Oil Canning Solution/Question?

[Reflex]

I have some oil canning on the bottom on my right wing as depicted by the red X in the drawing below. This appears to be somewhat common. The oil canning is slight, but it's there. After completing the wings on my -14 and having ample time to cool off and think about it, I've decided that I don't want to live with the oil canning.

After looking at countess methods of properly solving the problem, I don't believe it's possible to fix with a stiffener. Connecting to the ribs on each side of the bay is simply not possible. Today, I had an idea that I thought I'd run by you.

It almost makes me gasp.... but what about putting a very small flute(s) in the middle of the bay? It scars me to think about purposely putting a dent in a wing skin. Here's how it would work:

• Lightly abrade the skin where the flute(s) would be placed
• Lightly flute the middle of the bay to reduce the length of the skin between the ribs. The area would be about the size of a quarter.
• Repair the flute(s) with an epoxy/flox mixture

The aircraft will be painted so the flutes wouldn't show.

Am I nuts?

image ru

[Bavafa]

I can't see the pictures so not sure where the oil canning is in the wing.
Since you are suggesting fluting the skin, I assume it has not been riveted yet. So, are you sure you will have oil canning if riveted?

As far as I know the common area in the 14 that is prone to oil canning is the bottom and sides of the fuselage behind the baggage compartments.

[Reflex]

It's unfortunate that some folks can't see the picture. The wing is fully riveted or it would be a simple fix. Before riveting, I placed a cleco in every hole and had no oil canning issues. Followed the directions and riveted in the recommended sequence. I guess it's just unfortunate that it turned out this way.

For those that can't see the picture, the oil canning is taking place about 3 inches above the center inspection port. This bay is located just outside where the inside and outside skins overlap. Based on what I've seen on some of the builders web sites, it isn't uncommon for this to happen.

For fluting, I've machined what looks like a ball and socket and will weld them on to Vice Grip clamps. Those clamps would easily reach the center of the bay and when squeezed would reduce the material/distance equally in all directions.

Just a thought. Haven't tried it. Looking for input.

[Bavafa]

Could you use a section of J-channel to proseal to the skin and tie each end to it to each ribs by the way of a couple of rivets. The rivets could even be a pull rivet but 3" south of the access panel ought to give enough access to do this.

[PaulvS]

"After completing the wings on my -14 and having ample time to cool off and think about it, I've decided that I don't want to live with the oil canning"

Do you really need to do anything? (just saying) It is underneath the wing so it's not really in a highly visible area, so maybe only you will notice it. I'd worry that the repair could make it worse and then what will you do? Just saying!

[paul330]

I had some oil-canning on a couple of bays on my -10. It bothered me too. Now I couldn't even tell you which bays they were ........

Trust me, you'll have enough REAL problems to fix without worrying about this

[jliltd]

Reflex,

I admire your craftsmanship-like quest of perfection. If you are worried about out-plane distortion on the skin I wouldn't get too excited about it unless it was really large. This is merely a cosmetic item when parked on the ramp. In fact the skin between the ribs and spars (a bay) is actually designed to enter into what is known as "diagonal tension" where the skin oil cans and suddenly makes the panel/rib bay stronger by an equivalent diagonal warp becoming a virtual bar joist element firming up the load path. So before diagonal tension sets in there is a shear flow around the perimeter of the skin at the rivet lines. Once the load increases to a certain point it causes that skin "wrinkle" allowing the stressed skin to go into that diagonal tension mode where the shear load follows a direct stiffened diagonal path across the skin tying the substructure together nicely. So oil canning, as it were, is actually part of how stressed skin systems work.

Since this is on the bottom of the wing the predominate loads will be in tension under positive G and normal flight loads will smooth out the oil canning prior to any diagonal tension mode. So there should be no appreciable drumming of the skin in flight. And with the structural scale and loading skin fatigue cycles would be approaching the equivalent of infinity, or at least what that is for the lifetime of any small aircraft. The particular bay you are referring to is one of the smaller sized bays which helps.

On the bright side, on the bottom it will be out of sight when parked.

As far as your designed spherical bead fix, I don't know if it makes sense to make a permanent deflection in the skin in order to get rid of the oil canning. You are taking a structural system designed to stress the skin and then introducing a new permanent out of plane divot by purposely yielding the structural skin material at the same time. Then you go back and fill this with a ball of Bondo, adding weight with possible future maintenance. The cure may well be worse than the original symptoms. Your proposed deflection fix should not be confused with the linear beading or corrugations as seen on many GA airplane skins used to stiffen control surfaces while eliminate internal stiffeners.

If you want to stiffen the panel you could rivet or bond a doubler or small angle across the inside of the skin. You could also put a small amount of spray foam on the inside surface to stiffen the skin.

I could not see any photos in your post but I did see an isometric drawing of the skins with the skin area having a red X on it. Perhaps if you posted an actual photo of the oil canning it would be helpful.

I bet if you leave it an continue on and finish and fly the bird you would forget all about it, as alluded to by a previous poster.

[Jetmart]

Newbie here. I have never heard it discussed before but is there a concern at what temperatures the riveting is done at? Any concern with expansion and contraction with changes in temps? I realize that there is huge changes in temps when flying.

[f1rocket]

The first thing I would try is to simply proseal on a length of J-channel into the bay. That?s assuming you can get a hand in there.

[jliltd]

Originally Posted by Jetmart

Newbie here. I have never heard it discussed before but is there a concern at what temperatures the riveting is done at? Any concern with expansion and contraction with changes in temps? I realize that there is huge changes in temps when flying.

A proper question that. There is thermal expansion and contraction, each material having it's own ratio (coefficient of thermal expansion). This value is combined with the geometry of the parts to complete the picture. The larger the parts the more it matters.

Thread drift on that subject:

Boeing has "Summer" and "Winter" spars for some of it's long-winged aircraft. The assembly supervisors figured out long ago that if the as delivered spar length was either a bit too long or bit too short short (at the extreme ends of design tolerance) they would play heck in the assembly jigs with the tolerance stack up adjacent structure. So when an occasional bit short spar would show up in the Winter they would place it to the side in what was referred to as the "Summer Spar" stack. If a bit long spar showed up in the Summer it was placed aside in the "Winter Spar" stack. The spars were so long that the thermal expansion and contraction caused by the seasonal temperatures was just enough to get the spars to play well in the jigs, the long ones being used during cold temps and the short ones being used in the warm temps. The longer the piece of structure the more the total length change.

Jim