Autopilot

I Recently installed an MGL Avionics - Enigma EFIS (that will drive Trio autopilot servos) in my CH601 HDS that has 630 flight hours. My plan is to add two Trio servos (aileron, elevator) in the next month or so. I've been doing some research and taking measurements and came up with one issue so far.

The Trio servo comes with a control arm that has a 1.6 radius at its outermost attachment position. The servo has internal mechanical hard stops at +/- 75 deg. for a total travel of 150 deg. This gives a linear travel of 3.1 inches.

My elevator cable has a linear travel (stop to stop) of 4 inches. The installation needs to be setup so the control hard stops are contacted before the servo hard stops.

After discussing this with Trio (very helpful folks) it turns out that I need to extend the servo control arm by 5/8” to a radius of 2.23 inches which will result in a linear travel of 4.3 inches.

Worth keeping in mind is the fact that this 4.3 inches of travel is just to allow the elevator control to hits its hard stops when the servo is inactive.

When the servo is active, it only drives +/- 45 degrees.

Looking forward to hearing any discussions about current or proposed autopilot installations or issues.

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Permalink Reply by Bob Simmons on March 4, 2009 at 1:51pm
I'm planning on using the built-in autopilot capability of my Dynon FlightDEK-D180 for the autopilot in my CH 750. All you do is add two $750 servos and you have an autopilot. Unfortunately the CH 750 uses cable controls and so far Dynon only has servos for push tubes. They told me the cable servos are in the works though. Since I'll already have the D180, adding an integrated two axis autopilot for $1500 is hard to beat.
Permalink Reply by Bill Morelli on March 4, 2009 at 2:36pm
Bob,

Usually (as on certified aircraft) servos that drive a cable use a capstan with the control cable wrapped around it or a bridle cable wrapped around the capstan and then clamped to the control cable.

Trio has come up with an easier way to drive a cable using the same servo arm that drives a push tube. They have a push rod that attaches to the servo arm with a bolt and the other end of the rod has a clamp that attaches to the cable. Very simple and clever.

Seems like you could adapt this to your Dynon servo. I can send you a picture if you like.

I also will be getting an integrated autopilot (using my MGL Enigma) by just adding two servos. Great deal.

Bill
Permalink Reply by Bob Simmons on March 5, 2009 at 3:17pm
Sounds very interesting. I'll wait to see what Dynon comes up with for now, but I might indeed want to see that picture down the road.
Permalink Reply by Bob Simmons on July 9, 2009 at 4:48pm
Dynon now has a capstan servo, the SV32C.
Permalink Reply by Geoffrey Alexander on July 9, 2009 at 9:19pm
Bill. I am in the process of completing a ch-640. ( Anybody know what that is?) I am adding the servos using the MGL enigma to drive them. I didn't like the idea of attaching to the cables. The aileron servo is mounted under the rear floor board parallel to the main spar and uses a push-pull rod attached to the aileron bellcrank. The elevator servo will be mounted in the rear of the fuselage and will also use a push-pull rod mounted to the elevator bellcrank. By mounting this way I have avoided attaching anything to the cables and I can adjust the amount of travel by mounting the push-pull rod the necessary distance from the pivot point on the respective bellcranks.
Hope this gives you some ideas.
Permalink Reply by Bob Simmons on July 10, 2009 at 9:49am
Bill has a 601 and I'm pretty sure those don't have a bellcrank for the elevator.

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