Tail-shake risks assessment & mitigation by wind-tunnel tests on air-intake installation on a heavy-weight helicopter configuration

Abstract

In this work, the key role of the upper-deck design including engine installation as a potential source of tail-shake is at focus. The work is based on a Wind-Tunnel Test (WTT) campaign performed at the Airbus Helicopters’ Marignane wind tunnel facilities on a high-fidelity minibody fuselage at scale 1:3.5 representing a generic heavy-helicopter upper deck. Two different engine intake installations for a Power Unit (PU) have been investigated; in a first configuration, the air intake is implemented at the pylon-fairing trailing edge. The second configuration consists in positioning two air intakes on each side of the pylon fairing, close to the maximum cross-section location. Different measurement methods to evaluate aerodynamic interactions and wake sources are proposed. They consist in flow-separation assessments from surface oil flow visualizations, time-resolved particle image velocimetry (PIV) measurements, as well as unsteady skin-pressure measurements at the cowlings. Tail-shake related indicators are then proposed. Basically, a configuration which produces strong vortices characterized by a broadband spectral signature is believed to gather all the conditions for tail-shake to emerge. The flow over the baseline configuration (i.e. without air intake) is first analyzed for various combinations of angle of attack and sideslip, highlighting four different areas of flow separation at the cowlings. The complex flow topology around the upper deck is then assessed, which includes a spectral analysis of the flow unsteadiness in the time-resolved PIV planes. The influence of the air intakes (operating or not) is then evaluated. When located at the pylon-fairing trailing edge, the impact of the operating air intake on the engine-cowlings dynamic pressure and the flow-field topology is spectacular. The air intake is shown to be responsible for the generation of an intense broadband wake interacting with the pylon-fairing lip vortices, which is believed to be a potential severe source of tail-shake. The second air-intake configuration is also not favorable, because it requires enlarging the pylon fairing by 100 mm to integrate ducts from the inlets to the PU, which generates an intense wake similarly to a blunt body. At last, a mitigation mean is proposed for the first configuration. It demonstrates a significant reduction of the wake intensity and broadband signature at the source.