Physics of balloon flight, questions

Hi, My interest in balloons is new and as a 'balloon newbee' I am trying to understand how they fly. (I know, this is old hat to the rest of you, but I've got to start somewhere.)

I have two questions: 1) Why does the warm air in the balloon not spill into the surrounding air?

2) The calculation that I have seen that explains the lifting power of the balloon relies on the difference of densities between the internal and external environments. Is the lower density of the air inside a balloon achieved by the expansion of the balloon? Namely, does the heat increases the pressure which in turn expands the volume of the balloon which in turn increases the buoyancy?

Thanks for your forbearance for my asking what must be quite rudimentary questions. (If there is literature or web pages that you know of, that explains such questions, I would appreciate your forwarding the titles and the web addresses to me. )

Thanks.

Richard

It does. Even more importantly, the heat of the air is conducted through the balloon envelope, and radiates out into the environment. That's why, unless you keep heating the air inside the balloon, it eventually starts cooling and falling.

A slightly more direct way of looking at it is that heat, added to a (roughly) fixed volume of air, will cause that volume of air to attempt to expand. If the volume is restricted, pressure will increase, and if neither volume nor the quantity (or consistency) of air changes, neither does the density. If there are ways for the air to escape, or for the volume to expand, it will, and the air, occupying a larger volume (or the remaining air, occupying the same volume) will be less dense, causing the lifting power you mentioned. A hot-air balloon expands somewhat, and some of the air within escapes during heating, causing the change in density required for flight.

If, somehow, you had a 'balloon' which was both perfectly airtight and did not expand or contract at all, heating the air within it would have no effect on density and therefore it would not fly.

Richard :>

You ask interesting questions which I had to find the answers to myself not so long ago. Here's a non-data-dependent and non-maths-dependent explanation.

It's contained by the fabric of the envelope. The air next to the balloon's external surface /does/ heat up, as you would expect. The reason for the burner in flight is essentially to replace the heat lost through the fabric.

The balloon envelope is generally speaking designed to be a fixed size and shape.

Lift is achieved by the simple fact that hot air is thinner (less dense, less weight) than cold air because gases expand when hot.

To get the beasts airborne, the balloon is firstly inflated with cold air. This is then heated and the heat going in causes the air inside the envelope to expand. The excess air is then expelled through the hole in the bottom of the balloon.

What's left inside is hot air which is thinner than cold air, hence the whole envelope has buoyancy compared to the surrounding air.

Look at it like this. A cubic foot of air has a given weight (the weight of the gas) which of course varies with local temperature and air pressure and probably also moisture content.

Heat that volume of air (whatever it weighs) to around 100C and it expands. The original cubic foot of air now has /less/ gas in it. It's lighter. It rises.

Simple, innit?

There might be, but I don't know of them, so if anyone else has info, thanks in advance.

And yes, I know this explanation is simplified and experts can blow holes in it. That's not the point of this message.

I do believe this is why the air-craft, whether it be a balloon or an airship, whether it be filled with Helium or Hot Air - are all called 'Lighter Than Air' - craft. If you really want to 'hear' what some of the pros say, sign on to this Univ. of Colorado discussion group. Go to this site and request to be listed.