The position of the front (windscreen) of the canopy is where we found the problem to be--as if--the airflow around /over it created a lowered pressure/ turbulance/ whatever you want to call it -- at a point somwhere around the fin/rudder.the fin/ rudder and the plane went into an occilation due to lowered effectiveness of the fin--- that lowered pressure area. Anyway we could create this problem-- by simply positioning the cup along the square flat topped fuselage. It was quite interesting at the time.
Even on automobiles - this intersection and angle of the windscreen has shown to create problems
As I drive a ancient 4 banger now - I really have lost interest in th car "streamlining.'