Something New for Airliners: Propellers : High-Tech Design May Match Jets’ Speed, Slash Use of Fuel by 50%
It is July 18, 1992. The first PSA flight of the day to Los Angeles pulls away from a gate at San Diego’s Lindbergh Field. With propellers whirring, the state-of-the-art airliner races down the runway and climbs into the sky.
U.S. airlines took a seemingly backward step toward propeller-driven airliners this month at a Rohr Industries facility here. There, the National Aeronautics and Space Administration tested a futuristic, high-technology propeller that is scheduled to help power a business jet in a test flight over Georgia next year.
Propeller-driven aircraft were chased from the skies during the 1960s by larger and more comfortable jets that flew faster and higher. But proponents say the next generation of propeller-driven airliners will duplicate jet airliners’ comfort and speed--well over 500 miles per hour--yet cut fuel consumption by as much as 50%.
If in-flight tests planned during the next year are successful--and if fuel costs rise again--propeller-driven commercial airliners manufactured in the United States could return to the skies by as early as 1992.
Manufacturers are moving toward that goal. This fall, Boeing and General Electric plan to flight test a propfan by replacing one of a Boeing 727’s three jet engines with an advanced-technology, turbine-powered propeller engine.
And early next year, McDonnell Douglas will use a specially equipped MD-80 airplane to test both the ungeared GE propfan and a geared propfan being developed by the Allison Gas Turbine division of General Motors and Pratt & Whitney.
Although airliners will generally continue to look much the same as they do now, the propellers of tomorrow will not look like those of yesterday. Underscoring the differences of the new propulsion system, however, McDonnell Douglas prefers to refer to propfan engines as having “propulsors” instead of propellers.
The “ultra-high bypass engine” that Boeing and General Electric will test features two sets of swept-back, eight-bladed propellers. The two sets of blades are designed to rotate in opposite directions in order to “gain high efficiencies” that elude normally shaped blades used on smaller commuter and business planes, a Boeing spokesman said.
Even though safety dictates that test aircraft retain at least one Federal Aviation Agency-certified jet engine, proponents are confident that in-flight testing will prove that propeller-driven aircraft will once again play a major role in commercial aviation.
However, although airframe, engine and blade manufacturers already have spent hundreds of millions of dollars to commercialize the new propeller technology, industry analysts predict that propellers will not start spinning commercially in 1992 unless fuel prices rise or airline executives start snapping up the fuel-efficient A320 model jet airliner that Airbus plans to introduce in 1989.
If those market conditions exist, McDonnell Douglas and Boeing will “meet their dates or at least come close to them,” suggested Paul Nisbet, an industry analyst with Prudential-Bache Securities. “If all (factors) point toward an introduction, they could get them in production for delivery in 1992.”
Engineers Not Surprised
However, “I’d be inclined to think that it will slip a couple or three years,” Nisbet said. “Certainly, though, at some point the technology will come into play.”
Aerospace engineers, who have long known that advanced-design propellers could efficiently harness the power of jet engines, are not surprised that propellers will once again power commercial airliners.
Although small, commuter-type aircraft are powered by turbines that drive propellers--the NASA propfan tested by Rohr here was turned by a modified helicopter turbine engine--past propeller designs have not been able to move larger planes through the air fast enough to compete with jets.
The advanced research needed to do that was stalled because aerospace industry technology “just wasn’t advanced enough,” according to G. Keith Sievers, chief of NASA’s Advanced Turboprop Project Office at Lewis Research Center in Cleveland. “Even the best of the propeller-driven (commercial) planes flew at less than (about) 25,000 feet and didn’t move much above 0.6 Mach”--less than 450 m.p.h. at sea level.
Propeller research benefited from advances in aerodynamics, acoustics and structural design, but Sievers gives modern high-speed computers most of the credit for making breakthroughs possible.
“As they move, the (new-generation) propeller tips twist, turn and move around,” he said. “One of the tricks in this business is figuring out their hot running shape (as opposed to) their static cold shape. And that takes a tremendous amount of computing capability.”
It has proven to take a tremendous amount of money to design and build the new generation of propeller-driven aircraft. Even with likely future budget cuts, NASA alone will have spent more than $200 million on propeller research, and industry sources suggest that airframe and engine manufacturers will spend between $3 billion and $10 billion to develop a new, high-technology, propeller-driven plane.
But airline industry executives, their interest piqued by talk of 50% fuel reductions, are showing obvious interest: “These guys would kill their grandmothers for a 3% fuel reduction, so you can imagine what they’ll do for a 50% cut,” quipped a spokesman for one airframe manufacturer.
50% Less Fuel
In contrast to jet engines, which “get a plane from point A to point B by moving a small mass of air (through a turbine) at a high speed, our (propeller-driven plane) will move a large mass of air at a relatively slow speed,” McDonnell spokesman Bill Neal said. “It will do it by using as much as 50% less fuel than today’s jets. And even in today’s environment, fuel is still 20% of an airline’s total operating costs, and it’s likely to be higher.”
That kind of potential fuel saving has already attracted the attention of airlines that own McDonnell Douglas MD-80s and those that plan to buy the newer MD-88, which will be introduced next year. McDonnell Douglas engineers have been studying designs of both planes and believe that it will be possible to retrofit both with propfan engines.
“We’ll be watching (McDonnell Douglas’ tests) closely,” said Byron Miller, vice president of fleet operations for San Diego-based Pacific Southwest Airlines, which operates 29 MD-80s. “Obviously, with the kind of decreases in fuel consumption that propfans offer, a commercial airline has to be very interested in (the technology).”
Delta Air Lines, which will take delivery of 30 MD-88s (with an option for 50 more planes) starting next year, bypassed competing models because “McDonnell Douglas’ engineering folks told us that there’s a good possibility that the ‘fan’ could be put on the aircraft at a later date,” airline spokesman Bill Berry said. “(That) was the added carrot that made us go for the MD-88.”
While neither Boeing nor McDonnell Douglas has offered advanced propfan models for sale, both are studying airplane configurations that would replace existing jet-driven models. McDonnell Douglas has focused on a companion for its MD-80s and MD-88s, and Boeing is considering a twin-engine plane that would augment its popular 727 tri-jet.
However, “competitive factors” might prompt McDonnell Douglas to introduce its advanced propfan even if fuel prices remain stable and airline interest in the Airbus A320 remains lukewarm, analyst Nisbet said. “They’ve got the aircraft to put it on, and they can move on it with less cost than Boeing.”
More Tests May Be Needed
In addition to development costs, though, there is concern that the technology might not be fully proven. Although NASA’s advanced turboprop research program, which faces budget cuts beyond those instituted last year, has produced a “very success-oriented program. . . . We’re not quite there yet,” Sievers said. “And in this business, if you drop dead at the 90-meter mark of a 100-meter race, the record just doesn’t get set.”
But, in addition to unresolved concerns about noise and safety factors and some unproven technology, Nisbet pointed to “just plain market acceptance. . . . Are people going to want to go back to prop-driven planes? That’s more of a psychological than a technical question.”
