Week 6: Vertical Stabilizer & Right Wing

To start this week I continued prepping ribs for the right wing. I did run into one issue with the drawings where a couple of rivet item numbers were called out that were not listed in the instructions. One set was a similar bracket to other ribs, so I used the same rivet that was instructed there (4x15mm). The hole size for the other location was also intended for 4mm rivets, but the material was not as thick. The 4x10mm rivet looked like it would have plenty of grip length, so I used that and later confirmed with the factory that this was correct. They said they would correct the issue with the instructions.

This week I received the correct riv-nut mandrels, so I moved back to the vertical stabilizer assembly. I had to drill out all the holes to the correct size to get the riv-nuts to fit. Since these are metric parts and I don't have many metric drill bits, end undersized the hole slightly, then used a round file to enlarge it just enough to insert the riv-nuts. I put high strength loctite (277 ) on the body of the riv-nut before installing. This should help keep them from turning in the holes.

As suggested by another builder, I enlarge the bottom two holes on the rear spar to completely clear the riv-nut head. This let the doubler sit flush against the inside of the spar. The other riv-nuts did not need this, but I did notice that the clearance holes were not large enough for an M4 machine screw, so I drilled the clearance holes in the doubler and rear spar out to 3/16" to allow the screw to thread in cleanly. I also had to do this to one hole in the skin that will have a screw installed during final assembly.

With this done I could finish assembling the vertical stabilizer frame. Then I ran the wire I had ordered (2 wire, 18 gauge) for the beacon light, and added the appropriate edge protection. I then installed the skin, and after checking that the alignment was correct riveted it in place. I then re-checked the alignment, and after rigging the vertical stabilizer in an upright position temporarily rigged the rudder to verify they are both aligned correctly. This was very rewarding since it really looked like the back of an airplane!

With the vertical stabilizer done, I went back and finished installing the trim tab to the elevator assembly. I originally held off on this because I was short the correct size of rivet in that assemblies hardware kit. I did have enough left over from the vertical stabilizer that I did not need to request more. These had to be shortened to 5.5mm before I could rivet the hinge to the trim tab. At the advice of another builder, I also safety wired the trim tab hinge pin. After some trial and error I decided the best method was to tuck the wire into the trim tab since they both move together. Other than installing the beacon onto the elevator (which is on backorder), the empennage kit is now complete!

To finish the week I went back to prepping the right wing. On one of my last ribs, I accidentally bent the rib laterally which caused a crack at one of the relief notches. I thought this was going to mean replacing the part, but the factory instructed me to just extend the notch through the whole flange to remove the crack. The original design had the ribs fabricated this way, but the notch was shortened to add stiffness. This does lead to some cracking during the assembly process. They said this should not be an issue once the parts are put together. To correct this I drilled a hole at the bottom of the crack, then used a Dremel to connect the hole to the existing notch.

The next step in the wing assembly is to countersink most of the holes in the top and bottom of the main spar. This was a little nerve-racking since doing it incorrectly would be hard to reverse. After some careful experimentation and a lot of research I decided to countersink deep enough to allow a rivet to sit about .004" below the surface. Vans recommends .005" for the RV builds, but they use 100 degree flush rivets as opposed to the 120 degree flush pulled rivets on the Sling. It is better for the countersink to be a little shallow and have the skin not sit completely flush than to have a gap in the middle of the joint. If needed I can always go a little deeper when I start attaching the skins.

I should have 2-3 more days of countersinking, then I will be able to start assembling the structure of the wing.