Gap between fuel tank skin and interior ribs

[HighSchoolBuilders]

Hi all,

Please help!

I was wet riveting one of my fuel tank ribs, the sealant created a mess so I decided to clean it up and thinking to reapply the sealant to form the fillet.

After I clean the area, I notice there is a considerable gap between the rib and skin. There is a good amount of sealant oozed out as I rivet it, and I clecoed every single hole, and I only remove one cleco at a time as I rivet forward to aft.

I have been using the fay sealing method throughout, and decided to give wet riveting a try.

I am not sure if it is normal to have a gap to allow the sealant in between. Would love to hear any suggestion and pointers, thanks!

Hank

[HighSchoolBuilders]

The thing that worries me most, is that I only notice this gap because I happen to clean the area that would normally be covered entirely with sealant, and I wouldn't normally clean it up if I didn't make a big mess this time.

Theoretically, there COULD be gap on all my ribs....

[RV7ator]

Don't worry about the gap. As long as you have sealant around the rivets, the tank won't leak. Don't forget to apply a dab of sealant atop each shop head.

The gap could be for several reasons. 1. Too much sealant - it looks like you use considerably more than necessary, but better too much than too little. 2. Flanges not flat to begin with. 3. Rivets not set enough - it does look like some of the rivet shop heads could use a bit more bucking.

Several comments. It looks like you dimpled the skin holes that receive the baffle. These should be countersunk. Don't "undimple" if you have. It just makes the baffle more difficult to drop into place for assembly. Be sure to clean up any rib sealant from any future sealing areas, like the smear and dribbles onto the skin aft of the rib flange. You also don't want this area puckered up too much with excess sealant as it will inhibit the skin pulling down and seating on the baffle flange when you rivet it in place. To this end, I cleco the baffle in place while the rib/skin sealant cures. It helps hold the airfoil shape.

John Siebold
Boise, ID

[larrynew]

What John said in a picture. Be sure and remove (or prevent with tape) any sealant above the yellow line. It will interfere with the baffle sealing.



Also, isn't there supposed to be a gap left open at the corner like in the picture below? I can't tell in the original picture if it's there.

[DanH]

I dunno guys.....compromised structure is compromised structure. You wouldn't consider a big skin-to-rib gap acceptable anywhere else in the wing. And no, the sealant doesn't make a difference. At only 270 psi tensile (less after a hot fuel soak) and about 45 lbs/in peel the sealant is not a structural adhesive. Being only Shore 50A, it's also not stiff.

The sealant technical publications assume excess sealant is squeezed out during fit-up, with 0.001" to 0.003" being the assumed sealant thickness between riveted parts.

[BillC]

I agree with Dan on this. In addition, this gap will require that there be an identical gap when installing the fuel tank back plate (structural issue). In addition this will cause the skin on the tank to sit higher than the other wing skins and will need to be pulled down with the screws. I would run this by the experts (Van's support team) before proceeding.
JMHO

Bill

[HighSchoolBuilders]

Thanks guys, I just sent Vans an e-mail, will let you know what they say.

John, larry: Thanks, understand what you're saying about the excess sealant, thanks for the advice!

[HighSchoolBuilders]

Vans said it's okay only if there is sealant in between. In my case, I need to fill the gap up with sealant.

Thanks for all your responses!
Hank