Back to some real work today! With everything primed, the center section can start to go together for good. I started by dimpling all of the holes on the bottoms of the ribs (for the bottom skin). The forward most section of the seating area that is underneath your legs has a floor that is removable since the controls pass through the area, so instead of being riveted on it gets nutplates and screws. Nutplates are simple, but pretty fiddly to attach - each one has to be dimpled (as well as the rib flange) and then held in place while the rivets are squeezed. I attached the nutplates to the center two baggage area ribs as well. That area is used as a tunnel for controls and wiring, so again is screwed on.
Next up was riveting the seat ribs to the aft bulkhead.
In addition to the rivets, each rib also gets a bolt at the top and bottom. As far as I can tell, this isn't for strength, but purely because the top and bottom portion of each rib line up with the bulkhead web as well as the thick doubler (not sure what it's called, but the big chunk of aluminum that forms the I beam). A rivet would have to be extremely long to make it through all of this, so a bolt is used instead. There are quite a few places I've seen that do this. I got all of the bolts in and torqued them down.
Lining the seatback bulkhead, seat ribs and baggage ribs all up was easier said than done. Some seat ribs are riveted to the bulkhead by themselves, others are riveted in conjunction with a baggage rib, and yet others get spacers held in place before riveting. It took me a couple of ribs to figure out the secret sauce. Once I did, things went together fairly easily. I couldn't really get the squeezer to work in the tight quarters, so I just used a long rivet set on the gun and bent each rib out of the way to get at the rivets.
The outside seat ribs get two rivets and a bolt at the seatback bulkhead. As I was getting ready to put the right rib on, I noticed that the bolt hole was pretty badly out of round. I'm not sure how I didn't see that before. It's supposed to take a 3/16" bolt, but I decided to open the hole up to 1/4" instead. That size of bolt is overkill, but considering it goes through the bar that the rear wing spar bolts into, I'd rather have a tight fit than a smaller bolt that has play in it. Opening up the hole meant drilling the bars and bulkhead as well as the spacer and rib.
Just as I was getting ready to finally rivet that outside right rib on and put in the new bolt, I realized my mistake. I had placed the parts down, then spun the center section around so I could get at the outside easier. Soooo, the right became the left. Long story short, I originally upsized the correct side of the bulkhead (easy, because I was correcting an ugly hole), but I upsized the wrong rib. To fix that I had to also change the left side from 3/16" to 1/4" so everything would match up. Not a big deal, just time consuming.
The last thing I did for the night was put the forward section of the front bulkhead in place by clecoing on the vertical spacer that goes between it and the side skins. Next up will be flipping the whole center section over so I can rivet the bottom skins on.
