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Pratt & Whitney/Allison 578-DX
The Pratt & Whitney/Allison 578-DX was an experimental aircraft engine, a hybrid between a turbofan and a turboprop known as a propfan. The engine was designed in the 1980s to power proposed propfan aircraft such as the Boeing 7J7 and the MD-91 and MD-92 derivatives of the McDonnell Douglas MD-80. As of 2019, it is still one of only four different contra-rotating propfan engines to have flown in service or in flight testing.
The Allison Engine Company, a division of automobile manufacturer General Motors (GM), unveiled its Model 578 contra-rotating propfan engine at the Paris Air Show in mid-1985. The engine represented Allison's attempt to re-enter the commercial airline engine market, because after turboprop aircraft were overtaken in civil aviation in the 1950s and 1960s, Allison was mostly confined to selling military aircraft engines. Targeted to power passenger airliners seating between 100 and 160 passengers, the engine would be based on the core of the Allison T701 heavy lift helicopter turboshaft engine, which first ran in 1975. The engine would come in two sizes: a 10,000 shaft horsepower (7,500 kilowatts) variant with a 23:1 compression ratio and a three-stage boost compressor and power turbine added to the T701 core, and a 16,000 shaft hp (12,000 kW) variant with a 33:1 pressure ratio and a four-stage boost and power section. Since Allison estimated that a 100-seat plane required two 9,000–10,000 shaft hp (6,700–7,500 kW) engines and that a 150-seat plane needed two 13,000–15,000 shaft hp (9,700–11,200 kW) engines, the two sizes chosen should handle the 100–160 passenger aircraft market. Allison hoped to have its 578 engine ready for flight tests in late 1987.
Allison and United Technologies aircraft engine division Pratt & Whitney announced on March 14, 1986 that they would develop the engine as a joint venture. The agreement, which was signed on February 25, was reached because Pratt & Whitney would not be able to develop its own propfan engine by the planned 1992 certification date of the 7J7, and because it also did not have an engine core in the size range of 10,000–16,000 shaft hp (7,500–11,900 kW). Allison would develop the engine and perform ground testing, while Pratt & Whitney would integrate the propulsion system into the aircraft and manage the flight test program.
On February 23, 1987 in Detroit, Michigan, Pratt & Whitney and Allison officially formed a joint company named PW–Allison Engines. The new company claimed that compared a commercial jet engine, its propfan engines would consume 30-percent less fuel, require shorter runway lengths during takeoffs and landings, and be quieter (because of the slower propfan rotational speeds enabled by the 578's gearbox). The partnership would last 20 years and cover geared propfan development in the 8,200–16,000 shaft hp (6,100–11,900 kW) range, though the power range could be increased by 15 percent with a specific mechanical modification. The new company also expected Japanese and European engine makers to eventually join its program instead of creating competing propfan engines. PW–Allison Engines was to invest at least 500 million dollars to bring the engine into commercial production (depending on airline commitments to buy aircraft using its engines), and it assumed that the new company would take half of the market for new medium-sized airliners over the next ten years—a 13-to-18 billion dollar market, it estimated. Pratt & Whitney would handle the marketing to commercial customers, while Allison would do the marketing to military customers. Other members of the new company included GM subsidiaries Hughes Aircraft and Delco Electronics and United Technologies subsidiary Hamilton Standard, the propeller manufacturer that invented the propfan concept with the National Aeronautics and Space Administration (NASA) in the 1970s.
On December 20, 1985, Boeing and Allison agreed to test its new propfan engine for use on the Boeing 7J7. As of early 1986, Allison maintained that it could have its propfan engine certified by early 1990, although a few months later the company changed the certification target timeline to 1992. By early/mid 1987, Boeing had rejected using the PW–Allison engine for its 7J7. Although Boeing had not tested PW–Allison's propfan in flight, the airframer said that the PW–Allison engine's off-the-shelf core was not powerful enough to power the 150-seat plane. PW–Allison vigorously objected, claiming its engine's gearbox could handle 13,000 shaft hp (9,700 kW), the agreement between Pratt & Whitney and Allison covered up to 16,000 shaft hp (12,000 kW), and the gearbox design could go to 25,000 shaft hp (19,000 kW). The decision became moot in December 1987, when Boeing indefinitely suspended development of the 7J7 aircraft, four months after delaying the 7J7 availability date from 1992 to 1993.
