VISUALIZATION AND MEASUREMENT OF HELICOPTER ROTOR FLOW WITH SWEPT BACK TIP SHAPES AT HOVER FLIGHT USING THE FLOW VISUALIZATION GUN TIME LINE TECHNIQUE

The Flow Visualization Gun (FVG), a novel time line visualization tech nique, has been used to investigate the flow field of a helicopter rot or with swept back tip shape in hover flight condition. After introduc ing the FVG-technique, the paper presents some visualization photograp hs of the rotor blade tip vortices and the rotor downwash. Using ortho gonal sets of flow photographs and digital image analysis, the 3-dimen sional time line displacement within the flow and the tip vortex struc ture are determined. The data of 16 time line experiments are interpol ated in space and time to obtain velocity data on an evenly spaced 3-d imensional grid. Vorticity contour plots of the flow field show the co mplex arrangement of the tip vortices of the blade itself and of the p receding blade and some additional vorticity in the direct wake which may form a secondary vortex. Understanding and control of this vortici ty distribution is important for the design of new efficient tip shape s. While the FVG technique has been used so far for qualitative invest igation of complex flow patterns at local velocities of up to 20 m/s, a good comparison to laser velocimetry data validates the technique as an interesting tool for both qualitative and quantitative investigati on.