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
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.
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.
Conventional turboprops hit a wall when the combined forward speed of the airplane and the rotational speed of the propeller tips exceeds Mach 1, resulting in shock waves. The problem was that the rotational speed of the propeller was limited to only a few hundred rpms because the blade tips could exceed Mach 1 by only a small fraction; above that, efficiency plummets. But that speed was uncomfortably low for a turbine. A conventional turbine would have to be very large, with lots of stages, and each rotating stage would be followed by a fixed “stator,” turning the flow so it hit the next turbine wheel at the right angle to convey force.
What was unique in Adamson’s design, which had been refined by engineer K.O. Johnson, was that in profile, the counter-rotating turbine stages were interlaced; the direction in which each row of blades spun was the opposite of the direction of the stages immediately upstream and downstream of it. The design had no stators, and the relative velocity between each stage was doubled. Counter-rotation effectively doubled the turbine’s rpm, so the turbine could be made smaller, simpler, and more efficient.
But think about the mechanics. The turbine blades that drove the aft propeller were attached to a solid shaft in conventional bearings. The turbine stages driving the front propeller were riding outside the aft set and could not reach a central shaft. The turbine blades were attached at the tip to an outer case, which was carried on inter-stage bearings and a ring bearing at the rear of the nacelle. This design had to allow for thermal expansion and the load imbalances that would occur if a propeller blade broke off.
It was vital that the blades be lightweight so that the engine would survive if a blade separated. The UDF would have blades made from carbon fiber composite materials.
The new engine offered enormous potential but presented equally large risks. Rowe decided to fly a full-scale demonstrator in collaboration with Boeing, whose 727 test bed would fly in 1986. “I thought it was an engine of the future,” he says, “something we ought to pursue.” NASA headquarters ordered the Lewis center to support GE’s privately funded efforts even though Lewis was developing its own engine. While GE regarded the UDF as a technology program, Boeing presented it as the engine that would power its newest airplane: the 150-seat 7J7.
The 7J7 represented a changing of the guard at Boeing, the first project to be launched by a new generation of leaders: Program chief Jim Johnson reported to a rising vice president named Phil Condit. The goal, Johnson said in early 1986, was to deliver an airplane that cost less per seat than the 737.
Veteran executives were more cautious. Mulally says today that “anyone who had worked with propellers really wanted to see the concept validated”—they wanted to be convinced that the new engine would be reliable. At Boeing, Mulally recalls, the 737 team proposed an improved, longer-range 737 that would cost far less to develop than a new airplane. But Boeing promoted the 7J7 and its UDF with an enthusiasm that rings in Mulally’s voice almost two decades later. “It was a tremendous improvement,” he says. “We could have delivered that airplane.”
Johnson and Condit sold the 7J7 concept hard. Conboy says he took part in 50 presentations in 1987. In that year, Boeing settled on a larger design that used a little more fuel but offered six-abreast, twin-aisle coach seating, banishing the hated middle seat. The airplane was too large for the Allison-P&W 578-DX engine, and Boeing settled on GE’s planned production UDF-based prop-fan, the GE36-C25.
The UDF made its first flight on August 20, 1986, aboard a Boeing 727 test bed. The tests encouraged Prop-Fan and UDF proponents, demonstrating that noise was a problem but not an insurmountable one. A February 1987 Washington Post headline read: “The aircraft engine of the future has propellers on it.”
But not everyone was convinced. At the 1985 show, Jim Johnson wanted to pitch the 7J7 to Lufthansa’s technical director, Reinhardt Abraham. His endorsement would be a huge blow to the A320; Johnson directed Rudy Hillinga, Boeing’s chief salesman in Germany, to get Abraham to the Boeing 7J7 mockup at Paris.
But when Johnson showed Abraham a chart depicting the economic advantages of the 7J7 over the A320, the Lufthansa executive turned to Hillinga and said: “Rudy, get a photographer. We’ll sign this, and I’ll buy 20 of these aircraft if you can guarantee the figures.”
