Elevator Trim Servo Wiring and Test

HAPPY NEW YEAR, EVERYONE!!

With much of the tail cone already done by the builder I purchased it from I only a have a few options for things to work on until the next kit arrives (in a few months 😒). In preparation for attaching the tail feathers I decided to work on the elevator trim servo. In particular I wanted to (a) complete a bench test to ensure it works and (b) sort out how I was going to connect the wiring to the eventual fuselage and instrument panel.

The Ray Allen servo setup is pretty straightforward. The kit comes with the servo, a three position switch, and a position indicator display. The wiring diagram is also straightforward–connect the colors as shown.

MR. GREEN MEET MR. WHITE

Since I have previously dabbled in microelectronics I have no shortage of stuff for connecting components like this. One breadboard, a power supply and some hookup wire later…let there be light (emitting diodes)!

NO SMOKE! SO FAR, SO GOOD

I ran the servo through its complete range and after confirming the servo electronics I moved on to the hardware. The first step was to assemble the servo linkage and it was then I ran into my first real aircraft builder decision. The plans call for the three pieces of the linkage hardware to be rivetted together using two AN470AD4-7 rivets but for whatever reason the remaining hardware that came with the kit contained but a single rivet of this specification. Now, being new to this whole every-decision-you-make-could-be-life-or-death thing I sought out some advice from the “hive mind” of RV aircraft building: Van’s Air Force. One respondent suggested I just take a longer rivet and cut it down using a rivet cutter. While I admit it’s a great suggestion I don’t (yet) own a rivet cutter. Not wanted to delay the process by ordering a single rivet to be delivered via USPS during the Christmas crush I decided to use a AN470AD4-8 (just a mere 1/16″ longer than specified). I figured as long I managed to create a good shop head I would be fine. The Cleveland Aircraft Tools “Main Squeeze” rivet squeezer made easy work of that.

The next step was to deburr the trim bellcrank and assemble everything. That’s when I ran into conundrum #2. The bellcrank mounts were too close together and I could not fit all the called-for hardware in the gap between the two angle pieces. With the holes in the F-1095A servo bracket already drilled I decided to drill out the rivets in one of the brackets and fabricate a new one. Fortunately the kit came with some leftover 3/4×3/4 aluminum angle stock and this went pretty quick thanks to band saw, sander and grinder with Scotch-Brite wheel.

Because I had all the hardware already out I sorted out the positioning by first drilling the 1/4″ hole for the bellcrank hardware, connected the bellcrank, and then match-drilled the three mounting holes using the holes in the servo mount as a guide. In the end it probably shifted the remade mount outboard about 1/8″ but that was enough for the bellcrank to move freely. Four rivets later (I screwed up one and had to drill it out) and the bellcrank was in business.

SOME BELLS NEED SOME CRANKING!

At this point I couldn’t help but wire up the whole assembly and watch the bell, er, crank!

I am curious to see how this all goes together since the forward bellcrank attach point dips quite a bit below the bracket and seems like it would interfere but, hey, after all these years I assume someone would have fixed it if it were a problem.

The next step was to create an attach point for the wiring harness I planned to build. I wanted to be able to (somewhat) easily remove the servo assembly as needed and so there needed to be a way to disconnect the wiring running up front. The servo lead wires are pretty short and so you need to route them to a point quite close to the servo. Based on prior art from Mouser I decided to fashion a bracket that would hold a CPC (see below) from .020 aluminum sheet.

I started by fabricating a cardboard template that would provide enough space for the connector and a tab that will be used to rivet the harness mount to the servo mount. I also added flanges to provide stiffness to the part.

CARDBOARD–AN AIRPLANE BUILDER’S FRIEND

I transferred the cardboard dimensions to aluminum and drilled out holes for the connector using a step drill and the mounting holes (not quite getting them aligned straight but, heck, it’s buried in the back of the empennage after all). Then I cut, sanded and deburred the final piece (being careful to drill stress-relief holes at the bend points) and bent the flanges in a vise.

I then had to decide where on the servo bracket to mount it. It needed to be close to the servo but could not interfere with the mounting screws. I offset it enough to provide clearance and then test fit the entire assembly in the empennage.

With the hardware sorted and connected I moved to how I was going to mount and route the wiring. As I mentioned earlier, I got the ideal to use CPC (Circular Plastic Connectors) from Mouser, and so did some research on them. I liked the fact that they are pretty foolproof and easy to manipulate in tight spaces and so, after shopping around, I ordered a bunch of components from Arrow.com:

Now, all of those components are relatively inexpensive. What’s not inexpensive is the tool needed to crimp these particular connectors. At $200+ it seems like an extravagance at this stage but I see myself using these connectors a lot so I’m sure I’ll recoup my investment over time. (And, by the way, I also got the pin removal tool because I know I will screw something up at some point.) In addition to the specialty bits I also got some wire loom and silicone tape from Amazon and was ready to fabricate a wire hardness.

MAKING AIRPLANES, ONE WIRE AT A TIME

The first step was to cut the wire loom to length and thread the bare wires through. Of course I failed to recognize this as the first step and went ahead and attached the female sockets. This required carefully unrolling/re-rolling the loom around the wires. Argh…

Before I crimped the wires on the servo I tried practicing with some scrap wire, sacrificing a few connectors in the process. I found it a bit cumbersome to position the wire in the connector in the right way and also hold the two at the right spot in the tool. Also, the tool uses a ratcheting mechanism that doesn’t release until the connector is fully crimped so it’s nigh impossible to correct the alignment if you misalign something. Eventually I discovered that if I closed the tool until the first ratchet position I could maneuver a pin into the correct position and it would stay there. Then I could feed the wire into the open end and finish the crimp. In the end I got all five servo wires crimped without any screw-ups. Time to declare victory and go home!

CLOSE UP THEY LOOK LIKE TINY LIGHTSABER GRIPS

Actually I forged ahead and finished the servo-side wiring by attaching the wire loom and silicone tape, inserting the pins, and attaching the shell clamp.

To finish out the wiring tasks I created a temporary wire harness to use during the install/adjustment process and tested everything on the bench. I only had one brief moment of panic when the servo didn’t work before realizing I’d inserted one of wires into the wrong row on the breadboard!

IT ALMOST LOOKS SEMI-PROFESSIONAL!

The final elevator trim task was to finish the E-616-PP trim cable cover plates, to which are rivetted a set of cable brackets. The previous builder had purchased a set of milled cable brackets from iflyrv10.com, but only after match drilling the holes using the stock Van’s brackets so these ended up as scrap. (Actually I ended up using them as #30 countersink guides.) Van’s brackets are nothing more than a nut welded to a piece of steel and if I had built the tail from scratch I would have upgraded as well.

For a reason never fully explained, he had ordered one replacement plate so one of my first orders placed to Van’s was for a second. Since, at that point, I had not transferred the project over to my name I’m sure the folks processing the order were perplexed why a “non-builder” was ordering this very specific part. A few days later I had my part and was ready to complete the step…until I realized I had somehow neglected to include a rivet puller with my tool order. Fortunately this is a non-aircraft specific tool so a trip to the local Ace Hardware store rectified the problem.

The assembly process went pretty quickly. It would have gone faster but it was the first time I got to use my dimpling table so I spent considerable time making sure I had the right dies, adjusting the table height and ensuring I was dimpling on the correct side of the skin.

Now, on to bigger things!

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