Chin-scratching, fixing the aileron trim

Yeah, it was mostly a deep-thinking kind of night. As per usual, there’s been a lot of that lately, and for the past night or so, it’s all revolved around packaging stuff under the forward seat floor. I kind of already made one major decision earlier today, which was to ditch the idea of using the terminal strips for the wing interconnects. While they definitely make for some very organized connections, the downside is that they take up a lot of space, and that’s kind of at a premium in this area.

Instead, I’m just going to use inline interconnects in most places. For the landing, taxi, and pitot heat connections, I’ll go with handshake/knife connectors, which I can encase in heat shrink to make a semi-permanent connection. The autopilot network cable for the right wing can just use a DB9 connector, which is pretty simple, and the nav/strobe connections will just “detach” by extracting their pins from the DB15 connector on the Flyleds control board.

This decision means that I now only really have to deal with finding a spot for that control board. As part of worrying about this packaging, I decided it’d be prudent to temporarily reinstall the control column, just to help me visualize potential conflicts here – though my intent is not to mount anything in the center bay under the seats, since that’s where the column lives. I will need to run a wiring bundle through here though, but that can be snug against the floor and thus well out of the way.

I also decided to temporarily install the aileron trim servo and push tube as well, since that’s another potential obstacle. In order to see the worst-case scenario, I also ran the servo out to full extension, which in turn means the push tube sticks out as far on the right side as it ever will (and likely further than any real-world scenario). But when I got that in place, I noticed something was off. Let’s take a look at the photo:

(side note: this photo also shows how much space those terminal strips take up down there)

As a reminder, this trim system works via spring bias. A spring attaches to each of the eyelets on the trim tube, and each spring runs to a hook on one side of the column. Movement of the trim servo, then, tends to pull the column to one side or the other. The problem here is that the eyelets appear to be roughly equidistant from the control column – basically a nearly neutral position – but that shouldn’t be the case, since the servo is fully extended. 

Well, it turns out that I messed up when I fabricated that push tube. The drawings specify the location of the first eyelet hole relative to the forked end of the tube that mates with the servo, and the location of the second hole relative to the first. But I measured from the end of the tube for that second hole, and so it was about 1.5” too close to the servo.

The fix was simple – drill another hole. This does leave the tube with an extra unused hole, but I don’t see it as an issue here. If this was a highly stressed component – particularly if it was subject to significant bending loads – then I’d worry bout that hole weakening the thing, but here the tube will only be subject to very light tension/compression. No worries, and this is a lot easier than fabricating another tube – getting that fork right is rather painstaking.

Things look much better now. The second eyelet (on the bottom of the photo) is now out of view past the seat rib, and clearly much further from the column. In this position, the trim system would be exerting a pretty strong pull on the column, which is what we want with the servo at full extension:

In the other-news department, today I placed yet another decent size order with SteinAir. This time I’m getting the coax I need for all the antenna runs, plus some additional D-sub connectors, and some more solder sleeves, and the handshake terminals for the wing interconnects, and I forget what else. I also ordered some large gauge welding cable, which is what I’ll use for the really large wire runs for the starter and alternator circuits. I went with welding cable over Tefzel because it’s a lot more flexible and less prone to failure at the terminals. Quite a few RVs are running around with welding cable in this application, so it’s not a crazy choice on my part.

Next up, more packaging stuff. I’ve settled on a location for the Flyleds controller, and also decided to add holes for the lateral wiring runs under the control column, so I reckon I can go ahead and get those drilled. I probably ought to bite the bullet and get that bulkhead conduit hole drilled for the wiring run to the tail as well…kind of been putting that off.

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