In January 1986, Allison also signed a memorandum of understanding with McDonnell Douglas to perform flight tests on the Allison demonstrator engine, now known as the 578-DX, on an MD-80 testbed in late 1987. The flight test program was in preparation for McDonnell Douglas's ultra-high bypass (UHB) MD-91X derivative, a 114-seat airliner that had a planned service entry in the early 1990s. The demonstrator engine would convert 10,400 shaft hp (7,800 kW) into 19,000 pounds-force (85 kilonewtons) of thrust. One engine would be built for ground testing, while another would be used in flight tests. The 578-DX would follow in testing the competing General Electric GE36 UDF engine, which was scheduled for 75 hours of flight tests on the MD-80 testbed from May 1987 through August 1987; the 578-DX would complete its 75 hours of flight testing from December 1987 through February 1988. During the middle of each engine's test program, McDonnell Douglas would let airline representatives fly in the testbed aircraft to assess the passenger experience. McDonnell Douglas would begin installing one 578-DX demonstrator on its testbed in September 1987, while PW-Allison would perform ground tests with its other 578-DX demonstrator from mid-June 1987 to November 1987. Ground testing was to begin around August 1987, and PW–Allison was confident that its engine would be flight tested by early December.
A number of issues hindered the 578-DX, though. First, there was difficulty in manufacturing the thin, swept blades. Then, after one test engine was disassembled for inspection, the second engine ingested parts of the first engine into its compressor during testing. These problems delayed flight tests until the end of February 1988, and they delayed ground testing of the engine until December 19, 1987. In February 1988, PW–Allison postponed the service entry date until the end of 1993 at the earliest, as it said that the 578-DX was not powerful enough for the MD-92, and the required increase in engine power from 14,000 to 16,000 shaft hp (10,000 to 12,000 kW) could not be finished in time for the McDonnell Douglas's 1992 target date. PW–Allison moved the beginning of flight tests to May 1988, but the flight date target was missed again because of endurance problems from the electrical pitch-change motors, so flight testing was put off until August to October of 1988.
Amidst the flight test delays, McDonnell Douglas repeatedly expressed dismay that PW–Allison had not made a formal offer to place its engines into an MD-90 production program. The airframer wanted to offer its proposed derivatives to airlines with a choice of powerplant options, instead of advertising aircraft availability with just the competing GE36 UDF engine. However, PW–Allison refused to submit an offering to McDonnell Douglas and the airlines before its flight tests were completed and evaluated.
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Pratt & Whitney/Allison 578-DX
The Pratt & Whitney/Allison 578-DX was an experimental aircraft engine, a hybrid between a turbofan and a turboprop known as a propfan. The engine was designed in the 1980s to power proposed propfan aircraft such as the Boeing 7J7 and the MD-91 and MD-92 derivatives of the McDonnell Douglas MD-80. As of 2019, it is still one of only four different contra-rotating propfan engines to have flown in service or in flight testing.
The Allison Engine Company, a division of automobile manufacturer General Motors (GM), unveiled its Model 578 contra-rotating propfan engine at the Paris Air Show in mid-1985. The engine represented Allison's attempt to re-enter the commercial airline engine market, because after turboprop aircraft were overtaken in civil aviation in the 1950s and 1960s, Allison was mostly confined to selling military aircraft engines. Targeted to power passenger airliners seating between 100 and 160 passengers, the engine would be based on the core of the Allison T701 heavy lift helicopter turboshaft engine, which first ran in 1975. The engine would come in two sizes: a 10,000 shaft horsepower (7,500 kilowatts) variant with a 23:1 compression ratio and a three-stage boost compressor and power turbine added to the T701 core, and a 16,000 shaft hp (12,000 kW) variant with a 33:1 pressure ratio and a four-stage boost and power section. Since Allison estimated that a 100-seat plane required two 9,000–10,000 shaft hp (6,700–7,500 kW) engines and that a 150-seat plane needed two 13,000–15,000 shaft hp (9,700–11,200 kW) engines, the two sizes chosen should handle the 100–160 passenger aircraft market. Allison hoped to have its 578 engine ready for flight tests in late 1987.
Allison and United Technologies aircraft engine division Pratt & Whitney announced on March 14, 1986 that they would develop the engine as a joint venture. The agreement, which was signed on February 25, was reached because Pratt & Whitney would not be able to develop its own propfan engine by the planned 1992 certification date of the 7J7, and because it also did not have an engine core in the size range of 10,000–16,000 shaft hp (7,500–11,900 kW). Allison would develop the engine and perform ground testing, while Pratt & Whitney would integrate the propulsion system into the aircraft and manage the flight test program.
