Tank / Leading Edge Misalignment

[Kenneth]

I just finished up my first tank and was excited to get it fit on the wing. Unfortunately, it was pretty obvious from the start that the tank and leading edge skins were no longer in alignment on the top side of the joint. However, the bottom is perfect and there is no problem on that side.

Here is a picture of the joint with the screws backed out to show you the offset between the leading edge and tank skins.



I figured this could be fixed after finding this thread: Tank Pillowing Thread

After my first round of shimming using 5/8" x 5/8" shims between the joint plate and inboard leading edge rib, I had everything lined up perfectly when cleco'd together, but after riveting the gap got worse!

I've since reshimmed (nearly .080" in some places) and the fit is once again perfect prior to riveting.

Just to verify what I was doing was OK, I talked to Tom at Van's and he said he hadn't heard of this ever happening before and suggested that I remove the shims and try machine countersinking the dimples in the joint plate. I just don't think I got him to understand that the issue was not only when the screws were tightened, but that the misalignment causes the joint to deform (tank skin is bent down to match the leading edge skin.)

I don't want to discount his expertise and recommendation, but isn't my shimming technique acceptable also?

Thoughts? Do I have bigger issues I'm not seeing? It appears that the leading edge rib is just slightly offset down which would account for some of the mis-fit, but certainly not .080".

Thanks for any advice,
--Ken

[dan]

Do you see the opposite on the other side? If so, your Z brackets were drilled in such a way that it shifted the tank relative to the spar.

[Kenneth]

Originally Posted by dan

Do you see the opposite on the other side? If so, your Z brackets were drilled in such a way that it shifted the tank relative to the spar.

Nope, the bottom seam of the joint is in perfect alignment - same as it was when I drilled the Z-brackets using the method on your site (great work, by the way.)

I also thought that maybe there was excessive sealant between the rib and tank skin but that doesn't appear to be the case either.

Thanks,
--Ken

[jferraro17]

Double check that the main spar is level. Also, are you putting the z-bracket bolts in first, or the screws? I found that to be an issue when doing the screws first.

Good luck,

Joe

[LettersFromFlyoverCountry]

Does the size of the gap change as you go from leading to trailing edge?

[Kenneth]

Joe,

I doubt the spar isn't level, but I'll definately double check it. I have been putting in the center Z-bracket bolts before fitting any of the the skin screws figureing it'd be easier to get those bolts in that way. The misalignment exists even if no bolts or screws are installed, though.

Bob,

The gap did get worse the further you got toward the spar. Your recent editorial on whether something is "good enough" or should be repaired/replaced is causing me all this grief! You managed to put into words what has been rattling around in my head for too long.

At this point I'm thinking of completely removing the inboard rib and making sure its truly square and center in the leading edge. We'll see.

--Ken

[jferraro17]

It doesn't take much for that spar to screw up the alignment. Mine was "bubble" level, but until I started adjusting the center support, the skins wouldn't align...all the holes did, however!

Think of it this way...sagging spar will cause the nose of the skins to crimp onto each other,..bowing spar will cause the trailing edges to crimp and a gap or good fit on the nose.

Worth checking, as this is exactly what happened on my second wing.

Joe