The Taranis Non Linear Subtrim Gotcha

[Chris Bott - Moderator]

Either I'm late to the party on this one (probably) and everyone else knows about it, or I just spotted something that might just help someone.

I try not to use subtrim because it's generally best practice to have servos centred in the middle of their throw, leaving equal amounts of movement available either side.
That said it is often useful to use just a little subtrim to get a surface centred perfectly.

Who knew then, that by default, every time you use subtrim in a Taranis, you're making surface throw in one direction different to that in the other? - Asymmetric throws.

This problem (if it is a problem for you) is exacerbated by the use of Trims=>Subtrims. This is a function available at the bottom of the servos page, that turns your trim adjustments into subtrim and sets trims back to centre. (on the face of it, a very useful function).

So, if you suffer from asymmetric throw syndrome and cant figure out why, read on.

If we add say 20% subtrim to a channel, then (assuming 100% is set in mixes etc) the servo centre will move by 20% and we'll have 80% travel available in one direction and 120% available in the other.
So, guess what openTx does with this travel by default?

By default it maps 100% stick movement in one direction to 80% servo travel one way and 120% servo travel the other. If we put in "rates" or just change the mix amount to 50%, then for full stick travel we get 40% servo travel one way and 60% the other. (and by now are probably wondering why!)

To cure this (if it's not what we intended) there's a setting at the right hand end of the line on the servo page. Called "Linear Subtrim" Tick this box if on the PC, or change it to "=" on the Tx. Then, lo and behold, we have equal movement either side.

(Bear in mind now of course that in one direction we could be trying to drive the channel beyond it's hard end point now, so this could still appear to be assymetry.

The settings are probably better described in the paragraph about the Servo page, in this manual.