Helicopter noise reduction

BVI noise can occur on either the advancing or retreating side of the rotor disk and its directivity is characterized by the precise orientation of the interaction.

A tail-rotor which is recessed into the fairing of the tail (a fenestron) reduces the noise level directly below the aircraft, which is useful in urban areas.

In addition, the placement of the tail rotor within a shroud can prevent the formation of tip vortices.

In this yaw-control method, air is blown out of vents along the tail boom, producing thrust via the Coandă effect.

[2] Standard rotor blades are evenly spaced, and produce greater noise at a particular frequency and its harmonics.

Using varying degrees of spacing between the blades spreads the noise or acoustic signature of the rotor over a greater range of frequencies.