Model Balloons

This bit of Hoppy's web based spiel is about building model Hot Air Balloons. The balloons described here can be any size, but a practical maximum size is about 2m tall - beyond that they become a real pain to make because of their sheer size. Note, in the UK, I have seen references to the fact that any balloon with a linear dimension above 2m should be registered with the Civil aviation authority. So if like me, you want to be a good boy, then Keep them small!

Balloons below this size appear to be exempt from any other rules, but a bit of common sense never did anyone any harm:

  1. Don't fly them near busy roads - you're gonna look awfully silly if someone crashes due to 10 square meters of plastic obscuring their view
  2. Don't fly them near an airport - you're gonna look awfully silly if an aircraft gets smashed up because your balloon just got turned to confetti by a turbofan
  3. Be extra cautious in dry weather - the burner could make a nice pretty fire on the local moor if the balloon lands whilst still alight.
  4. It isn't my fault. I don't care what you did, it's your balloon, your problem. This is the CYOA (Cover your own ass) disclaimer. I refuse to be held responsible for you burning down your local town, or cutting off your finger.

A couple of Balloon photos. See, they do work! The balloons really need to be made from more than 6 pannels, as you can probably see the somewhat odd shape that they come out. Try making the pannels long and slender in order to make the balloon a more rounded shape. You can also see the effect of overheating the balloon - it shrivels! Liewise, if you spill fuel onto the ballon and fly it, then the balloon could catch fire, as er... observed in Cambridge one evening...

Construction

These balloons are made from plastic swing bin liners. They can either be liberated from the kitchen when The Boss is out, or obtained from the local supermarket. I got 40 bags for GBP1. They are very inexpensive, and should be easy to get hold of. They are made from very thin plastic. Look for the cheapest ones, the heavy duty ones suitable for putting bricks in are too thick, hence too heavy.

Things needed

Things to do

  1. Take 6 bags - Lets start with a small balloon!
  2. Slit the bags down their longest sides (cut along the seam), and cut off the bottoms. This should result in a stack of sheets, each about 1.5m x 0.8m. If your bags are too small, get some larger ones! Try to do this bit neatly, otherwise the balloon might not fit together properly.
  3. Fold each sheet in half along the longest edge and stack the sheets into a pile, with the folded edges aligned. Do this very carefully, and ensure that there are no air pockets trapped, or the balloon won't fit together properly.
  4. You should now have a stack of 6 folded sheets, about 1.5m x 0.4m. If you don't, I'll leave it to you to work out why not.
  5. Now, using the table of values below, mark onto the top sheet points which represent the edge of the balloon.
  6. Cut through all 6 sheets (12 layers of plastic), following the pattern on the top sheet. Put these 6 boat shaped pieces to one side, these form the panels of the balloon (gores).
  7. Take 2 opened out panels and lay one out flat onto a smooth table. Lay the second directly on top of the first, lining up the edges.
  8. Using the soldering iron (see later), fix the sheets together along the edge of half of the balloon. Leave the bottom open.
  9. Take the next sheet, and fix this along one of the free edges of the two joined sheets
  10. Fix all 6 sheets together, and then join the last 2 edges together. This completes the balloon. With a little careful thought about which way up the sheets go, you can get all the seams to appear on the outside, then the balloon can be turned inside out.
  11. Fasten a wire hoop around the base of the balloon. This is to attach the burner to, and to hold the bottom of the balloon open. Do this either with small pieces of tape or ideally by making a hem around the base of the balloon

Fixing sheets together

This should be done on an old table, otherwise you will end up with burn marks everywhere. Place some greaseproof paper on the table, and then put the two plastic sheets to be fixed together on top of it. Then place a double layer of greaseproof on top of this. Run the soldering iron slowly, using the flat edge, along the top layer of greaseproof paper. This causes the two sheets to melt together and the join is very strong. Make the join about 0.5-1 cm from the edge. Don't move too quickly or the sheets won't fix, and don't move too slowly or the sheets will just melt.

Table of values to make the gores

Mark the following points onto the top sheet of the stack of folded sheets, then using the sharp scissors, cut out the balloon pattern using these points as a guide of where to cut. Don't make any sharp corners. This should result in a collection of boat shaped sheets, as in the diagram. The measurements are not hugely critical, but make sure when you cut out the sheets that all the sheets are cut together as it's important that all the sheets are identical.
Distance from base (cm) Distance from the folded edge (cm)
0 12
10 14
20 16
30 18
40 20
50 22
60 24
70 26
80 27
90 24
100 22
110 17
120 12
130 7
140 0

A balloon gore

The standard balloon shape. Experiment with different shapes, size, and number of gores.

Testing the balloon.

Now the balloon is complete, use a hair drier to inflate the balloon. If the sheets are fixed together correctly, and there are no gaps in any of the joins then the balloon will fill up. If the hair drier is warm, then the balloon will float up to the ceiling! Do this inside the house or your balloon will fly away!

Flying the balloons.

This bit is easy. Fix a single piece of uninsulated solid core wire across a diameter of the hoop at the bottom of the balloon. Twist this wire around a ball of kitchen roll paper, about 2 sheets, so that it hangs just under the middle of the opening at the bottom. Now soak the paper in paraffin or meths - don't spill any on the balloon or it will pyre itself!
Now make sure that the wire hoop is circular, and holding the bottom of the balloon open. Lift the top end of the balloon off the ground, but keep the hoop on the ground so that the balloon stands upright - you really need two people for this bit. Apply a small quantity of fire to the paper, and make sure that the flame stays away from the plastic. The balloon will inflate itself, then just let go!

A small balloon like this ought to fly for at least 5 minutes, and several miles, so recovery is out of the question! If you are unlucky, then the wire hoop may melt away from the balloon and fall to the ground. Without any heat, the balloon will quickly descend. If you are very unlucky, then this falling hoop (with the burning paper) will land on something and cause a minature bonfire. If this something is a building, then this is likely to be extreemly bad news.

These balloons are quite small, so weight is very important, use as little wire as possible, and don't try to put too much kitchen roll on them.

How they work:

Recall your school days (if you are not old enough, then anticipate them instead), and look at one of the ideal gas laws. Pretend that the air in the balloon is at atmospheric pressure (constant), and at a uniform temperature.

At constant pressure, Volume / Temperature = Constant.

This implies that increasing the temperature will cause an increase in volume. Since the balloon is a fixed volume, if the air in the balloon is heated, then any excess air will come out of the bottom of the balloon. Since a volume of air has a mass, this causes the overall weight of the balloon and enclosed air to decrease. If the overall density of the balloon and air is less than that of the surrounding air, then the balloon will rise.
So...

Volumeballoon / Temperatureambient = Volumeheatedair / Temperatureheatedair. = constant

Since the heated air occupies a larger volume than that of the balloon, the mass of the balloon and enclosed air decreases.

Volumedifference = Volumeheatedair - Volumeballoon

So Massdifference = Densityair * Volumedifference

So lift = Massballoon - Densityair * Volumeballon ( Temperatureheatedair / Temperatureambient - 1)

So knowing that the density of air is 1.3Kgm-3, the volume of the ballon (about 0.2 m3 here) the ambient temperature, the temperature of the air in the balloon and the mass of the ballon, you can work out how much lift the ballon produces. As soon as the lift becomes positive, the balloon flies away. Note that the temperature should be measured in Kelvin (absolute temperature). Ie celsius temperature + 273.
This working ignores all sorts of things like the non constant density of air. It's an acceptable approximation though. It shows that a large balloon is easier to fly (you need to heat it less) as the mass of the balloon increases with area, but the lift increases with volume.