Going to mount the COM1 antenna under the rear passenger seat. Need to make a doubler plate as I'm working on those seat ribs.
Plan is to have a 0.032” doubler plate between seat ribs. I will also need a 0.025” filler plate to make up for the seat rib flange. Decide I would bend up the filler plate to attach to the rib that doesn't have a flange. 13/16" hole - AC43-13 Table 4-10 says 8.9 rivets per inch for 3/32" rivets in 0.032" 2024T3 plate, so 8 rivets min per side. In this case the rivets for the nutplates will count toward that.
Drew up a basic sketch of what I thought I'd do and then just started working directly with metal on the plane. Harvested 0.032” from the baggage door cut out of the side skin. Once I had the overall dimensions figured out I cut and bent the plates. Then marked edge distances and drilled a starter hole in the doubler to align with the rivet in the seat rib flange. Then just worked from there drilling and clecoing, and removing and marking additional holes and then drilling the filler. Eventually got to where I had all the holes drilled in the doubler and filler. Then removed the bottom skin from the skeleton and drilled the holes in the skin. Dimpled the filler and countersunk the doubler. Came out good. I'm going to alodine these parts to maximize conductivity.
3/14: I was not 100% happy with the doubler. Some of the holes had wandered a bit when I countersunk them. I was thinking it's ok, because they're only holding nutplates, but really these rivets are critical to transferring the load from the surrounding plate to the doubler around the hole. Decided it was easy enough to make a new one. Just back drilled everything using the holes that were already in the skins. Then was much more careful with the countersink - did it on the drill press with a solid aluminum angle to back up the hole. They say 0.032" is the minimum for countersinking a -3, but when you go 7 clicks past to accept a dimple, it is a little too thin.
3/14: Alodined these parts today. Used a zip lock bag. Only left then in a minute or so. Seems like they overcooked a little. Alodined the skin too before priming it.