I mounted the pitot heat controller onto an access panel. This will be the mid wing access panel, just inboard of the pitot tube. This was simple enough, just center in the panel, drill holes, countersink, install screws. I did run into some problems with cracks forming on the dimples on the large #8, and even #6 holes.
The first crack showed up while dimpling the mount holes using my #8 dies. A warning in the plans calls out the risk of cracks when dimpling large holes. This is compounded by the fact that the wing skin (and access panels) are about as think as you can dimple. All the stress of the dimpling process will crack any stress risers, so I found out.
This dimple cracked because I didn’t remove some scratches near the edge of the hole. When I was drilling the holes, some metal chips were trapped between layers and scratched the skin. I used scotch bright to remove them, but didn’t completely smooth out the scratches. This seems to have caused the crack on this hole. I set this panel aside and rebuilt using another (I have several in the kit). In the new panel I went ahead and drilled and dimpled all the holes, just in case I ran into another crack issue… which I did on the last hole. This hole was a #6 screw hole, and I guess I had an imperfection somewhere. The crack was not all the way through so I tried filing it out. This worked ok, so I decided to keep this panel.
I made some spacers to allow the skin dimples to nest without countersinking the controller box.
I tested the install with some screws and nylock nuts on hold it in place.
Then I primed the parts with some rattle can primer, put some locktight on, torqued it up and put some thread lock on.
Well done Neal sounds a bit tricky and frustrating but satisfying to get it finished
Yes, should have been a simple job. Good lessons learned in dealing with larger holes and thicker material.