Elevator Skin Holes Not Aligned

[jwilbur]

I was surprised this evening to find one of the pre-punched holes in one of the elevator skins out of alignment. It was a full 1/16" out. The other three skins were correct. Naturally, at first I assumed I had screwed something up (again) but not so this time.

I've got a series of pictures below which illustrate what I found and how I dealt with it. Has anyone else had this experience? Are there any suggestions on how to better deal with this should it happen again?

Figure #1, Clecos in the skin and a ruler show the alignment issue.

The problem was painfully clear while final-drilling. After thinking it through, I knew I would have to drill the hole where it was - one way or another, so I drilled into the rib, knowing I'd have to deal with it again.

Figure #2, The rib with the bad hole. The top-left of the mangled hole is where Van's expected the hole on the skin to be. The bottom-right of the mangled hole is where the hole in the skin actually was.

My solution was to take some scrap aluminum and make a little plate to go over the messed up hole. I first positioned it over the messed up part of the rib, clamped it, and match-drilled to the hole I knew was matched to the skin. I then match drilled another hole to hold the plate to the rib, deburred, dimpled and riveted the plate to the rib with a flush rivet.

Figure #3, Plate only with the hole matched to the skin.

Figure #4, Plate riveted to the rib

Figure #5, Rib with hole moved to match skin clecoed in place.

Last thing I did was to mark the outside of the skin to remind myself to use a longer rivet in that spot when the time comes.

 

[flion]

Your solution is excellent. That's a freak occurrence; the skin must have somehow moved as it was being punched. All my skins were fine and you noted that 3 of your 4 were good. However, this kind of thing has happened to builders in the past. For instance, when dimpling the ribs people have joined the 'Figure 8 Club' by accidentally punching a dimple die next to a drilled hole. The fix is similar to what you did.

I don't think anyone will notice the misalignment without, as you did, putting a ruler on the rivet lines. If you run into the situation again, you can use the same fix. If the offset is smaller, you can consider drilling #30 and using an 'oops' rivet. If it is somehow greater than minimum rivet spacing, then you can drill the proper rivet hole without needing to make the backup plate and the offset hole can be 1) ignored if it is in the structure (deburr it anyway), 2) drilled, dimpled and closed with a rivet if it is in the skin, or 3) ignored as in (1) except for filling if it is backed up by structure.

One caveat; I couldn't see from your pictures how you dealt with the figure 8 hole in the rib. That hole needed to be drilled out or have the 'points' removed with a rat-tail file and then deburred to remove any stress risers there. For completeness (so you aren't driving a rivet through a void caused by the enlarged hole between the skin and backing plate), you can then fill the hole with a structural adhesive - not something brittle that won't take the riveting - and re-drill once the adhesive has set.

 

[Phil]

One caveat; I couldn't see from your pictures how you dealt with the figure 8 hole in the rib. That hole needed to be drilled out or have the 'points' removed with a rat-tail file and then deburred to remove any stress risers there. For completeness (so you aren't driving a rivet through a void caused by the enlarged hole between the skin and backing plate), you can then fill the hole with a structural adhesive - not something brittle that won't take the riveting - and re-drill once the adhesive has set.

That's what I was thinking too. The figure 8 is definitely under the minimum edge distance. I'd consider shortening the rib so the hole is completely gone. Then use the plate to makeup for the section that you removed. You'd have to add a shim under the plate to make up for the gap caused by the missing original flange.

Phil

 

[jwilbur]

... I couldn't see from your pictures how you dealt with the figure 8 hole in the rib.

Thanks for the reassurance here. After drilling the properly aligned hole into the plate, I thoroughly deburred the hole in the plate and the figure-8 in the rib, smoothing the point. Then I deburred the point further after dimpling the rib. I was a little surprised actually at how much of the original hole was consumed in the dimple. I have a picture after the dimple, but it doesn't show very well so I didn't include it.

... The figure 8 is definitely under the minimum edge distance. I'd consider shortening the rib so the hole is completely gone. Then use the plate to makeup for the section that you removed. ...

I actually considered this, but decided it would accomplish the same goal (and be simpler) by extending the plate so that the hole in the plate met the min. edge distance. Figure #3 shows this.

 

[Phil]

The only issue I see with that is the possibility of the rib cracking out from underneath your plate. If that occurs your plate is useless and your edge distance on the plate doesn't matter.

The rib is the base material that the plate is attaching to. If the rib goes, then the plate might as well not even be there because it's not attached to anything.

That was the only reason I suggested removing the edge distance issue, shimming, and then putting the plate over the top. Your chances of a crack forming in the rib are completely put to bed.

Phil

 

[jwilbur]

The only issue I see with that is the possibility of the rib cracking out from underneath your plate. If that occurs your plate is useless and your edge distance on the plate doesn't matter.

The rib is the base material that the plate is attaching to. If the rib goes, then the plate might as well not even be there because it's not attached to anything.
Phil

Phil,

Thanks for taking the time. I appreciate it. This is all important to get exactly right and I haven't riveted anything to the skins yet so I have time to make changes. I want to understand your concern. If the rib were to crack, the most likely place is where the figure-8 is located - probably toward the close edge. But that part of the rib shouldn't be important anymore because of the plate. The plate is attached to the rib half-way between the figure-8 and the next good hole in the rib. Do you see something I don't? If the rib were to crack away under the plate, I'm not sure why the plate would be useless?

Thanks,

 

[Phil]

Yeah if it goes toward the end you're good.

I'm thinking if a crack starts on the opposite side of the figure 8 and radiates the opposite direction. If it heads that way then the plate is separated from the rib. (Ribs, plates, it's 7:20PM - now I'm hungry)

What I'm suggesting is removing the hole completely so the stress riser is completely gone and the crack never has an opportunity to get started.

The tail feathers on RV's seem to be a common place for cracks to form. I think it because they're generally pretty thin (.016), don't have ridged support structures (like the ridges bent into the Cessna elevators), and they're being pressed into airflow so it subjected to twisting.

All of those are only my opinion. But the tail feathers are a place that takes quite a bit of abuse. That's why I think it's easier to remove the stress riser now and not worry about what's going on inside the elevator in a place you are unable to inspect.

Like I said, all of this is just opinion on my part.

Phil