IIRC submarine screws use prime-numbered blades (5, 7, 9) to cut down on harmonics. Perhaps the same would be useful here, though I don't see many prime-numbered turboprops, which you'd expect to benefit from similar noise reduction. Related, I highly recommend HI Sutton's youtube channel if you're into submarines. https://www.youtube.com/watch?v=ugSEIiTZ1Pg
A magnus effect drone may have a decent chance at being quiet. IANAAeroEng but I'd guess that the moving part's interaction with the fluid can be made laminar or at least non-turbulent, and also unlike some propellers, the moving part shouldn't be approaching or interacting with the sound barrier. https://youtu.be/hlmvHfIAszo?t=16
However my money's on ionic propulsion drones since they have no moving parts to make noise. I wonder if the air accelerated through the grids produces an audible hiss. https://youtu.be/UGM4JXVB5FM?t=126
A magnus effect drone may have a decent chance at being quiet. IANAAeroEng but I'd guess that the moving part's interaction with the fluid can be made laminar or at least non-turbulent, and also unlike some propellers, the moving part shouldn't be approaching or interacting with the sound barrier. https://youtu.be/hlmvHfIAszo?t=16
However my money's on ionic propulsion drones since they have no moving parts to make noise. I wonder if the air accelerated through the grids produces an audible hiss. https://youtu.be/UGM4JXVB5FM?t=126