Editing Boeing B-47 Stratojet (section)

==Development==

===Origins===
The B-47 arose from an informal 1943 requirement for a [[Jet engine|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-engine]]d [[Boeing B-29 Superfortress|B-29 Superfortress]] equipped with four jet engines.<ref name = "Peacock AE p31">Peacock 1989, p. 31.</ref><ref name = "gungil 389">Gunston and Gilchrist 1993, pp. 38–39.</ref> In 1944, this initial concept evolved into a formal request-for-proposal to design a new bomber with a maximum speed of {{convert|550|mph|kn km/h|abbr=on}}, a cruise speed of {{convert|450|mph|kn km/h|abbr=on}}, a range of {{convert|3500|mi|nmi km|abbr=on}}, and a service ceiling of {{convert|45000|ft|sigfig=3|abbr=on}}.<ref name="Knaack p101">Knaack 1988, p. 101.</ref><ref name = "gungil 39">Gunston and Gilchrist 1993, p. 39.</ref>

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.<ref name="Peacock AE p33">Peacock 1989, p. 33.</ref><ref name = "gungil 39"/> The USAAF awarded study contracts to all four companies, requiring that North American and Convair concentrate on four-engined designs (to become [[North American B-45 Tornado|B-45]] and [[Convair XB-46|XB-46]]), while Boeing and Martin were to build six-engined aircraft (the B-47 and [[Martin XB-48|XB-48]]). The powerplant was to be [[GE-Aviation|General Electric's]] new [[General Electric J35|TG-180]] turbojet engine.<ref name="Peacock AE p33"/>

===Swept wings===
In May 1945, the [[United States Air Force Scientific Advisory Board|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 [[Robert Thomas Jones (engineer)|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.<ref>Von Karman,'' Aerodynamics: Selected Topics in the Light of their Historical Developments'', 1954.</ref><ref name = "gungil 3940">Gunston and Gilchrist 1993, pp. 39–40.</ref> Analysis by Boeing engineer Vic Ganzer suggested an optimum sweepback angle of about 35 degrees.<ref>Cook 1991, p. 152.</ref> Boeing's aeronautical engineers modified the Model 432 with swept wings and [[empennage|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.<ref name="Peacock AE p33"/><ref name="Knaack p102">Knaack 1988, p. 102.</ref><ref name = "gungil 40">Gunston and Gilchrist 1993, p. 40.</ref>

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.<ref name = "gungil 40"/> The Army Air Force liked this new configuration, so Boeing's engineers refined it, moving the outer engines further inboard to about {{frac|3|4}} 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",<ref>Cook 1991, p. 164.</ref> 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 (aviation)|rotation]] impossible, it was designed so that the aircraft rested on the ground at the proper angle for takeoff.<ref name="Peacock AE p33"/><ref name="Bowers Boeing p383">Bowers 1989, p. 383.</ref><ref name = "gungil 42">Gunston and Gilchrist 1993, p. 42.</ref>

Pleased with the refined Model 450 design, in April 1946, the USAAF ordered two prototypes, to be designated "XB-47".<ref name="Knaack p102-3">Knaack 1988, pp. 102–103.</ref> Assembly began in June 1947. The first XB-47 was rolled out on 12 September 1947,<ref name="Bowers Boeing p383"/> 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".<ref name = "gungil 43"/>

===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 [[Grant County International Airport|Moses Lake Airfield]] in central [[Washington (state)|Washington state]].<ref name="Yenne p158">Yenne 2002, p. 158.</ref><ref name="Peacock AE p34">Peacock 1989, p. 34.</ref> 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.<ref name="Natola 1724">Natola 2002, pp. 17–24.</ref>

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.<ref name="Natola 1724"/> [[Chuck Yeager]] also flew the XB-47, noting it was so aerodynamically clean that he had difficulty landing on the [[Edwards Air Force Base|Edwards lakebed]].<ref name="Yeager 177178">Yeager 1985, pp. 177–178.</ref> In February 1949, Russ Schleeh and Joe Howell "broke all coast-to-coast speed records" flying from [[Larson Air Force Base|Moses Lake AFB]] to [[Andrews Air Force Base|Andrews AFB]], averaging {{convert|607.8|mph|kn km/h}}.<ref name="Natola 3436">Natola 2002, pp. 34–36.</ref> During an early test flight, the canopy came off at high speed, killing pilot Scott Osler; the aircraft was safely landed by the copilot.<ref name="Flight Journal">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.</ref> The accident resulted in a canopy redesign and the hiring of pilot [[Alvin M. Johnston|Tex Johnston]] as chief test pilot.<ref>Johnston 2000, p. 135.</ref>

