Determining the Acoustic Far Field for Multirotor Aircraft

Guidelines exist for the far-field acoustic measurement of conventional helicopters on the basis of prior experience. However, it is not clear that this experience can be translated to novel aircraft configurations having multiple rotors operating at different rotation rates. This paper develops a computational approach for predicting the onset of the far field for rotorcraft, which can be used to inform far-field noise measurement procedures for multirotor aircraft. The proposed procedure is applied to several configurations, including a helicopter main rotor, several small multirotor uncrewed aircraft configurations, and a lift+cruise electrical vertical take-off and landing aircraft. The proposed approach is found to be consistent with the conventional wisdom for helicopter rotors. When applied to small multirotor aircraft with varied sizes and numbers of rotors, the far-field distance was found to scale well with vehicle diameter rather than rotor diameter. However, this far-field distance scaling did not hold for the larger multirotor lift+cruise aircraft, due to the significantly different operating conditions of the rotors. The far-field distance for the lift+cruise aircraft was consistent for both hover and level flight conditions. These results suggest that different far-field measurement guidelines may need to be developed for different classes of multirotor aircraft.