Helium lifting ability

A cubic foot of air at sea level under standard conditions weighs about 0.077 lb, so if you displaced that cubic foot of air with helium, the net lifting force would be slightly less than that (i.e.,

0.077 lb minus the weight of a cubic foot of helium, which I don't know the value of offhand). Of course, you also have to subtract the weight of the container.

Bert

ff wrote:

Sounds like another budding lawn chair pilot in the making.

I'm not sure but the simple way would be to get a 4oz fishing weight and see how many balloons you need to lift it and multiply this by 4.

Saturday mornings at auto dealerships they usually have a whole lot of balloons they might let you play with.

-- Roger Shoaf If you are not part of the solution, you are not dissolved in the solvent.

I believe a cubic foot of helium will lift about 28.2 grams. Multiply the volume times 28 and divide by 448 g/lb.

A 10' diameter weather ballo> Anyone know offhand without going to too much trouble, what is the ratio

A Google.com search on:

helium lift cubic foot

...found more than one thousand, three hundred web pages with the information you want. No trouble at all.

-- --Pete "Peter W. Meek"

I don't think there is an answer to your question but...

I think you are asking about Archimedes principle...

All bodies floating on or submerged in a fluid are buoyed up by a force exactly equal to the weight of the fluid they displace.

1 cubic foot of air= 0.0807 pounds at 32 F and 1 atm. 1 cubic foot of helium= 0.0111 (same conditions)

If you are planning a trip in a lawn chair that should provide the needed information.

I am unable to resist.

Anyone know offhand without going to too much trouble, what is the ratio > for volume of helium to lbs force against gravity ?

YES

Errol Groff Instructor, Machine Tool Department H.H. Ellis Tech

613 Upper Maple Street Danielson, CT 06239

860 774 8511 x1811

And -- of course -- one cubic foot of vacuum would lift the full

0.0807 pounds (again at STP), but the trick is finding a rigid container which doesn't weigh more than the lift. :-)

Be sure to file your flight plan. :-)

Enjoy, DoN.

If the sides are not rigid it's always *exactly* atmospheric pressure. If it expands either atmospheric pressure has dropped or the temperature of the balloon has gone up.

Here's a question for everyone. Explain *exactly* the mechanics of how and why a balloon floats :^)? Not as easy as one might think.

And a second one is why when you have a helium balloon in your car and put the brakes on to stop, the balloon will float to the back of the car instead of flying to the front like everything else. Again, what are the mechanics that make it do so?

John

Please note that my return address is wrong due to the amount of junk email I get. So please respond to this message through the newsgroup.

Thank you for sharing that information :-)

For those who were wondering, I'm not going flying in my lawn chair !! I have some R/C gear and was contemplating building a small blimp or dirigible. So I was wondering how large it would have to be to carry motor, battery, receiver and servos. Anyone ever tried this or seen one?

Fred

As I understand the physics, air is a fluid and the helium balloon being lighter than the air around it floats just as a drop of oil will float if released from the bottom of a volume of water.

Not as easy as one might think.

Again the air being heaver than the balloon the air has more mass. When this mass is in motion, pressure is equal on all sides of the balloon. When the brakes are applied the air inside the car keeps moving at the speed the car was going until it hits the windshield. As air bunches up in the front part of the car, there is more air pressure at the front of the car and less at the back so the pressure in the front pushes the lighter balloon backwards.

-- Roger Shoaf If you are not part of the solution, you are not dissolved in the solvent.

Never tried one but have seen numerous examples. Galliger (sp?, he of smashed water melons) used one in his act for a while.

There are at least a dozen R/C blimps available already. They have been around for years

This is not true unless there's a hole in the balloon to allow the inside and outside pressures to equalize. As long as the balloon is sealed, the internal pressure can be anywhere from ambient atmospheric pressure up to the maximum (gauge) pressure the balloon can withstand. The internal pressure is balanced not only by the external pressure, but also by stresses in the material of the balloon. The same is true for basketballs and pneumatic tires.

