The rotor wake above a tiltrotor airplane-model in hover

The mean and time-dependent rotor-wake flow over a XV-15 tiltrotor model in hover was studied using stereoscopic particle-image velocimetry. Results from the full-span model were compared to those obtained from a semispan model simulated using an image-plane. Additional data were measured in the wakes of single and twin rotors without the airframe present, with and without an image plane. Rotor wake contraction was considerably reduced over the semi-span model when compared to the full-span case. This is attributed to pressure increases above the fuselage due to the elimination of the lateral flow fluctuations by the solid image plane. In the absence of the image-plane, the correlation between pairs of tip vortices from opposing rotors was negative (i. e. vortices were moving in the same direction) near the centerline of the aircraft. In the absence of the airframe model, the wake flow of two side-by-side rotors was almost indistinguishable from that observed on a single rotor in conjunction with an image plane.