Superfly, the UCD .60 has two bad habits, in stock form, when rudder is applied. One is that on application, it drops the opposite wing. Second, it dives to the wheels. With the hight of the rudder, it had too much volume on top, causing it to twist. Therefore, the opposite wing drop. Reduced volume on top has eliminated opposite wing drop when rudder is applied. I am still at a loss for why it dives to the wheels, so I have the elevator slaved to the rudder to compensate. As you, I do not like to mix. The reason for the mix is that those same bad habits in the air will bite you real hard if you need rudder on a cross wind landing.
One thing that I have not tried yet is to applied rudder inverted. If, when inverted, on application, it dives to the canopy, then regardless of attitude, its diving to the earth, then that will lead me to believe that the rudder is still too large and is stalling the plane. If it goes to the wheels when inverted, then its back to square one.
I am still intrigued by this fault, but have not had enough time to deal with it. You have brought a good theory to light. BTW, I do not have flaps hooked up. If you are looking for perfection, a YS110FZ, with the tank on the CG will be a great start. After two years I am still amazed at this planes grace in the air. Joe