Republic XF-91 Thunderceptor

The Republic XF-91 Thunderceptor was a mixed-propulsion interceptor using a jet engine for most flight, and a cluster of four small rocket engines for added thrust during climb and interception. The design was largely obsolete by the time it was completed due to the rapidly increasing performance of contemporary jet engines, and was built to the extent of two prototypes only. One of these was the first US fighter to exceed Mach 1 in level flight.

The Thunderceptor design was one of two swept-wing modifications based on the original F-84 Thunderjet, the other being the F-84F Thunderstreak which happened later. A serious problem with most swept wing designs of the era was dangerous performance at low speeds and high angle of attack. The stagnant airflow over the wing tended to "slide" towards the wingtips, which caused them to stall before the rest of the wing at high angles of attack. In this situation the center of lift would rapidly shift forward relative to the center of mass, pitching the nose up and leading to an even greater angle of attack or, in extreme cases, end-over-end tumbling of the aircraft. Planes caught in this regime would often stall and crash, and a rash of such accidents on the F-86 Sabre led to the term Sabre dance. The most famous incident was the loss of F-100C-20-NA Super Sabre 54-1907 during an attempted emergency landing at Edwards AFB, California on January 10, 1956 which was caught by film cameras set up for an unrelated test. The pilot fought to retain control as he rode the knife-edge of the flight envelope but fell off on one wing, hit the ground and exploded with fatal results.

The Thunderceptor's most notable design feature was intended to address this problem. The wings were built to have considerably more chord (distance from front to back) at the tip than root, allowing them to generate more lift. This neatly addressed the problems with Sabre dance by delaying the point of stall on the tip to that of the entire wing. A side effect of this design was that the tips had more internal room, so the landing gear was mounted to retract outward with the wheels lying in the wingtips, using two small tires instead of one larger one. Another design change was the ability to vary the angle of incidence of the wing as a whole, tilting it up for low speed operations during takeoff and landing, and then "leveling it off" for high-speed flight and cruise. This allowed the fuselage to remain closer to level while landing, greatly improving visibility.

In keeping with its intended role as an interceptor, the nose was redesigned to incorporate radar, forcing them to move the air intake for the engine from its original nose-mounted position to a new intake below it. The fuselage was otherwise very similar to the F-84's. The first prototype did not include the radome, although this was a part of the second prototype.

The first prototype made its first flight on May 9, 1949, breaking the speed of sound in December 1951. It was later modified with a small radome for gunnery ranging (although not the "full" radome from the second prototype). The second prototype included the full radome and chin-mounted intake, but was otherwise similar. This airframe was later modified to use a V-tail for testing. With both the jet and rockets running, the plane could reach Mach 1.71, rather respectable for the early 1950s. Both prototypes were used for testing for several years.

As an interceptor the Thunderceptor was soon eclipsed by designs from other companies, but like the Thunderceptor none of these would go into production. The US Air Force decided to wait the short time needed to introduce newer and much more capable designs created as a part of the 1954 Interceptor project. The Thunderceptor, like the other interceptor designs of the era, had extremely short flight times on the order of 25 minutes, making them almost useless for protecting an area as large as the United States. The 1954 designs outperformed the F-91 in speed, range, loiter time, and included the radar and fire-control systems needed for night and all-weather operation. The era of the dedicated day fighter-type interceptor were over.

One XF-91, serial 46-680, is preserved at the National Museum of the United States Air Force. The second prototype, serial 46-681, was lost due to engine failure during take-off from Edwards AFB in the summer of 1951. Republic Aviation test pilot Carl Bellinger escaped from the aircraft just as the tail melted off — total flight time from take off to pilot evacuation was a mere ninety seconds. By the time fire apparatus arrived, driving seven miles across the dry lakebed, the airframe had been reduced to ashes. Chuck Yeager was flying chase in an F-86 Sabre on this brief flight.

XF-91 3-view

Specifications (XF-91 Thunderceptor)

General characteristics

• Crew: 1

Performance

• Thrust/weight: 0.60

Armament

• 4 x 20 mm cannon

References

Yeager, C. Janos, L. (1985). Yeager. Bantam Books. ISBN 0553050931.{{cite book}}: CS1 maint: multiple names: authors list (link)

External links

Descriptions

• XF-91 in U.S Centennial of Flight Commission
• XF-91 in sci.military
• XF-91 at the National Museum of the United States Air Force

Images

• XF-91b at www.aerophiles.com
• XF-91 at www.aerophiles.com

Related content

Related lists List of military aircraft of the United States - List of fighter aircraft