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GAD27 and roll trim direction puzzle

Xavier L

I'm New Here
I’m puzzled about the behavior of a Garmin GAD27 feeding roll trim information to a Ray Allen trim motor through a GSA28 roll servo. I find that I need to either cross wires compared to the Garmin documentation or tell the G3X Touch that the switch direction is reversed (although it is not) to make things work. I am curious to hear from others if you noticed the same thing, and of course to learn if I overlooked something.

The wiring has been done (and checked) to be exactly as suggested in the documentation. The “Roll Trim Switch Direction” is set to “Normal” and the “Roll Trim Motor Direction” is set to “Normal”.

In this setup, the pilot hat switch is connected to the GAD27 (1st connection), which sends power on two wires to the GSA28 servo (2nd connection). The servo then sends power to the Ray Allen trim motor (3rd connection).

1/ The pilot hat switch is connected to the GAD27. When I press the hat switch to the left, under “Configuration/System Information/ECS (GAD27)”, I see “Pilot Left”, as expected. Indeed, I checked that the “hat left switch” grounds Pin 28 (PILOT ROLL TRIM L) of the GAD27. So far, so good… but the plane rolls right. The problem must be somewhere else.

2/ When I press the hat switch to the left, under “Configuration/System Information/ROLL (GSA28)”, I see “Trim Input Roll Right”. Ha! I must have crossed wires from the GAD 27 to the roll servo. Well, after checking visually (wires of different colors, checked the wing root connector too), I can’t see any problem. I confirmed that with an Ohm meter, pins and a long test wire. Pin 9 (ROLL TRIM OUT 1) of the GAD27 is indeed connected to Pin 11 (TRIM IN 1) of the roll servo, and the return wire is wired according to Garmin documentation as well. Note that if I cross wires here compared to the Garmin documentation, all is well.

3/ As for the connection from the roll servo and the trim motor, when I press the “Roll Left” on-screen button under “Configuration/Trim/Roll Trim/Roll Trim Motor Movement/Test”, the plane rolls left. To me, this means that the wiring between roll servo and roll trim motor is correct.

To see if I had an internally mis-wired servo, I replaced it with the Garmin “GSA28 removal adaptor” which simply passes trim signals through. Same behavior, so the servo is not the problem. To see if I had an internally mis-wired GAD27, I borrowed another one. Same behavior! I also made sure that I did not swap ROLL TRIM POWER IN and Ground on the GAD27. As for my pitch trim, all is well.

G3Xpert kindly reviewed my schematics and loaded my configuration into their kiosk, and could not find any issue with either. They were not aware of the behavior reported here, hence my post.
 
Two settings

I just reconfigured my aileron trim.

there are two setting in the Garmin main configuration menu. One for trim motor direction and of for AP trim direction.
I had to play with these settings until I found the correct combination to make the motor and auto trim work correctky.
 
Xavier,

Don't forget that sorting out your trim motor wiring starts with powering off the GSA 28 servos and running the trim motors in the correct direction through the powered off autopilot servos.

Here is a simplified version of the steps explained in the G3X Installation Manual to configure/test your trim installation.
  1. Power off the GSA 28 servos.
  2. Use aircraft trim switches to run the trim motor. If the trim motor runs in the wrong direction, swap the trim motor wires at the control stick, at the servo on pins 11,12 (switch inputs), at the servo on pins 13,14 (trim motor drive wires), or at the trim motor itself.
  3. Verify that the trim now runs in the correct direction with the servos powered off.
  4. Apply power to the GSA 28 servos.
  5. Use the on screen controls on the servo configuration page to drive the trim motor for that servo.
  6. If trim motor runs in the wrong direction, change the trim motor direction setting on that page from Normal to Reverse (or Reverse to Normal if it is presently Reversed).
  7. Re-verify that the trim operates correctly in both the servo powered on and servo powered off conditions.
Steve
 
GAD27 and GSA28 roll trim polarity

Now that I am happily flying, here is a short summary and note of caution for future builders.

The GAD27 and GSA28 roll servos can independently control roll trim, but the roll right/roll left polarity convention they use is reversed (at least on the units I could check).

As a consequence, if you wired per plans, and your GAD27 roll setting is "Normal", expect to have to set your GSA28 roll to "Reverse". In any case, do check both settings before flying, as indicated in the manual. You don't want auto-trim to run-away in roll!

The pitch axis is fine, so you can have both pitch control devices set to "Normal".

Xavier
 
The GAD27 and GSA28 roll servos can independently control roll trim, but the roll right/roll left polarity convention they use is reversed (at least on the units I could check).

I don't think this generally holds... remember that other airplane types might use a servo installation that has the servo rotating the other way. The important thing is to verify your wiring and configuration all agree, which is pretty straightforward since the configuration interface gives you all the information you need.
 
If the roll trim pushes "the other way", the GAD27 would have to be set to Reverse and the GSA28 to Normal, because they still have opposite polarity conventions (on the roll axis).

But I agree, the take-away is to physically check trim direction with each device, then to configure them to reach desired results. We can't assume they behave the same.
 
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