Numerical Method for Predicting I.G.E. Hover Performance of a Lifting Rotor

Abstract

To clarify influences induced by non-uniform ground surface on I.G.E. hover performance of a rotor, a numerical prediction method is developed by combining a free-wake method with a panel method, where the most important feature is the ability to determine blade flapping motions to be consistent with the deformed wake geometry. The ground surface beneath the rotor is substituted for quadratic panels with unknown ground vortex strength which are determined by virtue of the non-penetration conditions at the collocation points. The rotor blades are meodeled by the lifting lines with a constant circulation which results in a wake structure to compose of deformed helical line vortices trailing from the blade tips. The numerical procedure is programmed as an interactive two-stage process, where the spatial arrangements of tip vortices are calculated at first by moving their nodal points with updated local velocities induced by the ground and trailing vortices and then proceed to the second stage where the equation of blade flapping motion is solved using averaged induced velocity distribution on the rotor disc. Iterations are executed until both the wake geometry and blade flapping motions are converged simultaneously. In this paper, we introduce typical numerical results obtained for a rotor hovering in close proximity above a uniformly inclined flat surface and discuss influences of the ground inclination angle and rotor height on the wake geometry, flowfields around the rotor and the rotor-induced torque. The ground interaction effect on the amplitude and phase angle of the blade flapping motion are also investigated, and their unique dependencies on the operating circumstances are clarified.