Hey Tim,

There are many variables envolved in answering your question. I'll see if I can list a few, and I'm sure others will list more:

Roll coupling:

- Wing dihedral is a biggy. The slip caused by the rudder brings the leading wing into a higher angle of attack, and the trailing wing into a lesser.......or negative AOA. The forward wing then produces more lift than the trailing and you get roll coupling in the direction of the rudder input.
- Wing placement in relation to the fuselage. High wing acts like dihedral, and low wing like anhedral.
- Rudder shape and location. If the rudder is not aerodynamically centered on the fuselage (and almost none are) its deflection will add some degree of roll.
- Horizontal stabilizer anhedral or dihedral. Same as the wing, but of course to a lesser extent.
- Fuselage shape. Since the top and bottom have different shapes, lift in a slip will not be the same around the plane's lateral CG, imposing a small roll tendency.

Pitch coupling:

- CG. A forward CG makes you add up trim, which is still there when you roll into a knife edge. Moving the CG back makes you trim down, increasing the tendency to pitch down in knife edge.
- Fuselage shape. The top may produce more lift in a slip than the bottom, pushing the nose to the wheels, or vise versa.
- Rudder shape and location. Again.
-Horizontal stabilizer position. It will be affected differently by the wing wash in level, inverted, or knife edge flight. In addition, deflection of the rudder may affect the lift to a larger degree on one side of the horizontal stab than the other.
- The affect of the spiral prop wash. May or may not be noticeable. Change the prop, and the affect may change.

Your probably getting the picture why each plane has different coupling characteristics. The easiest way to fix it is with a radio mix........as long as it's not too severe. Trying to mix out roll coupling in a high wing cessna model is an example of too severe. Aside from that, if you want the structural fixes, then:

Reduce dihedral to eliminate roll-out coupling, or lower the wing location.

Move the CG forward to eliminate pitch down coupling, or lower the location of the horizontal stab on the fuselage.

Good luck..........CJ