The effect of blade tip shapes on rotor in-ground-effect aerodynamics

High-speed videographic flow visualization and detailed two-component particle image velocimetry (PIV) measurements were conducted to examine the wake produced by a hovering single-bladed rotor as it interacted with a horizontal ground plane. The motivation of the work was to better understand the nature of the flow field at the ground and the possible aerodynamic mechanisms that create brownout dust clouds when rotorcraft take off and land over surfaces covered with loose sediment. Rotors with four different blade tips were tested: (1) A baseline rectangular tip, (2) A simple 20° swept tip, (3) A BERP-like tip, and (4) A slotted tip. Flow visualization was performed using a high-repetition rate Nd:YLF laser that illuminated appropriately seeded flows in radial planes, with imaging performed using a high-speed CMOS camera. PIV measurements were performed in regions near the blades and at the ground plane by using a Nd:YAG laser with a CCD camera. Measurements as functions of wake age were obtained to examine the morphology of the vortical rotor wake during its interaction with the ground. The results showed that the wake was subjected to powerful curvature and straining effects as it interacted with the ground plane and was deflected into a radially outward direction along the plane. Reintensification of the tip vortices during the interaction caused them to remain very distinct features in the flow near the ground to as old as six or more rotor revolutions. The unsteady outward flow over the ground plane was shown to have similarities to a classical turbulent wall jet, especially further away from the rotor. Flow measurements were obtained deep into the boundary layer region at the ground, and in some cases into the laminar sublayer. The results showed certain common flow features between the four blade tips, but also differences in the flows that may ultimately affect the problem of brownout. The slotted-tip was shown to be particularly effective in diffusing the tip vortices and reducing the overall intensity of the fluctuating aspects of the flow at the ground.