April 2, 2020

Rear Fuselage Structure

I took a short hiatus from building the Sling to work on a few adjacent projects. One of my other interests is 3D printing, and given some of the parts I made recently for my plane, I decided to make a few upgrades to my 3D printer. I’m completely replacing the hot end, electronics, and some of the motion components, and adding an enclosure to the frame to make it easier to print some of the stronger plastics like ABS, ASA and polycarbonate that print better under elevated ambient temperatures.

Major surgery to my 3D printer

Back to the airplane – as I scanned through Craig’s excellent fuselage build order, I realized that I have a lot of upcoming tasks that are dependent on having the rear fuselage ready. Back in June 2019, I discovered a crack in one of the rib sections that goes into the Rib 4 assembly, so I stopped working on the tail section and put in an order to TAF for a replacement part. Many months later I’m stilling waiting for a replacement, though it might be arriving this week.

I was able to work on the center fuselage in the interim, so it hasn’t been a problem, but now I decided I’m pressing ahead and will go as far as I can with the rear fuselage before I really need the replacement rib. I also just placed an order for the undercarriage kit, so I will be able to drop the fuselage onto wheels sometime this spring.

Overall, assembly of the rear fuselage structure was fairly easy, though I had to drill out a few rivets in order to slide the rear-most rib assembly in place (the one that is painted green in the photos below). I got as far as starting to install the wing step reinforcements before I started running into issues, but by that point the weekend was coming to a close.

The floor of the rear fuselage, which is about as far as I got last summer

I never finished putting together rib 4 assembly due to a crack in part RF-RIB-401-R; it’s the part in the next picture that is still bare metal. I riveted together the rest of the structure and left part 401-R clecoed in for now, until I get the replacement part. I think I can work around this until it is time to attach the upper skins, so I should be able to make good progress.

Rib 4 assembly
This is the crack in the rib
Ribs assemblies clecoed onto the floor assembly
Another view of the rear fuselage looking from the rear
Resting the fuselage on its side to rivet the ribs

The next few photos are show the first few steps of the wing step reinforcements installation. Access is kinda tight in this area; I needed to use a close-quarters rivet wedge. Initially, I installed the gussets in the next picture with 3.2mm rivets, but then I realized I neglected to install a reinforcement channel into the longeron along with the gussets. Then, I discovered the holes on the channel are drilled to 4.0mm, so I had to upsize the holes on the gusset and longeron to 4.0mm as well.

Installing reinforcement gussets for the wing step with 3.2mm rivets and an angle wedge

I had to make my own wedge for the larger mandrels on 4.0 and 4.8 millimeter rivets. I made one from a short piece of aluminum bar and filed an angle into the ends, then drilled appropriately-sized holes for the rivet mandrels.

Close-quarters rivet wedge from Cleaveland Tool Supply on top, works for 3.2mm rivets; and below is a home-made version for 4.0 and 4.8mm rivets

Here I am studying the manual and the parts to figure out how the reinforcement channels should be installed. Some (but not all) of the holes in the channel were drilled to 3.2mm but should be 4.0mm.

Placing the reinforcement channels with holes that don’t quite match

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