The International Cessna^® 170 Association

I'm looking at purchasing a '48 ragwing that was converted to metal covered wings in the 80s. I have a mechanic doing a pre-buy and he mentioned that he noticed oil canning on the bottom skins of the wings. He hasn't looked at it closely yet but doesn't think it is an airworthiness issue. My question is, is any oil canning in that area actually acceptable? I've never noticed it at all in any of the nosedragger Cessnas I've flown. I also have some time in a '46 140 with metallized wings. That covering job was pretty crappy (cosmetically, at least) and I didn't notice any oil canning there, either. Then again, I didn't notice what the mechanic noticed when I went to look at it, so maybe I'm just not observant enough.

A lot of Cessnas have oil canning.

Yes a lot of Cessna's have oil canning. As already stated, a metalized '48 is a creature all it's own with no other '48 or even Cessna to compare directly. So all you can say is this '48 has oil canning like lots of other Cessnas. Unless you or your mechanic think it an issue, it is probably not.

As others have said, the "oil-canning" is not uncommon even with factory built wings on other models. The metalized wings compound the problem for a couple reasons. If the geometry (twist) wasn't really close to spec when the aluminum was installed, then any washout adjustment after the wings are installed will cause the oil-canning. This alone would not be an airworthiness issue.
We see this often on Maule's. The dihedral is adjusted with the forward lift strut length, and the washout/twist is then adjusted with the aft lift strut length. Sometimes the skins "can" quite a bit when the twist is corrected to make the plane fly hands off.

Del is referring "generically" about twist in the wings of 170 models. In flight, the lower skins will likely "tighten up" and no longer oil-can.

There is no twist in the ragwing/metallized ragwing. Here are the specs:

Dihedral …………………………………………………………..0 degrees
Fabric wings have no built in twist.
Angle of incidence for the stabilizer . . . . . . . . . . . . . . . . . . . . . . . . –4 degrees
Length of struts – front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.47 inches
Length of struts – rear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106.58 inches

The length of the struts is measured from centerline to centerline of the bolt holes. The wing can be set in place with the strut installed, and any additional rigging required due to wing heaviness after test flight should be rigged out by lengthening the rear strut on the light wing one rotation at a time. In doing so, there is one caution; namely, double check the length of thread engagement in the strut adjustment. This can be done by checking the safety hole in the fitting made for that purpose. If a wire will not go through the hole, the thread length is satisfactory. This information was taken from Cessna service letter S.L.N. 49 dated 9-26-47 and a letter from Cliff Ives dated 8 December 1994.

George is correct. Forgive me speaking in generic terms. But I must add dihedral/anhedral and washout (or lack of either) is a specification, no matter the degree. If these tolerances are out of spec (manufacturers specification) in either direction from said specification, then either or both can cause the oil canning you see.