Measurement Techniques and Data Evaluation for VTOL Outwash Maps

The outwash flow of a VTOL aircraft in near-ground operation is a serious risk for surrounding objects or personnel, and has been investigated for a long time. The current paper contributes to this topic by revisiting quantitative outwash measurement techniques and evaluation strategies suitable for full-scale flight experiments. An array of purpose-built ultrasonic anemometers, a pitot tube rake, and fiber-film sensors were applied during outwash tests with a hovering Eurocopter EC135, complemented by accompanying numerical simulations with the Vorticity Transport Model (VTM). A focus is set on an analysis of the flow unsteadiness, revealing large fluctuations at low frequencies which require careful data post-processing. The fluctuations limit the application of pitot tubes to measure the mean flow due to their angular sensitivity, and ultrasonic anemometers are recommended as a particularly suitable and convenient measurement method with a sufficient frequency response. Furthermore, the experimental and numerical results are compared to semi-empirical outwash models, and the importance of simultaneous measurements of the ambient wind conditions is demonstrated.