Actually...........it's pretty complicated.

1) When the rudder deflects, the air tries to flow spanwise from the center, just like a very low aspect wing. The elevator acts as a fence, and recieves a bit of lift off the spanwise flow. In lay terms, a low placement of the elevator in relation to the vertical tail will cause a pitch to the canopy....A high elevator will cause a pitch to the gear, and there is a neutral location if you have the patience to find it.

2) Empenage shape affects the flow of air and can produce lift that causes a pitching moment. A curved upper turtledeck will create lift in a slip, pitching the airplane to the gear. If the top of the empenage is flat with sharp edges, it will tend to spoil lift in a slip, causing a pitch to the canopy (maybe). The roundness of the lower empenage will have the same effects, with opposite pitching tendencies. Cowl shape can have similar effects, but to a slightly degree, since the moment arm is shorter to the aircraft CG.

3) CG makes a difference. A forward CG will cause the plane to pitch to the canopy in knife edge. Think of it this way; when the plane is nose heavy, you have to trim up elevator to fly level in upright flight. Then when you roll into knife edge, the up trim pulls the nose to the canopy. A tail heavy plane works the opposite, and will tend to pitch to the gear in knife edge. There is a neutral point, but it may be outside the usable CG envelope for stable flight.

4) Wash from the wing. Depending on the placement of the tail and wing, the airflow from the wing may wash over the horizontal stabilizer, changing it's angle of attack. In knife edge, the wing is not producing lift, so the airflow over the tail will no longer be affected by wing wash.........that is if it was to begin with, and with some planes it is not.

5) Prop wash. Here is a topic for the aero guys. You have up or down thrust that can influence the horizontal stab. You also have the spiral airflow induced by the rotating prop that can influence the horizontal and vertical stab. Non of this is predictable...........by us anyway.

6) Airfoil shape. An asymetric wing airfoil will always tend to pitch to the canopy in knife edge. Hopefully your aerobatic plane has a symetric airfoil........so you can at least disregard this factor.

Totally confused?? There are more factors, but I think this is enough to throw out.

Luckily we don't need a working knowledge of all this to trim our planes. Try to adjust the pitching tendency by changing the CG. When you can't move the CG any more..........or don't want to for other reasons, then mix the pitch out. If it takes more than about 10% mix to fix it, then you can relocate the horizontal tail up or down to fix it (hard way) or slightly alter your wing incidence (easy way).

regards.........CJ