Question about gases and weight.

[Wing Zero]

I have a odd question, and I have NO idea where to start to explain the reason for it really. I have an idea I am working on, but I have a problem. My problem is, I need to know about how much volume, a helium filled ballow would take to lift about 3 pounds on weight. I am thinking about an airship, and need to know how much volume would lift how much weight. How much weight would a cubic foot of space lift in weight, etc... If anyone knows, please let me know! Thanks :) -james

[Purdue Aero Man]

What you are thinking about it buoyancy. Try this site or do a google search on it and you can find out a lot more.

http://www.pbs.org/wgbh/nova/lasalle/buoybasics.html

Basically, your airship needs to displace 3 lbs of air, not including the amount of air it needs to displace due to the weight of the helium inside.

I wrote up a little Excel worksheet for you to try it out. If anybody finds errors in it please let me know and I will correct it.

[steve lewin]

1 cubic meter lifts around 1 Kg

http://www.myairship.com/faq/index.html

HTH - Steve

[gouch]

Does the amount of pressure inside make a difference to the amount it could lift? Or to put it another way, If you had 1 cubic meter of helium at 2 bar would it be better to increase the pressure to say 4 bar to increase the bouyancy or lifting capability?

If I was to try and explain my own question, I would think that by having "more" of a lighter gas in a given volume it would make it lift more??

BUT then I'm thinking that by having more pressure the same volume of helium would be denser, therefore "heavier" ?, so it would lift less? or more accurately have to lift more of itself????

And compressing it to extreme pressures would increase the density to a point in which it is more than the outside air??? :confused: And the last time I remember lifting a full compressed helium bottle, it wasn't light ;)


This question came to me when I was reading steves link in which they talk of the modern non-rigid airships or "blimps" (Interesting to learn about where the word blimp came from too!)

Cheers
Paul

Now i'm confused :D

[steve lewin]

I'd say your second version is correct. Compressing a gas increases the density and hence the weight of the gas itself. Therefore it will lift less.

Steve

[gouch]

Thanks steve. I was tending to the 2nd version as well.

I'm guessing if you drew a curve that shows pressure over bouyancy it would get to a certain point of pressure before the curve starts coming back down?

[ShamelessMale]

Actually something with no pressure in it at all (a pure vacuum) would have the most bouyancy, since the weight of the gas inside would be zero. Of course something that would be strong enough to contain the vaccuum would be impractically heavy in the real world.

So the least pressure that will "inflate" your balloon (to displace a volume of air), will provide the most bouyancy.

However, having a small amount of pressurization means that the skin will become "stiff" and may not have to be so thick/strong/reinforced to keep its desired shape. I believe that is the real determining factor on the pressure to utilize...

[omega blood]

.

So the least pressure that will "inflate" your balloon (to displace a volume of air), will provide the most bouyancy.

...
So it would be better to use a gass bag instead of a rubber(latex) baloon because it would take more pressure to inflate a balloon than it would a bag.

[Sparky Paul]

Notice the high altitude balloons used for atmospheric research. On the ground, inflated, they're tall with a bulb of gas at the top, and floppy folds of plastic down to the payload. At operating altitude, they're spherical not because gas is added; it has all the gas at the release, the gas expands to get the final shape.
A rubberized balloon would not have the same capacity for load or altitude due to the loss of energy needed to stretch the rubber.
On the Mogul balloons of Roswell UFO fame, these were designed to get to altitude, collect the samples, descend to a lower altitude by valving gas, uopn releasing the sample payload they would then go back up uncontrolled to where they broke up due to the internal pressure.