G3X Touch - Autotrim during landing phase?

I have the G3X system with the GMC305 autopilot control panel, and two Garmin servos, wired with a VP ECB box. Auto trim is enabled and seems to be working correctly. I am trying to decide what is the best procedure / mode to be in when on final.

With the autopilot off (but with servos powered,) auto trim is still functional, but manual electric trim via the stick hat is disabled. I'm finding that stick forces are higher than when I trim manually. The auto trim seems to be maintaining a more neutral approach / AOA than I normally would if I were trimming manually - I dial in a little more nose up than the auto trim does.

So my question is what is the better procedure? Keep the auto trim enabled, or turn off the servos and fly with manual trim? I've landed both ways, but using more back stick pressure when the servos are on.

maus92 said:

I have the G3X system with the GMC305 autopilot control panel, and two Garmin servos, wired with a VP ECB box. Auto trim is enabled and seems to be working correctly. I am trying to decide what is the best procedure / mode to be in when on final.

With the autopilot off (but with servos powered,) auto trim is still functional, but manual electric trim via the stick hat is disabled. I'm finding that stick forces are higher than when I trim manually. The auto trim seems to be maintaining a more neutral approach / AOA than I normally would if I were trimming manually - I dial in a little more nose up than the auto trim does.

So my question is what is the better procedure? Keep the auto trim enabled, or turn off the servos and fly with manual trim? I've landed both ways, but using more back stick pressure when the servos are on.

Click to expand...

Hello Charley,

Perhaps there is some confusion regarding the terms "speed scheduling" and "auto-trim".

Auto-trim refers to a function which is only active when the autopilot is engaged. When the servos are flying the plane they sense the out-of-trim forces you cannot, and drive the trim motors on their own to keep the aircraft in trim.

Speed-scheduling refers to a function that is available when the autopilot is disengaged and the servos are powered. The servos monitor the aircraft speed and move the trim motors in response to your manual trim commands more slowly when the aircraft is flying fast, and more quickly when the aircraft is slowed down such as in the pattern. These speed-scheduled trim motor movement speeds are user configurable.

When the power is removed from the servos, speed scheduling is disabled and the trim commands are passed directly through the servos to the trim motors.

As explained here, auto trim is not functional when the autopilot is disengaged, and manual electric trim via the stick hat is never disabled, but if you attempt to manually trim the plane when the autopilot is engaged, the autopilot will disengage to honor your request.

It is always your job to trim the plane when the autopilot is disengaged regardless of whether the servos are powered on (and providing speed scheduling) or off and the trim motor drive is passed through without speed scheduling. In general, with these systems, your servos will always be powered, even when you are not using the autopilot.

Let us know if you have additional questions.

Thanks,
Steve

It sounds like you may have the variable speed function active in the VPX. This should be set to zero when using G3X servos. See P36 in the VPX manual. I think the voltage from the VPX is reduced when variable speed function is on and then the G3X doesn't see the voltage so it appears to be disabled.

Art

Thanks for the replies. I'll check the VP-X programming - I no longer have a Windows partition on my MacBook to run the VP-X config program, so I'll have to rig something up...