3 ms·

Sort of reminds me of the design of Navy submarine propellers (there known as 'screws'), optimizing for similar noise dampening properties. For a very long tim

Sort of reminds me of the design of Navy submarine propellers (there known as 'screws'), optimizing for similar noise dampening properties. For a very long time the design of modern submarine screws was one of the closest guarded secrets of submarine design, with submarines docked having cloth shrouds over the screw even in classified facilities. AFAIK, those weren't torodial, but they had similar long arcs of the blades. For more information: https://americanhistory.si.edu/subs/anglesdangles/taming.html https://americanhistory.si.edu/subs/anglesdangles/taming.htm...

WirelessGigabit 4y ago
That's funny, because I remember watching a documentary about some form of caterpillar drive.
- ChrisMarshallNY 4y ago
  That was the one that was "narrated" by Sean Connery?
  - froh 4y ago
    the hishtorically accurate shientific account of a rushian boat being reshcued?
    - BuckyBeaver 4y ago
      And all the while, the captain had a mouthful of shtew.
    - jonstewart 4y ago
      The order ish, engage the shilent drive.
      - dylan604 4y ago
        One ping only though
        donw 4y ago
        I would have liked to see Montana.
petee 4y ago
I believe they still shroud them. And I vaguely recall reading about a museum that received a retired prop design; they had to vertically mount it for display partially underground, as the Navy wouldn't let them cut the bottom blades since there was something still secret in the construction of the casting. My guess is it was hollow like jet fan blades
- dsfyu404ed 4y ago
  Even if the design principals are known the acoustics of a particular sub are not and can be inferred from the design.
mLuby 4y ago
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 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 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 https://youtu.be/UGM4JXVB5FM?t=126
- gonzo41 4y ago
  Ionic propulsion requires massive power. It's cool but you'll probably never see it in real life applications.
  - mLuby 4y ago
    Clarke's first law applies here. Maybe beamed power could help around a delivery hub.
- throw_away_8080 4y ago
  I love when primality is useful. Like cicadas. But 9 is not prime.
  - deleted 4y ago
    [deleted]
  - mLuby 4y ago
    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 https://youtu.be/ugSEIiTZ1Pg?t=386
mk_stjames 4y ago
Re: closely guarded submarine secrets, I always found this whole incident and the surrounding events just fascinating [1]; It involves giant machine tools, computer control systems, and a lot of international cold war politics. [1] https://en.wikipedia.org/wiki/Toshiba%E2%80%93Kongsberg_scandal https://en.wikipedia.org/wiki/Toshiba%E2%80%93Kongsberg_scan...
noveltyaccount 4y ago
From page 2 of that link: "Once they reach a certain speed, the blades begin to create a partial vacuum, which results in air bubbles. This is a state known as cavitation." How does a vacuum result in bubbles? Wouldn't the water just clap back and close the vacuum?
- pkage 4y ago
  Those bubbles do collapse, but the problem is that as they collapse they generate tiny little shockwaves. Apart from being noisy, these implosions can be quite destructive—the constant bubble collapses are fairly energetic, and cause cyclic stresses on materials. Over time, these can actually eat away at the surface of the propeller [0]. [0]: https://en.wikipedia.org/wiki/Cavitation https://en.wikipedia.org/wiki/Cavitation
- akincisor 4y ago
  Think of what happens to a fluid when exposed to a vacuum. The water surrounding the vacuum instantly vaporizes and creates bubbles of water vapor. Then these small vacuum bubbles collapse and create small shockwaves that create noise and damage the propeller/screw.
- deleted 4y ago
  [deleted]

WirelessGigabit 4y ago

That's funny, because I remember watching a documentary about some form of caterpillar drive.

ChrisMarshallNY 4y ago
That was the one that was "narrated" by Sean Connery?
- froh 4y ago
  the hishtorically accurate shientific account of a rushian boat being reshcued?
  - BuckyBeaver 4y ago
    And all the while, the captain had a mouthful of shtew.
  - jonstewart 4y ago
    The order ish, engage the shilent drive.
    - dylan604 4y ago
      One ping only though
      - donw 4y ago
        I would have liked to see Montana.

petee 4y ago

I believe they still shroud them. And I vaguely recall reading about a museum that received a retired prop design; they had to vertically mount it for display partially underground, as the Navy wouldn't let them cut the bottom blades since there was something still secret in the construction of the casting. My guess is it was hollow like jet fan blades

dsfyu404ed 4y ago
Even if the design principals are known the acoustics of a particular sub are not and can be inferred from the design.

mLuby 4y ago

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 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 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 https://youtu.be/UGM4JXVB5FM?t=126

gonzo41 4y ago
Ionic propulsion requires massive power. It's cool but you'll probably never see it in real life applications.
- mLuby 4y ago
  Clarke's first law applies here. Maybe beamed power could help around a delivery hub.
throw_away_8080 4y ago
I love when primality is useful. Like cicadas. But 9 is not prime.
- deleted 4y ago
  [deleted]
- mLuby 4y ago
  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 https://youtu.be/ugSEIiTZ1Pg?t=386

mk_stjames 4y ago

Re: closely guarded submarine secrets, I always found this whole incident and the surrounding events just fascinating [1]; It involves giant machine tools, computer control systems, and a lot of international cold war politics. [1] https://en.wikipedia.org/wiki/Toshiba%E2%80%93Kongsberg_scandal https://en.wikipedia.org/wiki/Toshiba%E2%80%93Kongsberg_scan...

noveltyaccount 4y ago

From page 2 of that link: "Once they reach a certain speed, the blades begin to create a partial vacuum, which results in air bubbles. This is a state known as cavitation." How does a vacuum result in bubbles? Wouldn't the water just clap back and close the vacuum?

pkage 4y ago
Those bubbles do collapse, but the problem is that as they collapse they generate tiny little shockwaves. Apart from being noisy, these implosions can be quite destructive—the constant bubble collapses are fairly energetic, and cause cyclic stresses on materials. Over time, these can actually eat away at the surface of the propeller [0]. [0]: https://en.wikipedia.org/wiki/Cavitation https://en.wikipedia.org/wiki/Cavitation
akincisor 4y ago
Think of what happens to a fluid when exposed to a vacuum. The water surrounding the vacuum instantly vaporizes and creates bubbles of water vapor. Then these small vacuum bubbles collapse and create small shockwaves that create noise and damage the propeller/screw.
deleted 4y ago
[deleted]