Boeing B-47 Stratojet


The Boeing B-47 Stratojet 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 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.
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, 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 Boeing's own 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 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. 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.
In December 1944, North American Aviation, Convair, Boeing and the Glenn L. 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. The USAAF awarded study contracts to all four companies, requiring that North American and Convair concentrate on four-engined designs, while Boeing and Martin were to build six-engined aircraft. 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. 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.
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.
Pleased with the refined Model 450 design, in April 1946, the USAAF ordered two prototypes, to be designated "XB-47". 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, 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, 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. 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.
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. In February 1949, Russ Schleeh and Joe Howell "broke all coast-to-coast speed records" flying from Moses Lake AFB to Andrews AFB, averaging. During an early test flight, the canopy came off at high speed, killing pilot Scott Osler; the aircraft was safely landed by the copilot. The accident resulted in a canopy redesign and the hiring of pilot Tex Johnston as chief test pilot.
The second XB-47 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. 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 generators" were added to prevent airflow separation.
Both XB-47 prototypes were test flown at Edwards AFB; the first XB-47 was disassembled and scrapped in 1954, making the second prototype the sole surviving XB-47. Upon retirement, XB-47 was restored and placed on display at the Octave Chanute Aerospace Museum in Rantoul, Illinois, remaining there until the museum announced its closure due to financial difficulties in April 2015. In late 2015, the Flight Test Historical Foundation began fundraising to purchase XB-47 for relocation to the Flight Test Museum at Edwards AFB. The purchase was completed in August 2016 and on 21 September 2016 the aircraft arrived at Edwards AFB for reassembly, restoration and eventual display at the Flight Test Museum.

X-model competitions

By mid-1948, the USAF's bomber competition had already been through one iteration, pitting the North American XB-45 against the Convair XB-46. The North American design won that round of the competition. As an interim measure, the USAF decided to put the North American bomber into production on a limited basis as the B-45 Tornado. The expectation was that B-45 production would be terminated if either of the remaining two designs in the competition, the Boeing XB-47 and the Martin XB-48, proved superior. It is sometimes claimed that the final production decision was made as a result of Boeing president Bill Allen inviting USAF General K.B. Wolfe, in charge of bomber production, for a ride in the XB-47. A formal contract for 10 aircraft was signed on 3 September 1948.

Production

VariantXB-47B-47AB-47BB-47ERB-47ERB-47HERB-47HRB-47K
Built210399134124032315

The total number of B-47s built was 2,042.

Design

Overview

The XB-47, which looked nothing like contemporary bombers, was described by Boyne as a "sleek, beautiful outcome that was highly advanced". The 35-degree swept wings were shoulder-mounted, the inboard turbojet engines mounted in twin pods, at about a third of the span, and the outboard engines singly near the wing tip. This arrangement reduced the bending moment at the wing roots, saving structural weight. The engines' mass acted as counter-flutter weights.
The wing airfoil was identified by Boeing as the BAC 145, also known as the NACA 64A12 mod airfoil. Wing flexibility was a concern, flexing as much as at the tip; major effort was expended to ensure that flight control could be maintained as the wing moved up and down; these worries proved to be mostly unfounded. Its maximum speed was limited to IAS to avoid control reversal, where aileron deflections would cause the wings to twist and produce a roll in the opposite direction to that desired by the pilot. The wings were fitted with a set of Fowler flaps that extended well behind the wing to enhance lift at slow speeds. The flight control surfaces were powered, augmenting the pilot's inputs and reducing the exertion required to overcome the forces involved.
The XB-47 was designed to carry a crew of three in a pressurized forward compartment: a pilot and copilot, in tandem, in a long fighter-style bubble canopy, and a navigator/bombardier in a compartment in the nose. The copilot doubled as tail gunner, and the navigator as bombardier. The bubble canopy, which provided a high level of visibility to the pilots, pitched up and slid backward; as the cockpit was high off the ground, the crew entered via a door and ladder on the underside of the nose. The extreme front of the nose was initially glazed for visual navigation and bomb sighting, but this requirement was soon deleted together with the glazing. Most production versions had a metal nose with no windows. A K-series bombsight provided integrated radar navigation and visual navigation, the optical portion extending through the nose in a small dome. For greater comfort, both heaters and refrigeration systems were present in the cockpit to manage the cockpit environment. There was little vibration compared to prior bombers powered by reciprocating engines.