Composite fuel tank question

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cluttonfred

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The Volksplanes use a composite fuel tank/forward fuselage decking originally designed for polyester resin and ordinary marine fiberglass cloth ("Alsynite" sheet still exists but now appears to be a fiberglass roofing manufacturer in Asia.) Here's the page from the now almost 50-year-old VP-2 plans:

Composite gurus, with what we know today about various resins and how they react with gasoline (including automotive fuel with potential alcohol content) what would be the best resin and fabric for this type of fuel tank today?

Bonus points if there is also a compatible sheet material readily available to use for the baffles and tank ends instead of making your own.

If the best answer is "just use polyester resin and ordinary fiberglass and slosh the tank with sealant" then that's fine, too.

If you're answer is, "Never mind composites, do it in aluminum and pop rivets," then we've already had a thread on that. ;-)

Thanks!
 

wsimpso1

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Glass cloth of similar type and vinylester resin. It doesn't mind ethanol at all, and usually has a higher Tg than the usual epoxy resins. Make the layup plies against the inside of the tank wet to preclude leaks.

Billski
 

Bill-Higdon

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The Volksplanes use a composite fuel tank/forward fuselage decking originally designed for polyester resin and ordinary marine fiberglass cloth ("Alsynite" sheet still exists but now appears to be a fiberglass roofing manufacturer in Asia.) Here's the page from the now almost 50-year-old VP-2 plans:

Composite gurus, with what we know today about various resins and how they react with gasoline (including automotive fuel with potential alcohol content) what would be the best resin and fabric for this type of fuel tank today?

Bonus points if there is also a compatible sheet material readily available to use for the baffles and tank ends instead of making your own.

If the best answer is "just use polyester resin and ordinary fiberglass and slosh the tank with sealant" then that's fine, too.

If you're answer is, "Never mind composites, do it in aluminum and pop rivets," then we've already had a thread on that. ;-)

Thanks!
There used to be a flat polyester glass sheet available at a lot of lumber yards back when the the VP-1 & 2 were designed one trade name was Alsynite IIRC.
 

cheapracer

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Considering how thin it's going to be, Kevlar would come out reasonably cheap?

Thinking of the impact resistance of Kevlar.
 

cluttonfred

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Thanks, all.

cheapracer, are there any issues with kevlar (or carbon) compatibility with vinylester resin? If so, I could definitely see using one or the other or both (thinking of that kevlar/carbon woven cloth) to get away with using fewer laminations (I hate working with composites).

VP1, what sort of foam was that and was it fuel-proof? Did you buy the sandwich already made or do the layup yourself? I'd love to see some pics of your tank, ideally in process and the final result.

Cheers,

Matthew
 

cheapracer

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cheapracer, are there any issues with kevlar (or carbon) compatibility with vinylester resin? If so, I could definitely see using one or the other or both (thinking of that kevlar/carbon woven cloth) to get away with using fewer laminations (I hate working with composites).
I honestly don't know, my talents are limited to simple VE parts and I have never done anything in Kevlar, sorry, I am just aware of it's attributes not if it's VE friendly, though i can't see any reason why not.

I have another wild cheap idea that I am going to try one day, perforated aluminium sheet with a layer of VE either side. I think in an impact the aluminium would spread the load over a greater area and maybe protect from puncturing. I may actually test it in a few weeks time, I am coming close to having to do my fuel tank.
 

Geraldc

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I used one layer of 100 gsm (about 3 1/2 oz) on inside and one layer 200 gsm kevlar with vinylester
 

lr27

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I've seen some of the ways fiberglass, carbon, and Kevlar composites fail. Given the choice, I'd prefer Kevlar or something like it for a fuel tank. At least if it isn't damaged by the fuel in question. (It's probably off topic, but I've seen the results of carbon composites shorting out high voltage AC. One part looked almost exactly the same, except all the epoxy was gone. Another burst open with lots of fibers sticking out like hair.)
 

Geraldc

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Sorry, I missed something, was this a VP tank? And inside and outside of...foam?
My tank is not for a VP but was using as example of materials used.
My tank is made with no foam only one layer fibreglass and one layer kevlar for the skin and
the same for bulkheads which are also baffles.The bottom and side in my tank are supported by aircraft
structure so this is strong enough.
Tank plug is formed out of a block of blue foam and covered in carton tape.
Tank bottom and sides are layed up on this plug .
Baffles have already been made on a flat surface.
Baffles are glued into bottom tank.
Top of tank is formed on same plug and then glued to bottom section.
 

wsimpso1

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Hmmm. The thin stuff folks are talking about scares me a bit. You folks had better have absolutely no way to make much pressure in these tanks. Let's lay out some conditions:

You want to know that:
  • a blocked vent and a fuel pump drawing from the tank at max vacuum will not collapse the tank enough to cause it to rupture someplace and dump fuel;
  • a blocked vent and a transfer pump filling the tank at max transfer pressure can not expand the tank enough to rupture someplace and dump fuel;
  • a full tank and max g in positive and negative directions will not overload the tank with head pressure through tank height;
  • a full tank in a spin will not overload the tank ends through centrifugal head pressure through tank spanwise dimension;
  • a full tank with aero load drawing on the outside skin of the tank can not expand the tank enough to rupture someplace and dump fuel;
  • a full tank at your crashworthiness limits can not be ruptured.
I am sure that you can come up with a few more. Establish the ones that apply to your plane, then check that you have enough tank wall strength and stiffness. Maybe you do or maybe you don't... I like SolidWorks for this stuff.

Billski
 

lr27

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At least a well made Kevlar tank is unlikely to leak much if collapsed.

With all those pressures, it sounds like a spherical tank would be best! Also weak fuel pumps.
 

Russell

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Please test products prior to using then in your fuel tank. A few months ago, the vinylester and ethanol question was asked on another group that I am a member … I wrote the below

When building my Glasair I was skeptical about the ability of vinylester and foam to stand up to fuel long term. I had the section of the wing that is cut out for the retractable gear. I cut that section into several one inch square pieces. The one inch was held as close as practical so I could measure it after testing to see if the size changed. One sample was placed in a jar of AvGas, a second in a jar of auto gas. After over a year both samples were removed and measured … I detected no change in either sample. I then pealed the glass from the foam to check the peal strength of the gas samples, I compared those with a sample that had not been subject to any chemicals … I could detect no peal strength differences in the three samples.

Fast forward about eight years. Our ever so wise government mandated that auto gas be contaminated with ethanol. I ran the same test using ethanol gas. After 24 hours the sample started to feel slimy, in 36 hours the glass fibers started to become free of the vinylester. I suspect that ethanol fuel would turn your Glasair wing to rubber in less than a week.
 

Russell

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One more thing, I have built several fiberglass tanks for water and for oil and an intake manifold. I found to prevent pin holes, use a layer or two of the fine fiberglass cloth (the 2oz. stuff commonly found in model planes) sandwiched between the structural fibers. The 2oz cloth has a fine weave that closes those tiny holes.
 
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