Abraham knew perfectly well that Johnson was in no position to do that. What Abraham really wanted was a stretched 737, to turn up the competitive heat on Airbus.
Abraham wasn’t the only skeptic. Conboy recalls that the airlines’ reactions were “mixed from day one. We’d talk to the planning people and they’d say ‘When can we have it?’ But we never got an enthusiastic response from the operations people.”
Gordon McKinzie, United Airlines’ manager for new technology, recalls that Boeing couldn’t settle on a design for the 7J7: “One week it was a single-aisle 90-passenger airplane, the next a 180-seat twin-aisle design. We saw things as being very fluid.” The aircraft was neither as fast nor as flexible as the 757, which was, in McKinzie’s view, “a great airplane.” At best, McKinzie felt, United would “have taken on some aircraft, not a large acquisition, just to feel our way along.”
Airbus’ chief planner, Adam Brown, still believes that Boeing hyped the 7J7 in a bid to disrupt the A320 program. At the 1985 Paris show, Airbus faced the inevitable question: Was the company still confident in the A320’s future? “We can go up against the ‘magic aeroplane,’ ” Brown answered, “and we can beat it.”
And beat it Airbus did. By the time Boeing decided to launch the stretch 737-400, Lufthansa had bought the A320. United and other airlines were anxious to start retiring their vast flocks of fuel-thirsty 727s. Northwest Airlines announced the first U.S. order for A320s—up to 100 airplanes—in late 1986. United followed suit within months.
Airbus stuck to its guns, Brown says today, because its studies showed that aft-engine aircraft were heavy, and maintenance costs would be higher. “The answer depended very much on the price of fuel,” says Brown. Only if the price of fuel remained high could the savings offset the greater price and complexity of a new aircraft. “With the projections that we were most comfortable with at the time, they couldn’t beat the A320.” And despite the forecasts, oil prices peaked in 1981. After the 1985 Paris show, Saudi Arabia, tired of losing market share while trying to stabilize the market on its own, and watching as the other OPEC members cheated, turned on the spigot. Jet fuel dropped to 85 cents a gallon.
But the biggest factor may have been unexpected developments at Boeing and GE. In 1981, the 737 was Boeing’s weakest seller, and the company’s objective was to sell 500 more and then close down the line. The 737-300 was launched with quieter engines as a quick fix, a move to help Boeing achieve that modest sales goal. Instead, 737-300 sales took off. By the end of 1987, Boeing had sold more than 1,000 of them, and each one had the previously slow-selling CFM56 engines from GE and its French partner, Snecma. The follow-on, -400, would use them as well. GE began to look at the CFM56 differently.
In 1987, the rival V2500 engine for the A320, which had beaten the CFM56 in early sales, ran into technical trouble. Its biggest customer, Lufthansa, switched to the CFM56. Suddenly, the CFM56 dominated a fast-growing market. GE’s Brian Rowe could read the signs. “When the CFM56 took off, we thought, What the hell? All we’d be doing [by launching the UDF] is killing our own business.” And while engineers expected that the UDF could be made reliable by earlier standards, turbofans were getting much, much better than that. “The biggest issue with the UDF,” Mulally says now, “was to make it a simple engine and get the reliability up and the maintenance down.”
At the end of August 1987, Boeing announced that the 7J7 had been postponed a year. (And Monty Python’s dead parrot was “just resting.”)
McDonnell Douglas tried to carry on with prop-fan development. It had the rear-engine MD-80, but it was losing ground to the A320 and 737. MDC fitted a UDF engine to an MD-80 in late 1987 and wanted to launch the UDF-powered MD-91 and -92 by July 1988. The company even saw a 300-aircraft market for a Navy patrol version of the MD-91. But GE wanted to see 100 to 150 airline orders before committing to the program. Recalls Conboy, “If people aren’t going to buy it, there’s not much you can do.”
