Long before the UCD was even thought of there were planes that pulled to the top or bottom or rolled during knife edge flight. Butchering the rudder will prove to be fruitless. The following is a quote from a post I wrote awhile back:

"I have two UCD 60's and a UCD 46. The UCD 60's do have a more severe coupling that varies with the amount of rudder deflection and speed. You either have to fly it out with your ailerons and elevator or mix it out with your computer radio.

This "coupling" is characteristic of low wing models that have no dihedral (flat wings) or too little dihedral. It seems that no one is addressing this and are concentrating on the rudder. In the past I've redesigned fins and rudders on other flat wing models but there was no significant change in flight characteristics. Contest pattern fliers used to get rid of this problem by sawing their foam wings in half on a band saw then re-gluing the wings together at a different dihedral angle. The correct dihedral will also help correct the tendency of the model to roll in the opposite direction of rudder deflection."

OK, thats what I said some time ago but I certainly didn't go chopping up my wings, I just live with it as is.