B-47D
In the late 1940s and early 1950s, a number of turbo-propjet engines were in development. In the simplest terms, a turbo-propjet engine is a gas turbine engine that uses its exhaust for thrust but also turns a propeller to provide high cruise speeds with better fuel economy. Gene Root from the RAND corporation had a theory that a smallcross-sectioned aircraft powered by turboprops could penetrate Soviet airspace, and he envisioned large fleets of such turboprop bombers.' Part of this reasoning was that turboprops had a lower fuel consumption than the then current jet engines. Therefore the USAF became interested, hoping for an increased unrefueled range at a high cruising speed of nearly 600 miles per hour.
Design of the XB-47D was initiated in February 1951, and 2 months later Boeing received a contract for the conversion of 2 B-47Bs. The Air Force pinned some hopes on gaining a high speed, long-range turboprop jet bomber from the project, but this was not its primary goal. The XB-47D was essentially developed to test a jet engine-prop combination and to provide data on the installation of turboprops in swept-wing aircraft.
The XB-47D closely resembled a B-47B, retaining the outboard J47-GE-23 jet engines, while a single Curtiss-Wright YT49-W-I engine, a turboprop version of the J65 Sapphire, occupied each of the inboard nacelles (in place of the paired J-47s). The XB-47D was 106 feet, 8 inches long (32.5 meters), 27 feet, 9 inches (8.5 m) high with a wing span of 116 ft. (35.3 m). Its maximum gross takeoff weight was 184,428 pounds (83,655 kilograms), and its maximum unrefueled range was 5,759 nautical miles (6627 kilometers). It had a three-man crew: pilot, copilot/gunner, and navigator/bombardier.
A successful technical inspection in January 1952 made it seem likely that an early 1953 first flight was possible. This, however, did not materialize. The Wright YT49-W-1 engines 9 (with a ducted-spinner airscrew) were plagued with problems, as were the Curtiss CT846S-A turboelectric propellors. The Curtiss-Wright prototype engine, with its 4-bladed propellers 15 feet in diameter, failed to pass the 50-hour qualification run. The Air Force then estimated that it would take another year before testing could resume. Continuing troubles with the engine-prop combination and shortages of government-furnished equipment delayed further progress.
Since 1951 Wright had been persevering with a shaftdrive engine derived from the Sapphire, retaining virtually the original compressor and combustion chamber but having a third turbine stage and a large reduction gearbox mounted centrally within the intake casting. It was being programmed for civil use, derated to maintain 6,500 s.h.p. to 25,000ft and running at top temperatures some 300 deg F under those of the fully rated engine. Curtiss Turbolectric airscrews for the derated T49 were of relatively light weight and need not be designed to exceptional peak efficiencies.
The first XB-47D was not flown until 26 August 1955, when the Boeing XB-47D took off from Boeing Field, Seattle. The second flew on 15 February 1956. Unlike the J47 version of the J65 Sapphire, the T49 turboprop version of the J65 Sapphire not only reached the hardware stage but also accomplished a fair amount of flying in a pair of XB-47Ds. The big single-shaft turboprops replaced the inboard twin-J47 nacelles, and drove large ducted-spinner propellers. Rated at about 6,500 e.s.h.p., the T49 was then the most powerful shaft-drive aero engine to have flown.
Further engine/propellor problems restricted flght testing to only 50 hours over a period of 18 months, however the aircraft reached 597mph at 13,500 ft, at that time a record for a propellor driven aircraft. However, this was accomplished at full-power settings and fuel consumption was excessive. Finally, as demonstrated, the XB-47D achieved a best cruise speed of 402 nautical miles per hour (462mph) at 42,400 feet (12,923.5 m) altitude. Thus, its all-jet-powered B-47 Stratojet counterpart was preferred. The aircraft's performance was comparable to that of a conventional B-47, and its reversible propellers shortened the landing roll, but the USAF did not follow up the idea. The XB-47D did not inspire confidence among those who flew it, and virtually all development ceased. One of the original aims of the program had been to produce a de-rated constant-power engine for commercial use.
Even though both aircraft accumulated a good many flying hours, no prototypes were ordered. Having served its basic purposes, the program never went beyond the experimental stage. At the conclusion of the program, the AF asked Boeing to ferry the two aircraft to Tinker AFB, where they would be converted back to bombers. Boeing replied that such a flight would need one or two stops to replace the engines and/or propellors. The AF forgot about the plan and the aircraft were instead scrapped at Seattle.
Boeing XB-55 (company designation Model 474) was a proposed Boeing aircraft designed to be a strategic bomber. The XB-55 was intended to be a replacement for the Boeing B-47 Stratojet in United States Air Force (USAF) service.
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