A team working on a silent, ion-propelled plane at MIT's Lincoln Laboratory found itself wondering whether prop noise in multirotors could be mitigated with differently-shaped propellers. "We came up with this initial concept of using a toroidal shape, this annular wing shape, to hopefully make a quieter propeller," More https://newatlas.com/aircraft/toroidal-quiet-propellers/?amp=true
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Max Berghouse