Fuel Tank Sealing Tips

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GESchwarz

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I would like to know how to close up an aluminum wet-wing fuel tank so that it will never, ever leak in a thousand years.

I have never done this before, but I think I have some pretty good ideas about how to do it. I want to run them past you veterans to see if you have any better ideas.

I have formed aluminum ribs that exhibit crimped flanges which mate to the inside of the skins. So this is an uneven surface. I think it might be a good idea to apply a coat of polysulfide sealant to the mating surface of the flange so as to fill the low spots of the crimps so that the resulting mating surface is more flat to the the skin to which it will be mated. This first coat would be allowed to cure before the actual mating. Before mating, a second coat of polysulfide would be applied.

Preping of the bare metal mating surfaces should be done with the same discipline to achieve a good bond as you would for a structural epoxy joint...Detergent clean, mechanical abraid, acid etch, DI water wash, rinse, and dry immediately prior to application of the sealant.

Every rivet hole that penetrates through the walls of the tank should receive a small bevel on both sides of the sheet. This bevel would provide a small volume of space for polysulfide be applied, that will not be wiped clean by the rivet as it passes through the hole. Upon bucking or pulling of the rivet, the rivet shank will expand into that space, the polysulfide will extrude out filling any leak in the immediate area of the rivet shank.

Before the final assembly of skins to the underlying ribs and spars which define the walls of the wet-wing fuel tank, the polysulfide is liberally applied to all mating surfaces. When the skin is put in place, it must be done so that it is not allowed to come back off. If this happens, the proseal will have been pushed out to the edges on either side of the joint, leaving very little polysulfide to fill any voids when it is layed down the second time, thus creating leak paths.

The skin should be well clecoed in place and rivited entirely before the polysulfide cures.

Mixing of the two parts of the polysulfide compound in the corect proportions must be done very accurately to ensure that it cures. It should be done at a temperature that is low enough so that it will not cure before all the riveting is completed. But as far as I know, polysulfide takes many hours or even days to cure. I am not sure if that is normal.

I have some gaps in the neighborhood of 1/4" at the corners of some ribs that I must filled. Can I fill those gaps with pieces of foam and seal them up with polysulfide? What kinds of foam can withstand exposure to automotive gasoline and avgas? Polyether Polyurethane, aka last-a-foam; Divinycell foam; Polyurethane foam?

That is all I can think of. I am soliciting your suggestions and criticism.
 

TFF

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This is my opinion. First get slow cure Proseal so there is no rush. Because it is a chemical reaction it needs to be working temp. Most of the tanks, ribs are set in place wet then rivets riveted wet. You don't want a pad of the stuff, you want it to fill small voids. 1/4 gaps are too big to fill with Proseal and guarantee no leaks down the road. You might get away for a while and I am sure there are planes filled that way but not good. I would not be backing it up with foam even if fuel proof. You Might have to have some formed metal patches made up matching the riveting. Putting a bead around all the inside or outside edges and on top of the rivet buck heads should only be backup sealing. I hope you give some updates on your plane.
 

gtae07

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I will post more in the morning, but a couple quick things:

Lots of good examples from the RV world; rwad VAF and some builder logs.

Sealant goes on onego. Don't apply it to a flange, cure it, then try to apply more and mate it up.

For big gaps a filler plate on the inside 0r outside.
 

BBerson

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I would build the tank bottom and sides (no top) without any sealant. Then on the inside seal the corners with 2" fiberglass tape and polysulphide like a fiberglass tank would be made with epoxy instead of polysulphide. Then bond the top on. Of course this would need to be part of the wing design from the beginning. :)
 

GESchwarz

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I would build the tank bottom and sides (no top) without any sealant. Then on the inside seal the corners with 2" fiberglass tape and polysulphide like a fiberglass tank would be made with epoxy instead of polysulphide. Then bond the top on. Of course this would need to be part of the wing design from the beginning. :)
Why would you not apply sealant in the lower joints during assembly? Ultimately, that is where a leak must pass to the outside.
 

BBerson

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Why would you not apply sealant in the lower joints during assembly? Ultimately, that is where a leak must pass to the outside.
Because it's messy and hard to get a good seal that won't leak after the airframe flexes. I would use a removable tank top so access is easy 10 years later. With the top off the critical lower joints can be easily filleted and reinforced with 2" tape and polysulphide.
A leak in the top gasket isn't so bad. Because airplanes set unused 99% of the time and wouldn't leak if half filled. A few drips in flight are no big deal. Cessnas have large removable gasketed inspection holes on top for access. Same for some Grumman Goose float tanks. It wouldn't be much different to make a complete top and removable gasket. The Cherokee wing tanks are completely removable with a hundred screws. Anything is possible.
 

