hot air and vapour

In the preparation meeting for an installation of Les Petits Débrouillards in the Parc de la Libération, Valenton, I offered to try to make both a hot-air balloon and a Phut-Phut Boat.  I’d never made these, and had only a week in which to find the time to do it in…

A hot-air balloon

This is what I imagined:

hot-air balloon

and this is what I achieved (for anonymyty of participants, I’ve applied oil-paint effects to some images):


montgolfiere n16h15crazy-filmedValentonlaunch par Harriott

The ambient temperature in the park that day was 32°C, and there was a slight breeze too, so the conditions were not at all good for flight. On a still evening a few nights before, about 23°C, I got a more positive lift-off:


montgolfiere-k20h21-alc-burn-flight par Harriott

Okay, an interlude for safety considerations. The skin of these balloons is thin plastic (or paper), and flamable, so first consideration was: no children within 6 metres!  I was, however, encouraged and impressed by the indoor flights posted by Science Toy Maker, so I went ahead with my outdoor models, but I also did various burn tests, and discovered that thin plastic shrivels rapidly away from flames, but if it does catch alight, it drips, at which point it’s dangerous. All of my flights were with tethered balloons that I could easily keep control of.

How does it fly?

1m³ of air at 20°C (293K) weighs about 1.2kg, and at 80°C (353K), it weighs about 200g less, so if we can somehow get the air in the bag hot enough, fast enough, and keep the weight of the whole thing as low as possible, it will float, being in total lighter than the ambient air around it.

The ideal gas law tells us that the amount of a constant volume (and pressure) of gas is inversely proportional to the absolute temperature. So we can roughly state that a 30°C temperature increase will cause a 10% reduction in amount of gas in the bag, ie a 10% reduction in weight.

I could only get flight using extremely thin plastic bags. The air was heated by burning crepe paper soaked in domestic cleaning alcohol, and held in place below the opening of the bag by 1.2mm thick cross-wires.

The best conditions are cold days, with no wind.

I did try, in my experiments, a much stronger heat source (with a heavier balloon-frame):


j 21h58 trangia then torch 8m par Harriott

Other examples

Science Toy Maker’s indoor candle-powered balloons are excellent, and quite safe with a well-behaved group, but you need to find the super-lightweight bags.  Jared Hottenstein shows the same thing.

I got my technique from Cobbler Videos, but I wouldn’t let a balloon float off free!

Paper Sky Lanterns

Kongming lanterns are beautiful, and require more care to make.  jollicorp shows how, and Animaplates shows how to make one montgolfiere-style.

A phut-phut boat

I’d seen these made out of pop-cans in India, and I had a quick go at making one.  I first tried something super-simple using just straws, a folded piece of an old beer-can (that I picked up), a soup carton, blue-tack, and a glue-gun to seal up the heater.  Of course the glue melted when I applied a candle flame.  So I tried again, gluing two lengths of carbon-fibre tubing that I happened to have into the metal heat-engine with chemical metal (a kind of epoxy glue, used for repairing engines).  Yes it worked, just:


phut-phut l slow movement par Harriott

The main reason it doesn’t move more forcefully is, I think, the carbon-fibre tubing, which is providing too much insulation.  The engine works by flashing drops of water inside the metal engine into steam, and that steam needs to recondense quickly so that water can be sucked back in.

Science Toy Maker provides precise instructions for making a good motor.

We didn’t use my boat at Valenton, which is maybe just as well – as it is, it wouldn’t have helped to teach much science…

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