Martin XB-51

https://en.wikipedia.org/wiki/Martin_XB-51

The Martin XB-51 was an American trijet ground-attack aircraft. It was designed
in 1945 and made its maiden flight in 1949. It was originally designed as a
bomber for the United States Army Air Forces under specification V-8237-1 and
was designated XA-45. The "A" ground-attack classification was eliminated the
next year, and the XB-51 designation was assigned instead. The requirement was
for low-level bombing and close support. The XB-51 lost out in evaluation to the
English Electric Canberra which - built by Martin - entered service as the
Martin B-57 Canberra.

The resulting unorthodox design, first flying on 28 October 1949, was (unusually
for a combat aircraft) fitted with three engines, General Electric J47s in this
case: one at the extreme tail with an intake at the base of the tailfin, and two
underneath the forward fuselage in pods. The innovative, variable incidence
wings, swept at 35° and with 6° anhedral, were equipped with leading edge slats
and full-width flaps. Spoilers gave most of the roll control and undersized
ailerons provided feel for the pilot. The combination of variable incidence
adjustment and slotted flaps allowed for a shorter takeoff run. Four 954 lb
(4.24 kN) thrust Rocket-Assisted Take Off (RATO) bottles with a 14-second burn
duration could be fitted to the rear fuselage to improve takeoff performance.
Spectacular launches were a feature of later test flights.

The main landing gear consisted of dual sets of wheels in tandem in the
fuselage, similar to the Boeing B-47 Stratojet, with outrigger wheels at the
wingtips (originally proved on a modified Martin B-26 Marauder named "Middle
River Stump Jumper"). The B-51 was a large but aerodynamically "clean" design
which incorporated nearly all major systems internally. The aircraft was fitted
with a rotating bomb bay, a Martin trademark; bombs could also be carried
externally up to a maximum load of 10,400 lb (4,700 kg), although the specified
basic mission required only a 4,000 lb (1,814 kg) bombload. Eight 20 mm (.79 in)
cannon mounted in the nose would have been installed in production aircraft.


Role
Bomber

Manufacturer
Glenn L. Martin Company

First flight
28 October 1949

Retired
25 March 1956

Status
Canceled in 1952

Primary user
United States Air Force

Number built
2

Unit cost

US$12.6 million for the program

In 1950, the United States Air Force issued a new requirement based on early
Korean war experience for a night intruder/bomber to replace the Douglas A-26
Invader. The XB-51 was entered, as well as the Avro Canada CF-100 and the
English Electric Canberra; the Canberra and the XB-51 emerged as the favorites.
The XB-51 was a highly maneuverable aircraft at low level, and substantially
faster than the Canberra (its "turn-of-speed" was faster than most fighter
aircraft of the era). However, its load limiting factor of only 3.67 g (36 m/s2)
restricted tight turns, and the XB-51's endurance was substantially poorer than
the Canberra's; this latter proved to be the deciding factor. Additionally, the
tandem main gear plus outriggers of the XB-51 were thought unsuitable for the
requirement to fly from emergency forward airfields.

The Canberra was selected for procurement and the XB-51 was officially cancelled
by the USAF. However, Martin was selected to build 250 Canberras under license,
with the designation Martin B-57. Furthermore, Martin's rotating bomb bay was
incorporated into the B-57. A "Super Canberra", including other XB-51 features,
such as swept wings and tailplanes, was proposed. This project – although it
promised much better speed and performance – never reached the prototype stage,
mainly because the many changes would have taken too long to implement and test,
before it could be put into production.

Specifications (XB-51)

General characteristics
Crew: 2
Length: 85 ft 1 in (25.9 m)
Wingspan: 53 ft 1 in (16.2 m)
Height: 17 ft 4 in (5.3 m)
Wing area: 548 ft² (50.9 m²)
Empty weight: 30,906 lb (14,019 kg)
Loaded weight: 57,874 lb (26,251 kg)
Max. takeoff weight: 62,452 lb (28,328 kg)
Powerplant: 3 × General Electric J47-GE-13 turbojets, 5,200 lbf (23 kN) each

Performance
Maximum speed: 644 mph (560 kn, 1,037 km/h) at sea level
Range: 1,075 mi (934 nmi, 1,730 km)
Ferry range: 1,444 mi (1,255 nmi, 2,324 km)
Service ceiling: 41,750 ft (12,725 m)
Rate of climb: 6,600 ft/min (33.5 m/s)
Wing loading: 105.6 lb/ft² (515.6 kg/m²)
Thrust/weight: 0.27

Armament

Guns: 8 × 20 mm M24 cannon (0.79 in) cannon with 1,280 rounds
Rockets: 8 × High Velocity Aerial Rockets (HVAR) or
Bombs: Up to 10,400 lb (4,720 kg) carried internally





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