page_title: Propfan
section_title: Flight test programs
hierarchical_section_title: Propfan / Development / 1970s–1980s / Flight test programs
caption_attribution_description: The Pratt &amp; Whitney-Allison 578–DX geared propfan demonstrator engine, installed on an MD-80 testbed aircraft.
context_page_description: A propfan, also called an open rotor engine, or unducted fan, is a type of aircraft engine related in concept to both the turboprop and turbofan, but distinct from both. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop. A propfan is typically designed with a large number of short, highly twisted blades, similar to a turbofan's bypass compressor. For this reason, the propfan has been variously described as an "unducted fan" or an "ultra-high-bypass turbofan."
context_section_description: Hamilton Standard, the only remaining large American manufacturer of aircraft propellers, developed the propfan concept in the early 1970s. Numerous design variations of the propfan were tested by Hamilton Standard, in conjunction with NASA in this decade. This testing led to the Propfan Test Assessment (PTA) program, where Lockheed-Georgia proposed modifying a Gulfstream II to act as in-flight testbed for the propfan concept, while McDonnell Douglas proposed modifying a DC-9 for the same purpose. NASA chose the Lockheed proposal, where the aircraft had a nacelle added to the left wing, containing a 6,000 horsepower (4,500 kilowatts) Allison 570 turboprop engine (derived from the XT701 turboshaft developed for the Boeing Vertol XCH-62 heavy lift helicopter). The engine used an eight-bladed, 9-foot diameter (2.7-metre; 110-inch; 270-centimetre), single-rotation Hamilton Standard SR-7 propfan as its propulsor. The test engine, which was named the Allison 501-M78, had a thrust rating of 9,000 lbf (40 kN), and it was first operated in flight on March 28, 1987. The extensive test program, which cost about $56 million, racked up 73 flights and over 133 hours of flight time before finishing on March 25, 1988, although most of the flight testing was done in 1987. In 1989, however, the testbed aircraft returned to the air from April 3 through April 14 to measure ground noise levels during en-route flying. The engine was removed after that, and the aircraft was converted to a space shuttle training aircraft later that year.
  The GE36 Unducted Fan (UDF), from the American engine maker General Electric (GE) with 35-percent participation from French partner Snecma (now Safran), was a variation on the original propfan concept and resembled a pusher configuration piston engine. GE's UDF had a novel direct-drive arrangement, where the reduction gearbox was replaced by a low-speed seven-stage free turbine. One set of turbine rotors drove the forward set of propellers, while the rear set was driven by the other set of rotors which rotated in the opposite direction. The turbine had 14 blade rows with seven stages. Each stage was a pair of contra-rotating rows. Airframers, who had been wary of issue-prone gearboxes since the 1950s, liked GE's gearless version of the propfan: Boeing intended to offer GE's pusher UDF engine on the 7J7 platform (which would have had a cruise speed of Mach 0.83), and McDonnell Douglas was going to do likewise on their MD-94X airliner. The GE36 was first flight tested mounted on the #3 engine station of a Boeing 727-100 on August 20, 1986. The GE36 UDF for the 7J7 was planned to have a thrust of 25,000 pounds-force (110 kN), but GE claimed that in general its UDF concept could cover a thrust range of 9,000 to 75,000 lbf (40 to 334 kN), so a UDF engine could possibly match or surpass the thrust of the CF6, GE's family of widebody engines at that time.
  McDonnell Douglas developed a proof-of-concept aircraft by modifying its company-owned MD-80, which is suited for propfans due to its aft fuselage-mounted engines (like its DC-9 ancestor), in preparation for the possible propfan-powered MD-91 and MD-92 derivatives and a possible MD-94X clean-sheet aircraft. They removed the JT8D turbofan engine from the left side of the fuselage and replaced it with the GE36.  Test flights began in May 1987, initially out of Mojave, California, which proved the airworthiness, aerodynamic characteristics, and noise signature of the design.  Following the initial tests, a first-class cabin was installed inside the aft fuselage and airline executives were offered the opportunity to experience the UDF-powered aircraft first-hand.  The test and marketing flights of the GE-outfitted demonstrator aircraft concluded in 1988, exhibiting a 30% reduction in fuel consumption over turbo-fan powered MD-80, full Stage 3 noise compliance, and low levels of interior noise/vibration. The GE36 would have the same 25,000 lbf (110 kN) thrust on the MD-92X, but the same engine would be derated to 22,000 lbf (98 kN) thrust for the smaller MD-91X. The MD-80
caption: The PW–Allison 578-DX engine installed on the same MD-80 testbed. The contra-rotating, geared propfan engine is 11.6 ft (3.5 m) in diameter, with six blades in front and six blades in back.
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