Prediction of Phase-Offset Effects on the Tonal Noise Produced by Contra-Rotating Propellers

Abstract

Contra-rotating propellers (CRP) are widely employed in multi-rotor vehicles due to their aerodynamic efficiency and compact design. In practical operations, the rotational speed of the propellers can fluctuate about its mean value. These fluctuations lead to azimuthal misalignment between the two propellers, commonly referred to as phase offset. This paper investigates the effect of this phase offset on the tonal noise characteristics of a CRP configuration. A semi-analytical framework is presented to predict tonal noise generated due to unsteady loading arising from potential field interactions. The method requires as input the spanwise distribution of steady aerodynamic loads. The proposed framework is validated by comparing noise predictions with experimental data and is subsequently used to investigate the effects of phase offset. The results show that the odd harmonics of the blade passing frequency are more sensitive to variations in phase offset, whereas the even harmonics remain largely unaffected. Furthermore, the overall sound pressure level shows maximum variation with phase offset in the propeller rotational plane. The study also highlights the potential of the phase offset parameter as a means of incorporating uncertainty due to rotational speed fluctuations into the semi-analytical method proposed for tonal noise prediction.