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Tank Pillowing

RV7Factory

RV7Factory

Chief Obfuscation Officer
Since this seems to be an issue not specfic to any model I will post this here.

Despite my best efforts during construction (it was perfect before riveting), I have some slight pillowing of the tank skins between the first two screws which attach it to the splice plate. I understand this a fairly common issue, and I have searched but not found a resolution to my particular case.

Here is what I got...
061026_005.jpg


You can see the tank skin pillowed about the thickness of the skin between the first two screws. The spar isn't sagging, there is no excess sealant in this area interfreing with the splice plate, the tank is screwed/bolted to the spar, and it occurs on both wings in the same spot but nowhere else.

I have been able to improve it somewhat by forming the tank skin by hand, but other than filler ;), does anyone have any suggestions on how to further improve the fit?

Thanks,
 
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Shrink it

I had the same problem and just grabbed the nearest shrinker and very very gently brought the pillows home. Be careful and only a small amount at a time. Practice on some scrap and see just how much this thing can do.
 
Have you tried pushing it down with the screws just a little loose and then tightening?
If that doesn't work maybe slightly enlarge the holes for the screws in the direction of the pillow and then push down and retighten?

I guess if they're on for good (knock on wood!) you could put a little proseal in there and then clamp it down while the proseal dries...

Thomas
 
Bill, can you tell more about a shrinker. I'm not familier with that tool or procedure. Thanks, Mike
 
mine did this

mine did this as well, i have yet to try to fix it. i did remove the nutplates and arch them. helped a small amount but still have some . seems to me its more like a flat spot where the nutplates are but i wondered how many were like this.
 
RV7Factory said:
.......some slight pillowing of the tank skins between the first two screws which attach it to the splice plate......any suggestions on how to further improve the fit?
Click to expand...
Brad,
You could approach the problem from an alternative perspective.
Here is one possibility.
061026005editted1rb5.jpg
 
Shrinker Info

The shrinker is a device that will actually grab the metal and shove it together or if you switch jaws you can do the opposite. It is usually table mounted and is very simple. You put the metal into the jaws and pull on the handle. As the jaws come together they grab the metal and litterally compact it in the area that has been grabbed. It is terribly effective and only a small amount of work will move a lot of metal. You do not want to work your skin a whole bunch and whatever you do dont over do it because you want to keep the strength in the metal. Dont be afraid to use it just be careful. I have seen these tools in several of the suppoliers catalogs. For me it was the purfect fix. I used it in the same area that you have and along the trailing edge of the tank. No shims or proseal to mess with and the tank matches the wing nicely.
 
I like Rick's solution of using shims on the leading edge skin abutting the tank, but here's a couple of things you could try, first:
Get some aluminum tape and put on a layer all around the flange supporting the tank edge. The ply is thin, but by doing this, you will fill up the extra space all around the radius of the tank and this may cause the pillowing to disappear. I'm guessing you could probably even try two layers of tape without seeing a noticeable edge on the tank skin, expecially if you have the edge chamfered.
Also, either in conjunction with the above or separately, try tightening down the screws on the top of the wing first. If nothing else, you might be able to move the pillow effect to the bottom where it is less noticeable.
 
Thanks for all the suggestions!!! I appreciate it.

Rick6a, as usual, you go above and beyond with your assistance. Thanks!

I will try a few of the less invasive things first and see if I see any difference. If I don't see any improvement, then I will go to the shim, which ironically was something I already did on one wing to improve the fit at the aft end of the tank.

I'll post my findings here soon.
 
After trying a few of the suggestions, with moderate results, I decided to go the shim route. It worked great. Thanks Rick!!!

AFTER...
061113_001.jpg
 
Yup! I used .025 and sanded (beveled/tapered) it down on both ends so it was thinner at the ends than in the middle. I made a paper template for drilling the holes and after dimpling I then inserted it (from inside the LE) between the slice plate and the rib and re-riveted. The width of the shim was about as a wide as the rib flange, but maximum width isn't super critical because it is behind the splice plate. As Rick outlined, this brought the LE skin and splice plate out a little further to more closely match the curve of the tank skin.

Edit: In some ways I wish I had gone a little further down the wing (aft) with the shim, say maybe 1 or 2 more rivets, as I still have some VERY slight pillowing between the 2nd and 3rd screws. At about half the thickness of the skin maybe less, it is so slight that I didn't want to push my luck.
 
