Tiny waviness/bubbles/surface imperfections in fabric near leading edge

[kubark42]

What are these imperfections in the fabric surface? I always assumed they were due to the fabric application process, but someone suggested they might also be early indicators of surface (filiform) corrosion in the aluminum skin near the leading edge. This surface characteristic extends in a fairly consistent way more or less along both lengths of the wing. They're very small, like tiny goose bumps and not at all like blisters.

 

[Bob Turner]

PK screws?

 

[kubark42]

PK screws?

I had to look up what those are. I learned something new today!

Are you suggesting that I might be seeing the PK screw heads underneath the fabric, or that the P-K screws might be causing galvanic corrosion of the aluminum? The former seems unlikely because of the visible extents.

Is this kind of surface quality abnormal, or is it just a normal fact of life for fabric-covered airplanes and Citabrias, perhaps from the late 1990s?

 

[Bruce]

Jumping in but without pictures it’s more of a guessing game, unless my pictures aren’t showing up.
What year is the aircraft, when was it covered, is it stored in a damp environment? Some questions I would ask to build the story. Are these bubbles do they move when you press them or solid?

 

[kubark42]

unless my pictures aren’t showing up

So strange, the pic disappeared. I had linked to it originally, but now I just uploaded it directly. Let's see if it works better this way.

Built/covered in 1998; always hangared (if you're to believe the prior owners); the chicken skin doesn't move, it's solidly attached.

 

[Joe]

American Champion service letter 449 has some advice on the subject. If it is original fabric I'd say the knots are corrosion. If it has been recovered it is possible that they didn't clean up the leading edge very well.

 

[kubark42]

From Service Letter 449:

Wing Skins Leading edge skin material is alclad 2024-T3 aluminum sheet of .020 or .025 thickness. Corrosion in the form of pitting under the fabric covering may occur. Aircraft manufactured in 2001 and later have an etching wash primer applied to the leading edge skins. Aircraft manufactured prior to 2001 do not have leading edge skin corrosion protection beyond the use of alclad material. Corrosion tends to begins near openings in the fabric (i.e. stall switch, inspection covers) and where items are secured with PK screws (i.e. wing tips, speed fairings, stall switch, fuel vent, inspection covers).

In moderate and severe operating zones stainless PK screws should be replaced with equivalent zinc plated steel PK screws.

Monitor the leading edge skin condition. Corrosion will slowly cause the fabric and paint to raise into blisters - the fabric and paint condition will appear more severe that the actual condition of the skin. The raised areas should be identified by size; large (.010 to .030 inches in height) and small (less than .010 inches in height). The combined spanwise length of areas with large blisters should not exceed 50% of individual wing span.

If blisters are greater than .030 inches in height, if large blisters density exceeds 1 per square inch, or if the leading edge skin condition is otherwise in question remove a 3.0 inch diameter section of fabric to gauge the extent of corrosion. Pitting should not exceed a depth of .005 inches - more severe pitting should be addressed by repair or replacement of the leading edge skins. Repair fabric with 8.0 inch diameter patch per airplane maintenance manual.

At the time of wing recovering corrosion protection should be applied to the leading edge skins. If recovering per American Champion specification CFI-1 using 30-NF glue, apply PPG ESU400 or equivalent wash primer to leading edge skins. If covering by another method - refer to covering system approval holder to determine compatible corrosion protection.

So it sounds like that is indeed surface corrosion, but very mild.

They mention to "apply corrosion inhibitors after washing", but don't give any indication of what such an inhibitor would be. Is standard paint wax good enough or is there a more useful compound we could apply?

 

[Bob Turner]

My J3 leading edges date from 1946, and have never had any corrosion. I parked outside a total of maybe a quarter century, four miles from the Pacific Ocean. Seems to me the combination of Alclad and a liberal coat of nitrate dope or Poly Brush would inhibit this ugly stuff?

I never wash my aircraft. Grease comes off with WD-40; bugs and dust with Lemon Pledge. Rough stuff with #7 polishing compound. Which reminds me - the Decathlon gets "Pledged" today.

 

[kubark42]

"apply corrosion inhibitors after washing"

Dale at ACA says that that section is for the strut attachment fittings, not for the LE skins.

He also confirms that this is no big deal so long as it is being monitored and not allowed to get out of control. Basically fix it at the next recovering.