Military

T41

The Allison TF41-A-1 turbofan was a license built Rolls Royce RB162-256 Spey. The Rolls Royce TF41-A-2 engine built, with modifications under license by Allison division of General Motors. The TF41, a non-afterburner engine, had a thrust of 15,000 pounds which was a considerable increase over the TF30-P-8 and -408. The original power plant of the A-7 was a nonafterburning version of the Pratt & Whitney TF30 turbofan. This is the same engine that, equipped with an afterburner, powers both the F-111 and the F-14. Beginning with the A-7D, however, the more powerful Allison TF41-A-1 turbofan was installed. An American-made version of the British Rolls-Royce Spey, the TF41-A-1 has a bypass ratio of 0.7 and uses a five-stage fan.

The TF41 engine is an axial flow gas turbine engine with a round fixed geometry inlet, five stage low pressure compressor including a three-stage low bypass ratio fan, and a variable geometry eleven-stage high pressure compressor. The compressor rotors are driven by two-stage low and high pressure turbines through a coaxial shaft. The combustion section consists of ten can-annular axial flow combustors. A bypass duct directs fan discharge air to an annular mixing station which is blended with turbine exhaust gasses upstream of the fixed area convergent exhaust nozzle. The engine has no afterburner.

Ling-Temco-Vought Aerospace Corporation manufactured the A-7Ds over an eight-year period, producing 459 of the Short Little Ugly Fellows, or SLUFs, as their pilots called them. They were also known as "Little Hummers." Originally ordered for the Navy in early 1964, the Air Force accepted its first A-7D in late 1968 to replace the A-1 Skyraiders, F-100 Supersabres and F-105 Thunderchiefs in the tactical attack role. What distinguished the Air Force variant from its Navy parent were the Allison TF41 turbofan engine [of 14,250 pounds thrust] and a 20 mm Vulcan rapid-fire cannon, capable of firing 6,000 rounds per minute.

The A-7C was initially intended to be a two-seat training version of the A-7B. When this plan was not pursued, the A- 7C designation served as a "stop-gap" assigned to those aircraft accepted with the improvements intended for aircraft accepted as A-7E but lacking the Rolls Royce TF41-A-2 engine intended for the A-7E. All A-7Cs were powered by either the Pratt & Whitney TF30-P-8 or -408.

The A-7E made its combat debut when VA-146 and VA-147 deployed in April 1970 in America (CVA 66). The A-7E was similar to A-7B but with improved naval weapons delivery system, the AVQ-7B Head-Up Display, the ASN-91 Tactical Computer, the APQ-126 Forward Looking Radar, the ASN-90 Inertial Measurement Set and one 20 mm M61Al gun instead of two 20 mm MK-12 guns.

Management of the A-7 at Tinker began in 1966 while maintenance operations started six years later. From 1972 to 1988, the Oklahoma City Air Logistics Center performed programmed depot maintenance, analytical condition inspection or fly-in modifications on nearly 1,500 aircraft. Management and repair of the TF41 engine continued until September 1991. The December 1995 AE3007 tests re-established Allison as an AEDC test customer. The last AEDC test of an Allison engine, the TF41, had taken place in 1979.

Due to the high speed and high rotational energy in engine disks, cracks in disks can propagate quickly and lead to uncontained engine failures. Techniques to reliably detect disk cracks are extremely important and improvements to these techniques with increased crack detection sensitivity are needed to increase flight safety. Present day engines are disassembled and disks are inspected using one or more non-destructive evaluation techniques to detect cracks during major overhauls. These techniques are costly, are performed at discrete times during the disk run history, and are not always effective in detecting cracks. Techniques are currently being developed to detect disk cracks during actual operation of the engine.

Although rare in occurrence, a disk rupture in an aircraft engine can lead to a catastrophic failure. In 1989, United Airlines flight 232 crashed during an attempted landing at Sioux Gateway Airport, Iowa. The separation, fragmentation, and forceful discharge of the stage one fan rotor from the number two engine led to leaks in all three hydraulic systems, resulting in loss of flight controls. The failure was due to a fatigue crack in a critical area of the fan disk. Here, 111 fatalities occurred. Another example of a catastrophic disk failure was Delta Air Lines flight 1288. For this, a crack in the front compressor disk hub propagated to fracture during takeoff roll at Pensacola Regional Airport, Florida. This caused an engine burst where debris penetrated the fuselage. Two passengers were killed and two others were seriously injured.

In 2004 a cooperative program was conducted between the FAA, U.S. Navy, NASA, and the U.S. Air Force to evaluate crack detection techniques in a seeded fault engine test. The first stage fan of a TF41 engine with a seeded fault was run in a full scale engine test facility. Various disk crack detection systems were installed on the disk and monitored real time. Post-engine-test cycles were accumulated on the TF41 disk in a spin pit to further measure crack growth and disk strain. The crack in the TF41 first stage fan disk grew during the engine test, but significantly less than that predicted from fracture mechanics analysis. Techniques to detect disk crack using center of mass shift seemed feasible in the engine test environment. Techniques to detect disk crack using blade deflections was not effective in this test due to blade wander.




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