Diff - a better way

Mike Shellim 30 Dec 2013, last updated 13 June 2019

Aileron differential or 'diff' is used to counter adverse yaw, by reducing the movement of the downgoing aileron. In this article I'll explain why the behaviour in OpenTx is not ideal, and provide a workaround.

The issue

Consider a model with separate aileron servos on channels 1 and 2. Here's how you might specify 50% diff on each aileron (in the Mixers menu):

CH1 (aileron 1)

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

CH2 (aileron 2)

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

As long as the the trim is in the centre, diff works as expected. However the following occurs if trim is applied:

Diff behaviour is reversed as the stick is moved away from centre in the opposite direction to the applied trim. Diff works correctly only when the stick is moved enough to cancel the trim.
With the aileron stick at centre, altering the value of diff causes one of the ailerons to move, thereby inducing an unwanted roll command.

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

The workaround

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

CH1 (aileron 1)

Source=Ail, Diff=50%, NoTrim

Source=TrmA, wt=25

CH2

...

The workaround is incorporated in all the Pro templates.

Demo

The demo below contains examples with and without the fix.

diff_fix.zip
for OpenTx version 2 or greater

Load the .EEPE file into OpenTx Companion, open the sim, and apply full aileron trim. Then compare the outputs as you move the aileron stick.

CHs 1,2 show the default behaviour
CHs 3,4 show the workaround