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Anyone else have a crack like this?

Brantel

Brantel

Well Known Member
I have a friend with an RV6A that started life as a quickbuild kit. I think he has somewhere around 300hrs on it now.

He has developed what looks like a crack in his top inboard wing skin along the rear spar rivets about two thirds the way out the flap area. The flaw looks like it spans about three to four rivets.

The flaw runs parallel to the rear spar rivet line and is slightly in front of the rivet holes. The flaw runs across the rivet heads but not thru the centerline of the rivets, it is slightly forward of that.

I do not have a picture yet and hopefully he can send me one soon and I will post it.

We cannot tell if this is just a crack in the paint or is a real crack. The paint is thick in this area and it is base coat/clear coat.

Anyone seen this before???
 
I've got a base/clear paint job and notice cracks in the same area. I took the paint off on a place underneath to make sure it wasn't a skin crack and it wasn't. I won't say ignore it but don't go rebuilding the airplane either. It's probably just paint cracking where the skin flexes.
 
Here are some pics. Keep in mind that these are all of the same defect....

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fef5ur.jpg
 
Realizing that the photos are very close-up shots, it would seem to me that if you push down on one side of the "crack", (the side further away from the rivets,) you could determine if it really is a crack by watching closely and even by feel.

Then if it is a crack, which it sure appears to be, then you could stop-drill at each end as a first step.
 
Low stress area

This is an area of the wing that has very low stress in the skin. Some shear stress, but really nothing compared to what the skin is capable of, so I can not imagine why the skin would crack there.

I would check the fit of the flaps. If the nose skin of the flap is bearing fairly hard on the wing skin when the flap extends and retracts, it may be flexing the skin and drag spar channel a little, and this could crack the paint.
 
scsmith said:
This is an area of the wing that has very low stress in the skin. Some shear stress, but really nothing compared to what the skin is capable of, so I can not imagine why the skin would crack there.

I would check the fit of the flaps. If the nose skin of the flap is bearing fairly hard on the wing skin when the flap extends and retracts, it may be flexing the skin and drag spar channel a little, and this could crack the paint.
Click to expand...

I agree with Steve - you could be getting some flexing from the flaps. But I also agree with Walt - I think I'd examine underneath the paint, just to make sure. This looks an awful lot like some cracks I've been looking at in a structure at work - right through some rivet holes.

I bet it's paint, but it would nice to be sure.

Paul
 
It is very hard to push this area down. On the front side, the flaw is still above the flange of the spar so when you push, it is against the spar flange and won't move.

PCHunt said:
Realizing that the photos are very close-up shots, it would seem to me that if you push down on one side of the "crack", (the side further away from the rivets,) you could determine if it really is a crack by watching closely and even by feel.

Then if it is a crack, which it sure appears to be, then you could stop-drill at each end as a first step.
Click to expand...
 
The fact that it is not originating on a hole makes me think it is not a skin crack.

I cant imagine how flight loads would cause this area to crack and if it did, we would see it all over the place with other aircraft.

The flaps do lift the aft edge of the skin up about 1/16 when the flaps are retracted. 1/16th over a span of 3+ inches (from the aft edge of the skin to the rivet line) does not to me seem like that would do much damage in just 300hrs. Also this looks like it would crack aft of the rivet line if that is what is causing the problem.

He definately wants to understand the problem but is dreading having to fix the paint if he strips the paint and dye test it.

He was excited to hear that Sid had the same thing but found out it was not a real problem. Now he is going to be disappointed to see all the suggestions to investigate further. There really is only one way to know for sure and that is to strip it down and dye test it.

Anyone have any good suggestions on how to remove the paint above this flaw without damaging the skin so bad that you cannot tell if it is a real crack or not? Stripper or Sand?

scsmith said:
This is an area of the wing that has very low stress in the skin. Some shear stress, but really nothing compared to what the skin is capable of, so I can not imagine why the skin would crack there.

I would check the fit of the flaps. If the nose skin of the flap is bearing fairly hard on the wing skin when the flap extends and retracts, it may be flexing the skin and drag spar channel a little, and this could crack the paint.
Click to expand...

Ironflight said:
I agree with Steve - you could be getting some flexing from the flaps. But I also agree with Walt - I think I'd examine underneath the paint, just to make sure. This looks an awful lot like some cracks I've been looking at in a structure at work - right through some rivet holes.

