Vertical stabilizer conduit mod

As I mentioned before in Empennage Mods, I made the changes required to run wires through the vertical stabilizer. For now this will be an empty conduit, but at some point in the future I may decide to run wires for e.g. a camera.

This was strongly based on Mouser's mod, with a few changes: I didn't add a plate to the bottom, I'll rivet the top doubler to the bottom of the top rib and I'm only running one conduit (I'm assuming that whatever power I need will not be enough to produce interference on data cables).

Before I did anything, I went through the Standard Aircraft Handbook - which didn't say anything about conduits - and the Aviation Mechanic Handbook - which lays out the rules for how much you can fill the conduit and their minimum bending radius. I'm not filling conduits now, but the bending radius got me thinking how I'd route it from the bottom of the stabilizer into the fuselage, so I decided to look at how the connection is made - as seen on manual page 11-6, the vertical stabilizer sits over part F-1014, near two big holes. To make the smallest (biggest radius) bend possible, the best thing seems to be routing closer to the VS spar (the forward side of the aft hole through the ribs):

For the bottom rib, unlike Mouser's, I decided not to use a mounting plate/doubler, since that nose rib has to be removed and replaced during assembly, and I'd rather not add much slack to the conduit (the above book does say that the reasonable slack is 1/2"). I also considered adding one of the MIL-style circular connectors at the top, but as cool as it'd look, decided against it for simplicity, sanity and weight.

I started by marking the corners of the flanges of the top rib over a thicker piece of aluminum (blue markings below), then drew lines and the respective rounded corners to cut the doubler from. Resulting dimensions are annotated below:

Markings for the doubler
I cut the doubler into the rough shape I had outlined, then found its center (midpoint on each edge) to mark the point of the center (conduit) hole. I also marked rough locations of the locations for AN470-AD4 rivets near the corners:

I drilled the rivet holes with a 1/8" bit, then drilled the center hole and enlarged it to 5/8" with the unibit. It fit nicely under the rib as intended:

I then clamped them together and match-drilled the doubler holes into the rib, using a #30 bit for the rivet holes, and the same unibit for the conduit hole.

At this point I grabbed the conduit to check that it fit through the hole, which it didn't - silly me, 5/8" is the measurement for the inside of the conduit, so I went back and enlarged the conduit holes further to about 1/16" short of the external diameter (to give a tight fit in addition to the silicone glue to hold it in place). With this, the conduit fits just right.

For the lower ribs, the work was simpler - just adding something to hold the conduit, for which I followed Mouser's suggestion and used the Panduit LHMS-S5-C cable ties. I held them into the lightening hole, marked the hole position, then drilled it to #30:

Finally, I deburred all the edges and holes then cleco'ed the whole thing together to see what it looked like, and I'm happy with the results:

Conduit passing through the vertical stabilizer
Doubler to support the conduit

Conduit sticking out the top (needs some finish after assembly)

Time lapse:

Total vertical stabilizer time: 27.7h

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