GESchwarz

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It wouldn't be much different to make a complete top and removable gasket. The Cherokee wing tanks are completely removable with a hundred screws. .
That's a hell of lot of nut plates. How do you maintain structural integrity of a stressed skin wing if an entire section is removable? I would need nutplates on all the rib flanges too. Rivnuts are easier and would work too. It is still a lot of work.
 

BBerson

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That's a hell of lot of nut plates. How do you maintain structural integrity of a stressed skin wing if an entire section is removable? I would need nutplates on all the rib flanges too. Rivnuts are easier and would work too. It is still a lot of work.
Like I said, an entire section of the Cherokee stressed skin wing is fuel tank and removable.
 

mcrae0104

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Welcome back, GESchwarz. Glad to see you posting again, and would love to see an update on your project thread.
 

GESchwarz

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Welcome back, GESchwarz. Glad to see you posting again, and would love to see an update on your project thread.
Thanks Mcrae. Life gets in the way. 2.5 years ago I separated from my wife of 28 years and I have been launching my new business, Soft Landings Home Inspection. Only lately have I gotten back on the plane in a significant way. I'll gather some pictures together and post them with a status report maybe next week sometime. Getting close to skinning my wings. I turn 60 in a few days so I really have to get humping on this project and get it done.
 

gtae07

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I would like to know how to close up an aluminum wet-wing fuel tank so that it will never, ever leak in a thousand years.
There's no way to guarantee that. Closest you can get is industry-standard practice that works pretty well.

I have never done this before, but I think I have some pretty good ideas about how to do it. I want to run them past you veterans to see if you have any better ideas.
I've built six RV fuel tanks. Two are flying, four have passed leak checks on the ground. I actually kind of enjoyed the process; it's more fun (relatively) than doing all the rest of the structure work. But my heart's really in the electrical and systems stuff.

Before you start, wear old clothes; sealant never comes out of them. Get lots of disposable gloves (Sam's has the best deal running at 400 for ~$16). Put two layers of gloves on--when you need to change them, they go on easier over another glove than bare sweaty hands. Work in a cool environment and use fans if necessary--dripping sweat into your clean, about-to-be-sealed tank is no fun.

I have formed aluminum ribs that exhibit crimped flanges which mate to the inside of the skins. So this is an uneven surface. I think it might be a good idea to apply a coat of polysulfide sealant to the mating surface of the flange so as to fill the low spots of the crimps so that the resulting mating surface is more flat to the the skin to which it will be mated. This first coat would be allowed to cure before the actual mating. Before mating, a second coat of polysulfide would be applied.
Apply the sealant to the flange of the rib (I prefer using a popsicle stick to apply like frosting; others swear by the expensive injector guns), and install it wet. Don't try to do one coat to fill the flutes and let it dry. If you apply the sealant properly it will fill the flutes. You'll get squeezeout along the rib that you'll smooth into a fillet later.

Once you put the rib in place, put a cleco in every hole. Then, remove one cleco at a time, use a toothpick to put a tiny dab of sealant back in the hole, insert the rivet, and set it. Do this while the sealant is wet--some people swear by letting it set up first, but that only works if you can get good clamping pressure along the joint. At least on the big airplanes I work on at my day job, spring clecos are not considered adequate for this.

Once riveted, smooth the squeezeout into a fillet along the rib/skin joint. Last, using a tool or a gloved fingertip, put a little blob of sealant on the shop head of each rivet to encapsulate it. You can use excess squeezeout here.

Any squeezeout on the exterior skin then gets wiped up. Some people will use MEK now, others will wipe it with dry paper towels and wait till the sealant cures before using a solvent. Either way, don't let the solvent sit on the rivets; wipe it and let it dry quickly. Same for smudges on the inside of the tank; be careful not to get solvent on the "good" sealant.

Preping of the bare metal mating surfaces should be done with the same discipline to achieve a good bond as you would for a structural epoxy joint...Detergent clean, mechanical abraid, acid etch, DI water wash, rinse, and dry immediately prior to application of the sealant.
Usual practice is to abrade (scotchbrite etc.) then thoroughly clean all parts and tools with MEK, acetone, or naphtha until no traces of anything are left. WEAR A RESPIRATOR! MEK is really nasty stuff but it works best, if you can get it (Lowe's carries it here in Georgia but I think the PRCa has banned the stuff). I even soak the rivets in MEK and keep a stock of "clean" rivets for this, just to be sure. Your method would work too, I guess.

Also, clean your tools (rivet set, bucking bars) with solvent before and after, and watch for rivet gun exhaust. The fine oily mist it sends out (you do lube your air tools, right?) is probably fine on most of the rest of the airplane but in your tank, that's bad. I wrapped the exhaust with paper towels and a rubber band.