GE and MDC flew the MD-80 to the Farnborough airshow in September 1988, but the effort was already out of steam now that McDonnell Douglas was building the MD-90 with a conventional engine. “We all shook hands and said that it was a valiant effort,” says Conboy. MDC did fly the Allison-P&W engine on the MD-80 in 1989. “The operation was successful,” says Al Novick, who was part of the Allison-P&W team, “but the patient died.”
The UDF demonstrator is in the engine collection of the Smithsonian’s National Air and Space Museum, and the Allison-P&W prototype is in a company training center in Indianapolis. But nobody involved in the prop-fan or UDF writes the experience off.
GE put the UDF’s blade technology directly into the GE90, its most powerful commercial engine. “We trained a lot of good guys,” Rowe says. One was Mike Benzakein, then the company’s leading technologist. GE is still looking at counter-rotating turbines and fans, and came close to proposing such an engine for Boeing’s new 7E7.
Colman Shattuck, the program’s leader at Hamilton Standard (now Hamilton Sundstrand), says that the project was “one of the best times of my career” and that it spurred today’s all-composite propeller technology. The firm’s French subsidiary, Ratier-Figeac, makes the multi-blade propeller for the Airbus A400M military transport.
For Allison, the benefit had little to do with technology. “We got involved publicly with Boeing and Douglas,” says Novick, “and it helped us tremendously in getting back into the commercial business.” Embraer picked an Allison turbofan, the AE 3007, for its EMB-145 regional jet, and Rolls-Royce, which acquired Allison in 1995, cranks turbofans out by the boatload.
Mulally and Condit became the leaders of the next major Boeing project, the 777. It had little resemblance to the 7J7, but the underlying technologies and disciplines—computer-aided design and manufacture, and integrated electronics—were similar. Mulally quotes Boeing chairman Thornton “T.” Wilson as saying that the 7J7 “was the best investment in aircraft development that Boeing ever made” and adds that “we could not have done the 777 without the 7J7.” Condit went on to head Boeing until he resigned in late 2003, accepting responsibility for ethical and performance problems.
At the 2001 Paris Air Show, Mulally whipped the covers off another radical, rear-engine airplane, the tail-first Sonic Cruiser. In an echo of 1985, Boeing promised vast improvements over anything the competition could do. Like the 7J7, the Sonic Cruiser failed to ignite customer interest, and Boeing is pushing the 7E7, now redesignated the 787. Both aircraft offer more comfort with a lower fuel burn, Mulally says. “The cool story about the 7J7,” Mulally says today, “is that it’s exactly the same [idea] as the 7E7, but at a smaller size.”
The Airbus A320 went on to become one of the most successful airliners in current production, second only to the 737 in total unit sales. Airbus’ Adam Brown also compares the 787 to the 7J7. “The issue is whether the advanced-technology combination can give you a big enough gain in efficiency to supersede what’s already on the market,” he says.
And once again, the key to the fortunes of both companies could be the price of a barrel of
Sidebar: What’s a propfan (and what isn’t)?
The prop-fan and UDF are unique among propellers because of their speed and power loading—the amount of power driving a propeller of given diameter. The thin blades and sweepback improved efficiency at transonic speeds, just as they did on an airplane wing. The airplane was subsonic, but the prop tips hit Mach 1.1 in a helical path.
Russia’s Tu-95 bomber and its airliner derivative, the Tu-114, were designed in the 1950s and had jet-like swept wings. The turboprop-powered Tupolevs could sprint at Mach 0.78, but had to cruise at around Mach 0.7 for best range. Their 15,000-hp engines drove 18-foot counter-rotating propellers, requiring tall landing gear to keep the tips off the runways.
The Ukrainian Antonov An-70 and the yet-to-fly Airbus A400M cruise at up to Mach 0.72, about as fast as the jet-powered C-17 airlifter, but slower than commercial jets. They use large-diameter propellers, not prop-fans.
There is little interest in true high-speed propellers today. The latest conventional turbofans are more efficient than the engines of the mid-1980s, thanks to new fan aerodynamics and materials, so there is less to be gained by a move to a UDF-type engine. It’s also questionable whether the prop-fan could meet current international noise rules.