Experimental Aeroacoustic Assessment of a Tip-Joined Blade Propeller

Abstract

This study presents an experimental aeroacoustic evaluation of a tip-joined blade (TJB) propeller and compares its performance with conventional two- and four-bladed configurations under similar operating conditions. Acoustic measurements are conducted in the anechoic wind tunnel at Delft University of Technology using an eight-microphone far-field directivity arc. All propellers are designed to deliver equivalent thrust while maintaining identical diameter and chord distributions, enabling a consistent comparison at an advance ratio of J = 0.4 and a rotational speed of 4,000 rpm. The TJB propeller satisfies the thrust requirement but exhibits a propulsive efficiency approximately 4% lower than the four-bladed configuration, primarily due to increased torque associated with the closed-loop geometry. The acoustic results show that the TJB does not provide a uniform reduction across the entire noise spectrum. Broadband noise levels are reduced relative to the four-bladed propeller and fall below those of the two-bladed configuration above approximately 9 kHz, yielding broadband overall sound pressure levels comparable to the two-bladed baseline. In contrast, tonal levels are increased, with the blade-passing frequency peak exceeding those of the two- and four-bladed propellers by approximately 3 dB and up to 17 dB, respectively, at the θ = 90° observer position. Consequently, the total overall sound pressure level of the TJB propeller lies between those of the two-bladed and four-bladed propellers. These findings indicate that the TJB geometry provides effective broadband noise mitigation while exhibiting increased tonal components, highlighting both the potential and limitations of tip-joined blade concepts for propeller noise reduction.