Aeroacoustic analysis of tip-joined blade propeller

Abstract

This study presents an experimental aeroacoustic investigation of a tip-joined blade (TJB) propeller, compared with 2- and 4-bladed configurations under identical operating conditions and matched thrust. The influence of blade azimuthal angles (30∘, 60∘, and 90∘) was also analyzed to precisely quantify the acoustic benefits of the TJB propeller. Experiments were conducted in an anechoic wind tunnel to quantify and separate tonal and broadband contributions in the acoustic spectra at various radiation angles. At 4,000 rpm and an advance ratio of J=0.4, the TJB produced equivalent thrust to the baseline 2-bladed propeller, with only a 2.8% difference. Under these conditions, it exhibited the strongest first tonal peak at 133 Hz, 2 dB higher than the baseline propeller, while broadband noise above 9 kHz was the lowest among all configurations. The TJB, with slightly reduced propulsive efficiency (56.7% for the 4-bladed 90∘ to 54.5% for the TJB), showed the highest tonal overall sound pressure levels (OASPL) across all directivity angles. This behavior is attributed to aerodynamic interactions between the forward and rearward blade segments, similar to the scissor 4-bladed 30∘ configuration, which also displayed elevated tonal levels. However, the broadband levels of the TJB remained lower than those of the 4-bladed configurations and comparable to the 2-bladed propeller. These trends are associated with reduced trailing-edge noise resulting from the sweeping of the blade elements necessary to form the joined-blade structure. Overall, the results indicate that the TJB propeller provides broadband noise reduction benefits, despite no observed advantage in tonal noise.