A computational study of helicopter rotor wakes and noise generated during transient maneuvers

In this study, three types of flight maneuvers, namely arrested descent, turns, and roll-reversal maneuvers, were examined using a rotorcraft aeroacoustics prediction model. Components of the model comprised a utility helicopter flight dynamics model, a time-accurate free-vortex rotor wake model, and a maneuvering rotor noise prediction code. All rotorcraft motions and rigid body blade motions were included in the respective models. This study emphasized the rotor wake geometry and its development during various flight maneuvers. In a transient maneuver, the rotor wake behavior is shown to be relatively complicated and in some cases the tip vortices "bundle" together. The interaction of the tip vortices with the rotor blades can create a significant increase in levels of the impulsive noise, but primarily when the interaction is nearly parallel to the blade. When a bundle of tip vortices interacts with the rotor in a near-parallel interaction, a dramatic increase in impulsive noise is observed over a large region under the rotor.