Propeller tonal noise reductions through synchrophasing: Mechanisms and performance

Abstract

This paper presents an analytical investigation into the mechanisms and effectiveness of propeller synchrophasing in which noise reductions from a number of co-planar identical propellers are obtained by setting their blade azimuthal positions at any instant in time to be separated by a fixed angle. In this paper we demonstrate that noise reductions obtained through propeller synchrophasing arises from the destructive interference between acoustic spinning modes that are locked to the propeller as it rotates. We demonstrate that the main factor in determining levels of noise reduction is the separation distance between the centres of rotation compared to the acoustic wavelength at the blade passing frequency of interest. Simple analytic expressions are developed to predict the azimuthal directivity and sound power reduction for two co-rotating and counter-rotating propellers, which will be shown to be in close agreement with measured data. The principles of propeller synchrophasing identified for the two propeller case are generalised to multiple propellers, and an iterative scheme for identifying the optimum synchro phase angles will be presented. This paper will demonstrate that propeller synchronising is only effective for relatively small propellers operating at low tip Mach numbers and is ineffective for contra-rotating propellers, as observed in previous studies.