CHAPTER 41 - Aft and Bottom Skins
Once all the dust had settled from moving, I could begin to work on skinning the bottom fuselage. With the tailcone in place, I layed in the aft bottom skin and immediately made my first mistake. I went ahead and drilled it to the stringers. When I added the aft side skins and strapped them down, I found that the fuselage had twisted almost imperceptibly but enough to place one of the skins in jeopardy for edge clearance. If I had not already drilled the bottom skin, I could have easily shifted it (about 1/4") and the fuselage structure so that everything would work. Twisting the structure by that slight amount did not measurably affect the level of the longerons. As it sat, the side skin sat on the bottom stringer with enough clearance that I could have drilled it to the stringer with plenty of edge clearance - but not using the holes I had already drilled. In the end, I drilled halfway between each existing hole, giving edge clearance to the side skin. As the situation improved toward the front of the skin, I only had to do this for a dozen holes. The other skin was fine, so I just shook my head and continued. As you can see in the first photo, I elected to ignore the Orndorff video and trim my side skins down.
Next I moved to the inspection access for the elevator pushrod. I measured the space between the bulkheads and stringers and, leaving a little room for the doubler and nutplates, decided on a maximum access size. I used Paint to create a template drawing that consisted of three rectangles; the outside dimension of the doubler, the inside dimension of the doubler, and the access hole midway between. The circle drawing function was perfect for rounding off the corners. I printed off several copies of the template and trimmed them to size. Then I used spray adhesive to attach them to aluminum sheet and made my blanks. The photo shows a doubler blank waiting to have the inner part removed and using the full template. The other blank (on the left) uses the center dimension and is a cover plate. I used the actual cover plates to marks the cutouts on the skins so that the fit would be as close as possible. Once the blanks were trimmed (that was a lot of filing!), the templates were removed using acetone. The next photo shows a doubler in place, held by rivets through the skin at top and bottom and sharing rivets with the bulkheads fore and aft. The next photo shows the view from outside and you can see that the extra thickness of the doubler at the bulkheads does not affect how the skin lays. The next photo shows the cover in place for a nice tight fit. The covers were then drilled through the doublers and nutplates added.
Drilling the forward bottom skins in place was very straightforward. However, I did run into a couple of problem areas. When drilling the joint between the forwardmost skin and the two skins behind it, don't follow the rivet spacing blindly. Instead, be sure to take into account the seat rib locations. I ended up putting a couple of rivets uncomfortably close to the seat rib webs. Also, when drilling the rivets along the firewall flange, be sure to set back any rivets next to structural brackets where the stiffeners go. Moving those rivets aft by the thickness of the bracket will make bucking them much easier.