Thunderchief

The USAF decided to purchase the F-105 Thunderchief instead of producing the F-107A after giving it some serious thought

A mid-1950s improvement on the popular F-100 Super Sabre was the North American F-107A Ultra Sabre. The F-107A was distinguished by its engine air intake located above the cockpit, all-moving vertical fin, and system (known as a Variable Area Intake Duct) that automatically adjusted the quantity of air supplied to the jet engine.

The F-107A (serial number 55-5118), the first of three prototypes, flew in September 1956 and reached Mach 1. A few months later, F-107 flew at Mach 2 (twice the speed of sound).

Notably, the F-107A had issues “tuning” the intake at a high number of Mach during its flight test campaign, an issue that had essentially afflicted the whole Century Series. Several undesirable flying properties were discovered during the subsequent, more thorough study. The automatic variable inlet duct system developed a recurring issue as the flight test campaign went on, limiting the maximum altitude for sustained flying to 51,000 feet.

The U.S. Air Force (USAF) did not finally decide to cancel the F-107 and purchase the F-105 Thunderchief until March 1957, as Ted Spitzmiller explains in his book Century Series, The USAF Quest Air Supremacy 1950-1960.

The National Advisory Committee for Aeronautics (NACA), the forerunner of NASA (National Aeronautics and Space Administration), then leased the first and third F-107A prototypes for use in high-speed flight research, with the third F-107A’s variable intake serving as one of the primary investigation points. A fixed intake subsequently replaced the variable-area inlet duct, which reduced the top speed to Mach 1.2. Nonetheless, the variable-area inlet duct proved to be a recurrent cause of mechanical issues.

According to 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 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 take-off when he encountered control problems. The plane ground looped, blowing both main tires, and a 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 purchased by the Pima Air Museum in Tucson while the second prototype (F-107A 55-5119, which was employed for weapons testing with both conventional and atomic bombs) is on display at the Air Force Museum in Dayton, Ohio.

Photo by U.S. Air Force

Additional source: National Museum of the U.S. Air Force