Allowable wing skin corrosion?

[1001001]

I have dug around various references and haven't found a solid answer to this question. I plan to call Van's tech support about this, but builder support isn't open until later. Maybe someone will see this and have an answer.

For various reasons I'm just getting around to preparing the wing lower skins, and found that an animal (probably a squirrel) got into the crate that had the wing skins in it. At the inboard edge of one of the big skins, there is some corrosion due to the animal relieving itself, and the liquid getting under the blue protective film. There are small areas of pitting corrosion around this end of the skin. I partially cleaned these up by gently wet sanding and measured the depth of the pits using a dial depth gauge. They are a lot shallower than they look, but still a few thousandths deep.

AC 43.13-1B says what to do about pitting corrosion, but leaves the thickness margins to the manufacturer's judgment, and doesn't mention an acceptable loss of thickness. Anyone know how deep (in terms of % total thickness) is acceptable for a corrosion repair on sheet alclad aluminum? I'd really hate to have to buy and have shipped a new skin (this is one of the largest skins in the entire kit), but will if I have to. Also, anyone know if Van's can roll the skins up and pack them in a smaller box to avoid huge shipping charges for oversize packages?

 

[fixnflyguy]

pics? Type RV?

Probably nothing to worry about, but more detail may help the evaluation. I'm in this specialty for heavy jet aircraft and deal with it daily. 10-15% is typically "allowed" by the OEM manuals , however, if its at the end of the sheet, it may actually be in the overhang that is very low load carrying anyway. I usually clean that type light corrosion with red/blue Scotchbrite pads and alodine and prime..likely you just have filliform corrosion that started at the edge of the sheet between the Alclad ,a thin pure aluminum coating and the actual 2024-T3 alloy sheet .

 

[ZachMiller]

I've also done repair engineering on heavies (and some turboprops) in a past life. As said, 5-15% is the typical allowable reduction in thickness, typically with a list of caveats. OEMs will even tabulate allowables for specific parts and areas. I'd say you're probably safe with 5% pretty much anywhere.

If there are no OEM allowables, you can always reference the manufacturing limits for wrought aluminum. Looks like it is ?.0015 for below .039 thick and ?.002 for .039-.079. Reference here, page 25:

https://online.kaiseraluminum.com/d...cument/1010/Kaiser_Aluminum_Sheet___Plate.pdf

I had the exact same situation happen on one of my horizontal skins. It didn't look great, but a little sanding to remove the pits only removed .002 max from the skin. I touched up with Alodine and primed and I built on.

 

[1001001]

Probably nothing to worry about, but more detail may help the evaluation. I'm in this specialty for heavy jet aircraft and deal with it daily. 10-15% is typically "allowed" by the OEM manuals , however, if its at the end of the sheet, it may actually be in the overhang that is very low load carrying anyway. I usually clean that type light corrosion with red/blue Scotchbrite pads and alodine and prime..likely you just have filliform corrosion that started at the edge of the sheet between the Alclad ,a thin pure aluminum coating and the actual 2024-T3 alloy sheet .

The sheet is 0.025" alclad, and most of the pits are about 0.005" deep at the deepest, so that's about 20%. It's in the area where there is a double row of rivets where the outboard skin overlaps the inboard, and around the edge of the joggled inspection plate cutout. I'm a little concerned about their proximity to the edges, but none of them are really too close to rivet holes. Oddly, the pits are not near the edges where the liquid came in, but further inside the plastic. I'm guessing this is because the liquid was able to evaporate quickly at the edge, but once it had migrated inboard, it was held against the skin for much longer without evaporation by the plastic film. This is definitely not regular filiform corrosion from moisture/condensation intrusion, there was a noticeable residue and urine odor, with crystals of what I assume to be urea. It was concentrated around areas where there were punched holes and edges in the plastic.

 

[PhatRV]

What are you building? If you are building a 9 or 10, then the bottom skin is in tension all the time, except for standing on the ground. The area you mentioned is almost a doubler where the two sheets of aluminum meet at a double rivet line. The stress on the wingskin at this joint is low. Also, there is an instruction that you should bevel the sheet metal edge at this section so both sheets can be faired when joined together. If you are still worry about it, order a replacement because it's a small shipping cost for a peace of mind.

