Flow Measurement Techniques for Rotor Wake Characterization on Free-Flying Helicopters in Ground Effect

Abstract

Helicopter operation close to the ground is of interest to the rotorcraft community as the rotor wake can lift up sediment particles from the ground, causing deteriorated visual conditions for the pilot - a phenomenon that is referred to as brownout. The impact of unsteady maneuvering flight on brownout is still unclear and quantitative flow field measurements are needed to better understand the underlying physical effects. The present work investigates particle image velocimetry (PIV) and the background-oriented Schlieren (BOS) technique for studying the rotor wakes of free-flying helicopters close to the ground. Two flight tests with a model helicopter and a full-scale BO 105 helicopter were conducted and time-resolved stereoscopic PIV and BOS data were acquired during steady and unsteady flight maneuvers in ground effect. This paper discusses the measurement setups realized in these flight tests, technical challenges of implementing the measurement methods in a flight test environment, and example flow field results. The presented measurement data and evaluated results demonstrate the successful first application of PIV, time-resolved BOS, and combination of both measurement techniques on free-flying model- and full-scale helicopters. This novel application of full-scale flow measurements enables a more complete investigation of flow phenomena in the wake of free-flying helicopters close to the ground and will contribute to better understanding rotor wake interactions such as brownout.