13.9.2 Auto Pilot Pitch Servo Mounting

This entry is part 35 of 67 in the series 13 - Electrical / Instruments

I’ve already installed the autopilot roll servo in the back of the cabin. Now it’s time to install the pitch servo.

First, I have to figure out the best location. After talking with Malcolm, I decided to put it on the right side of the canard.

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I made a mock bellcrank out of cardboard to determine how long the bellcrank              needs to be. My goal was to try and keep it from infringing on the avionics shelf area.

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After playing around with this bellcrank, the linkage, and some basic math, I determined that it wasn’t going to be a very big bellcrank.

I talked with Malcolm and he recommended cutting a notch in the canard so the bellcrank doesn’t have to move so far aft.

I contacted my friend Robert who fabricated the parts for my nose gear door mechanism and sent him the drawings for the bellcrank.  As usual, he came up with some really good changes.  While we were talking, I mentioned my frustration about drilling holes through the center of a shaft. He said “I can fix that”.  I figured that he would tell me some secret about how to drill holes.

About a week later, he called and I went to pick the bellcrank. Not only did he make me a bellcrank, but also a fixture to clamp it in place  and align the drill bit!  Sweet!!!

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Here’s a dry fit in about the location I think the servo will end up.

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Here’s the bellcrank clamped in place with the drill fixture.  I miscalculated the length of the bellcrank arm and had to drill another hole in the bellcrank arm. Once I was happy with the location, I drilled out the holes. And they were perfectly through the center of the torque tube.
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I made a pair of hardpoints out of aluminum and used structural adhesive to bond them in place.
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To make sure that I’m not going to go over-center on the servo, I made a protractor that allows me to measure the rotation of the bellcrank. With this, I was able to determine that I was about 10 degrees under the maximum rotation without going over-center.

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Next, I trimmed off the excess metal of the bellcrank and painted the parts.
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Another dry fit checking the motion.
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Covering the hardpoints with 2xBID
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Final installation (I still have to replace the phillips head screws and replace them with drilled head AN3 bolts and safety wire them).
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