May 30, 2008

Nose skin problem. More dimpling - 6.0 hrs.

I've been having such a time with these aileron nose skins, trying to get them to fit right. And I finally figured out what the problem is. The curve on the nose skins' bottom surface has been overbent at the factory. This causes a "ski jump" effect right under the nose of the ailerons on a skin that's supposed to lie flat. This also explains why I had a hard time getting it clecoed together. I spent several hours today trying to work out a solution to this problem. It was very frustrating.

When the light is just right, you can see how overbent this part is near the top of the picture above.

Here's a shot from another angle.

Now on the ends, where it clecoes to the nose ribs and the counterweight pipe, it's forced to lie flat because of the support underneath the skin.

But out in the middle where there's no support under the skin, look how deformed this is. This is the bottom of this aileron, and it's supposed to be flat! It's all because this part was overbent at the factory. I have to figure out a way to "unbend" it.

Long story short -- I didn't get a satisfactory response from the factory. I'm concerned not only about how this looks, but this skin would be under constant stress. And I'm also concerned about how much drag might be induced into the airstream sliding along the underneath surface of the wing, but they basically said it wouldn't be a problem. But this is unacceptable to me. So I decided to find a way to unbend it, or straighten it out a little bit. Boy oh boy... until you've ever tried to unbend one of these skins a little bit, you have no idea how stubborn this material is! I don't have a brake or any kind of metalworking equipment for this kind of thing, so I had to try several things by hand. First, I put on a leather glove and simply tried to "push" the crease open by pushing and sliding my hand along it inside the skin. No good. I used more and more force until I was pushing as hard as I'm capable of pushing on it, and it would spring right back into this shape. I finally ended up using a wood dowel about 1" in diameter (a good ol' closet hanger rod). I wrapped a rag around the end so it would slide on the metal, and pushed it along the crease inside, with the end of the skin braced against the wall so I could really use some force. Then I had to go to something a little larger in diameter. I worked on both of these skins for several hours, moaning and screaming and cussing, and it gradually worked this thing open a little bit at a time until I finally had something I considered acceptable. Wringing in sweat, I finally moved on. I didn't take any pictures. I wasn't in the mood for photography. I was just glad to get it done.

This picture may give you an idea of how nice it came out. The bottom surface of the nose skin is facing us in the picture above. This is the jig I made to countersink the counterweight pipes, and dimple the nose skin for the pop rivets that hold it all together. I stole this idea from someone's web site. It's very simple. A 2 x 6 has some drill bits masking-taped to the edge, and some 1 x 1 blocks screwed to the bottom edge so I can clamp it to the workbench. The pipe lays on top and the drill bits keep it from rolling off. The nose skin is then laid down on the pipe and lined up with the holes so they can be dimpled, using the countersunk pipe as a sort of female dimple die.

The nose skin is laid down over the pipe and clecoed in place.

For dimpling, this is the rod out of the C-Frame dimpling tool. A dimple die obviously fits in the end, and the other end can be hammered on.

Dimpling in action. This worked really well. The dimples weren't perfect, but running a deburring bit in each one (below) cleaned them up real nice.

A cleaned-up dimple, and a trial fit with one of the pop-rivets that will go in, after priming is finished. This is also a fairly good shot of how nice my nose skins looked after all the work I did to unbend them a bit.

<< Previous | Home | Next >>

Contact me: swayze "at" (replace "at" with the @ sign... no spaces... you know the deal)