Helicopter external noise radiation in turning flight: Theory and experiment

The external noise radiated by a Bell 206 Helicopter in steady, turning flight is analyzed using two different measurement systems: a boom mounted system attached to the helicopter and a ground measurement system oriented perpendicular to the helicopter's flight path. The resulting data set is unique in that it combines both, simultaneous in-flight noise measurements with ground noise measurements. Noise comparisons are made, using both microphone systems, for a range of flight conditions including: steady straight flight and steady turning flight both in level flight and in descents. It is shown that Bell 206B Blade Vortex Interaction (BVI) external noise gradually increases in level flight at moderate bank angles and with increasing rates of descent. A new analytical procedure is presented to develop Rotor Noise Model (RNM) noise hemispheres for turning flight, for a helicopter following a helical trajectory. The hemispheres are developed using the ground microphone array and depict the directivity of the noise. The method is applied to the Bell 206B data showing the potential of the analytical procedure, but that a more careful placement of the ground microphones is needed to insure well-developed hemispheres. Low ambient winds and careful piloting are also necessary for robust and accurate noise hemisphere development.