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June 2008 edition

Brown-out – flow visualisation using Fluent VBM

by A.N. Modha, T.A. Blaylock, W.Y.F. Chan, QinetiQ U.K. Ltd

Helicopter brown-out presents a considerable challenge to safe operation near the ground. During brown-out, a sand cloud engulfs the helicopter and causes the pilot to lose visual awareness of the operating environment which can lead to accidents. This paper presents the results of a study of the flow field when a helicopter flies close to the ground. The aim is to improve the understanding of the aerodynamics of the phenomenon. A simulation of rotor wake using CFD calculation is presented. The solver used is Fluent with the rotor flow calculated using the Virtual Blade Model (VBM). The purpose of this study is to examine the effectiveness of using commercial CFD software. The study demonstrates the existence of a substantial ground vortex which is thought to cause brown-out. This article is an edited version of the paper prepared by the authors and does not include the figures showing the results of the experiments.

When a helicopter is flying near the ground in sandy terrain, as would be found in semi-arid or arid environments, the wake of the helicopter rotor kicks up a cloud of sand in a phenomenon called brown-out, so called as it obscures the pilot's view. It is analogous to the white-out condition that occurs in snow/ice conditions. The particle cloud blinds the pilot's view and so reduces the visual cue....

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