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 (occurring 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.
This kind of reminds me of an idea I bounced off Charlie Smith while planning a macroengineered maximization of electric transport in an economy where the elex-gened CO2 all gets turned into algae the sale of which pays for the re-engineering of everything to be run on virtually zero-emission coal fire:
Out here in the sticks, we do still have municipal airports with runways that are long enough to, at 3-g electromagnetic rail acceleration, support pretty impressive glide distances – at least long enough for commutes to Omaha.
I’ve got mixed feelings about these technologies that decentralize population. On the one hand, I’d just as soon the urban areas get real, and start leading the drive toward ocean thence space colonies, ultimately rewilding the planet. On the other hand, urban areas are getting less real all the time and people need to escape.
In a comment on 2023-01-14, I quoted Robert Heinlein from a 1973 interview on the conflict between high technology and personal liberty and the need (at least historically) for concentration of large numbers of people in urban settings to create technology. I’ve nothing to add to that comment, so I’ll just link to it here.