We completed the installation of all the nutplates on the Right Main Spar.....
....and installed the two Aileron Bellcrank Brackets. Just like on the Left Main Spar, the bolts that hold the brackets were already installed in the Spar Assembly from Van’s.
Now, both Spars are almost complete. We need to countersink the nutplate screw attach holes and torque the eight bolts holding on the four Aileron Bellcrank Brackets.
We will start this process by countersinking the larger Fuel Tank Attach screw holes. In Part 6, I added two excerpts from the plans defining the maximum inside and outside diameters of the coutnersink. The maximum outside diameter of the countersink is .370 (9.4 mm) and maximum inside diameter of the countersink is .220 (5.6 mm).
Here is the excerpt:
With these maximum diameters in mind, I found the middle distance of .295 (7.5 mm). I made the initial countersink (using a #30 countersink cutter in the cage as directed in the plans) to the middle figure and it looked very shallow. The Fuel Tank Skins are .032 thick, so I found a scrap piece of aluminum of the same thickness, drilled it, dimpled it with a #8 screw dimple die, and checked the fit.....it was shallow.
I was interested what other builder’s were doing, so I started looking at other builder’s websites. Shawn’s site provides some details on how he accomplished his countersinking, as he countersunk his larger holes to a diameter of .355. Keeping the maximum of .370 in mind, I started to enlarge the diameter of the hole a little bit at a time. After each “layer” of aluminum was removed, I checked it against my scrap template and the guage below. Eventually, I settled at a hole diameter of .350. That fit pretty good with the scrap template I dimpled. Now, we started on the rest of the holes.....continuously checking the diameter.
Here is the first completed Main Spar Flange with the nutplate screw holes countersunk to a diameter of .350.
Just for information: I did find my cordless drill made a “better hole” than my air drill. I have a variable speed Sioux air drill, but for some reason (probably operator error) it tended to chatter a lot and not leave a round countersink. The cordless drill cut much smoother and made a pretty round hole. If I had to, I used my deburring tool to make a few turns at the very end to make sure the hole was round. I think it worked out pretty well in the end.
Due to the depth of the coutnersink versus the thickness of the Flange, as Shawn mentioned, there is a knife edge created. Because I chose to install the nutplates BEFORE the countersinking, I’m not able to “clean up” the knife edge from the opposite side of the Flange. It’s not really all that bad, but some of the inner circles are not completely round.
Anywho, we hope to finish the countersinking in the Spars tomorrow and they will be complete. Once we are finished, I will use a small airbrush to Akzo prime the holes we countersunk in the Flanges.