TIP VORTEX GEOMETRY OF A HOVERING HELICOPTER ROTOR IN GROUND EFFECT

The wide-field shadowgraph method has been used to photograph the tip vortices of a hovering helicopter rotor in ground effect. The shadowgr aphs were used to obtain quantitative measurements of the rotor tip vo rtex geometry both in and out of ground effect. Many important phenome na are visible in the rotor wake using this method. These include the variation in descent and contraction rates of the tip vortices in grou nd effect, and the interaction between tip vortices in the far wake. T he tip vortex geometry from the shadowgraphs is compared with the tip vortex geometry predicted using a free wake hover performance analysis . The free wake analysis accurately predicts the tip vortex geometry b oth in and out of ground effect. Performance data from the test is com pared with the performance predicted using several methods, including the free wake analysis. All methods provided reasonable predictions of the helicopter performance in ground effect.