English: Three North American F-100A-5-NA Super Sabre fighters (s/n 52-5770, 52-5773, 52-5778) at the National Advisory Committee for Aeronautics in 1955. On the left is NACA High-Speed Flight Station’s F-100A (52-5778) with a modified vertical fin. On the right is the USAF F-100A (52-5773) with the original vertical fin configuration. NACA added a larger vertical fin to the airplane in December 1954, adding 10 percent more surface area. Later North American installed an even larger fin, having 27 percent greater area, as well as wingtip extensions. These modifications solved the dangerous directional stability and roll coupling problems that the F-100 was experiencing. (text: NASA)