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RV7Factory said:
..used .025 and sanded (beveled/tapered) it down on both ends so it was thinner at the ends than in the middle....
Click to expand...
Brad,

Good deal. Sounds like you knew exactly what you had to do. You could probably do the same job again in under a half hour. :rolleyes: Hint: Next time you could forego the step using a template and just insert the undrilled shim stock into place then spot the rivet locations on it with a Sharpie. After temporarily removing the shim stock, you would then drill through those Sharpie marks up to a generous size...more than enough to clear the dimples. You have some leeway here. The holes drilled into the shim stock are not at all critical...just the thickness of the shim. But you already knew that. From your picture, it looks like you mated the skins perfectly. Nice.
 
Good practice??

Rick6a said:
Brad,

Good deal. Sounds like you knew exactly what you had to do. You could probably do the same job again in under a half hour. :rolleyes: Hint: Next time you could forego the step using a template and just insert the undrilled shim stock into place then spot the rivet locations on it with a Sharpie. After temporarily removing the shim stock, you would then drill through those Sharpie marks up to a generous size...more than enough to clear the dimples. You have some leeway here. The holes drilled into the shim stock are not at all critical...just the thickness of the shim. But you already knew that. From your picture, it looks like you mated the skins perfectly. Nice.
Click to expand...

Rick... is your hint a good idea?

As I read the description, the shim was placed between two dimpled pieces that were riveted together - the rib and the splice plate on the non-tank side. An oversize hole in a middle piece of a riveted stack does not really sound like a good idea. It should be match drilled and dimpled...

This was standard technique on the old, non-matched, non-quick built kits.

gil in Tucson - with many shims inserted between skins and ribs... :)
 
Thanks Rick, and you're right, I got the other wing done in half the time tonight. I hadn't read your post before starting so I followed the same process as last night... paper template and dimpled the holes. Now both wings look near perfect.

My earlier comment about still having slight pillowing between screws #2 and 3... forget I even said anything! After giving it another look tonight I can't imagine how the fit could be any better. What was I thinking? :D
 
Fixed my pillowing

I followed your lead and fixed my pillowing tonight. Incidentally mine was in exactly the same place. You can overdo it however. Mine may not have been as bad and only needed a shim the length of two rivet holes(plus appropriate ED). Thanks for the tip. :)
 
I have the same exact problem.

I am going to attempt to do the shim in place method described above but I have a question?

How the heck did you get that shim to go in there? Is it not very tight? I can't see how to get the shim to slide into place while fighting the dimples. If I recall, I dimpled the skin, countersunk the splice plate and left the rib alone. (I did it this way because my holes I drilled in the rib ended up too close to the web of the rib to get a dimple die in there)

I want to get this figured out before I go and drill out rivets that are not in a easy place to buck. (My leading edge is already on)
 
Brantel said:
I have the same exact problem.
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Brian;

Very common. I ran into the same deal on both tanks in exactly the same location and followed my own advice. I believe it is caused by the too generous screw spacing at a location with a notable curve to it. To support my theory, notice how close the rivets are in the leading edge adjacent to the pillowing. If you drill out enough rivets, you can wedge a shim in with not much difficulty. As I mentioned before, it is not necessary to dimple the shim...you can if you want to but it is not really necessary. In your case, with the leading edge already riveted on, it will be a bit harder to reach in there and buck the rivets but it is quite doable, certainly with a little help.

There is another (possible) way to fix the problem but I rejected doing it myself for cosmetic reasons. You could lay out another #8 screw hole centered between two existing screw holes in the area of the pillowing and crank out the gap with a little help from the extra screw.
 
No dimples

Rick6a said:
Brian;

....
As I mentioned before, it is not necessary to dimple the shim...you can if you want to but it is not really necessary.
......
Click to expand...

Rick... see previous post #14.....

This sounds like a bad structural practise to me... it would create "air gaps" in stacked layers of aluminum....
The internal dimple would be forced up into the hole on the shim, placing strange stresses in the material.

Sketch it out... :)

gil in Tucson
 
Picking Nits?

az_gila said:
Rick... see previous post #14.....gil in Tucson
Click to expand...
Gil,

I have in fact noted post #14 and respectfully reject its technically correct position as being a bit overblown relative to this particular situation. Engineering theory and practical reality can be two different things. This is not after all, a critical application. All we really want to do is address a mostly cosmetic issue by elevating the leading edge skin to the level of the fuel tank skin by fabricating a small shim that will pick up just two 3/32" holes. Of course you can dimple the holes if you want. Go for it if that gives you the warm and fuzzies. In practice though, I'll wager that doing so will make insertion of the shim a bit more problematic...practically speaking. But if you elect not to dimple the shim, I can promise that after shooting the rivets, that shim is not going anywhere and the skin surface will be perfectly smooth. In side by side comparison, I defy anyone to detect any real world difference in the outcome between otherwise identical shims one dimpled, one not.
 
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