MGL Avionics User Forum

I have a Gen 1 XTreme EFIS with MGL servos in my Zenith STOL 750. I flew for a long time without slats and my cruise was about 80 kts and the AP did OK. Later, I installed the slats, bigger tires, etc., dropping my cruise to about 72 kts. With the AP engaged, it now banks back and forth and it is very annoying - the only thing that has changed is that my cruise speeds are lower. Is this plane's configuration now just too slow for an autopilot?
As far as the AP's configuration, the bank servo's pushrod is connected to the most distal hole on the servo arm, target rate of turn is 3.0, servo torque 80%, servo magnitude 20 (the manual says it goes to 30, but it only goes to 20 on my setup page), and heading control medium (although it still does the same thing with fast or slow settings).
John
N750A

Servo magnitude at 20 is seriously high as is the target rate of turn.

The magnitude dictates how much servo movement to apply for a given desired correction. If that's too much it will over-control - in particular if you tell it to aim for 3 degrees per second turns.

Try 2.5 degrees per second and a reasonable servo magnitude usually is around 12 to 15 or so.

Also have a look at your servo shear screw and make sure it has not loosened - same goes for the big grub screw that goes in the side of the boss. If these are not fixed with something like a drop of blue locktight or similar they will come loose after a while - that starts to introduce play which means the AP is unable to perform small corrections.

Overcontrol due to high magnitude and control issues due to play tend to result in quite similar effects on the AP's ability to set your aircraft solidly on a heading and keep it there with just the right amount of correction required.