Experimental wake characterization of free-flying helicopters in ground effect

Kurzfassung

The rotor wake of a helicopter operating in ground effect is a complex three-dimensional flow field that can impact the operational safety of the aircraft, for example, by interacting with loose ground sediment. The aim of the present thesis is to analyze the structure of the wake generated by helicopters operating close to the ground and to address effects related to both varying flight states and rotor scales. Two experiments were conducted using optical flow field measurement techniques to capture the wakes of both a sub-scale and a full-scale helicopter in free flight. The focus of the model helicopter experiment was the quantification of the rotor wake in different flight states. Both quasi-steady and unsteady maneuvering flights have been investigated using time-resolved stereoscopic particle image velocimetry (PIV). The observed changing flow patterns, such as recirculation and ground vortex flow, in forward flight at low advance ratios were found to be in good agreement with existing wind tunnel data. Parameters describing general wake structures and individual blade tip vortices showed a significant dependence on the forward flight velocity. Landing approaches were performed and showed large vortical structures close to the rotor disk, which contained distinctly higher velocities and momentum fluxes than expected from quasi-steady conditions at the same advance ratios. A vertical takeoff maneuver with a rapid increase of collective pitch led to the bundling of blade tip vortices into a "starting vortex", with circulation values up to 6 times higher than for an individual blade tip vortex. To assess the scalability of the results regarding full-scale helicopters in realistic flight conditions, the wake of a free-flying Bo 105 test helicopter was measured. A time-resolved background oriented schlieren (BOS) system was used to visualize blade tip vortices for a large portion of the rotor wake, and a stereoscopic PIV system provided spatially resolved velocity data close to the ground. The high sensitivity of the BOS system enabled the detection of vortices up to an age of ψ = 630◦. Different instability mechanisms, such as long-wave, short-wave, and pairing instabilities with varying intensities have been observed in different flight conditions. A quantitative analysis of vortex locations showed a periodic variation due to interactions of consecutive vortices resulting in vortex pairing. The instantaneous velocity data was used to detect individual blade tip vortices with ages above ψ > 450° close to the ground and to extract vortex parameters, with the most concentrated vortices being detected during takeoff maneuvers. General wake patterns and characteristics of the wake outwash close to the ground were investigated by means of averaged velocity fields and were found to be in good agreement with the model helicopter experiments. As a result, sub-scale investigations seem to be a valid tool to predict overall wake parameters, despite the increased complexity of the blade tip vortex system of a full-scale helicopter.