Noncompactness corrections to the Brooks, Pope, and Marcolini self-noise model for small rotor noise predictiona)

Accurate prediction of high-frequency broadband noise is essential for assessing the environmental impact of small uncrewed aerial systems (sUAS). The widely adopted Brooks, Pope, and Marcolini (BPM) airfoil self-noise model has been validated for rotorcraft and wind turbine applications but shows limitations for sUAS-scale rotors, partly due to its assumption of acoustic compactness (chord smaller than the wavelength of sound). This paper introduces a modification to the BPM model to account for noncompactness by incorporating an alternative directivity function and an exact numerical formulation based on Amiet's aeroacoustic transfer function (non-dimensional radiation integral). The modified models were applied to predict the noise from a hexacopter hovering at 20 and 40 feet and compared with outdoor measurements from a ground microphone grid. For all ground microphones, the median A-weighted SPL prediction error with the original BPM model was reduced from 8.7 dBA to 1.0 dBA at 20 feet and from 2.7 dBA to 1.8 dBA at 40 feet using the modified formulation. The modified models also showed substantially improved one-third octave spectral agreement, demonstrating the importance of noncompactness corrections for accurate sUAS trailing edge broadband noise prediction using the BPM model.