THEORETICAL-STUDY OF THE ACOUSTIC BENEFIT OF AN OPEN ROTOR WITH UNEVEN BLADE SPACINGS

Reducing rotor noise by spacing blades unequally is a rather old concept, but it seems again to be coming to the fore. The theoretical method followed here is based only upon a simple mathematical analysis of phase-shifted periodic functions, so it is quite general and can be applied to any kind of acoustic sources. The main result is that the overall sound level is independent of the blade spacing, provided the spectrum generated by a single blade is flat enough. This means that the gain may be only subjective and appear only in A-weighted sound levels or PNdB. The shapes of the radiated spectra are discussed, and they are well validated by tests published by another laboratory. It is shown that the tones can be shifted toward the low-frequency range, but also toward the higher frequencies, which may be interesting at long distance, because of the atmospheric absorption.