ORIGINAL: Tall Paul
Certainly the simplest fix.
I'm presuming "ailerons full up" means the side that usually would go up to command a turn to that side doesn't move any further up, while the other side goes down, lessening the drag on the downgoing side so the up-aileron's drag brings the nose around.
Yes, I'm no aerodynamic expert so I don't know exactly why, but there is no weird yaw effect as I had when using flapperons that were both down.
I'm sure someone will explain it...
And someone will contradict him, and so on and so on...