Autopilot Yaw Servo Install

[SMGlaeser]

I'm working on AP servo installation. I found plenty of ideas and photos of pitch and aileron servo installs, but haven't been able to find any yaw servo install photos (plenty of ideas, but I need a photo). I've ruled out a capstan setup, the Garmin servo with a capstan setup allows for 2.6" of travel in each direction and I measured my rudder cables move just over 3" in each direction. As I write this, I am second guessing my measurement... Regardless, I came up with a push rod installation method and want to see if anyone out there has experience to share.

My setup (mocked up in the photo):

• Servo bracket welded to the last fuselage station
• Servo mounted between the two aluminum fairing strips, access through inspection covers
• The push rod is above the lower aluminum fairing strip, does not interfere with the rudder cable exit
• Will use a rod end mounted to another hole drilled in the control horn, geometry will avoid interfering with the rudder cable or over-centering the servo control arm

[spinningwrench]

I did a capstan style arrangement just behind the baggage compartment. My calculations and experimentation showed I had sufficient travel of the capstan to make it work on my Patrol. As a basis I copied the installation style from Husky certified aircraft which I believe Garmin publishes.

[Tailwheelflyer]

I did a firewall mounted capstan by creating a loop between the rudder petals. Seems to be working so far but we will be flying in the next few weeks. You can see my setup at 12:15 in the video. https://youtu.be/EaXvexrLYOM

[rodsmith]

I made provisions for the same setup, brackets for the pulleys and servo and wired connector. Will initially see how the autopilot works as 2 axis. Looking forward to hearing your experience with it.

[SMGlaeser]

That is a really clever setup. Have you done a W&B yet to know if weight in the tail is good or bad? Based on the prototype, expect weight in the tail will be a good thing.

[500AGL]

Couple of things to pass along from Garmin, and what I am doing for yaw.
A few select Install manual pdf's attached.


From Garmin's Install Manual. **WARNING Do not use crank arm attachments to actuate cable driven controls. A Capstan attachment should be used for this purpose. Note that Steve's example above is a pushrod going all the way to the rudder horn, which is different that attaching to a cable. Only thought on this is it will be running outside alongside the cable, so look at the fabric opening, travel and swing.

My opinions :
Essentially a Servo Crank Arm and a Capstan have the same travel, per Garmin. 2.6" on the Capstan vs 2.95" when using a long arm crank. Using a standard crank arm (more torque) the range is limited to 2.3".
In the experimental world of course, you can experiment and be a test pilot to experience a range greater than specified, but that's on you.

More personal opinion is that a capstan will provide a greater range at a consistent torque load, in a safer manner then a crank arm. In particular if your bridle cable routes are 180* from each other. (See pdf attachment) I will also include a photo of what I am installing for a yaw servo.

Don't misinterpert my crank arm statement above. Crank arms are perfect for the right application. I am using them for both pitch and roll, as they both attach to a linkage or bellcrank.

[rodsmith]

Good info, I like your setup.

[spinningwrench]

I made a custom crank arm for my yaw axis that is inbetween the two offered by Garmin which will optimize the full rotation of the Garmin servo.

[rv8bldr]

I put the YD servo in the tail and drive a pushrod directly to the rudder arm. Very clean and simple to setup (and take out if I don't like it).


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[Tailwheelflyer]

Just to fill everyone in my yaw damp servo has performed flawlessly on the firewall install and the autopilot flies without any help.

[schu]

Do you have really poor yaw stability without actively driving the pedals and without the YD on? Seems like the lacking springs would make it less stable, though you AP isn’t working against the springs like in Mark’s design.

[Tailwheelflyer]

I have the springs supplied with the kit installed at the pedals but it flys straight, hands off feet on the floor even without the autopilot running. No trim tabs on the rudder, I was pleasantly surprised.

[Bcone1381]

I have the springs supplied with the kit installed at the pedals but it flys straight, hands off feet on the floor even without the autopilot running. No trim tabs on the rudder, I was pleasantly surprised.

Did it fly like that on the first flight, or did you need to fine tune the rigging?

[Tailwheelflyer]

Had a very slow role to the left at cruise. Shimmed the upper aileron mounts on the left and that fixed it.