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Back in the Saddle Again

April 25th, 2013
Tank Skin Woes

Tank Skin Woes

Time to fess up. I have been a little discouraged and busy and the plane has taken a hit as far as work. I was chugging along with the tanks last year when I read a post on VAF that talked about the #8 screw dimples cracking on tank skins. I thought that was weird as I did not remember that happening on mine. So I went out to the shop to confirm and to my dismay, I was also a victim. I was crushed. I had just Pro Sealed the stiffeners, fuel flange, and drain fitting on one skin and it was all for naught?

I stewed about it, beat my head against the wall, groaned a bit, and overall just was deflated. May seem like an overreaction to some, but it was a little disheartening. It did not help that the number of dimples that seemed to be affected was pretty high by my standards. So I turned out the lights and walked away. That was months ago. Then the holidays hit, weather got cold, life got busy.

I discussed my options with some fellow builders. One even asked his engineering department about it and they had some suggestions. I contemplated ordering new skins and undoing the filler flange (as that part is as much as a skin) and starting over. Other options were presented, so I mulled them all over. Vans said that this can happen, but that they would build on and that they would likely not move…much. They also suggested that the hole could be drilled a few thousandths bigger to catch the crack, but not to really worry about it. Right…do they know who they are talking to here? The friends engineering department agreed with Vans on the enlarging the hole. The bearing and strength is at the dimple shoulder, not the hole, so it was inconsequential to the structure what the hole size was as long as the head of the screw had a good material base to sandwich.

Ultimately, I decided I would give enlarging the holes a shot, and if all else failed, I could always get new skins. One of my builder friends tossed me a 0.2081″ double margin bit and said, “start here.”┬áNow it was a matter of just doing it.

Today, a fellow builder from Colorado posted on VAF that he would be in town and would like to visit another project. Reading that, and thinking what the heck, I tossed out an invitation to visit. This would at least get me out to the shop and talking RV’s. We set up a time and I went out to the shop after work figuring that I would at least clean up the shop for visitors. Once I was out there, I decided to try dressing one hole and see what it looked like. Nicely enough, it actually took the crack out and once I deburred it and cleaned it up, looked OK. So I moved on to the next hole. After an hour, I had dressed and cleaned all of the problem holes. For the most part, they all cleaned up nicely. There a still a few that have a remnant of a crack, but with a needle file, could easily be polished out. The hole would look a little strange, but would never be visible under the screw.

For those that are wondering about the size of the hole still, rest assured, even after drilling out what was originally a 0.166″ hole, that was then stretched bigger by the dimple die, to 0.2081″, the material left is still slightly more than the shoulder of the spar countersink (remember how they knife edge in the spar). So all I removed was the overhanging material from the stack-up of the tank skin, spar web, and nut plate. I also made a test piece from 0.032″ to confirm how it seats in the spar dimples and to confirm the holes and can report that it works and looks great. So perhaps I dodged a bullet here.

What did I learn here?

  1. Don’t use a #19 bit for the pilot holes on a #8 screw dimple. Use a #17 or even a #16 bit. The amount of material stretching is too much on the #19 hole. The pilot will still center on the die as you compress them and the hole stretches to a hole bigger than the #19.
  2. Make sure to back the holes you are enlarging with a piece of wood behind them. When I enlarged the skin holes, I just let the bit run free in the hole. I think this allowed the bit to wander and chatter.
  3. Take your time on larger dimples.

In the end, I think I have good skins and will build on. Other than this confession, you will not know it had to be fixed, and the plane will never see the difference in strength.

After it was all said and done, my visitor, rockwoodrv9a, stopped by and we chatted for an hour or so. Real nice Nice guy! I hope he found the visit worth while. I did, as it got me working again. Now to keep the momentum going.


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