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 Air Force Base|Chanute AFB]] where it was used as a maintenance and familiarization aircraft.<ref>{{Cite web |url=http://b-47.com |title=The B-47 Stratojet Association |website=The B-47 Stratojet Association |language=en-US |access-date=8 December 2016}}</ref> The second prototype was equipped with more powerful General Electric [[General Electric J47|J47-GE-3]] turbojets with {{convert|5200|lbf|kN|abbr=on|lk=in}} of static thrust each.<ref name="Yenne p165">Yenne 2002, p. 165.</ref><ref name=tjpwpwr>{{cite news |url=https://news.google.com/newspapers?id=XsxeAAAAIBAJ&sjid=szIMAAAAIBAJ&pg=6446%2C3174477 |work=Lewiston Morning Tribune |location=(Idaho) |agency=Associated Press |title=Boeing turbo-jet packed with power |date=October 9, 1949 |page=1}}</ref> The J47 or "TG-190" was a redesigned version of the TG-180/J35; the first prototype was later retrofitted with these engines.{{Citation needed|date=August 2019}}

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]]s" were added to prevent [[Flow separation|airflow separation]].<ref>The Road to the 707 The Inside Story of Designing the 707, William H. Cook 1991, {{ISBN|0 9629605 0 0}}, p.194-198</ref>

Both XB-47 prototypes were test flown at Edwards AFB; the first XB-47 (46-065) was disassembled and scrapped in 1954, making the second prototype (46-066) the sole surviving XB-47. Upon retirement, XB-47 (46-066) 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.<ref>{{Cite web |url=http://www.paxtonrecord.net/news/politics-and-government/2015-04-23/chanute-air-museum-closing-rantoul.html |title=Chanute Air Museum closing in Rantoul {{!}} Paxton Record |website=paxtonrecord.net |access-date=8 December 2016 |archive-date=31 October 2016 |archive-url=https://web.archive.org/web/20161031172922/http://www.paxtonrecord.net/news/politics-and-government/2015-04-23/chanute-air-museum-closing-rantoul.html |url-status=dead }}</ref> In late 2015, the Flight Test Historical Foundation<ref>{{Cite web |url=http://afftcmuseum.org/foundation/ |title=Flight Test Historical Foundation |access-date=8 December 2016 |archive-url=https://web.archive.org/web/20160911190328/http://afftcmuseum.org/foundation/ |archive-date=11 September 2016 |url-status=dead }}</ref> began fundraising to purchase XB-47 (46-066) for relocation to the [[Air Force Flight Test Museum|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.<ref>{{Cite news |url=https://www.edwards.af.mil/News/Article-Display/Article/951797/experimental-bomber-returns-home-after-six-decades/ |title=Experimental bomber returns home after six decades |newspaper=Edwards Air Force Base |access-date=8 December 2016}}</ref>

===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 [[William McPherson Allen|Bill Allen]] inviting USAF General K.B. Wolfe, in charge of bomber production, for a ride in the XB-47.<ref name="Yenne p160">Yenne 2002, p. 160.</ref> A formal contract for 10 aircraft was signed on 3 September 1948.<ref name="Knaack p107">Knaack 1988, p. 107.</ref><ref name = "gungil 4344">Gunston and Gilchrist 1993, pp. 43–44.</ref>

===Production===
[[File:B-47As, Boeing Airplane Co. Plant II, North Apron, Wichita, KS, 1951.jpg|thumb|Seven B-47A at Boeing's [[Wichita, Kansas|Wichita]] plant, January 1951]]
{| class=wikitable style="text-align:center"
|-
! Variant
| XB-47
| B-47A
| B-47B
| B-47E
| RB-47E
| RB-47H
| ERB-47H
| RB-47K
|-
! Built
| 2
| 10
| 399
| 1341
| 240
| 32
| 3
| 15
|}

The total number of B-47s built was 2,042.<ref>{{cite web |url=https://www.grissomairmuseum.com/exhibits/gallery/bombers/plane-4 |title=B-47 Stratojet total production number 2 |url-status=dead |access-date=12 July 2021 |archive-date=6 January 2022 |archive-url=https://web.archive.org/web/20220106120751/https://www.grissomairmuseum.com/exhibits/gallery/bombers/plane-4 }}</ref>