This question has bugged me for a while. Many equally stable pattern planes have the stab in differing vertical positions, and I've wondered why.

This summer, I kit-bashed an "old reliable" Ultra Sport .40, with the idea of reducing the pitch coupling, especially in knife edge.

I moved the stab down about 1 inch, to place it on the thrust line, rebuilt a lighter vertical fin/rudder from sheeted sticks, and changed the rudder hinge line from angled to upright. (90 degrees to the waterline) I also reduced the frontal area of the plane, by removing the turtledeck and rebuilding it with a lighter, lower one.

This greatly reduced the coupling. I had assumed that the stab re-positioning was the cure, but from reading the above posts, it may have been ALL the changes in combination.

If you want to see the article and photos (just one page) it's at http://www.nextcraft.com/us40bash.html