AN EXPERIMENTAL INVESTIGATION OF BLADE-VORTEX INTERACTION AT NORMAL INCIDENCE

Blade-vortex interaction (BVI) near and away from the blade tip was experimentally investigated for normal incidence where the vortex is generally parallel to the rotor axis. The experiment was designed to be representative of the chopping of helicopter main rotor tip vortices by the tail rotor. Tip Mach number, radial BVI station, and freestream velocity were varied. Fluctuating blade pressures, far-field sound pressure level and directivity, velocity field of the vortex, and BVI angles were measured. BVI produced impulsive noise and impulsive pressures near the blade leading edge. The radiation pattern for BVI away from the tip was that of a dipole, with the direction of minimum radiation, previously found to be in the plane of the blade, rotated considerably in the direction of negative angle of attack. For BVI near the tip, the intensity of the interaction was reduced and the radiation pattern was more complex. Away from the tip, the peak amplitude of fluctuating pressure on the suction side of the blade (near the leading edge) was larger than that on the pressure side, as expected due to the effect of compressibility. This trend was reversed near the tip.