I have been messing with my GP ultimate. I have moved the CG back to improve its snapping / tumbling characteristics. It is back far enough that the model is unstable in pitch, not a lot but others that have flown it have noticed it. This makes it tumble really well but.. I have tried knife edge and it has horrible coupling. I was going to mix it out but would have required 3/4 of the available up travel of elevator on high rate to stop it. I did not want to mix this much in.
This got me thinking what effect the CG had on knife edge. So whats the answer?

The answer is the same as in pitch. The closer your CG is to the AC of the plane, the less stability and more control authority you have. The relationship to elevator is not straight forward:

1) You will need less rudder input to yaw the plane because of the neutral/negative stability.

2) Because of 1, you will need less overall lift in the knife-edge (required lift, roughly = weight of plane + force to counteract down force by the rudder/fin- which is now less). Because of this, you might have noticed you can maintain the knife-edge at slower speeds or with less angle of attack (of the fuselage).

3) If you fly the knife-edge at slower speeds, your wings will produce less lift with the elevator neutral- this will cause the plane to fall in the longitudinal axis and require more up elevator relative to your prior setup.

4) If you fly the knife-edge at the same, higher speed but with less AOA, you probably won't notice the effect described in 3), but your wings will have a different attitude to the wind (top wing will not be as hidden from the wind...), so you will require less aileron input to keep the plane from rolling in the direction opposite the bank.

5) Because of the lower required rudder input, you will also have less rudder-induced roll (the direct variety, not the roll coupling as in 4), which means less required aileron input to keep the plane from rolling in the direction of the bank. This is an opposite force to that in 4, so with both forces reduced, you may not notice any/much change in required aileron input.



Hope this is thoroughly confusing, so that you just forget it and go fly Seriously, hope this helps- others will probably explain with more clarity.

Your plane is tail heavy enough to require down trim for level flight. Roll to knife edge and you still have down trim, which has to be compensated with a mix. My Tail heavy Cap 232 requires 11% up elevator for good speed knife edge but full throw rudder requires ALOT of up elevator.

As for 3/4 elevator max throw for normal knife edge, I am puzzled...