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J47

In 1948 GE tested the first afterburning turbojet engine in the world, the J47. GE began developing the J47 from the earlier J35. The J47 would power several of the new front-line military aircraft, including the F-86 Sabre Jet. Production of the J47 turbojet engine was completed by GE in early 1956. It was the notable powerplant of B-47, F-86D (also E, F, K, and L models), XF-91 Thunderceptor and FJ-2.

North American Aviation, Inc., manufactured the original F86 series airplane as an all metal, single-place, high-performance day fighter. A General Electric J47 series axial flow turbojet engine that developed 7,400 pounds of thrust at sea level powered it. The airplane incorporated a swept wing design with tricycle retractable landing gear, slotted flaps, leading edge slats, fuselage mounted speed brakes, interconnected elevator and stabilizer, and an irreversible hydraulic control system.

Over-temperature operation of the J47 engine was a major phase of the F-86 improvement program. Flight tests conducted by North American Aviation proved that the operation of the engine at higher than normal temperatures increased the high-altitude rate of climb by 2,000 feet per minute. Further tests by the Power Plant Laboratory also provided the same conclusions, but the laboratory's engineers warned of rapid turbine blade deterioration resulting after ten minutes' operation. Their primary concern was that the advantages would be outweighed by increased maintenance and supply problems, which were already severe in Korea.

The Republic XF-91 Thunderceptor was a single-place fighter, powered by a General Electric J47-GE-17 turbojet engine. The wing had a sweep angle of 40 degrees at the 50-percent-chord line and had inverse taper and variable incidence. The root chord (airplane center line) had a dimension of 95 inches while the wing tip chord measured 154.5 inches. The wing incidence was variable in flight through a range from -2 degrees to 5.65 degrees.

On April 21, 1951 the experimental Chase XC-123A, powered by four J47 turbojet engines, made its first flight. Designed as a troop and cargo transport for the Air Force, the XC-123A was fitted with four turbojet engines, installed as pairs in pods.

The B-47 with six J47-GE-25A turbojets at 6000 lb of thrust (dry), or 7200 lb of thrust (using water injection for takeoff) provided a 20-percent increase in thrust necessary to get off the runway. Still today, B52-B, powered by eight PW J-57-19 turbojet engines, produce 12 000 lb of thrust with water injection, and the water-injected 747-200 is an old yet well proven system.

The engine incorporated a hydro-mechanical fuel control for the main combustion chamber, and an electronic (vacuum tube) fuel control for the afterburner; unfortunately, the reliability of the electronic control was poor due the problem-prone vacuum tube technology in a harsh operating environment like a jet engine.

The original control law for the J47 was designed using only frequency response techniques. The engine underwent altitude testing at NACA (National Advisory Committee for Aeronautics) Lewis Laboratory (now NASA Glenn Research Center). During the testing, it was found that the noise in the speed sensor, coupled with the high gain of the speed governor, caused the engine to behave like a limit cycle. To solve this problem, GE and NACA engineers worked together and applied the time-domain step response analysis method. The problem was soon fixed by reducing the control gain at altitude. This industry-government cooperation experience has established a good foundation for building a knowledge base for engine controls.




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