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bladders...

Bladders must be the biggest headache for homebuilt inflatable kites. There are a few options...

commercial bladders

Airtime sell a range of oversize bladders but they are expensive. Oversize bladders are good because all the stresses are transmitted to the dacron sleeve and punctures are less likely.

Some kite shops have stocks of bladder kits for older kites that they will sell off cheaply. I bought two Storm 1 bladder sets (16 sqm and 12 sqm) for £35 each including VAT and shipping from powerkiteshop. At these prices it isn't worth making my own bladders.

homemade

My first bladders were made of polythene layflat tubing and my kite flew once using these. They leak. I tried many different valves and sealing techniques but it's not the right material. The only plus is it's cheap so you won't feel too bad about the money you spent on it!

I tried a PVC lilo and made a successful strut bladder. I glued the PVC with a toluene based glue (Careful - toluene is toxic) but the PVC is heavy. Ideally you want PU (Polyurethane) but it's impossible to get hold of in small quantities.

I did get a sample of PU from Lord Corporation in the UK, and it's big enough to make two strut bladders. Maybe with enough samples you could make a full set! :-)

Then I tried Nylon Vacuum Bagging layflat. It is also cheap and much more promising, but harder to seal the ends. Simply tying a knot in the end of the tube works, but can leak slightly. Here's how I made some successful nylon bladders:

Get some film. I used Elastomax from Aerovac.

Cover your work surface with newspaper!!

Get some good quality CA glue. (ZAP-CA from a model shop)

Cut the tubing with a soldering iron. Cut it 2cm bigger than you want the finished bladder to be. Don't forget to make the bladder slightly oversize! My first strut bladder was just a little too small because I forgot this!

The soldering iron heat seals the edge so that the glue cannot get inside the bladder (but it's not a very good seal which is why I use glue as well). If the glue gets inside it will stick the two layers of film together! Run the soldering iron up and down a few times to get the seal as good as possible. I used a 15W iron, others have reported that their soldering irons just burn holes in the film...

Fold over 1 cm of film all along the heat sealed edge, and crease the film by running your fingernail along it.

Dribble some glue along the crease, and fold the film over.

This glue is very thin and will run down the crease. You don't need a lot of glue, most of it ends up on your fingers anyway!! I haven't tried a thicker CA like ZAP-A-GAP, might have some advantages.

Don't press and hold the film with your fingers or they will stick to it!!!! Instead run your fingers back and forth along the seam until the glue goes off. If you want you can put a piece of newspaper on top of the seam but this sticks to the bladder and makes a mess. This just looks ugly, doesn't affect the glue joint... In fact I think you get a better joint like this because you can press hard on the newspaper with a ruler or a long batten. The glue takes a few seconds to go off, and then it keeps curing for a little while afterwards.

I don't think CA sticks to polyethene so it might work putting a piece of that on top instead of newspaper. Haven't tried it though.

Takes a little practice - do a test seam first!

Fold the film over again and glue as before. The original heat sealed edge should now be glued inside the second fold you made. I did my first bladder this way and it was airtight, no holes to seal.

I tapered the ends of my LE bladder using a different method and this was a disaster - lots of little leaks. (The 'Bad' method in the picture below) I think the problem was partly that the LE bladder had already been used in the kite and so the film was creased... The method I used for the LE bladder was to heat seal as before and then glue a long strip of film over the top of the sealed edge. It didn't work very well, and I don't recommend it, but in theory I suppose you could get a neater joint this way... I might try this again with a new piece of film.

For valves I glued on armband valves... I am going to add a second valve to the LE with the non return flap cut off so I can deflate it easily.

Don't glue your fingers (or anything else!!) together... :-)

As I mentioned I tapered the last metre or so of my LE bladder so that there is less film bunched up at the ends. I don't know if this has made any difference - probably not. But it did show me that it is difficult to get a good seam on a bladder that has been used in a kite. This is because the film gets creased and it's harder to get it to sit flat. For a good glue joint you want it as crisp and flat as possible. So If you start off with no taper in the LE, don't cut it to taper the ends after it's been in the kite like I did. I had to make a second LE bladder ;-( The second one I made has tapered ends and they are airtight.

I wouldn't bother tapering the ends of the LE...

Contact me! redhot@freeuk.com