XF-91 Thunderceptor

infobox Aircraft
name =XF-91 Thunderceptor
type =Interceptor
manufacturer =Republic Aviation

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caption =
designer =
first flight =9 May avyear|1949
introduced =
retired =
status =Cancelled
primary user =
more users =
produced =
number built =2
unit cost =US$11.6 million for the programKnaack, Marcelle Size. "Encyclopedia of US Air Force Aircraft and Missile Systems: Volume 1, Post-World War II Fighters, 1945-1973". Washington, DC: Office of Air Force History, 1978. ISBN 0-912799-59-5.]
developed from= F-84 Thunderjet
variants with their own articles =

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.

Design and development

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 10 January 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 problem of 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 fitted to the second prototype.

Testing and evaluation

The first prototype made its first flight on 9 May 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 XF-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.

urvivors

The second prototype, "46-681", had an engine failure during takeoff 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 was a mere ninety seconds. By the time fire apparatus arrived, driving seven miles across the dry lake bed, the tail section had been reduced to ashes. 46-681 was then fitted with a V or "butterfly" tail (more common on Beechcraft of the era) and was flight tested with this configuration. It was later used at Edwards AFB as a crash-crew training simulator, then scrapped.

The first prototype, serial "46-680", is preserved at the National Museum of the United States Air Force.

pecifications (XF-91 Thunderceptor)

aircraft specifications

plane or copter?=plane
jet or prop?=jet
crew=1
length main=43 ft 3 in
length alt=9.52 m
span main=31 ft 3 in
span alt=13.18 m
height main=18 ft 1 in
height alt=5.51 m
area main= 320 ft²
area alt= 29.73 m²

empty weight main=14,140 lb
empty weight alt=6,410 kg
loaded weight main=18,600 lb
loaded weight alt=8,400 kg
max takeoff weight main=28,300 lb
max takeoff weight alt=12,840 kg

engine (jet)= General Electric J47-GE-7 (later GE-17)
type of jet= axial-flow turbojet
number of jets=1
thrust main= 5,200 lbs dry, 6,900 lbs with afterburner and water injection
thrust alt= 30,6 kN))

Powerplant: 4x Reaction Motors XLR11-RM-9 rockets, 1,500 lbf (7 kN

max speed main=984 mph
max speed alt=1,584 km/h

range main=1,170 mi
range alt=1,880 km

ceiling main=50,000 to 55,000 ft
ceiling alt=15,200 to 16,800 m

climb rate main= 47,500 ft in 2.5 minutes
climb rate alt= 14,500 m

loading main= 58.12 lb/ft²
loading alt= 283 kg/m²

thrust/weight= 0.60

armament=
* 4 x 20 mm cannon

ee also

aircontent

related=

similar aircraft=
*Saunders-Roe SR.53
*Saunders-Roe SR.177

lists=
*List of military aircraft of the United States
*List of fighter aircraft

see also=

References

Notes

Bibliography

* Pace, Steve. " Republic XF-91 Thunderceptor Rocket Fighter" (Air Force Legends N.210). Simi Valley, Ca: Steve Ginter Books, 2000. ISBN 0-942612-91-4.
* Winchester, Jim. "The World's Worst Aircraft: From Pioneering Failures to Multimillion Dollar Disasters". London: Amber Books Ltd., 2005. ISBN 1-904687-34-2.
* Yeager, Chuck and Janos, Leo. "Yeager: An Autobiography". New York: Bantam Books, 1986. ISBN 0-553-25674-2.

External links

* [http://www.centennialofflight.gov/essay/Aerospace/Republic/Aero43G13.htm XF-91 in U.S Centennial of Flight Commission]
* [http://www.ibiblio.org/pub/academic/history/marshall/military/airforce/f_series_fighters/f91 XF-91 in sci.military]
* [http://www.nationalmuseum.af.mil/shared/media/photodb/photos/060728-F-1234S-045.jpgThe two XF-91s at Edwards AFB in 1953 after modifications]
* [http://www.aerofiles.com/repub-xf91b.jpgXF-91b at www.aerophiles.com]
* [http://www.aerofiles.com/repub-xf91.jpgXF-91 at www.aerophiles.com]
* [http://1000aircraftphotos.com/Fairchild/5419.htm Several pictures of the XF-91 "46-680"]