Expo"nential" is a curve function that is used to control "how much" a control surface, or whatever is linked to that particluar channel moves in relation to the amount you move the stick at a certain point in the sticks range of movement. Usually Expo will only allow you to make one input, but some mixes have 5 point curves that allow you to manipulate the curve so that the arc varies at different points along the range of movement. Most people just use this on helis and throttle curves, but they are in there.


Expo allows you increase or decrease how much the surface moves relative to the center point at a given point in the sticks movement. With zero expo every increment that you move the stick, rotates the servo shaft the exact same amount. When you start to ajust the expo you create a "curve" in the way the servo responds to your stick movement.

What that does practically is change how the plane responds to what you are doing with the sticks. Most thumbs flyer that want to be really smooth add a little expo, like -25 (which is +25 if you fly JR stuff) to soften up the center so that in the range of the stick where you want to be really smooth you do not jerk the plane around with tiny movements.
So to adjust expo for low rates you fly the airplane and see where the plane is jerky and seems too touchy and you add expo until the plane responds smoothly, but not sluggishly to subtle control inputs. Part of getting this right is to set your low rate throw so that you just barely have enough throw to execute the most demadind manuever you do on low rates and no more.

Then for High or 3D rates typically what I try to do is adjust the Expo so that when I am on high rates the plane "Feels" like it is on low rates when I am making small changes but when I want big movement the plane still moves. An easy way to do this is to actually look at the curve on the Dual Rate screen and flip your rate switch back and forth, you will see the curve change as you flip back and forth between low and high rates. Just adjust the high rate expo so that the center of the curve out to about 20% of the range of movement are the same. OF course they will seperate as you move out further on the curve because the end points are different on low rates and high rates.

Once you do that you should just about be able to make an entire flight on high rates and never notice it on takeoff or landing. Before I figured this out there was no way I would ever accidentally land on high rates because the plane would be too twitchy. Now I do it all the time because high and low rates feel so much alike around center stick.


Generally, I use about 25-30% expo on low rates for everything but Rudder which is about 35-50%.
On high rates I am about 75% on rudder and everything else is about 40-50%.