Can A Balloon Hover?

[71silverfox]

After noticing that a balloon filled with helium floats up, and a balloon filled with air falls to the ground, my question is this -

If a balloon is filled with the correct amount of both helium and air - does it hover? (does not float or fall)

[heathfield]

Yes. But a helium balloon can be kept at constant height by venting off, and refilling from the helium tanks on board. This is doubtless what the likes of Branston did on his cross-Atlantic attempt. No point in trying to mix the helium with air to obtain a constant altitude.

[71silverfox]

Thanks, but I was thinking more of a small balloon (or beach ball for example)
I want to know if it would 'hover' in mid air

[chrisgel]

Not a scientist but I would think it would be extremely difficult to get the balance "just right" and of course if you did then it would only be "just right" for the altitude you were at at the time.
If you carried the balloon to the top of a block of flats for example the balance would no longer be correct.
So technically possible probably but practically it would be very difficult.

[jim360]

Even a small balloon should stay at the same height with the right mixture of helium and air -- in fact, even the ones that float upwards will, eventually, stop rising and just stay at the same height. It's just that you can't see where this height is.

The requirement is that the density/ pressure of gas in the balloon matches the density/ pressure outside. Once this happens, and assuming a perfectly airtight balloon, then it should stay at the same height.

In practice this is unlikely to happen because the balloon is not perfectly airtight, and anyway pressure and density aren't just functions of height. But in a controlled and stable atmosphere the balloon should hover at some height that will depend on its density of gas.

[Peter Pedant]

Come on Jim - QM has rotted your brain

Let's do a gedanken experiment.....

If you think of a balloon shaped volume of air - well that doesnt move up or down and so enclosing it shouldnt either - - - if the skin weighs nothing.

O God so re-jig: imagine a balloon filled with a volume of something that stays in the same place. Then for nothing to move then the weight of the balloon skin and the light airy stuff whould be the same as the volume of air it takes place of (displaces)

If it reminds you of Archmiedes it should do.

or of the derivation of the simple stokes equation (no not navier-stokes that has differentials and other hard things)

I think also Millikan used the idea in his estimations of the charge of the electron - the oil drop was made to hover by an electric charge rather than filling it with light stuff.

[jim360]

Well if you allow for the weight of a balloon skin then just fill it with a mix of air that is just slightly less dense than the outside atmosphere, but since balloon skins can be very light it doesn't seem worth adjusting for it too precisely.

[lardhelmet]

Good grief - I must be tired. Thought it said "Can a balloon hoover"?

[seadogg]

Presumably the air/helium in the balloon remains at constant humidity compared with the ambient air. Also the heating from solar radiation will affect the balloon in a different way. According to Boyles Law the stability of gas within the balloon will become unequal to that of the surrounding air and it will rise or fall.

[Peter Pedant]

jim clearly you arent a party man

Lardhelmet - I am amazed Ed has not zero'd your contribution

When I noted that I had misread "beasts of the field"
as the front of women in a field - it hardly lasted 30s

[Graham-W]

I've done it loads of times.
Take a small helium filled balloon and attach a short length of sellotape to it. Hang a piece of paper onto the sellotape which is heavy enough to stop the balloon from rising. Gradually reduce the weight of paper by tearing pieces off. You will reach a point where the balloon just begins to rise. Add small pieces of paper 'til you get to an equilibrium and the balloon hovers.
Good fun for the children and an introduction to science.

[jomifl]

The elasticity of the envelope would be a significant factor.