Every rivet hole that penetrates through the walls of the tank should receive a small bevel on both sides of the sheet. This bevel would provide a small volume of space for polysulfide be applied, that will not be wiped clean by the rivet as it passes through the hole. Upon bucking or pulling of the rivet, the rivet shank will expand into that space, the polysulfide will extrude out filling any leak in the immediate area of the rivet shank.
Not necessary. Just do your standard deburr and dimple/countersink routine. All you'd be doing there is weakening the skin and creating more fatigue crack opportunities for no gain.

Before the final assembly of skins to the underlying ribs and spars which define the walls of the wet-wing fuel tank, the polysulfide is liberally applied to all mating surfaces. When the skin is put in place, it must be done so that it is not allowed to come back off. If this happens, the proseal will have been pushed out to the edges on either side of the joint, leaving very little polysulfide to fill any voids when it is layed down the second time, thus creating leak paths.
I've found it easier to work one or two ribs at a time--you don't have to do the whole tank in one go. Mix up enough sealant (see below) for a rib or two, put them in, and mix more if you want to keep working. I don't know the details of your design, but if possible leave off the last part that will "close" the tank and do that on a separate session.
If you really need to pull a part back out, you can; apply a little more sealant to the flange before reinserting it. It's best to avoid having to do that though.

The skin should be well clecoed in place and rivited entirely before the polysulfide cures.
Whatever parts you're installing should be riveted within the working time of the sealant (see below). But you don't have to do the whole tank at once.

Mixing of the two parts of the polysulfide compound in the corect proportions must be done very accurately to ensure that it cures. It should be done at a temperature that is low enough so that it will not cure before all the riveting is completed. But as far as I know, polysulfide takes many hours or even days to cure. I am not sure if that is normal.
I mixed my sealant on plastic plates. Got a cheap "tea" spoon (long handle) for scooping sealant, and measured with a cheap Chinese digital scale off Amazon. Put the plate on the scale, zero it, add a blob of sealant (40-60 grams or so). Note the figure, zero the scale, and add the correct amount of hardener with a toothpick. Check your sealant to be sure but AMS-8802 sealant typically mixes 10:1 by mass. Mix until no light or dark streaks are visible, and be sure to get the contact areas on the plate very well. I mixed using popsicle sticks.
The sealant has a working time and a cure time. Working time is approximately denoted on the sealant itself--you'll see a designation such as B-2 or A-1/2. The letter tells you the viscosity; A sealant is runny like pancake syrup, B is goopy and more like warm-ish caramel. The number is the nominal working time in hours, and there's a direct relationship to full cure time too. I used B-4 on my tanks and it gave me longer working time, but the tanks I did in the winter took almost two weeks to fully cure.
As you work you'll eventually notice that the sealant is starting to get harder to work and spread, and that's your sign that it's about time to stop working with it. That's why it's better to work with smaller amounts and do one or two parts at a time; you won't be as rushed. Full cure will take a week or two; the sealant will be like hard rubber.

I have some gaps in the neighborhood of 1/4" at the corners of some ribs that I must filled. Can I fill those gaps with pieces of foam and seal them up with polysulfide? What kinds of foam can withstand exposure to automotive gasoline and avgas? Polyether Polyurethane, aka last-a-foam; Divinycell foam; Polyurethane foam?
Don't use foam, first.
I assume you're talking about relief radii at the corners and maybe tight-radius parts like around the leading edge of a rib? Don't worry about those on any ribs in the middle of the tank; only the end ribs matter for this. On the RV's, you fill the nose rib area with a metal doubler shaped to that area. The tank attach bracket on the inboard end also fills that gap. You could do the same at corner locations. A small .020-.025 plate with two or three rivets holding it on to the rib (seal them like the rest of the tank) should work fine.


A few other things:

Better people than I will mask off the inside of the tank (leaving room for squeezeout). I tended to get smudge marks all over the place inside that I had to clean up later after the sealant cured.

If you don't already have your sealant, buy it from Van's. Their pricing is half the price of the "generic" at Spruce, and a quarter of the price of name-brand Proseal. I haven't found it cheaper anywhere else. They only carry B-2, though, and it comes in quarts (as opposed to the pints everyone else sells).

Don't be stingy with sealant. Yes, as Billski says "weight is the enemy" but trying to use just the bare minimum (there's a guy on VAF that says "just put a little donut of sealant around each hole" :lex:) is much more likely to lead to leaks. It's worth the slight extra weight to not have to re-seal a tank.
 

gtae07

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That's a hell of lot of nut plates. How do you maintain structural integrity of a stressed skin wing if an entire section is removable? I would need nutplates on all the rib flanges too. Rivnuts are easier and would work too. It is still a lot of work.
Oops, missed this.

