Dave,

If you want to achieve differential, a simple way is set the control arms off 90 degrees relative to the servo. That said, typically the servo control arms are installed when the servo is in the neutral position 90 degrees or midpoint in their travel. If you install the arm a little toward the leading edge, or possibly trailing edge in some cases I suppose, the servo arm will travel less in one direction vis-a-vis the other. Try it out and you will get the idea. Differential should make a high wing trainer a better flier.

Good Luck!

DaveB