Composite Fuel Tanks
This section is seriously frowned apon by Sequoia Aircraft corp. as Alfred considers this a modification and he doesn't want other people being an alternative source of information. Therefor, please view this information in the spirit it is displayed. I make no warranties etc. and don't wish this to come back to me via Sequoia.
Sequoia sell a metal fuel tank system. They have done a hell of a lot of testing to get a front tank that works and shouldn't split open. Unfortunately, it is reflected in the price. This was WAY out of my league, price wise. So what did I do??
Well first Up I tried many sheet metal companies. They didn't want to know. It wasn't that they couldn't do it, but that they wouldn't do it. It had nothing to do with liability either.. we don't have that problem.... yet. They just saw it as too complex and made the assumption that it wasn't cost effective to make. I eventually found a firm, and got them to make the rear tank. (start simple) The end cost was also very expensive although 1/4 the cost of Sequoia. For the front tank I started making the damn thing myself. I formed all the aluminium parts and got a guy to stich weld them together while I checked the fit. A couple of welds broke. If it was completed, it probably would not have, but it put me off. After reading the problems Sequoia had with their tanks and all the mods and additions they had to make to stop them splitting apart I was nervous about the front tank. Also the hassle that Steve Wilkinson had, sealing a small hole that generated in a weld.
There had to be a better way.
After much reaserch I came back to the composite tank. I had given it brief consideration a long time ago but dismissed it. My main concern at the time was the electrical bonding, or rather, lack of it in a composite tank. Now I think I have a suitable work around for that problem and I feel that all the other benefits far outweigh the downsides. The way I see it, the composite tank is strong, will resist cracking, doesn't have welds to fail, is VERY cheap, doesn't require any fancy welding of aluminium and I should be able to make it myself. So I was off... scrap the aluminium tank I had half made.
First, what resin to use? I did a lot of research. SafeTpoxy seemed very resistant to Fuel. It isn't made any more but has a close cousin EZpoxy. It would mean importing a hazardous substance and the stuff has some very nasty chemicals in it. I looked further.
A lot of epoxies didn't spec the required fuel resistance so I moved away from epoxy despite the most desirable slow setting of epoxy. I looked at poyester resin. That had reasonable fuel corrosion resistance in low temperatures but still didn't fit the bill. Lastly I looked at vinylester resin. Here it was. A resin with plenty of fuel resistance. I looked at various types and decided to try Dow Chemical's Dericane 411-350. A quick email to Dow had a very helpful chap phone me back with all the info I needed. The chemical is available here !! I can get smaller quantities, it's pre-promoted and its cheap. Best of all, I wasn't doing anything new. Service stations use underground gas tanks made of the stuff and, I was told, auto gas is more corrosive that Avgas !! FANTASTIC!!
Next I got a product sheet to get an idea of what to do.
The recommended method is to have 2 layers of matt fowed by one layer of BID cloth for the structural part, followed by one layer of matt. All layers must have a lot more than the normal quantity of resin to make it seal. The mold should also have resin spread over it prior to any cloth to ensure complete wetting of the cloth on the inside of the tank.
I decided I would use 2 layers of matt and one layer of BID and stop there, mainly for weight reasons.
I made a large polystyrene mold and covered it with plastic to prevent the stuff from sticking... cutting a long story short, here is the result. If I do it again I will modify my method but I think the end result is fine.
Here is the completed tank. It's not the pretiest thing you'll ever see, but then again, neither is the alluminium one !!
All in all I am quite happy with it for a first attempt.
28th Dec 2002
|The rear tank was made before I
decided to make the front tank from composite. I just had it tested (It leaked) and
repaired. Here it is, freshly installed.
You might be able to make out my ELT antenna under the control runs.
22 Jun 2003
|Something else that may interest is the fuel valve I'm using. I ordered an Andair fuel valve and got them to make a custom face plate for it. The valve is awesome. It is silky smooth with very positive stops and it protects the off position by requiring you to pull the lock pin out to select off. I can absolutely recommend this valve. It is the best I've ever seen.|