Wake Displacement Modifications to Reduce Rotorcraft Blade-Vortex Interaction Noise

摘要

Increasing the vertical separation between the rotor and its wake can reduce the strength of blade-vortex interaction (BVI) noise. A simple, first-order analytical model is developed to estimate the vertical distance between the rotor and the wake at any location of interest in the rotor disk. This model is used as an indicator to evaluate the potential effects of added forces, descent angle, advance ratio, and thrust coefficient on the separation between the wake and the rotor with the aim of identifying strategies that maximize this distance at a particular rotor disk location. The analysis focuses on landing approaches for helicopters and tiltrotors operating near helicopter configuration. The most appropriate strategies to increase this separation at the location of interest are found to depend on whether the wake is initially above or below the rotor. Added drag, added downforce, shallower descents, lower advance ratios, and higher thrust coefficients are shown to be desirable if the wake is initially below the rotor plane at the location of interest; this is the case of tiltrotor approaches and some helicopter descent operations. On the other hand, added thrust, added lift, steeper descents, higher advance ratios, and lower thrust coefficients are usually more appropriate if the wake is initially above the rotor plane at the location of interest; this is the case for other helicopter descent operations.