Aerodynamics and Acoustics of Asymmetric Propellers With Uneven Blade Spacing

The presented work aims to establish whether asymmetric propellers with uneven blade spacing offer any acoustic benefits over a conventional propeller with identical blade geometry. An experimental investigation was conducted to measure static thrust and characterize and compare the acoustic signatures of three asymmetric propellers with different blade spacings to an equivalent conventional propeller. The main observation was that, similar to unevenly spaced symmetric propellers, the asymmetric uneven blade spacing introduced a higher number of tones with lower amplitudes, spreading the acoustic energy over the frequency domain. Additionally, the altered blade spacing resulted in a noise composition shift from tonal to broadband in a higher frequency band, especially for smaller blade spacing angles. Calculated OASPL was higher for propellers with smaller spacings at equivalent operating points, while the propeller with 90-degree blade spacing had a similar OASPL to the baseline propeller. Static thrust tests indicated a performance penalty associated with reduced blade spacing. Overall, the metrics presented in the current analysis suggested no clear benefit of asymmetric propellers on acoustic performance. However, the altered sound quality may lead to reduced annoyance, subject to further psychoacoustic investigation.