PREDICTION OF UNSTEADY PRESSURE AND VELOCITY OVER A ROTORCRAFT IN FORWARD FLIGHT

The problem of predicting the periodic three-dimensional vortex-dominated flowfield around a rotorcraft in low-speed forward flight is studied using potential-flow methods, whose results are compared with surface pressure measurements and flow velocity measurements. The test case used is a two-bladed teetering rotor above a hemisphere-cylinder airframe in a wind tunnel. The dominant features of this problem are modeled by a lifting-line/lifting-surface rotor model with a free wake distorting in the presence of the airframe. The airframe flowfield is modeled using a source/doublet panel method. The instantaneous flowfield is computed at specified intervals of rotor azimuth, with the unsteady effects of blade motion added to the formulation. As a simpler alternative, modeling the energy addition at the rotor using actuator segments led to sucessful prediction of the time-averaged pressure field and the periodic velocity variations along the spine of the airframe. When a fully unsteady potential formulation is used, however, large differences appear between measured and computed periodic velocity at the sides of the airframe. These are due to inadequate understanding of interaction of the rotor tip vortices with the airframe surface. When measured velocity data are used in the vortex interaction region, the surface pressure is predicted quite well, even at the sides.