Acoustic Performance of Co- and Counter-Rotating Synchronized Propellers

The understanding and reduction of noise from the new generation of electric propeller-driven aircraft, such as those adopted in Electric Distributed Propulsion (DEP) configurations, is becoming of great importance to the success of these concepts. The study presented in this paper aims to explore the noise suppression capabilities of DEP systems by changing rotation direction and separation distance under both static thrust and in-flow conditions through the implementation of electronic synchronization of the propellers. The noise characteristics of two-bladed propellers were analyzed by conducting experimental measurements on two adjacent, electronically synchronized propellers. The acoustic tests were conducted in an aeroacoustic wind tunnel, using a bespoke rig resembling a DEP system. The experiments involved varying the relative phase angles between the two propellers from 0° to 90°. Each propeller had a diameter of 9 inches and a pitch of 5 inches, operating at a constant rotational speed of 5000 rpm. The noise measurements were taken for co-rotating and counter-rotating propellers, examining two center-to-center separation distances under two advance ratios of 0 and 0.47. The key findings revealed a significant correlation between noise reduction and increasing relative phase angle, particularly at 90°, across all directivity angles and regardless of propeller rotation direction, advance ratio, and separation distance. The results illustrated significant reductions in noise directivity and tonal noise at the blade pass frequency. The experimental results also revealed that phase synchronization offers a slight advantage for co-rotating propellers over counter-rotating propellers at an advance ratio of 0.47. The separation distance between the two propellers appears to have a minimal impact on the noise level at the first blade pass frequency using the phase synchronization method. Finally, the experiments demonstrated that phase synchronization can provide significant benefits in noise control for DEP configuration.