On February 23, 1987 in Detroit, Michigan, Pratt & Whitney and Allison officially formed a joint company named PW–Allison Engines. The new company claimed that compared a commercial jet engine, its propfan engines would consume 30-percent less fuel, require shorter runway lengths during takeoffs and landings, and be quieter (because of the slower propfan rotational speeds enabled by the 578's gearbox). The partnership would last 20 years and cover geared propfan development in the 8,200–16,000 shaft hp (6,100–11,900 kW) range, though the power range could be increased by 15 percent with a specific mechanical modification. The new company also expected Japanese and European engine makers to eventually join its program instead of creating competing propfan engines. PW–Allison Engines was to invest at least 500 million dollars to bring the engine into commercial production (depending on airline commitments to buy aircraft using its engines), and it assumed that the new company would take half of the market for new medium-sized airliners over the next ten years—a 13-to-18 billion dollar market, it estimated. Pratt & Whitney would handle the marketing to commercial customers, while Allison would do the marketing to military customers. Other members of the new company included GM subsidiaries Hughes Aircraft and Delco Electronics and United Technologies subsidiary Hamilton Standard, the propeller manufacturer that invented the propfan concept with the National Aeronautics and Space Administration (NASA) in the 1970s.
On December 20, 1985, Boeing and Allison agreed to test its new propfan engine for use on the Boeing 7J7. As of early 1986, Allison maintained that it could have its propfan engine certified by early 1990, although a few months later the company changed the certification target timeline to 1992. By early/mid 1987, Boeing had rejected using the PW–Allison engine for its 7J7. Although Boeing had not tested PW–Allison's propfan in flight, the airframer said that the PW–Allison engine's off-the-shelf core was not powerful enough to power the 150-seat plane. PW–Allison vigorously objected, claiming its engine's gearbox could handle 13,000 shaft hp (9,700 kW), the agreement between Pratt & Whitney and Allison covered up to 16,000 shaft hp (12,000 kW), and the gearbox design could go to 25,000 shaft hp (19,000 kW). The decision became moot in December 1987, when Boeing indefinitely suspended development of the 7J7 aircraft, four months after delaying the 7J7 availability date from 1992 to 1993.
In January 1986, Allison also signed a memorandum of understanding with McDonnell Douglas to perform flight tests on the Allison demonstrator engine, now known as the 578-DX, on an MD-80 testbed in late 1987. The flight test program was in preparation for McDonnell Douglas's ultra-high bypass (UHB) MD-91X derivative, a 114-seat airliner that had a planned service entry in the early 1990s. The demonstrator engine would convert 10,400 shaft hp (7,800 kW) into 19,000 pounds-force (85 kilonewtons) of thrust. One engine would be built for ground testing, while another would be used in flight tests. The 578-DX would follow in testing the competing General Electric GE36 UDF engine, which was scheduled for 75 hours of flight tests on the MD-80 testbed from May 1987 through August 1987; the 578-DX would complete its 75 hours of flight testing from December 1987 through February 1988. During the middle of each engine's test program, McDonnell Douglas would let airline representatives fly in the testbed aircraft to assess the passenger experience. McDonnell Douglas would begin installing one 578-DX demonstrator on its testbed in September 1987, while PW-Allison would perform ground tests with its other 578-DX demonstrator from mid-June 1987 to November 1987. Ground testing was to begin around August 1987, and PW–Allison was confident that its engine would be flight tested by early December.
A number of issues hindered the 578-DX, though. First, there was difficulty in manufacturing the thin, swept blades. Then, after one test engine was disassembled for inspection, the second engine ingested parts of the first engine into its compressor during testing. These problems delayed flight tests until the end of February 1988, and they delayed ground testing of the engine until December 19, 1987. In February 1988, PW–Allison postponed the service entry date until the end of 1993 at the earliest, as it said that the 578-DX was not powerful enough for the MD-92, and the required increase in engine power from 14,000 to 16,000 shaft hp (10,000 to 12,000 kW) could not be finished in time for the McDonnell Douglas's 1992 target date. PW–Allison moved the beginning of flight tests to May 1988, but the flight date target was missed again because of endurance problems from the electrical pitch-change motors, so flight testing was put off until August to October of 1988.
Amidst the flight test delays, McDonnell Douglas repeatedly expressed dismay that PW–Allison had not made a formal offer to place its engines into an MD-90 production program. The airframer wanted to offer its proposed derivatives to airlines with a choice of powerplant options, instead of advertising aircraft availability with just the competing GE36 UDF engine. However, PW–Allison refused to submit an offering to McDonnell Douglas and the airlines before its flight tests were completed and evaluated.
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