Saturday, February 21, 2009

Rudder issues

I'm having rudder issues. One of the more ticklish bits of building this particular airplane is the trailing edge of the rudder, one of the last things to touch the air as the plane goes by. You have to get it straight, and ideally you need to have it nice and, well, together. The trailing edge consists of two rudder skins and a wedge-shaped piece of aluminum that all meet at a point. The instructions say to put fuel tank sealant or some kind of glue on the wedge, glue it all together, clamp it, leave it alone for 48 hours, and then go through the ticklish process of riveting it so it comes out nice and straight.

Now, this is the second time I've built this rudder. The first time around, I got it all finished and hung it on the wall of the workshop to get it out of the way. (I was in a rather small workshop at the time; space was precious.) A few weeks later, the string I'd used to hang it broke, the rudder took a catastrophic fall, and the trailing edge got crunched rather badly. After much wailing and gnashing of teeth, I ordered new parts.

I recently finished the rebuild, and it was time to do the trailing edge again. The last time I did this, I used fuel tank sealant and had a whole bunch of the stuff left over, that I wound up tossing because, let's face it, how often are you going to use fuel tank sealant around the house? (Fuel tank sealant comes in two parts. Once you mix them, it'll eventually set. I ordered a small quantity, but it came in a container where you had to mix the whole thing, and the whole thing was enough to do a whole fuel tank and quite a bit more than I needed. You can store mixed sealant in the freezer for up to about six months, but at the speed this project's progressing, six months won't cut it.)

So this time I tried T-88, an epoxy used in aircraft construction. Here you can see it clamped together so the glue will set. That little aluminum bit at the left is a test piece. All seemed to go reasonably well. I gave the glue a full week to set and the test piece seemed pretty solid, so I started working on it. That's where I ran into problems.

Here you can see a shot of the new trailing edge, on top, and the old one on the bottom. The epoxy separated as I set the rivets. It turns out T-88 on aluminum just doesn't have the peel strength fuel tank sealant does. When I tried peeling the test piece apart, I was able to pop it as well. I've written the tech support folks at the kit manufacturer to find out how serious a problem this is.

The trailing edge itself came out pretty straight. It wasn't perfect, but there weren't any major problems. I've contacted tech support about the width of the trailing edge. We'll see if I need to drill out the rivets and redo it.
Posted by Picasa

No comments: