> it runs around a steep banked turn with the tail hanging low and the engine pointed to the "top" of the circle cone of the turn. That's a sign of too small a vertical tail <

I have been troubled by this problem with a small bipe when I only used ailerons. Mixing the rudder to ailerons does help things out. Will adding to the fin area help or must the rudder also be larger? And if this mod is done, will rudder input still be necessary for a decent turn?