Hi, All,

I have a question.
What happens if I change the rudder shape to have more area below the horizontal stab line?

the pink line in the picture, I may reduce the area above the line a little.

I would like to know Level flight, KE flight results etc.

thanks,

What does the plane do now that you want to change?
The vertical area relative to the wing has more to do with pulling to the canopy or the belly in KE, and nose-up or down in turns.

Just like what you said above.
Plane pitches to belly hard during KE, level flight when using rudder.

I am trying to remove mixing because up elevator mixing messes up roundness when rudder correction is necessary. (I don't fly very good )

One of the fixes in a typical pattern plane trim list would be to reflex the ailerons, since mixing is not desirable.
Or move the c.g. forward, or decrease wing incidence..
Listed in order of difficulty...
Lowering the rudder area might work, but will take a lot of fitting and flying to find what can be done.
Doing that might improve the stall manuvers also.

You may have a bit better yaw control in high alpha maneuvers. The reason is because the lower part of the rudder will be out of the wake of the horiz stab.

Tall Paul,

By reflexing the ailerons, do you mean to lower both ailerons?

Reflex means raise.
In essence, if such things need to be done, then the plane should be redone from square one, to eliminate the problems.
This is the primary reason pattern planes tend to look like they came from the same mold. There's one optimum solution to the pattern situation (or any other), so the successful designs tend to evolve towards the same configuration.
A different name on side is the only real difference between most of them.
You might fly the thing a lot, to get used to what it can do, and what you want it do, and bash it to do that.
Then build a serious version with all the improvements.