Measurement of model helicopter rotor flow velocities with a laser Doppler velocimeter

The velocity data of primary interest to the rotor aerodynamicist are the instantaneous local velocities at the blades and in the plane of the rotor disk. The accurate measurement of these velocities has always presented the rotor experimentalist with a dilemma - how to measure the flow at the rotor disk without interfering with it and without concern for the physical impact of the blades when passing the measurement point. It has been demonstrated in recent experimental work that the dilemma may be solved by use of laser Doppler velocimeter (LDV) equipment. In September 1973, LDV techniques were applied to a model rotor in a wind tunnel, and velocities were measured at several radial and vertical positions in the close proximity to a passing blade. This is believed to be the first successful application of an LDV system to measure rotor downwash velocities in a wind tunnel.