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air pressure bouency

Old 04-02-2013, 03:15 AM
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X1AaronW
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Default air pressure bouency

would increasing or decreasing the air pressure in a sub effect its bouency? Or maybe changing the gases to say argon?
Old 04-02-2013, 06:17 PM
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Default RE: air pressure bouency

No.
Changing the gas concentration or make-up changes it's weight so little that it is not an effective way to control buoyancy.
A SCUBA tank is slightly lighter when empty but it was compressed to 3000psi !

Adding weight inside the watertight compartment is easier by pumping water into an inner bag or by filling a screw driven piston are more common methods.
Old 04-02-2013, 06:38 PM
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X1AaronW
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Default RE: air pressure bouency

I have seen this on alot of subs. I like it alot but im just coming in to subs so I want to stay simple on my first. after I have a feel for it and get used to the way a sub behaves I will add a windshield wiper pump and a bag. Also I want to keep the buoyancy down and the weight down but the concept of a light submarine just seems more and more impossible even with a free flooding body. I would like it to be fast too so when I want I can make it jump out the water. So basically im not going scale. Im trying to make a stunt sub lol.
Old 04-11-2013, 07:34 PM
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Default RE: air pressure bouency


ORIGINAL: X1AaronW

... I will add a windshield wiper pump and a bag. .... Im trying to make a stunt sub lol.
Windshield washer pumps won't work since they are centrifugal and would allow the pressurized bag to immediately vent back through the pump when it stopped. It needs to be a peristaltic pump.
If you want a stunt sub then it does not need a static diving system at all. Make it skinny, put a powerful motor on it and make it barely positively buoyant. It can then dive dynamically using dive planes but will surface if it loses power or radio contact. A limit to the power you can apply, however, is the torque roll induced by the screw. A pair of counter-rotating props solves this problem.
Old 04-12-2013, 01:13 AM
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X1AaronW
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Default RE: air pressure bouency

I actually mean dynamic diving. Got my words confused sorry. Another problem I am coming across is how can I make a submarine that is light yet low buoyancy? From what I can see its impossible.
Old 04-21-2013, 07:12 AM
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Greg W
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Default RE: air pressure bouency

They only way to keep the sub light and minamal bouyancy is to keep the dry portion of the sub as small as possible. that volume is what the wieght has to overcome. What is the size of the sub you were hoping to build?
Old 04-21-2013, 08:27 AM
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X1AaronW
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Default RE: air pressure bouency

Rougly 1.5ft with a 2inch diameter.
Old 04-21-2013, 01:29 PM
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Greg W
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Default RE: air pressure bouency

if you calculate the volume of the cylinder you can convert to wieght in fresh water. I could be wrong but my calcs are 18" * 3.14 *1*1 = 56.52 cubic inches using an online converter 56.5cu inches =.24459 gals. water is roughly 8.2 lbs per gallon so .24459 *8.2= 2lbs so your sub needs to be lighter than 2 lbs to be positivly bouyant. If it is heavier you will need to add foam outside the cylinder but inside the hull to increase the bouyancy.this seems right in my head but my gut isn't so sure. if you have your cylinder made you could float it in a tub of water and see how much wieght it takes to sink it. Good luck. Greg
Old 04-21-2013, 03:25 PM
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X1AaronW
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Default RE: air pressure bouency

thanks for he numbers man I appreciate it. Now another question. Is it a bad idea to use an outrunner brushless motor in water? Thats what im working with for now. It will be total submerged.
Old 04-22-2013, 03:31 PM
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Greg W
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Default RE: air pressure bouency

Sorry but I have no wxperience with brushless motors in the water. I have read things saying it will work and others saying it won't
Old 04-22-2013, 03:41 PM
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X1AaronW
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Default RE: air pressure bouency

well I appreciate the help man. Thank you.

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