Time variation of rotor broadband noise

Rotor broadband noise studies typically analyze the broadband spectrum over a time scale on the order of the rotor period. However, since the time scale of turbulence generating the broadband noise is much smaller than the rotor period, it is physically-valid to construct a spectrum using a time period much less than the rotor period. The objective of this paper is to explore the importance of this unique viewpoint on rotor broadband noise prediction. Modulation of the blade self-noise spectrum during a blade passage or rotor period was observed for rotors in hover and forward flight-in both noise predictions and flight test data. Even for axisymmetric deterministic loading, modulation of the spectrum was still observed, due to convective amplification, retarded-time effects, and the blade position changes relative to the observer. Time variation of noise directivity was found to be dominant towards the advancing side of the rotor, and within ±30^◦ of the rotor disk plane. Overall, the time variation of the broadband self-noise spectrum was found to be important in the test cases, and is expected to be an important consideration for helicopters and urban air mobility (UAM) aircraft.