Rotor Aerodynamics in Ground Effect at Low Advance Ratios

TLDR

The study used flow visualization and force/moment measurements across varying advance ratios, heights, and collective pitches, conducting both steady‑state and translational‑acceleration experiments. Three flow regimes were identified: a recirculating flow at low advance ratios, a horseshoe vortex at intermediate ratios, and a fully downstream wake at high ratios, with significant deviations from classical ground‑effect theory and pronounced effects of translational acceleration on rotor forces and moments, indicating that ground‑effect magnitude depends on flight path. Author: (Author).

Abstract

Abstract : The results of an experimental study of the aerodynamic characteristics of a helicopter rotor operating in ground effect at low advance ratios are presented. Flow visualization studies were conducted along with measurement of the forces and moments acting on the rotor as a function of advance ratio, height above ground and collective pitch. Steady state experiments as well as non-steady experiments involving translational acceleration were conducted. Three distinct flow regimes were noted from the flow visualization studies. At the low end of the advance ratio range a recirculating flow was present, at intermediate advance ratios a horseshoe shaped vorted formed under the rotor, and at the high end of the advance ratio range studied, the rotor wake flows entirely downstream. At test conditions were the recirculating flow or ground vortex is present there are marked departures from classical ground effect theory. Translational acceleration was found to have a significant effect on the rotor forces and moments indicating that some appreciable time is required for the establishment of the ground effect flow field when a recirculating flow or ground vortex exists in the steady state. The results indicate that magnitude of the ground effects experienced by a helicopter are sensitive to the flight path of the helicopter. (Author)