This is generally true, but it doesn't mean that the final pressure is equal to the initial pressure, or that either one is equal to the ambient atmospheric pressure. The expansion will relieve some of the increased pressure differential, but not necessarily all of it. The amount will depend on the stress/strain characteristics of the balloon material.

The forces due to gravitational acceleration acting on the mass of air in the atmosphere results in a vertical pressure gradient (lower pressure at higher altitudes), which means the pressure acting on the top part of the balloon is lower than the pressure acting on the bottom part. Integrating those pressures over the surface of the balloon gives a net upward force (aka buoyant force). If the weight of the balloon is less than the buoyant force, the balloon will rise; if the weight is more than the buoyant force, it will sink; if the weight is exactly equal to the buoyant force, the balloon will stay where it is (but it's an unstable equilibrium).

The same as above, except that now we're talking about an acceleration due to braking instead of due to gravity, resulting in a horizontal pressure gradient instead of a vertical gradient. The balloon's mass multiplied by this acceleration presumably results in a horizontal "weight" which is less than the horizontal "buoyant" force, so the balloon "rises" (i.e., moves in the direction of the acceleration).

Bert

Pressure curve for toy balloon:

Danish (?) government study on toy balloons: Maybe those Mylar balloons aren't Mylar: Miscellaneous facts about latex balloons: R, Tom Q.

But, mechanically, "how" does it float. You almost answered it below.

It took a little time for a friend and I to realize how a balloon floats. I think about half a Big Mac at McDonalds one day. You're correct here. It really puzzled me for awhile when I first noticed this phenonemen (sp). I have a lot of fun with kids when I tell them this, they usually go beserk and get real excited when they see it happen.

The reason why a balloon floats in still air is because of the pressure gradient between the top and the bottom of the balloon. Most people don't think there is enough of a gradient to make a balloon float but there is. They usually think of pressure drops between sea level and the top of a mountain, not one over just a foot of height. If the pressure was the same it would sink.

John

Please note that my return address is wrong due to the amount of junk email I get. So please respond to this message through the newsgroup.

That's eggsackly the same answer I give folks who "ask in the negative", as when they barge in on me with, "You haven't seen Gwendolyn have you?" My answer is, "YES, I haven't."

Jeff

-- Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"Success is getting what you like; Happiness is liking what you get."

It's higher as the balloon expands or there would be negative work being done to inflate the balloon against the elasticity of the rubber.

It can be anything up to what the material will withstand. It can't be lower than atmospheric though because the internal and external pressure will equalise back to atmospheric via a change in volume of the balloon. Once the balloon (given the relatively inelastic material) has reached its maximum volume the internal pressure will rise as more gas is pumped in. The lifting capability will then decrease though as the mass rises for no corresponding gain in volume and boyancy.

For a balloon made of inelastic material the maximum lift occurs when the internal pressure is the same as external and the ratio of volume to mass is highest. i.e. fully inflated until the pressure just starts to rise above atmospheric. For a balloon made of elastic material the maximum lift also occurs when the ratio of volume to mass is highest, but the pressure for that depends on the strength and elasticity of the material.

Dave Baker - Puma Race Engines

"How's life Norm?" "Not for the squeamish, Coach" (Cheers, 1982)

These are readilly available. Have been for several years. Try one of the Indoor Flyer R/C magazines for ad's for these.

Above is a link to a google search for radio control airships. About

5000 links. Some are sure to be pertinant. Have fun!

Cheers Trevor Jones

In article , snipped-for-privacy@aol.comma says... as the balloon expands. I think it's the same with soap bubbles.

I think he meant to say that the differential work done to inflate the first ten percent is smaller than the differential work to go from, say, 80 to 90 percent capacity.

But still net postive everywhere on the curve.

This is pretty obvious if you blow up ballons for a kid's party. Getting it to start is the toughest!

Jim

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