RV wing tanks (leading edge) are removable too. The tanks are basically the D-section; curved leading edge skin, ribs, and a "baffle" closing out the back portion that sits ahead of the spar a bit. On older models there's a row of screws along the outboard end, two rows along the top and bottom where it joins the spar, and an angle bracket with bolt at the inboard leading edge. Newer models use one row of screws along the spar instead of two, but have brackets at the ribs that tie into the spar webs (you access the bolts through inspection panels under the wing).

Yes, it's a lot of nutplates. But it makes inspection, repair, and construction a whole lot easier.
 

GESchwarz

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Wow, gtae, that is just the sort of instructions I am looking for. Thank you for your generous input.

My tank is not in the D section but rather between the spars. I did not want it in the crush zone during a crash. I am a nut about crashworthiness. I may have more questions as time goes on. Thank you all.
 
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gtae07

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What problems will I have because my tank is integral to the wing structure, between the spars, and not removable?
Hrm. That's how lots of airplanes do it (including pretty much all transport-category aircraft), so you won't be the first.

If you ever do have to make repairs or do anything like that inside the tank, access will be more difficult and you won't have as easy of a place to cut an access hole (the typical RV method, if needed, is a hole through the "baffle" at the back of the tank). Most likely you'll have to go through as skin, then put a ring doubler in and make a panel. Big airplanes do more repairs inside the wing and have distributed sensors for fuel quantity so they need access, which is almost always through access panels down the length of the bottom of the wing.

I don't know how you're doing fuel quantity and such like that, but RVs have an access panel on the inboard rib that the fuel pickup (if not using the flop tube system for inverted flight) and the fuel quantity float (if using that) will mount to. It makes servicing them easier if/when you have to do it. Current practice is to just seal it on since the supplied cork gasket tends to swell and not work as well. Just make sure your sealant is cured before you add fuel--my dad's former hangar-mate didn't wait long enough and had a big mess to clean up.

At your intermediate ribs, make sure you have some kind of gaps at the lower corners (by the spar) and a couple holes along the lower flange so fuel can pass through even at low fuel levels. Also make sure you have some kind of vent holes or passages high up so you don't trap air in the tank.

Only other thing I could see being an issue would be riveting the second skin on. At some point, unless you build some access hatches in each bay or on the end ribs, you're going to have to go to blind rivets. They make closed-end rivets and Van's uses them for the rib-to-baffle joint, but they're "non-structural" as blind fasteners go so you'll have to be very conservative with the loads.


One other thing you could do if you want, during your prep, would be to alodine the interior parts. Don't prime; there is a special fuel tank paint that large aircraft use but I don't know how well it will deal with gasoline and ethanol. But alodine will be fine. Wait till it's dry then solvent clean. Once the tank's dry and leak-tested you could paint the outside if you want.
 

Toobuilder

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One of the more successful RV wing tank builders detailed his system on VAF and its about as close to a step by step as you can get. Of course it helps that he made his living as a material applicator building fuel tanks for military fighters. His method was diferent in the respect that after sealing and then clekoing it all together, he walked away for a day. After it all cured, he'd pull a cleeko, debur, apply a dab of sealant and shoot the rivet. He claims he never had a leak and his sealing looked like artwork.
 

TFF

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Some Mooneys leak and some don't. The bad ones end up with refit bladders. With anything you have to build access for down the road. Or a plan on how to repair. I worked for a regional airline for about a decade. Plenty of tank repairs. Some planes never leaked, some were repaired and never gave a problem, and then there were the small few we would park over drainage grates so pilots could not see the puddles. RVs have tanks instead of wet wings because worse come to worse you can easily build another tank instead of another wing.
 

GESchwarz

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His method was diferent in the respect that after sealing and then clekoing it all together, he walked away for a day. After it all cured, he'd pull a cleeko, debur, apply a dab of sealant and shoot the rivet. He claims he never had a leak and his sealing looked like artwork.
When you squeeze a bond joint too tightly, the adhesive or sealant can be pushed completely out of the joint, and there is your leak path. That is one reason why I like the idea of applying sealant with a very fine trowel, like a 80tpi hacksaw blade or something like that. Let it cure, then apply a second coat immediately prior to assembly.

I am using the same fuel level sending unit that Van's uses. It will penetrate the tank through the inboard most rib. I may install access panels if someday I have a leak that needs repairs.

To they put the access panels on the bottom skin so that they do not get water inturusion (if the panels were on top)?
 
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