Wrapping up the lower shelf

Hooray for shelves! OK, I didn’t finish them yesterday like I thought, but that’s cause I ended up not doing airplane things yesterday. So I maintain that this counts, you just have to interpret “tomorrow” as “the next workday.”

The first part of the day was just figuring out how to get these quarter-turn fasteners installed. I attempted to find some documentation for these Friday night, and about all I ran across were some engineering drawings. So I did some mocking up and tinkering today just to wrap my head around how these should work. The overall idea is obvious: two rivet holes to attach the base to its mounting surface, and one larger one in the center to accommodate the actual fastener.

What’s interesting is that the specs on McMaster-Carr seem to indicate that the center hole should be a different size for the two panels – but that makes no sense, as the fastener has to go through both panels. So I just started experimenting with scrap. A 1/4” hole was a good size to accommodate the fastener shaft with a bit of wiggle room.

Next up was how to actually locate that center hole. I couldn’t just use a base as a drill guide, since the center hole isn’t really round. Instead, I worked out the rivet hole spacing, and just made myself a drill guide using a piece of scrap angle. With this, I could drill three #40 holes in a piece, then enlarge the center hole to 1/4”. I did a test run drilling a set of holes in another piece of scrap angle, and it worked perfectly.

Next I drilled a 1/4” hole in a strip of sheet, and used the fastener to attach them. This worked, but didn’t hold the pieces tightly together. This wasn’t automatically an issue, though; in practice, there’s a little retainer that goes on the fastener, which is what keeps it captive in the panel. I measured one of those and found that it was .040” thick. Just right for me to use a piece of scrap hinge as a pseudo-retainer to test the final fit. (I couldn’t just use the retainer; they’re a one-time use thing, and I had no spares)

So finally, after probably an hour and a half, I was satisfied that I could actually mount these things. Next I had to actually fit them. The challenge here was accurately drilling the center hole in assembly between the shelf and mount brackets, in position in the airplane, clamped together tightly. What I ended up doing was first laying out those holes in the shelf. To do this, I installed the shelf in the plane (for, I dunno, the fourth or fifth time) and marked the outlines of the four brackets. Then the shelf came out again and I marked the counterpoints for each bracket, then drilled a #40 hole at each spot. Then the shelf went back in again, got clamped in place, and I drilled the center holes through the brackets. Finally, it all came back out again, I used my drill guide on the brackets, enlarged holes as necessary, countersunk for flush rivets, and then it was time for primer.

By this time it had been raining steadily for a while, so I ended up shooting primer on the brackets inside the hanger. It’d been windy all day, so ventilation wasn’t really an issue:

While the primer was drying, I went to work finalizing the shelf itself. I still hadn’t final-trimmed it to length, so there was that, and I also wanted to add a small flange to the aft side. The sheet has a bit of a curve to it, which has been annoying while fitting it. In practice, the hinge would keep one side straight, and the quarter-turns ought to take care of the other, but I really wanted to have it be naturally straight. So I laid out a 1/2” flange, did the cuts necessary to set that up, cleaned up the edges, and got the flange bent.

By the time that was done, the primer was dry, and I got the bases riveted to the brackets and clecoed those back into the fuselage. That made it time for a real live test fit, albeit without the fasteners actually captive in the shelf. And…it fit! Whew. So I installed the retainers (necessitating removing and then reinstalling the shelf again), and gave it another test fit.

The only thing I didn’t like was that the fasteners didn’t quite seem super snug – it took hardly any force whatsoever to turn them. It seemed to me that, in this state, they could very possibly vibrate loose in flight, which would not be a good thing with the shelf being right above the stick. I already had a loose plan for this from my earlier experimentation – I went into one of my O-ring kits, got four little 3/8” ID rings, and put one on each fastener above the retainer. With those in place, the fasteners fit nice and tight. At this point, i was satisfied enough with everything to take the final step of riveting the brackets to the brace.

So finally: pictures! Here’s the finished shelf stowed, with the brackets still clecoed. The wings on the fasteners are just visible, as is the flange in the aft end of the shelf:

And the shelf lowered, showing a little more detail on the brackets and bases:

Next, it was time for some live mockups. After having worked on placing the electrical distribution stuff on cardboard cutouts for a while, I could finally look at it in place:

This was a worthwhile exercise. The E-bus alternate feed relay (top right) has vertically-oriented terminals. It’s not obvious from the photo, but the forward upper brace is in somewhat close proximity. I think that large wires going into and out of the relay would be problematic with this layout. Fortunately, this seems easily solvable just by reorienting the battery bus a bit.

A part of the layout tinkering, I laid out some cable guides in their rough locations, and then decided to go to town annotating a photo of the setup to demonstrate the planned wire routing. All wires enter/exit the shelf near the hinge line, necessary to allow the thing to pivot properly. Power feeds enter on the left, while all the individual circuit lines exit on the right.

The red line represents switched power from the master contactor. Normally, this line will be supplying almost everything on the airplane, with the exception of a couple things on the battery bus. The switched power feed first goes to the main bus fuse block on the left, and then continues to the E-bus fuse block, going through a Schottky diode. More on that in a second as well.

The blue line represents an always-hot line, directly from the battery – where it will be protected by an ANL fuse. The line feeds to the battery bus, then continues to the E-bus alternate feed relay. This relay is controlled by a switch in the cockpit, and it allows for feeding the E-bus directly from the battery bus. In the case of a master contactor failure, or any sort of electrical fault with the main bus feed, this will allow keeping minimum equipment online using just the ship’s battery. This situation is also the reason for the large diode between the main and E-bus; without this, closing the alternate feed would also energize the main bus, which is not desirable for a few different reason.

Finally, the green lines represent all the individual wires from circuits, making a nice little bundle. That bundle will also include a switched ground wire that will control the E-bus alternate feed relay.

(now the only question is: will it look anywhere this neat when I’m done with it?)

(also that turned into a lot more of a lecture than I intended)

Av shelf

So now…what next? I think I need to start working up a fastener inventory for actually mounting all this stuff. Between that and the need to get the ELT ordered, I ought to have justification for a Spruce order. I wonder if I should also go ahead and order the large-gauge wire needed for the bus feeds here. With those in hand, I think I can finalize the component locations here and get stuff mounted.

In the meantime, I guess now that I have avionics on hand, I can fabricate the ADAHRS mount in the aft fuse; that’s pretty independent of other stuff, and I could also finalize the static system routing – by which I mean cutting one more piece of tubing probably a couple feet long…

I think I’m going to stop typing now.

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