Effect of Sloped Terrain on In-Ground-Effect Hover Performance for an Isolated Rotor

Abstract

The present work conducted performance testing using a laboratory-scale isolated rotor operating over a ground plane mounted to a six degree-of-freedom motion platform to simulate in-ground-effect operations over sloped terrain. The rotor utilized a pair of 1:13.24 scale OH-58C blades, and performance measurements were collected using a six-axis load cell to which the rotor was mounted. Seven ground plane angles ranging from 0–18 deg, five collective blade pitches ranging from 0–8 deg, and 15 hub heights ranging from a nondimensional hub height, z/R, of 0.6 to 2.0 were tested. Additionally, the rotor was operated out-of-ground-effect for collective blade pitches ranging from 0–12 deg in increments of 1 deg in order to compare in-ground-effect and out-of-ground-effect hover performance. In-ground-effect hover over sloped terrain was found to have over a 7% reduction in performance as compared to hover over level terrain at low hub heights and large ground plane angles. In-ground-effect hover over sloped terrain was also found to require 2% more power than hover out-of-ground-effect at high hub heights and large ground plane angles. Finally, a semi-empirical model for hover performanceover sloped terrain was developed on the basis of the classic Cheeseman and Bennett ground effect model for level terrain. The coefficients obtained from this model were found to change in a consistent manner as both ground plane angle and blade loading coefficient changed, which suggests that the model could be used for future performance predictions for hover over sloped terrain.