ORIGINAL: BarracudaHockey
It takes about 20 to 25 percent expo to make the control response linear as a round servo output has the opposite built in, that is, you're automatically more sensitive at center than at the extremes.
But you are forgetting that our control surfaces move around a hinge line in an arc, thus compensating for the non-linear movement at the servo. Arc servo wheel movement, translated to linear movement of the pushrod, translated to non-linear movement at the control surface. Bottom line: Control surfaces move linear with TX stick movement - maybe not 1:1, but linear and proportional. 15% generates 15%, until you add your own expo.
Kurt