The Short, Happy Life of the Prop-fan

Meet the engine that became embroiled in round one of Boeing v. Airbus, a fight fueled by the cost of oil

Boeing’s 150-seat 7J7 concept (left) would meld prop-fan technology and lightweight composite structure to deliver big gains in fuel efficiency. (The Boeing Company)
Air & Space Magazine

LOOMING LIKE AN ALIEN CRAFT ABOVE THE GENERAL ELECTRIC STAND at the 1985 Paris Air Show was an egg-shaped engine nacelle that had, mounted on its smaller end, two rows of scimitar-like propeller blades 12 feet in diameter. The GE people called this new prop-fan engine an UnDucted Fan, and it was the most radical feature of the proposed Boeing 7J7. “2,500 Days” read the Boeing brochures that papered the company’s stand at Paris—within seven years, Boeing proposed to create around this engine a revolutionary 150-seat airliner that would burn only half as much fuel as the new and yet-to-fly Airbus A320, which was about the same size.

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But by the time a real UDF flew at an airshow in September 1988, the much-hyped project was dead, a victim of misread history and a changing economic climate. For a brief period, though, prop-fans, a new class of engines that marked a return to propeller blades, but of an advanced type, held center stage as the saviors of the airline industry.

Prop-fans originated, in part, in two wars in the Middle East. The 1967 Six-Day War resulted in Israel’s annexation of vast areas of neighboring Arab territory. The Arabs struck back, launching the 1973 Yom Kippur war, this time imposing an oil embargo on the United States, Europe, and Japan. The embargo drove fuel prices to new heights with little warning, and airlines suffered staggering losses. The stage was set for technology to come to the rescue.

In the United States, NASA’s Lewis Research Center in Cleveland, Ohio, is the center for propulsion research. Dan Mikkelson, an engineer there, knew that the secret to an ultra-efficient engine was an extreme bypass ratio, a number describing the proportion of cold air volume driven rearward by the engine’s fan to the volume of hot gases coming from the compressor-turbine core. A propeller would be more fuel-efficient than any jet, but propellers couldn’t operate at high Mach numbers, and passengers would not want to go back to eight-hour transcontinental flights. Mikkelson and Carl Rohrbach, a veteran engineer at prop maker Hamilton Standard, “thrashed ideas back and forth,” Mikkelson recalls, and worked out the broad outlines of a radical propeller that would hold its own against a jet, enabling an aircraft to cruise at up to Mach 0.8.

Out of this emerged the Advanced Turboprop Project. Announced in an October 1975 technical paper, it promised massive fuel savings over a conventional engine: 30 to 35 percent, says Mikkelson—and many people hated it. “The old guys within the airlines were deaf to it,” Mikkelson says. “They remembered the old days with piston engines, with blades falling off.” When fears arose among airline officials that the word “turboprop” would meet with consumer resistance, the term “prop-fan” was used in a poll of United Airlines passengers. It worked: 50 percent of the respondents said they’d fly on a prop-fan-powered airliner. Hamilton Standard and NASA continued to support the project, but it moved forward in slow, careful steps, and it was not until 1981 that Hamilton Standard received a contract to fly a full-size working version.

In 1980 and ’81, following the Iranian revolution and the Iran-Iraq war, fuel prices made another painful jump, one that most oil market experts thought would be more than temporary. Bob Conboy, a market analyst who joined GE from Pratt & Whitney in 1980, recalls, “We had decided that fuel was going to rise to $2 or $2.20 per gallon by the mid- to late 1980s.”

In 1981, Art Adamson, GE’s head of advanced design, formed a team backed by Brian Rowe, senior vice president in charge of GE’s aircraft engine unit, to explore more efficient engine designs. At the time, the company’s CFM56 turbofan engine was being threatened by the new V2500, developed by International Aero Engines (comprising Pratt & Whitney, Rolls-Royce, Japanese Aero Engines, and Germany’s MTU) and promoted as more efficient. “We never believed it,” Rowe recalls. But the perception was that GE’s technology was obsolete. Adamson’s team produced what Conboy calls “a really innovative design, an example of what drives the whole industry.”

Unveiled in 1983, GE’s innovative UDF took NASA by complete surprise, but it galvanized interest in the new propellers. It was bigger and more powerful than the NASA engine, but it would fly earlier, in late 1986. After Boeing’s ’85 Paris offensive, Pratt & Whitney, Hamilton Standard, and Allison teamed up to offer the 578-DX, based on the NASA research. Both teams offered propellers with two rows of blades spinning in opposite directions to reduce losses due to “swirl”—energy wasted in imparting spin to the air behind the airplane. Both would be installed on the airplane’s tail, not under the wings, to allow room for the propeller disc and to keep noise out of the cabin. “The rear row of blades has to chop through the wakes of the front row,” says Hamilton Standard’s Colman Shattuck, an engineer on Rohrbach’s team. “It’s a very good noise generator.”

The big difference between the two designs was how the propellers were driven. The core of a turbine spins at tens of thousands of revolutions per minute, and to transfer power to a propeller or fan, traditional design relied on some form of gearing. The Allison-P&W team saw no problem with driving the radical new propellers via a 13-to-1 reduction gearbox, similar to the ones they’d used for years on the venerable T56 turboprop, which powered the Air Force’s Lockheed C-130. But Rowe and GE disliked gearboxes, which were heavy and costly to maintain.

Boeing’s Alan Mulally, now president of the company’s commercial airplanes division, headed engineering on the 7J7 project from start to finish; he calls Adamson’s solution “really, really cool,” and it’s hard to disagree. The UDF blades were powered directly and gearlessly by a turbine, driven by hot gas from the engine. The two rows of propeller blades were each anchored to multiple rows of turbine blades.


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