Helicopter propellers are typically even louder, since the tips are going supersonic (due to the diameter of the propellers). The trademark "chop chop chop" sou

Helicopter propellers are typically even louder, since the tips are going supersonic (due to the diameter of the propellers). The trademark "chop chop chop" sounds, as far as I know, are caused by sonic booms. The tech you're linking to is about minimizing the source of sound for the sonic boom. Airplane and drone props are entirely subsonic. They make a lot of noise when the pitch is significantly different than the speed that the airplane "should" going at. In other words: if you imagine a propeller cutting through butter, there is a certain distance it would go in one rotation. Propellers are loud when they're going significantly over or under that distance. Also - I'm just a hobbyist, so my explanation may not be correct. This effect is especially noticeable with fixed-pitch propellers used for hovering. For reference, check out these videos. Note that airplanes have 1D variable pitch. [1] http://youtu.be/e8nYZ-fMSHg?t=1m5s http://youtu.be/e8nYZ-fMSHg?t=1m5s - note the sound at 1m12s [2] http://youtu.be/QB6vtE3rIrQ?t=9m15s http://youtu.be/QB6vtE3rIrQ?t=9m15s - note the sounds at 9:25 and 9:53. For more examples, see https://www.youtube.com/results?search_query=propeller+taxiing https://www.youtube.com/results?search_query=propeller+taxii...