Unsteady Aerodynamics of Rotorcraft in Ground Effect

Abstract

This paper brings together experimental and computational work done in the study of low speed ground effect of rotorcraft. The paper recaps previous work done experimentally to study the unsteadiness in the rotorcraft flowfield using flow visualization and hot wire measurements. These experimental results are compared with computational results obtained using a classical vortex method coupled with an image !system to simulate the ground. The comparisons are adequate; however, the computations use a lifting-line approach leading to some discrepancies in time traces of velocities in the wake. The paper also presents experimental results obtained in measuring fuselage loads in ground effect. I. Introduction The flowfield around a rotorcraft flying close to the ground presents a fluid dynamics problem where large fluctuations are observed under nominallj quasi-steady flight conditions, separated by seemingly-random intervals. In ground effect (IGE) conditions, the wake i)f a helicopter rotor interacts with the ground which causes significant perturbation to the flow near the rotor blades, is well as the rest of the craft. Interactions between the main rotor wake and the ground have been associated with :he formation and passage of a ground vortex in transitional flight. The basic question that arises I rom the reported flight test results, is whether 1. The unsteadiness arises from the craft interacting with different regions of an otherwise quasi-steady flowfield, due to changes in wind direc tion, ground clearance or aircraft speed; 2. Or whether long-period fluc tuations are generated in an otherwise periodic flowfield under fixed flight conditions. Detailed experimental studies of the rotor wake and ground vortex were performed with an isolated model rotor above a static ground plane at low advance ratio, and various ground heights. This study aims to quantify time scaies of unsteadiness by bringing together results from pulsed laser sheet flow visualization, hotwire anemometry, and fuselage force measurement. In pl evious work with this experimental set-up, it was shown that the wake was steady enough in the absence of ground effect, to enable clear quantification of the unsteadiness caused by ground effect. This unsteadiness was quantified using laser sheet imaging of vortex dynamics. It was then shown that large transient velocity fluctuations occurred with long intervals, in the ground-vortex and in the rotor inflow regions. The present paper investigates a decision point in the investigation of unsteady ground effect. It is argued that transients could occur due to two basically different situations, or a combination of these situations