An Analysis of Helicopter Blade-Vortex Interaction Noise with Flight Path or Attitude Modification

This study examines the effects of flight-path modification, and attitude modification through the introduction of an X-force, on helicopter blade-vortex interaction 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, and the aeroacoustic code WOPWOP is used to calculate the resulting BVI noise. The results in the present study indicate that while regions of high BVI noise may be qualitatively identified using simple momentum theory based analyses (as previously done in the literature), the maximum noise levels, as predicted using the free-wake and WOPWOP, are at different flight conditions 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 interactions become dominant, even as others recede in importance. Thus, fairly significant changes in shaft angle may produce relatively small reductions in the total BVI noise. Similar observations can be made for changes in advance ratio. It is seen that as different interactions become dominant, the directionality of the total radiated noise changes significantly. Introducing an X-force in the drag direction tilts the rotor disk forward, increases the downwash due to the free-stream and pushes the vortices lower, relative to the baseline. In contrast, a propulsive X-force moves the vortices upward, relative to the baseline. Once again, different interactions are seen to be dominant for different levels of X-Force; and this changes depending on the flight path. Thus, while a drag force was found to reduce BVI noise for a certain flight path, a propulsive force was found to be beneficial for another flight path, and at yet another flight path, neither drag nor propulsive forces (of reasonable magnitude) produced any significant reductions. Previous momentum theory based analyses, which suggested that either a drag or a propulsive force could change the inflow conditions and reduce BVI noise had not considered the emergence of “other” BVI events that were not acoustically significant for the baseline aircraft (no X-force). This makes exploiting the X-force control concept more challenging (with the magnitude and direction of the “optimal” X-Force varying with change in approach path), even though reductions of more than 10dB in peak BVI noise levels certainly appear to be possible.