Elevator skin cracks over spar

Garage Guy · #1 07-30-2009, 12:22 PM

At about 520 tach hours, our 180hp RV-6 is developing cracks in the 0.016 elevator skins. These cracks are at the leading edge, not the trailing edge. I want to stop drill them while I think about building new elevators out of .020, but the ends of the cracks are right over the spar flange:

Anybody have advice for stop drilling cracks in this location? How important is it to avoid drilling into the spar? And what's the best way to avoid doing that?

Thanks in advance for any help,

--Paul

Rick6a · #2 07-30-2009, 03:59 PM

Quote:

Originally Posted by Garage Guy

Anybody have advice for stop drilling cracks in this location? How important is it to avoid drilling into the spar? And what's the best way to avoid doing that?.....

Seeking "advice" from others can be a mixed bag. How do you separate the wheat from the chaff? This looks to me like a "damned if you do, damned if you don't" scenario. Would it be best to wait until the cracks migrate beyond the spar flange? That would be hard for me to accept. Since nobody has offered up a reasonable solution so far, this is what I'd likely do if my airplane. There is no way you can avoid slightly touching the spar flange with the drill bit by stop drilling a #50 hole just beyond the end of the cracks. The trick would be to drill though the skin just deep enough to generate a clean hole, yet be deft enough to only leave a slight tick mark on the spar flange. Since a routine technique in sheet metal repair is to blend out all sharp edges, I can't see how producing a very slight tick in the spar flange with the tip of the drill bit would be a show stopper. Still, that is merely my opinion. Its best to contact Van's to see what they say. Let us know what their suggested fix is.

Mel · #3 07-30-2009, 04:09 PM

For what it's worth, never stop drill "at the end of the crack".
Always drill just beyond the crack and let the crack migrate into the hole.

Russ McCutcheon · #4 07-30-2009, 04:20 PM

You may get buy with changing the vibration characteristics of the skin, Some locals have stopped cracks in there 16 thou skins buy putting clear packing tape over the problem areas, after a couple of years the cracks are not growing.

rv72004 · #5 07-30-2009, 11:05 PM

I am sure Vans has a answer for this. I agree with the advice on this post but would still want a reply from Vans before I did anything.

PCHunt · #6 07-30-2009, 11:28 PM

Quote:

Originally Posted by Rick6a

..........There is no way you can avoid slightly touching the spar flange with the drill bit by stop drilling a #50 hole just beyond the end of the cracks. The trick would be to drill though the skin just deep enough to generate a clean hole, yet be deft enough to only leave a slight tick mark on the spar flange............

Rick's advice is good: see what "Van's best" have to say. Having said that, the very last piece of metal in my plane that I would want to have any defects in is a spar. I would rather have one crack in a skin, and watch it grow a bit, than to have a starting place for a crack in a major structural member that I could not ever watch. If a crack were to develop in the spar, there would be no way to ever see it coming.

plehrke · #7 07-31-2009, 04:27 AM

Very interesting place for a crack to have started in the skin. I have one at the end rivet (just behind the spar) of the rib. Stopped drilled it and it has not grown. Will need to eventually replace the h-tail as stop drilling the crack it a temp fix and is never the long term solution.

Did you build the tail? Did you use a glob of RTV at the end to help dampen out the vibration? Two ways you could have gotten a crack forming over the spar is the hole was not prepared right (had a burr of nick in the hole) or if you had a blob of RTV there it shifted the vibration from the end of the rib to the end of the blob. Maybe the blob needs to tie (bond) the rib to the spar. At the big aircraft company (day job) we would tie off the rib to the spar to keep this from happening (the story I could tell of this topic being an issue . . . . maybe in a few years. . . . . )

Garage Guy · #8 07-31-2009, 09:56 AM

OK, I called Vans builders assistance and got this advice:

Quote:

I got several cracks at that location on my RV-4 at about 200 hours and stop drilled them. Now 800 hours and no problem. Don't worry about drilling into the spar flange. It is the spar web that gives strength. When drilling, you can either drill all the way through the flange or stop when you know you have drilled through the skin.

To answer other folks' questions in this thread... Talking to the original builder of our RV-6, he did not use Proseal or RTV on the empennage control surface stiffeners. Early on, he rebuilt the rudder with .020 skins because of cracks and it has been fine since.

--Paul

cjbouldin · #9 08-05-2009, 08:55 AM

Quote:

Originally Posted by Mel

For what it's worth, never stop drill "at the end of the crack".
Always drill just beyond the crack and let the crack migrate into the hole.

Just curious, why would that be preferable instead of just drilling at the end of the hole?

islandmonkey · #10 08-05-2009, 09:18 AM

Quote:

Originally Posted by cjbouldin

Just curious, why would that be preferable instead of just drilling at the end of the hole?

Because a crack is like an iceberg. There is the bit you can see and the micoscopic bit you cannot see that extends beyond the visible end of the crack. Drilling a stop hole on an extended line from where the crack visibly ends, should stop the crack extending any further. However keep an eye on the crack to make sure it does not grow beyond the stop hole.