Diff - a better way

Mike Shellim 30 Dec 2013, last updated 18 Dec 2015

Aileron differential or 'diff' is used to counter adverse yaw, by reducing the movement of the downgoing aileron.

Here's how you specify 50% aileron diff for a model with separate aileron servos on channels 1 and 2:

CH1 (an aileron)

Srce=Ail, wt=100, Diff=50%

 

CH2 (an aileron)

Srce=Ail, wt=-100 Diff=50%

In this article I'll explain why the behaviour in OpenTx is not ideal, and provide a workaround.

The issue

Diff works as expected, as long as the the trim is in the centre. However if you have some aileron trim applied, the following happens:

Neither of these effects is desirable (though, interestingly, both the Multiplex Cockpit SX and 4000 show similar behaviour).

The Cause

By default, OpenTx applies the trim offset first, followed by diff.

Workaround

The workaround involves taking trim out of the aileron mix, and adding it back in as a separate offset.

Here is the standard aileron mix with diff and trim enabled:

1: Source=Ail, Diff=50%

Here is the workaround:

1: Source=Ail, Diff=50%, NoTrim

2: Source=TrmA, wt=25

Line 1 is the same as before, but with trim removed.
Lines 2 adds the trim back in.

The result is:

Demo

The demo below contains examples with and without fix, and works with OpenTx version 2 and above.

Download demo (ZIP)

Load the .EEPE file into OpenTx Companion, open the sim, and apply full aileron trim.

Compare the outputs as you move the aileron stick.

Canned setups

The diff workaround is incorporated in all my canned setups.