I also had the problem of the boom/fuse joint cracking. In my case it was a fatigue problem. There is a stress concentration at the sharp corner at the bottom of the pylon where the boom intersects the fuse.
To fix the problem, I put a balsa fillet between the pylon and the boom. The fillet has about a one inch radius. I then wrapped the pylon/boom/fillet with three layers of 3k bi-directional carbon fiber to reinforce the joint. Two layers would have probably been enough. I positioned the carbon using 3M77. I put a layer of finishing glass over the whole bit and wetted it out with West Systems epoxy.
I had to fix my fuse after it cracked. I used an external reinforcement of 5oz glass that covers the lower bottom of the pylon and wraps around the forward 2 inches of the tail boom. This is also plenty strong and hasn't showed signs of fatigue.
The cracks were under my previous reinforcement and was not externally visible unless you looked really carefully. I noticed at a contest that I needed to put more and more up elevator trim on the plane and that I had difficulty executing a zoom. It wasn't until I got homw that I noticed the cracks and could see that the tail boom was flopping around. All that was preventing it from sliding out was the screws for my skeg that went all the way through the reinforcement, the pylon skin and the tail boom. Reason mumer 3 for using a skeg!
Reinforcing the Luckenbach fuse/boom joint
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Posted - 11/19/2003 : 5:31:39 PM
