RV-3 top skin rear spar cracking

I want some group wisdom on cracking top skin repair (both wings, similar cracking).



Background, I purchased this RV-3 (no mods, just a straight RV-3) this year, I've owned an RV-3A (CN1&2) before and I knew what I was getting into. I saw these cracks when I bought the plane and I purchased it knowing I would need to do this repair. The damage might be caused by any or all of; over-stressing the wings without rear-spar mod, extending the flaps at too high of an airspeed (these cracks are right in the middle of the flaps), or a vibration mode in the rear spar.

The cause doesn't really matter, they're there and they are on both wings (4 rivets 24" out from the root), which means it's stress.

I've stripped the paint off the top skins and confirmed the cracks are not just the paint, it's in the skin. I've also boroscoped the rear spar from the inside and taken pictures from the outside, there's no sign of rear spar damage. Also I boroscoped te main spar and I do not see any signs of over-stress or damage. So the damage is limited to the top skin only and it's on both wings in roughly the same place.

On my RV-3, this top skin goes from the root to the tip 97" x 27", the material cost doesn't bother me, but man the riveting and the jigging. I'd rather just patch it. I need to pull a skin regardless to get to the spar for the mod.

The Actual Question:
Is it acceptable practice to put a patch under cracked skin?

If not, I will probably cut the inboard skin and do a joint similar to the RV4's skins somewhere out by the aileron bell crank.

thoughts? suggestions?

Thanks!
-Bruce

You know what I'm going to say... contact Van's support.

Dave

Yeah, this really just me putting it down... I probably should just call them.

They may suggest installing the CN-1/-2 upgrades, too, of course.

WHen you find out, please report back - thanks!

Dave
RV-3B now working on the cowl, when I'm not frivolously wasting time by going flying in the other airplane, like I did today.

Cracks

Looking at the pictures,

I would check the depth of the countersinking.

I have had similar pain of repairing my RV 3 that was built well originally but had been poorly maintained by previous owners, so I understand your dilemma.

Rob
RV3 G-BVDC

David Paule said:

They may suggest installing the CN-1/-2 upgrades, too, of course.

WHen you find out, please report back - thanks!

Dave
RV-3B now working on the cowl, when I'm not frivolously wasting time by going flying in the other airplane, like I did today.

Click to expand...

I'm doing both CNs already. I also asked if I can do it from the top, cause that's already open.

inside view of top skins

This is interesting and plays into Rob's countersinking post. Also notice there's a 5th crack I couldn't see on top.



Also, I sent an email to Van's with all this info and if they recommend my repairs or have any additional ideas.

Thanks all!

-Bruce

In the latest picture of the underside it looks like there is a stress riser around the shoulders of the dimples. I wonder if the original dimple dies may have left a sharp ring that contributed to the fatigue cracks?

Don't know the thickness of the skin, but possible that they were machine countersunk in error?

Jpm757 said:

Don't know the thickness of the skin, but possible that they were machine countersunk in error?

Click to expand...

See my last picture, they're dimpled. It's 0.032

In the commercial world you would do an external patch and that requires completely cutting out the damaged section of skin, putting in a filler with the same thickness and material as the skin, then putting a doubler that's one gauge higher thickness over the entire area (i.e. 0.032 skin requires a 0.040 doubler) with enough room for a double row/column of fasteners. Machine countersink the doubler. If you need blind rivets, use cherry rivets. AFAIK the smallest cherry you can get is a 4 (1/8 in.), so you need to be careful not to knife edge the doubler. You might need to go to 0.050 in. in that case.

I'm not sure how you would do an internal doubler with the spar in the way. The important thing is to maintain a load path that will transfer the load from the skin into the doubler and back into the skin without over stressing the fasteners and causing further skin cracking later.

There are some calculations to back this up that I didn't do, but this is very similar to repairs I've written for non-pressurized areas.