Auto-trim misaligns ailerons with asymmetric deflection

[rubenp02]

Bug report

Issue details

When using ailerons connected to different channels and that are configured to have a different range in the up vs. down position (say, 25º up vs just 15º down, which is useful in some designs to reduce adverse yaw when rolling), the auto-trim functionality may incorrectly adjust both ailerons in the same direction toward the center of the PWM range. This leads to an unintended pitch trim instead of a proper roll trim.

Version
4.5.7

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[X] Plane
[ ] Rover
[ ] Submarine

Airframe type
Flying wing

[IamPete1]

This leads to an unintended pitch trim

That should be corrected by the pitch autotrim.

What you may be seeing is that the autotrim will only trim to in +-10% from the average of min and max. So if one output is against that limit on the the other will be trimmed so you get asymmetric results.

This sort of thing is very hard to do, we have to try and work out the intent of the person who configured the aircraft. We can't tell from parameters alone.

[rubenp02]

The problem is the assumption that the average of min and max is the neutral position for the control surface, which doesn't hold for control surfaces that are set up to be asymmetric. I understand that the current parameters and the way auto-trim is done don't allow for a simple fix to this, but I think it's a pretty big issue for a feature that the wiki recommends turning on. Even the parameter description just says "the automatic trim won't go more than 20% away from a centered trim", which may be interpreted as a physically centered control surface instead of the PWM midpoint.

[IamPete1]

The limits are called out on the wiki with a example (https://ardupilot.org/plane/docs/auto-trim.html).

If you have a better wording we could update it.