Complex sentence: Flight testing revealed severe aerodynamic problems, mostly caused by the airfoil section and thickness of the wing, but the vertical stabilizer had to be redesigned to smooth out the airflow at the intersection of the horizontal and vertical stabilizers.
Split into simpler sentences: Flight testing revealed severe aerodynamic problems, mostly caused by the airfoil section and thickness of the wing. The vertical stabilizer also had to be redesigned to smooth out the airflow at the intersection of the horizontal and vertical stabilizers.