Fitting the F-711 bulkhead2h
September 14, 2022
Today I finished trimming the J-channels and stumbled upon mismatched parts.
I finished the J-channels by marking them against the actual location of the F-706 bulkhead.
However, when I started fitting the parts better to the skins (fluting the bulkheads, and adjusting the flanges), I found that I have a pretty big mismatch between the holes of the F-711 bulkhead and the skin.
There are two vertical lines of rivets on the F-711 bulkhead (formed by two halves, the F-711A and F-711B), and in my case, the distance between them was larger than the distance between matching holes on the skin.
This is what a mismatch looks like on both sides of the fuselage.
I measured the distance between the holes on the bulkhead and the skin and got about a ~0.030" difference.
Still figuring out what to do. My first thought was to use NAS1241 rivets (they are 1/64" oversize, for a total of 7/64"). If I split the difference for the offset, I should get ~0.015" larger holes, which would get me exactly to the NAS1241AD3 rivet size.
However, the more I thought about it, the less I liked that idea. It uses "wrong" rivets, some already marginal edge distances will get even smaller. Also, there is no guarantee that 7/64" will clean the holes. The risk is that it will get worse, and I will have to go for the AN426AD4 rivets, which will make the edge distance even worse.
I asked Van's Aircraft about my NAS1241 idea. Also, I asked if they can make me a part without the holes. Still waiting for the response.
Someone on the VAF suggested replacing the flanges, which got me into some thinking. I can replace the flanges, but riveting them to the part would require removing the F-711C bars and maybe using the flush rivets (so they sit under the bars). Or instead, I can extend the flanges and get them under the bars, so they reuse the same AN470 rivets the bars are attached to. At this point, I might as well duplicate the top part of the F-711 instead of making separate flanges. At this point, it would make sense to do the bottom part, too, so I can use these 8 rivets on the bottom to attach the new part to the rest of the F-711. Then I might just duplicate the whole part since it is not that complicated.
So this is my backup plan for now. I think it should not be too hard to trace the existing part. It is a square-ish part, with a rounded bottom (which I can trace from the part itself).