B-47E Stratojet

The Boeing B-47 Stratojet (Boeing company designation Model 450) is a retired American long- range, six-engined, turbojet-powered strategic bomber designed to fly at high subsonic speed and at high altitude to avoid enemy interceptor aircraft. The primary mission of the B-47 was as a nuclear bomber capable of striking targets within the Soviet Union.

Development of the B-47 can be traced back to a requirement expressed by the United States Army Air Forces (USAAF) in 1943 for a reconnaissance bomber that harnessed newly developed jet propulsion. Another key innovation adopted during the development process was the swept wing, drawing upon captured German research. With its engines carried in nacelles underneath the wing, the B-47 represented a major innovation in post-World War II combat jet design, and contributed to the development of modern jet airliners. Suitably impressed, in April 1946, the USAAF ordered two prototypes, designated "XB-47"; on 17 December 1947, the first prototype performed its maiden flight. Facing off competition such as the North American XB-45, Convair XB-46 and Martin XB-48, a formal contract for 10 B-47A bombers was signed on 3 September 1948. This would be soon followed by much larger contracts.

During 1951, the B-47 entered operational service with the United States Air Force's Strategic Air Command (SAC), becoming a mainstay of its bomber strength by the late 1950s. Over 2,000 were manufactured to meet the Air Force's demands, driven by the tensions of the Cold War. The B-47 was in service as a strategic bomber until 1965, at which point it had largely been supplanted by more capable aircraft, such as the Boeing B-52 Stratofortress. The B-47 was also adapted to perform a number of other roles and functions, including photographic reconnaissance, electronic intelligence, and weather reconnaissance. While never seeing combat as a bomber, reconnaissance RB-47s would occasionally come under fire near or within Soviet air space. The type remained in service as a reconnaissance aircraft until 1969. A few served as flying testbeds up until 1977.

Development

Origins

The B-47 arose from an informal 1943 requirement for a jet-powered reconnaissance bomber, drawn up by the United States Army Air Forces (USAAF) to prompt manufacturers to start research into jet bombers. Boeing was among several companies to respond to the request; one of its designs, the Model 424, was basically a scaled-down version of the piston-engined B-29 Superfortress equipped with four jet engines.Peacock 1989, p. 31.Gunston and Gilchrist 1993, pp. 38–39. In 1944, this initial concept evolved into a formal request-for-proposal to design a new bomber with a maximum speed of , a cruise speed of , a range of , and a service ceiling of .Knaack 1988, p. 101.Gunston and Gilchrist 1993, p. 39.

In December 1944, North American Aviation, Convair, Boeing and the Glenn Martin Company submitted proposals for the new long-range jet bomber. Wind tunnel testing had shown that the drag from the engine installation of the Model 424 was too high, so Boeing's entry was a revised design, the Model 432, with the four engines buried in the forward fuselage.Peacock 1989, p. 33. The USAAF awarded study contracts to all four companies, requiring that North American and Convair concentrate on four-engined designs (to become B-45 and XB-46), while Boeing and Martin were to build six-engined aircraft (the B-47 and XB-48). The powerplant was to be General Electric's new TG-180 turbojet engine.

Swept wings

In May 1945, the von Kármán mission of the Army Air Forces inspected the secret German aeronautics laboratory near Braunschweig. Von Kármán's team included the chief of the technical staff at Boeing, George S. Schairer. He had heard about the controversial swept-wing theory of R. T. Jones at Langley, but seeing German models of swept-wing aircraft and extensive supersonic wind-tunnel data, the concept was decisively confirmed. He wired his home office: "Stop the bomber design" and changed the wing design.Gunston and Gilchrist 1993, pp. 39–40. Analysis by Boeing engineer Vic Ganzer suggested an optimum sweepback angle of about 35 degrees. Boeing's aeronautical engineers modified the Model 432 with swept wings and tail to produce the "Model 448", which was presented to the USAAF in September 1945. It retained the four TG-180 jet engines in its forward fuselage, with two more TG-180s in the rear fuselage. The flush-mounted air intakes for the rear engines were inadequate while the USAAF considered the engine installation within the fuselage to be a fire hazard.Knaack 1988, p. 102.Gunston and Gilchrist 1993, p. 40.

