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aluminum fuel tanks, welded vs. riveted/prosealed.

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gschuld

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Joined
May 9, 2007
Messages
422
Location
Toms River, New Jersey
I will need to make two stub wing fuel tanks for my KR project. I am fortunate enough to have a professional marine aluminum fuel tank and sportfish tower builder as a good friend of mine. I can get the material at cost, and I can have free run of his shop(bending brakes, etc. to cut, shape, and prep welded style tanks for him to weld up for me once they are ready(I've already built a few tank with him in the past this way for a few boat projects). Other than the wholesale cost of the materials, it will cost me nothing for top quality welded tanks. Welded tanks use thicker material than riveted and prosealed tanks though. My tank would be roughly 22" x 20" and follow a laminar flow wingshape chordwise, about 8" tall at the fwd end tapering to 5" at the back end(about 12 gallons). I figure that using .090 aluminum in one continuous wrap with shaped end pieces and a single center support/baffle would be the most logical construction with the least amount of weld seams(plus the anti slosh foam). Assuming .090 is a reasonable thickness, I come up with just under 12 lbs per tank. This is nearly identical to a similar sized molded (Moeller)marine polyethylene tank of the same interior volume(though it obviously wouldn't fit since it's reactangular). I am not sure what thickness would be appropriate for a riveted/prosealed tank, but I'll guess that the thickness would be roughly half the .090 of the welded. If that is the case, having the thicker welded tanks could cost me an extra 12lbs overall. For a 1350 target gross weight two place plane, that is a decent amount of extra weight.

Is it fair to say that going with the thicker welded fuel tanks will likely result in stronger, more puncture/crash resistant tanks? That would certainly help to justify the extra weight. I also would have the peace of mind knowing that the tanks were done up by a pro who has built hundreds of aluminum fuel tanks for the abusive marine applications. Neither he nor I have any experience with riveted/proseal type tanks so that is not a plus, though I'm sure I could figure out how to do it competently.

Any thoughts?

George
 

PTAirco

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Joined
Sep 20, 2003
Messages
3,548
Location
Corona CA
I like welded tanks - but with 0.090" the weight becomes an issue, especially for a KR.

I had some tanks welded, that were 0.040" and the guy who did the welding (TIG) was more used to 0.125" (marine tanks too), but he did a great job. I think from a strength point of view you can go overboard - fuel lines will likely rupture in any serious crash, for example, even if the tanks stay intact. I think my 0.040" tanks will do as well as any. I foamed them in place, which should help protect them.

I always thought Nomex-type flying suits would be a useful safety precaution if one is concerned about cockpit fires.
 

gschuld

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Joined
May 9, 2007
Messages
422
Location
Toms River, New Jersey
I didn't even consider going with such light gauge aluminum for welding. 0.040? Wow! Hell I'd be down to just about 6lbs per tank. That sounds great to me as long as that is considered strong enough. I know if it can be done well, my guy can do it. Hmm, are there a lot of welded 0.040 fuel tanks out there? I'm curious where you came up with that thickness choice?

Being as small as my tanks are dimension wise, I have serious doubt that they will be subjected to any twisting or racking forces being that they will be located between laminated wood (and carbon reinforced) stub wing spars.

George
 

Jeremy

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Joined
Feb 23, 2003
Messages
75
Location
Salisbury, England
I agree, 0.090" is going to be a bit heavy. I've made welded wing tanks in the past from 1mm (0.040") and found them to be fine. The technique I used was to fold the tank up (with hand formed end plates that conform to the wing section) and pop rivet it together, using alloy rivets. The tank was then welded up and the rivet heads welded over. This works really well, as it minimises the distortion from welding. The only thing to remember is to drive out all the rivet steel cores before welding and rattle them out of the filler hole.

I made up forms for the end plates from MDF and just sandwiched alloy sheet between them and beat it to shape. Really quick and easy to do and needs no special tools. The top, bottom, front and back skin was simply wrapped around, with gentle folds at the corners and an overlap of around 12mm (1/2") at the trailing edge, pop riveted together, then welded over.

I machined up top-hat profile alloy bosses to take the threaded fittings and just welded them in place.

all told it was a pretty easy job and has lasted a few years now with no problems.

Jeremy
 

hiryu

Member
Joined
Mar 28, 2006
Messages
15
FWIW Tony Bingelis discusses riveted and sealed tanks using .032
 

PTAirco

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Sep 20, 2003
Messages
3,548
Location
Corona CA
I can't believe I don't have a picture of the finished tanks, but here they are in progress. They fit into the leading edges of a high wing, hold about 8 gallons each.

Being this thin, they do need a lot of support, the rear edge fits snugly into the angle section of the spar caps and the rest is supported by two part foam squirted into it. May I never have the misfortune of having to remove them. (Actually, it can be done in extremis.)

Your welder will be aghast if you ask him to weld this kind of thickness. You need to design the joints for it - no simple butt joints; the stuff warps so much while welding they will never line up. I bent simple flanges at all joints to make lap joints and it worked ok. He even managed to weld the fairly heavy screw-in flanges for the fitting without too much problem.

I think with enough support and a good welder it is doable, but I would swallow a little extra weight and go to maybe 0.050" or 0.065" next time, just to make it a little more forgiving. My airplane is a very lightweight job, so I was watching every ounce.
 

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gschuld

Well-Known Member
Joined
May 9, 2007
Messages
422
Location
Toms River, New Jersey
Thanks for the thoughts. I will certainly kep this in mind and perhaps do a litle seam experimenting with different gauge sheets of aluminum and see what I'll be really comfortable with.

As a side note, though I intend to run LL AV gas, I would like to make the fuel system as ethanol/mogas capable as possible from the get-go. You just never know what will be forced down our throats in the future:ermm:. Just trying to be prepared just in case. Aluminum seems to have a decent reputation of being fairly impervious to ethanol. Does that sound about right?

George
 

Midniteoyl

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Joined
Sep 3, 2003
Messages
2,406
Location
Indiana
With the riveted tank, what's the added weight of the proseal (and rivets... ;))? Seems to me that a .065 wall welded tank would be the same, if not a little lighter after all is said and done.

Besides, proseal can fail and leak sooner than a weld.
 
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