An experimental characterization of phase delay effect on the noise emitted by propellers in tandem configuration

The paper presents part of the experimental activities in the framework of GARTEUR Action Group RC/AG-26 to study the acoustic and aerodynamic characteristics of small rotors configurations, including the influence of the rotor-rotor interactions. The study aims to investigate the effect of phase delay on the noise emitted by two co-rotating propellers in a side-by-side configuration, focusing on the effect of interference. For this purpose, a measurement campaign was performed within an acoustic wind tunnel at constant rotational speed by varying the relative propeller phase shift. Subsequently, the results were compared with those corresponding to an isolated propeller operating under the same conditions. For the purposes of this study, the two propellers can be considered as two independent noise sources. Variations from the isolated case can be attributed to interference phenomena, whether destructive or constructive.

One key aspect of the present paper is the implementation and test of a synchrophaser to impose a constant phase delay between the propellers. This system acts as a feedback control system, ensuring the correct relative positioning of the propellers. The analysis principally aims to understand the effects on the tonal component, specifically to determine the feasibility of generating destructive interference to reduce noise annoyance for people. The results show that the interference between the propellers changes significantly for different phase angle values. It is also observed that the acoustic impact is highly dependent on the listener’s position, making it very challenging to define a single, unified noise control strategy. The present study could pave the way for the development of optimization algorithms aimed at finding the best configuration from an acoustic perspective.