ORIGINAL: alan0899
...G'day Mate,
Differential actually reduces the amount of down aileron, & leaves the up aileron as is.
The method described in the trim chart is easier to understand. You fly the plane towards yourself, pull up until vertical, release sticks, then half roll, then you are looking at the top of the plane.
That way, Left is left & right is right.
The above method is specialized for pure aerobatic planes, and can be used to find the right amount of differential to use for Comp-Arfs and such that use top-hinged ailerons. The aerodynamic differential that exists as a result of the non-symmetrical hinge location (or wing incidence, etc) will cause heading deviations that can most easily be seen and corrected using the above test. It is NOT a suitable test for the amount of differential to use for a Cub or a PT-19. For the latter airplane types I fly straight away from me in level flight at a moderate cruise speed, and use rudder at lower speeds to make up the difference.
Differential is any case with different amounts of up and down. It doesn't have to only refer to reducing the down. What if I wasn't using all the throw and I chose to increase the up? The effect would be the same, and it would still be differential aileron throw.