RE: The Why of Clark-Y
If I may interject some information for consideration, I built a 1/4 scale Albatros D.Va with scale undercambered airfoil, (very thin and sharp LE) about the time Proctor came out with their kit. It it is a joy to fly and performs very predictably. Ailerons are very responsive but still requires a bit of rudder.
The second plane, a 1/4 scale Roland C.II, also with scale airfoil (thicker undercambered foil with a more blunt LE) , originally flew with noticeable adverse yaw and required substantial differential aileron throws (very little down and max up) and a lot of rudder to perform well. Now flies more as it should - my impressions this far.