the main influence on the characteristics of the tip vortex system. The topology of the vortex, the interconnected shear layer, and the position of the vortex with respect to the blade tip appear to be highly pitch dependent. An analysis of the blade tip vortices is carried out, focusing on the swirl velocity, core radius, and circulation distribution. The results are compared to previous investigations at the same test facility and a numerical simulation.