Good catch and thanks!

Well, with rudder to elevator coupling, often as you increase the rudder throw,, say for high alpha knife edge or a knife edge loop you need proportionally more (or less) elevator correction. So from no rudder input to maximum rudder input the needed correction is not linear. What you can end up with is a plane that flies nice and true in high speed flat knife edge but as you increase the rudder throw to do a knife edge loop or a low speed high alpha knife edge the elevator correction just is not enough with increased rudder throw. So what you do is go to a pmix that allows a 5 point mix and you set the mix so that you get exactly the elevator deflection you want for a given increment of rudder input. On a 9C this is PMix 7. What you typically see is 5 points. The center, or #3 point is the neutral point where the rudder is at rest. Points 2 and 4 are where you set the mix at lower deflections, 2 is for left rudder and 4 is for right rudder. 1 and 5 is where you set the mix for higher rudder deflection, 1 being left and 4 right. This allows you to set a non linear R-E mix that works just like a throttle or pitch curve on a helicopter. That way whether you are flying a nice flat normal knife edge, or digging in with the rudder the plane will track straight(er). Its almost impossible to get it perfect but it sure makes things easier.

The mix may look something like this
1. -10
2. -4
3. 0
4. -3 (because left and right coupling may or may not be the same)
5. -8

Again, this further improves the planes general flight characteristics, even moreso in high alpha flight where you really use full deflection rudder.