 

[1001001]

What are you building? If you are building a 9 or 10, then the bottom skin is in tension all the time, except for standing on the ground. Easy to measure and find out though. The area you mentioned is almost a doubler where the two sheets of aluminum meet at a double rivet line. The stress on the wingskin at this joint is low. Also, there is an instruction that you should bevel the sheet metal edge at this section so both sheets can be faired when joined together. If you are still worry about it, order a replacement because it's a small shipping cost for a peace of mind.

It's a -10. The other thing I am not sure of is if any of these pitted areas line up on both sides of the skin, which would make the thickness reduction even greater. Your point about the thinning of the skin to make a more aesthetic joint in this area is a good one; the manual doesn't specify how much thinning is allowable here, but it might save me some money!

I'm curious about your statement that the 9 and 10 bottom skins are under tension, but implying the other RV models are not. I understand why they would be tensioned in flight, but not why the other models wouldn't be. Does it have something to do with a different structural design for aerobatics?

I agree, it's a small cost for peace of mind, but I still don't want to pay for a big sheet shipment. I'm hoping they can roll the skin up and ship a tube or smaller box.

Thanks for the replies everyone!

 

[PhatRV]

It's a -10. The other thing I am not sure of is if any of these pitted areas line up on both sides of the skin, which would make the thickness reduction even greater. Your point about the thinning of the skin to make a more aesthetic joint in this area is a good one; the manual doesn't specify how much thinning is allowable here, but it might save me some money!

I'm curious about your statement that the 9 and 10 bottom skins are under tension, but implying the other RV models are not. I understand why they would be tensioned in flight, but not why the other models wouldn't be. Does it have something to do with a different structural design for aerobatics?

I agree, it's a small cost for peace of mind, but I still don't want to pay for a big sheet shipment. I'm hoping they can roll the skin up and ship a tube or smaller box.

Thanks for the replies everyone!

I was referring to the 10 flying right side up all the time so the bottom wing will be in tension. I think most wings are designed for stiffness and to prevent skin buckling at the top where the skin is at compression. Not sure if Van designed his wing like this but I think it is a general practice. So that means the bottom skin thickness can be thinner than the top skin. Look at your spar too. The bottom spar cap is smaller than the top one. I don't know the thickness of the rv10 skin but if you add up the inner and outter sheets, the combined thickness is quite substantial at the overlap area.

 

[1001001]

I talked to Builder Support this morning, and was told that (as expected) corrosion allowances really depend on the location and stress level in a part. Van's claims that they don't have specific information for a wing skin, as a Part 23 manufacturer might.

In experimental aviation, it comes down to builder judgment.

So, new skin ordered.

 

[Mel]

I talked to Builder Support this morning, and was told that (as expected) corrosion allowances really depend on the location and stress level in a part. Van's claims that they don't have specific information for a wing skin, as a Part 23 manufacturer might.
In experimental aviation, it comes down to builder judgment.
So, new skin ordered.

I think this is a wise choice. Everything may be fine, but it would always be in the back of your mind.

 

[ZachMiller]

Good call, knowing the location and depth of the damage.

Structurally, a wing skin lap is not a doubled up area (at least not in the primary load direction, inbd/outbd), it's an area where all of the load in one skin is being transferred to the other skin. Therefore the stresses can be quite high and not evenly distributed. The design of the RV is not particularly like the jets I've worked on, as on the jets the lower wing skins essentially extend through the fuselage with machined fittings, i.e. the center wing skin is fully attached to the outboard wing skins. Therefore the load in the lower wing skin on the RV is probably much lower compared to an airliner due to the difference in design philosophies. That doesn't mean the load in the joint is that low, however, someone would have to do some thorough analysis to determine that. There is no way I'd risk it on such a critical location on a critical part, not for what a new skin costs.

For reference, I dealt with damage to fuselage skin laps a few times on airliners. It's not apples to oranges, since airliners are pressurized which makes the primary fuselage structure much more susceptible to fatigue, but the OEM manuals allow very little rework in these areas. Almost every time we saw damage here we ended up involving the OEM for stress analysis and often we needed to install doublers to reinforce the area.

That's my opinion anyway. I think replacing it is the right call.

 

[1001001]

Structurally, a wing skin lap is not a doubled up area (at least not in the primary load direction, inbd/outbd), it's an area where all of the load in one skin is being transferred to the other skin. Therefore the stresses can be quite high and not evenly distributed.

This was *exactly* my thinking--what is a joint, anyway, but a place to transfer stresses? I was also concerned about the pitting damage near the inspection plate cutout, where stresses are concentrated and routed around the edge.

Thanks for all the helpful responses, everyone!