F-107 Ultra Sabre: all-weather fighter-bomber
The F-107 was designed for a US Air Force (USAF) competition to select a new all-weather fighter-bomber.
The design originated as a redesigned F-100, but it quickly became apparent that the Air Force’s request for an internal bomb bay would require an all-new design. North American’s design incorporated a unique overhead jet intake to allow for carrying weapons partially submerged in the aircraft’s belly.
The first flight of the F-107 was on Sep. 10, 1956. The F-107’s competitor for the Air Force contract was the Republic F-105.
During its flight test campaign, the F-107A experienced problems with “tuning” the intake at high number of Mach as had occurred with virtually all the Century Series. Later more in-depth evaluation found some unsuitable flying qualities. However, as the flight test campaign progressed the automatic variable inlet duct system became a chronic problem; thus, the maximum altitude for sustained flight was limited to 51,000 feet.
As explained by Ted Spitzmiller in his book book Century Series, The USAF Quest Air Supremacy 1950-1960, it was not until March 1957 that the USAF made the final decision to cancel the F-107 and instead buying the F-105 Thunderchief.
The first and third F-107A prototypes were then leased to the NACA (National Advisory Committee for Aeronautics), predecessor to NASA (National Aeronautics and Space Administration), for high-speed flight research – the variable intake on the third F-107A being one of the prime investigation points. However, the variable-area inlet duct proved a constant source of mechanical problems, and it was eventually made a fixed intake that limited the top speed to Mach 1.2.
According Spitzmiller “a second area of interest to NACA and the USAF was the evaluation of a “side-stick” installed on the right side of the F-107A cockpit in addition to the traditional center position. The North American Aviation X-15 rocket plane, then in the final stages of design, would use a ‘hand-controller’ that cradled the pilot’s right arm so that the high acceleration forces imparted by the rocket engine would not cause unintended control input. The traditional center stick was for use after burnout of the rocket engine.”
As Spitzmiller explains “the first prototype had so many problems that NACA eventually grounded it and used t as a spares donor for the other two. The third plane, which was turned over to NACA in February 1958 for the variable duct research and side-stick evaluation, completed forty flights before an incident ended its flying days. Test Pilot Scott Crossfield, who was preparing to fly the X-15, was piloting 55-5120 [F-107A third prototype] during a take-off when he encountered control problems.
“The plane ground looped, blowing both main tires, and a small small brake fire resulted. Although the airplane was not badly damaged, it was decided not to invest in repairs. Crossfield indicated that the desired attributes of the side stick had been demonstrated and that no further testing was needed. The plane was cut up for use as a fire-fighting training form.”
Airframe 55-5118 was acquired by the Pima Air Museum in Tucson, while the second prototype (F-107A 55-5119 which was used for weapons testing with both conventional and atomic bombs) can be seen at the Air Force Museum in Dayton, Ohio.
Century Series, The USAF Quest Air Supremacy 1950-1960 is published by Schiffer Publishing and is available to order here.
Photo credit: U.S. Air Force