This study examines the effects of flight‐path modification and attitude modification through the introduction of an X‐force on helicopter blade‐vortex interaction (BVI) noise in low‐speed descent. A free‐wake analysis is used in conjunction with a
rotor aeroelastic model, to accurately determine the positions of the vortices relative to the blades and the unsteady impulsive BVI loading. The aeroacoustic code WOPWOP is used to calculate the resulting BVI noise. The results in the present study indicate that while regimes of high BVI
noise show general qualitative correlation to regimes of zero inflow from simple momentum theory based analyses, the maximum noise levels are actually seen at different advance ratio and rotor disk tilt combinations from those at which momentum theory predicted zero mean inflow. As the rotor
shaft angle is varied (corresponding to change in descent rate) different advancing side BVI events become dominant, even as others recede in importance. Thus, small to moderate changes in shaft angle have little effect in reducing the total BVI noise. It is also seen that as different BVI
events become dominant, the directionality of the total radiated noise changes. An X‐force in the drag direction tilts the rotor disk forward, convects the vortices lower relative to the baseline. In contrast, a propulsive X‐force convects the vertices upward relative to the
baseline. For a high‐noise baseline condition, with one or more parallel vortices in the rotor plane either a propulsive or drag X‐force does not, in equal measure, reduce noise. This is again related to different interactions dominating for different levels of X‐force;
these changes depend on the flight path. Thus, while a drag force was found to reduce BVI noise for a 0.1875 advance ratio moderate glide path, a propulsive force was found to be beneficial for the same advance ratio and a steeper glide path. This makes exploiting the X‐force control
concept more challenging. Even so, reductions of more than 10 dB in peak BVI noise levels certainly appear to be possible using X‐force levels of the order of about 5% of the vehicle weight.
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Document Type: Research Article
Rotorcraft Center of Excellence, Department of Aerospace Engineering, The Pennsylvania State University, University Park, PA
Publication date: 2005-04-01
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