Hailed for its fuel-saving potential when it was developed 19 years ago, the concept never quite got off the ground

In 1988 the launch of the unducted fan (UDF) was virtually a given. At the Farnborough air show, while the GE36 demonstrator engine buzzed overhead on its McDonnell Douglas MD-81 ultra-high bypass (UHB) demo testbed, GE's general manager of commercial operations at the time, Ron Welsch, told Flight International: "It's not a case of if it's a case of when."

GE chief engineer Brian Rowe was just as bullish. "There are a lot of interested people out there. We intend to sell it, and we intend the programme to go ahead in the next couple of years."

ge36 demonstrator engine    
The GE36 demonstrator engine set the Farnborough air show buzzing in 1988 - and then disappeared

The UDF/UHB demonstrations in the late 1980s traced their roots to the OPEC oil embargo of 1973 and the rude awakening it gave the USA to its vulnerability to internationally controlled oil supplies. The increased price of oil affected the airline industry in particular. At the time of the crisis the USA imported 6 million barrels of oil a day, 64% of which came from OPEC.

Although US airlines began their own fuel-saving measures, reducing consumption by over a billion gallons a year, any advantages were wiped out as jet fuel prices jumped from 12¢ to more than $1 per gallon. The result was that total yearly fuel costs increased by $1 billion, or triple the earnings of the airlines. Before 1972, fuel accounted for 25% of carriers' total direct operating costs, but during the crisis it shot up to more than 50%. Today it makes up almost 40% of direct operating costs.

Political pressure turned on NASA which, in February 1975, formed the Intercenter Aircraft Fuel Conservation Technology Task Force to explore potential options. Out of this came a set of aeronautics projects including the Energy Efficient Engine (E3), which would generate advanced technology that would feature in the GE90, and the advanced turboprop (ATP). NASA believed an ATP could reduce fuel consumption by up to 30% over existing turbofan engines with comparable performance at speeds up to Mach 0.8 and altitudes up to 30,000ft (9,150m).

Despite the fuel saving predictions, public and industry perception of propellers and concerns over the technical challenges delayed the go-ahead of the ATP until 1978. NASA worked with Allison and Pratt & Whitney on a joint concept, and awarded Hamilton Standard advanced blade development contracts. In 1981 Hamilton began to design a large-scale, single-rotating, composite blade set.

In the meantime, unknown to NASA, GE was developing its own design: a gearless, counter-rotating pusher system. As GE's development was based on commercial considerations, most of its decision-making hinged on the publicity value of shifting the image away from propellers to propfans.

NASA was shown GE's UDF design in 1983 as preparations were under way to begin large-scale demonstrations of the NASA industry team ATP concept. This combined Hamilton Standard's SR-7A propfan with an Allison turboshaft and gearbox, and was tested in 1986. The complete engine, with an eight-bladed unit, flew on a modified Gulfstream II in 1987.

The P&W/Allison propfan team was also established in 1986. It spun off many of the technical and design elements from the joint NASA-industry studies. It combined an Allison 571 power section (based on an industrial and marine gas generator), a new low-pressure compressor, a new gear system and propfan module derived from the ATP work, a new nacelle and a full authority digital electronic control system derived from the PW2037.


The resulting 578-DX demonstrator included two rows of 2.95m (11.6ft) diameter blades: six in the front row (counter-clockwise rotation viewed from the rear) and six in the aft row (clockwise).

GE meanwhile flew its demonstrator in 1986 on a 727 testbed, and actively pursued potential applications on future Boeing and McDonnell Douglas projects, including the 7J7 and the MD-90 - both aft-engine configurations and the only suitable new models to which the UDF could be fitted. The demonstrator engine, based on a F404 gas generator, was also mounted on the same MD-81 that would later fly the P&W/Allison engine, and flown in mid-1987. By now backed with NASA Lewis participation, the flight tests revealed important results, including key issues such as acoustic signature and performance.

GE declared the demonstration showed that "without an acoustically attenuated duct around the fan, the community noise levels and internal cabin noise levels of such an unducted fan configuration would be acceptable and certifiable".

Performance was shown to be in the "high propulsive efficiency levels of the 1990s" and in a production version such as the planned GE36-C25 envisaged for a planned MD-92 would have been in the 0.5 (total fuel/payload lb) range versus around 0.8 for the CFM56-powered Airbus A320 and 1.3 for the JT8D-217 powered MD-80.

The P&W/Allison 578-DX tests, meanwhile, began "flawlessly" in April 1989, though by now the world had changed, and with it the prospects for propfans. Fuel prices were coming down and without the imperative of higher operating costs, neither the will nor the reasons existed for propfans to push forward.

The signs were there the year before, when Welsch admitted that at 65¢ per gallon, the fuel price was too low to justify the UDF. "If fuel were at a buck or so a gallon, they'd be clamouring," he said.

P&W geared turbofan 
© Pratt & Whitney   
P&W is adamant that the geared turbofan will lead the next generation engine race

No business came in for either offering and, even as the P&W/Allison demonstrator prepared for take-off in the Mojave desert, the decisions that would kill any chance for a commercial production propfan (in the West at least) in the 20th century were being taken down at the coast in Long Beach.

In 1989 McDonnell Douglas admitted it was studying the International Aero Engines V2500 in place of a UHB option for the MD-90. The decision was "an either/or situation," and reflected concerns the airlines were showing "about the technology risks with the UHB", said the company at the time.

Airlines certainly appeared reluctant to gamble on the fuel efficiencies while oil prices remained low and McDonnell Douglas sounded the death knell for the UHB later that year when it confirmed selection of the V2500. Parallel efforts to launch a GE UDF-powered T-tail airliner project named the MPC-75 between Germany and China also failed around that time, leaving the various propfan concepts with nowhere to go except museums.

"If fuel were at a buck or so a gallon, they'd be clamouring." Ron Welsch, GE's former manager of commercial ops

Related links:

Flight's engine directory
Green sky thinging - engine concepts for the future

Source: Flight International