For military missions, the helicopter is a versatile aircraft with many benefits. However, the high levels of noise radiation enable large acoustic detection ranges. For mission success, knowledge of acoustic radiation and propagation is paramount. This paper describes the results of acoustic measurements for a helicopter that is typically used for operations by the German special forces. The measurements include identification of the global noise radiation during different flight conditions, determination of the time between acoustic detection and arrival at the target location, and determination of the acoustic detection distance. Different approaches and departure procedures were executed, and the acoustic radiation was measured. Results indicate that during approaches, the most noise is generated either during the transition from steady level flight to descent/deceleration or at the end of the flare procedure. For the helicopter considered here, a left turn departure generates more noise at a target location then a right turn departure.
W.F.J. Olsman (Wed,) studied this question.