RE: Expo

Servos produce a circular motion.

They push rods that move in a linear motion.

This means that as you move the TX away from stick center, there is the GREATEST relative movement of the pushrods, as you get towards the END of stick travel you have the LEAST movement of the pushrod.... ( think Sine waves on their side ... ).

So setting the Expo UP TO 50% helps equalize the pushrod movement throughout the stick movement.

HOWEVER once you go BEYOND 50% you are buying into trouble.

Why?

Beyond 50% you are effectively throwing all of the pushrod movement towards the extremes of stick movement on the TX.

This results in a fairly dead, center stick area, and the a sudden and imprecise movement of the surfaces as you get further out.

That in turn causes the flyer to tend to over correct.

Do this with your rudder and you'll have a hard time getting your plane to track down the runway at take off...

e.g. you compensate for left yaw with a little right rudder... nothing seems to happen, so you give it more, still not too much occurs so you give it more...
NOW the plane veers hard right, so you slam the stick left... and you end up over correcting again.