Theoretical study of the acoustic benefit of an open rotor with uneven blade spacings

Theoretical arguments are presented supporting the idea of reducing subjective noise levels of a rotor by irregular blade spacings, based on a simple mathematical analysis of phase-shifted periodic functions. It is found that the overall sound level remains constant whatever the blade spacings may be, provided the spectrum generated by a single blade is flat enough. This means that the gain is subjective and appear only in A-weighted sound levels of PNdB; it increases if the number of blades decreases, while holding the same rotor performance. It is shown that the sound tones can be shifted outward the low-frequency range, but also toward the higher frequencies, which may be important at long distance because of the atmospheric absorption.