Reduction of Tonal Propeller Noise by Means of Uneven Blade Spacing

An experimental study was conducted on the noise characteristics of four-bladed general aviation propellers with uneven blade spacing. The subscale propeller designs were inspired by the fourbladed McCauley propellers used on the Beechcraft King Air 350 series aircraft. The 4-inch diameter (1:22.5 scale) propellers were manufactured using high-resolution stereolithography and were powered by a high performance, radio controlled brushless electric motor. Acoustic measurements were taken with a 24-microphone array. The use of uneven blade spacing created additional tones over which the acoustic intensity was distributed. Large amounts of acoustic intensity were shifted into the lowest frequency tone (occuring at half of the blade passage frequency of the propeller with evenly spaced blades), resulting in reductions of A-weighted overall sound pressure levels of up to 5 dB for polar angles near 90. These reduction are partly offset by increases in A-weighted overall sound pressure levels of up to 4 dB at polar angles less than 50. Although the theory used to predict propeller noise does not show good agreement with experiments, it does show this trend of increasing noise at low polar angles. Since noise at low polar angles are weighted less in noise metrics such as flyover noise, the use of uneven blade spacing has potential for providing noise reduction without adding excessive complexity to propeller design