Wing skin oil canning

[mototopo]

Hi all,
I just completed my (first) tank and I'm moving on the upper wing skin(s). I have still not installed the tank because I'm waiting for the proseal to dry before testing the tank for leak, but I clecoed the top skins for a test.

Well, all the hole on skins and structure lines up perfectly, double checked the structure on the jig and it looks straigh... but I noticed a certain amount of oil canning on the 2 most outer bays.

I contacted Van's and Gus told me that is not uncommon... sometimes it looks better when riveted, sometimes changing the bend in the rib flange slightly make the skin tighter but that is only a cosmetic problem and I can leave it as is or, if I prefer, I can rivet or bond a few stiffeneres between the ribs.

Someone else experienced the same problem? How do you fix it?

Is possible that, even if everything looks square and straight, not installing the tank can cause this problem?

Thank you for your help.

[Jamie]

Hi Claudio.

It's really impossible to know for sure if it will oil can until it's riveted. I had the same problem with my fuselage when it was upside down.

If I were you I would just double-check to make sure the wing is as straight as possible and then rivet it.

Some people will undoubtedly respond to your query and suggest heating the skin with a heatgun while you rivet it. I honestly don't think that has any effect whatsoever (except maybe weakening your aluminum if you get it too hot) but some folks swear by it.

[flion]

Heating the parts helps reduce the oil-canning on summer days and the like. Making sure the rib and spar flanges are properly bent is more important, IMHO. I've seen a lot of fluting instructions that have you flute until the rib or whatever lays flat. Well, pick it up and it's still flexible and flimsy. But it's possible to have the part lay flat and the flanges be under- or over-bent. It's not too late to address the problem when the parts are together (in fact, riveting the skeleton can cause some problems); you can use a string to check that all the flanges are 'normal' to the skin. Or, a large piece of scrap sheet will help, especially at the spar flanges which are hard to check with the string method. When you rivet, you don't want the skin pulling the flanges to it or the flanges pushing the skins away. Finally, don't sweat the oil-canning too much. Even the tightest skin will flex a bit. What you don't want is such a severe condition that it 'snaps' while flexing, making that distinctive oil-can 'ka-puck, ka-puck' sound.

[mototopo]

Thank you!

I'll check again for the correct bend on the rib flanges.
By the way, the "ka-puk" sound is lighly audible... the skin flex about 5 mm (don't have my unit converter on hand right now ) and I would like to avoid this on the finished wing.

[mototopo]

Ok, following the suggestion from Patrick, I removed the outer skin and re-checked rib flanges for the correct bend. I noticed a small under-bend on the first three ribs (counting form outboard). I had to slightly bend the flanges a little more and add a little touch-up with the fluting plier in a couple of spot.

Re-clecoing the skin with a cleco in every other hole, I noticed the improvement was really visible, but a little of oil canning was still there... I reclecoed the skin with a cleco in every hole and I ended up with a skin tight like a drum!!!

So, I'm (almost!) sure I will have no more problem once the skin will be riveted!