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A comment on Toroidal propeller allows a drone to operate more quietly [pdf]. Parent comment

Re:Toroidal propeller allows a drone to operate more quietly... by mLuby on Sunday January 29, 2023 @05:47PM (#34573583)
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

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Re:Toroidal propeller allows a drone to operate more quietly... by gonzo41 on Sunday January 29, 2023 @06:30PM (#34573935)
Ionic propulsion requires massive power. It's cool but you'll probably never see it in real life applications.
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Re:Toroidal propeller allows a drone to operate more quietly... by mLuby on Sunday January 29, 2023 @08:15PM (#34574722)
Clarke's first law applies here. Maybe beamed power could help around a delivery hub.
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Re:Toroidal propeller allows a drone to operate more quietly... by throw_away_8080 on Sunday January 29, 2023 @09:38PM (#34575231)
I love when primality is useful. Like cicadas. But 9 is not prime.
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Re:Toroidal propeller allows a drone to operate more quietly... by mLuby on Sunday January 29, 2023 @10:45PM (#34575547)
Whoops I meant to write 11, not 9. I haven't actually heard of an 11-bladed screw, though here's a picture of an 11-bladed pumpjet. https://youtu.be/ugSEIiTZ1Pg?t=386
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Re:Toroidal propeller allows a drone to operate more quietly... by wruza on Monday January 30, 2023 @03:15AM (#34577142)
In wartime it is, and also equals to pi squared.
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Re:Toroidal propeller allows a drone to operate more quietly... by ilyt on Monday January 30, 2023 @12:24AM (#34576094)
>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,

Might be just odd-numbered ones, not neccesarily prime. But other interesting case is how car tyres have variable feature size on the tyre so the produced sound is more spread over the spectrum and appear quieter

> 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

That would be literally shocking (and changes properties heavily based on humidity) so I'd think use would be limited, it's not exactly great way to lift anything heavy

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Re:Toroidal propeller allows a drone to operate more quietly... by sgtnoodle on Monday January 30, 2023 @01:50AM (#34576613)
That ionic propulsion drone/startup you linked to is a scam. Their drone has a ducted fan in the center that's providing the thrust.
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