The engines were moved to streamlined pylon-mounted pods under the wings, leading to the next iteration, the ''Model 450'', which featured two TG-180s in a twin pod mounted on a pylon about a third of the way outboard on each wing, plus another engine at each wingtip. The Army Air Force liked this new configuration, so Boeing's engineers refined it, moving the outer engines further inboard to about of the wingspan. The thin wings provided no space for tricycle main gear to retract so it would have needed a considerable bulge in the fuselage aft of the bomb bay for lateral stability. The only way to get a bomb-bay long enough for an A-bomb was to use a "bicycle landing gear", the two main gear assemblies arranged in a tandem configuration and outrigger struts fitted to the inboard engine pods. As the landing gear arrangement made rotation impossible, it was designed so that the aircraft rested on the ground at the proper angle for takeoff.Bowers 1989, p. 383.Gunston and Gilchrist 1993, p. 42.

Pleased with the refined Model 450 design, in April 1946, the USAAF ordered two prototypes, to be designated "XB-47".Knaack 1988, pp. 102–103. Assembly began in June 1947. The first XB-47 was rolled out on 12 September 1947, a few days before the USAAF became a separate service, the United States Air Force (USAF), on 18 September 1947. According to aviation authors Bill Gunston and Peter Gilchrist, Boeing subjected the first prototype to "one of the most comprehensive ground-test programmes ever undertaken".

Flight test phase

The XB-47 prototype flew its first flight on 17 December 1947 (the anniversary of the Wright Brothers' first four flights on 17 December 1903), with test pilots Robert Robbins and Scott Osler at the controls. It lasted 27 minutes, flying from Boeing Field in Seattle to Moses Lake Airfield in central Washington state.Yenne 2002, p. 158.Peacock 1989, p. 34. While not experiencing major problems, the emergency hot wire system was needed to raise the flaps and the engine fire warning indicators falsely illuminated. Robbins stated that it had good flight characteristics.Natola 2002, pp. 17–24.

Robbins had been skeptical about the XB-47, saying that before his first flight he had prayed, "Oh God, please help me through the next two hours." Robbins soon realized that he had an extraordinary aircraft. Chuck Yeager also flew the XB-47, noting it was so aerodynamically clean that he had difficulty landing on the Edwards lakebed.Yeager 1985, pp. 177–178. In February 1949, Russ Schleeh and Joe Howell "broke all coast-to-coast speed records" flying from Moses Lake Air Force Base to Andrews Air Force Base, averaging .Natola 2002, pp. 34–36. During an early test flight, the canopy came off at high speed, killing pilot Scott Osler; the aircraft was safely landed by the copilot.Boyne, Walter J. "Flying the B-47: An inside look at the USAF's first jet bomber".''Flight Journal'', April 2002. Retrieved 31 March 2010. The accident resulted in a canopy redesign and the hiring of pilot Tex Johnston as chief test pilot.

The second XB-47 (46-066) prototype first flew on 21 July 1948 and, following its delivery to the USAF in December of that year, served as a flying test bed until 1954. Its final destination was Chanute AFB where it was used as a maintenance and familiarization aircraft. The second prototype was equipped with more powerful General Electric J47-GE-3 turbojets with of static thrust each.Yenne 2002, p. 165. The J47 or "TG-190" was a redesigned version of the TG-180/J35. The first prototype was later retrofitted with these engines.

Flight testing of the prototypes was careful and methodical since the design was new in so many ways. They initially suffered from " Dutch roll", an instability that caused it to weave in widening "S" turns, remedied by the addition of a " yaw damper" control system to automatically deflect the rudder to damp out the weaving motion. Wind tunnel tests had shown it would pitch up at maximum speed due to wing stall on the outboard section of the wing. This was confirmed during flight tests so small vanes called "vortex generator