I started today by finishing the installation of the landing light housing1. I removed the clamps and checked the glue, which had dried well. After that, I countersunk and pulled the rivets in the top row of holes I had drilled earlier, as described in the manual. This finished the installation.
After this, the next several steps all pertain to installing the wingtip. Instead of starting with these, I moved to section 572, installing the corner nutplates for the fuel tank. This is completely independent of the wingtip work and had to wait until now because they install through the leading edge skin and the rear tank extension skin. This step was a bit tricky for a number of reasons. The first of these is that there isn’t a lot of margin with where they are installed. This is because they install on the bottom of the nose and tail ribs. The tail ribs are wide enough that this is mostly fine but the nose ribs are actually narrower than the nutplates so they sit slightly on top of the bend. This works out okay but they must be very centered. This is hard to do because the ribs are covered by the leading edge skins.
What further complicates this is that the position of the nutplates given in the manual is wrong. It instructs you to line them up with some other lines of rivets; however, if you do this they will hit the side of the rib and not fit. Furthermore, the “lower” hole that retains the nutplates (not through the skins, only the capstrip) will not be far enough away from the edge of the skin and will therefore interfere with the edge and not work out. Fortunately for me, Craig had already encountered these things on his wing and gave me a heads up.
I addressed these issues by drilling the center of the hole for my nutplates 7/32” from the edge of the sheet metal (or slightly less). This gave enough clearance to not have to trim a cutout for the second nutplate retention rivet (although I still had to carefully to that with a Dremel on one of them). I also carefully marked the inside and outside edge of the tail ribs before choosing the location to drill and made sure it was mostly centered (cheating slightly to the outside of the rib/away from the bend). This gave plenty of space and was farther away from the center/bend of the rib than was described in the manual. Finally, for the nutplates in the leading edge, the manual is mostly right that these are aligned with the rivets in the capstrips although I still measured carefully and adjusted slightly. I still erred slightly towards the edge of the rib and made sure to pilot these with a #55 and carefully check their position from the underside before committing to a larger bit.
Another thing that made these tricky was the fact that the center hole for the nutplates needs to be #10 sized but large twist bits do a bad job in sheet metal. I was able to work around this but it wasn’t necessarily pretty. I first made sure I was using a drill stop on every bit since this was above the spar and also taped a bit of backing material to the top of the spar under each hole location (wood, scrap aluminum, etc.) just in case. I also made sure the free edge of the sheet metal near where I was drilling was clamped down. After that, I drilled the hole with a #55 and, assumign the position looked good, I stepped up to a #40. This was all okay and the holes were reasonable at this point. I then tried to step up to a #20 bit. This usually “tore” the edges and required that I break off some large burrs with a pair of pliers. I also ran a #30 bit through the hole at this point to try to dull the burrs a bit more. At this point, I was usually able to get the #20 to go but sometimes I needed to get it slighted very slightly out of the hole so that it had some momentum by the time it contacted. There were usually large burrs between layers at this point but between the chip chaser, a dental pick, and a blow gun I was able to remove them. Finally, I used a #10 reamer to enlarge the #20 hole to the final size. This doesn’t catch at all and enlarges the hole cleanly, leaving a much nicer edge. I did one final pass at burrs in the hole at this point before calling it done.
With the center hole drilled, I then had to drill the #40 holes for the retention rivets on each side. I first checked the nutplate on the underside of the rib to ensure that it fit and that a centered hole would work. With that verified, I drew a center line through the hole and used the nutplate drilling jig to drill the first hole on the leading edge skin/tank extension skin side of the hole (I learned that this ordering was important). Unfortunately, flipping the jig around didn’t work because of the curve in the skin and the uneven levels (the indexing pin wouldn’t go into the first hole drilled). Instead, I cleco’d the nutplate onto the top of the skin and lined up the other hole with the line I drew earlier. From here, I match drilled the hole through the nutplate, being careful to ensure that the drill was square. I was then able to deburr all holes one last time, cleco the nutplate to the underside (in its final position), and pull the rivets. The front two nutplated needed a bit of a squeeze with a pair of plier wrenches to flatten out the bend in the nose rib enough to make the rivet reach but it worked out just fine. With all of this done for all four corners, section 57 of the manual was done too2.