While experimental studies typically ensure the proper scaling of the advance ratio, testing under ambient conditions often leads to a reduced free-stream Mach number and, if the advance ratio is maintained, to a reduced tip Mach number. This research analyzed the sensitivity of a propeller's far-field tonal noise emissions to variations in helical tip Mach number when operating at a non-zero angle of attack and experiencing wake encounter, using low-fidelity methods. The chosen aeroacoustic solver, based on Hanson's time-domain theory, allowed to capture physical insights via waveform analyses.
The analyses conducted showed that variations in tonal noise amplitude, directivity, and spectral content due to a flow non-uniformity depend on the propeller helical tip Mach number. An enhanced influence of steady noise sources at higher Mach numbers was observed. Waveform analyses suggest that, as Mach number increases, the effect of temporal variations in the acoustic planform—determined by blade element positions at their retarded times—on unsteady loading noise diminishes relative to steady loading noise, thereby amplifying the influence of the latter. These results highlight that propeller aeroacoustic data obtained at low helical tip Mach numbers in experiments cannot be directly scaled to full-scale propellers operating at high Mach numbers.