I bet it's paint, but it would nice to be sure.

Paul
Click to expand...

Sid Lambert said:
Wow, I agree. These look very jagged. A paint crack will look very straight and uniform. Check this ASAP.
Click to expand...
 
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Brantel said:
Anyone have any good suggestions on how to remove the paint above this flaw without damaging the skin so bad that you cannot tell if it is a real crack or not? Stripper or Sand?
Click to expand...

Just tape off the suspect area and chemically strip the paint. I would guess about 1/2" wide by the length of the crack would be sufficient.
This will make the paint touch up area pretty small.
 
Brantel said:
.......He definately wants to understand the problem but is dreading having to fix the paint if he strips the paing and dye test it.
......
Click to expand...

doesn't matter if it's just paint or actual skin crack, the need for investigation is the same. If it's only a paint problem it still needs taken care of, as the paint is not sticking properly and possibility of corrosion underneath.
 
Its certainly not flight loads; the paint around the rivet heads isn't cracked either. Possibly got stepped on. FWIW, I'd also recommend chem stripping the paint and inspecting the underlying aluminum.
 
Though, I doubt the crack is from flight loads, it is possible that it is from a pre-load. When I riveted my rear spar I thought to my self that the rear spar angle is not exactly the same as the angle of the skin laying on top. When riveted there is definitely a small "stress riser" there. I have seen this on several other RV's as well.

That being said, I'd be curious to find out if that is actually a crack in the skin or just the paint. Either way, I don't believe the plane is airworthy the way it is. It sure looks like a crack to me.

P.S. You can probe the "crack" with a heavy needle or razor and possibly remove enough paint to get to the skin. A dye check can be conclusive with just a 1/4 or so of exposed crack. If there is a crack you will need to then find the ends of it to determine your repair dimension. AC 43.13!
 
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Just a thought

The top flange of the rear spar is bent at 82 degrees. The bottom flange of the rear spar is bent at 95.5 degrees. These rear spars can be very easily built upsidedown. If that is the case the flanges of the spar will not match the airfoil causing stress to the skin. This was talked about in one of the rvators. Just a thought. Might be worth checking
 
UPDATE:

My friend could not stand it after reading these post....He went ahead and stripped the paint and the skin is definately cracked.

He is now looking for suggestions on how to fix this.

There is at least one reference to this issue in the archives by svanarts:

http://www.vansairforce.com/community/showthread.php?t=5730&highlight=skin

Anyone have any better suggestions than Vans did about this issue?

Makes me wonder how many more have these cracks and people just ignore them? Afterall, this is a QB wing that is built just like the other 2424 others flying.

By the way, he used MEK to strip the base coat/clear coat and primer. He said the paint came off like nothing but the primer took some rubbing.
 
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Sorry to hear. It sounds like he has a few cracks from your first post. If there are enough cracks I would consider reskinning since that crack is sitting on top of the rear spar.

Next time I'm at the hangar I will take a picture of paint cracks so people can see the difference.
 
Unfortunately, he will have to do a "scab patch" repair (AKA external repair doubler). AC 43.13 has the repair process. Basically, it'll be 2 rows of rivets around the cracked area. Use a doubler one gauge thicker than the original skin. I would cut out the damage and not just stop drill. A thin stainless steel plate under the skin will protect the spar while you use a die grinder to remove the crack. I would use a thin layer of pro seal under the doubler although it's not required. Champher the edges of the double and it'll look as good as possible. Alternately, you could try a flush repair. It'd be more complicated and I a not sure that it would work in this location.
 
Nope, just this one.

Sid Lambert said:
Sorry to hear. It sounds like he has a few cracks from your first post. If there are enough cracks I would consider reskinning since that crack is sitting on top of the rear spar.

Next time I'm at the hangar I will take a picture of paint cracks so people can see the difference.
Click to expand...
 
I've seen exactly these same type of cracks on the tops of Cessna wings caused by flaps that were rigged to over retract. Since they are supposed to snug to the top of the flap track slots, I always assumed there was an error in the wing trailing skins & closeout. On an RV, you would either have a high flap or an over retracted flap. One's easier to fix than the other.
 
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