NYTimes: A Cleaner, Leaner Jet Age Has Arrived

四月 9, 2008

Published: April 9, 2008

JET engines are now so reliable that a pilot can go an entire career without seeing one fail. Autopilots are so good that some airlines have set up their cockpits to emit a loud beep every few minutes, to make sure the crew is still awake. And navigation is so accurate that landings can be timed to the second.

So what’s left to worry about in aviation?

In a word, fuel.

Jet fuel is now the largest expense for most airlines, and for American carriers each penny increase in price per gallon costs nearly $200 million a year. The industry is also becoming increasingly nervous about what happens when that fuel is burned. Aviation is responsible for about 2 percent of global emissions of greenhouse gases, and that share will rise as air travel continues to grow.

So the industry is scrambling to build greener airplanes — to save weight and improve engine efficiency, with an eye toward reducing operating costs and emissions.

In the short term, a revolution in jet engines is about to occur, with radically different designs that use gears to cut fuel consumption, noise and pollutants. And those new engines will power planes built more and more with carbon composite materials, which are lighter and may also be safer than the aluminum they replace.

In the longer term, the fuel itself may change; scientists are looking for an aviation version of ethanol, something that can be made from plants rather than petroleum.

The newest aircraft will also swap out many of the conventional hydraulic systems that control flaps, slats and other parts and replace them with electric motors, saving weight. In addition, new aircraft will use motors to pressurize the cabin for the same reason.

The geared jet engine will enter commercial airline service around 2013, if all goes well. Here’s how it works:

In a conventional engine, a big fan sucks air into a combustion chamber, where it is compressed, mixed with fuel and ignited. The expanding gas then blows out the back of the engine, producing thrust that pushes the plane forward.

But before the gas exits, it turns a second set of blades, which is mechanically connected to the fan in the front, to make it turn.

The problem, though, is that in current designs the two turn at the same number of revolutions per minute. But the fan in front is bigger, so the blade tips move so fast they are noisy and inefficient.

Pratt & Whitney, the engine company based in East Hartford, Conn., is testing an engine with gears arranged so that the fan in front turns at just one-third the speed of the blades in back. The innovation allows the use of a very big fan in the front, which will move more air at a slightly slower speed. “You can either make thrust by moving more air, or taking air and putting more fuel in it, to accelerate the air,” said Robert J. Saia, vice president of Pratt for new engine models. The geared turbofan does the former, he said. In addition, he said, with air moving more slowly, fewer internal parts are needed, and an engine that will produce 15,000 pounds of thrust will weigh about 500 pounds less than a conventional engine of the same power.

Mitsubishi has selected the engine for its new regional jet, and All Nippon Airways has agreed to buy 15 of them. Bombardier, in Montreal, has listed the geared turbofan as a choice for a new line of regional jets it will build.

Those new regional jets and other new planes will also use more composites. Boeing’s new 787, expected to enter service next year, will be 50 percent composites, by weight, compared with just 12 percent in its last new airplane, the 777, which was certified in 1995.

Composites have been around for years, and for aircraft are usually handmade. At the factory, the process starts with sheets of carbon fibers. In conventional manufacture, workers use shears to cut the sheets to the desired shape, layering them in a mold. Then the mold is put in an oven, and a resin, impregnated in the fibers, binds them together.

Boeing looked at that process and decided against it. “We knew that from a cost standpoint, if you had to lay down 100,000 pounds of composite material by hand, it would be very expensive,” said Tom Cogan, chief project engineer of the 787. But it has found a way to spread the carbon fiber on frames with a machine.

Carbon fiber has a weight advantage of about 20 percent over aluminum, although Boeing gave back a little bit of the gain by putting in bigger windows. The stronger composite frame of the plane makes the change possible.

Using composites to improve passenger comfort is a trend in aviation. Companies building private jets have switched to composites but used the weight advantage to build a bigger plane, rather than a plane of the same size with a lighter weight.

Corrosion is a problem on aluminum airplanes, because they pick up moist air on the ground, and the water condenses on the metal as the plane cools in the upper atmosphere. But composites do not rust, and they are not subject to metal fatigue, although they are subject to other deterioration. Still, Boeing expects planes made with more composites to have lower maintenance costs, as well as lower weight.

The 787 has another innovation that is becoming common in new designs: electric motors instead of hydraulic pumps. At Honeywell, a major manufacturer of aircraft parts, Robert H. Smith, vice president of advanced technology, described systems that will run on electricity as opposed to hydraulic fluid, as is the case today.

Among examples he cited are the brakes on a 787 and the thrust reversers on the Airbus A380. The thrust reversers are mechanical parts that slide into position to reverse the direction of the jet engine’s blast, used by pilots when they are landing. Some pneumatic systems are also being replaced; for example, the system that keeps the wings free of ice on conventional jets does so by squeezing hot air, borrowed from the jet engine, through holes in the wing. But on the 787, that work will be done by electricity. The cabin atmosphere will be pressurized by an electric pump.

Electricity is “safer, lighter and greener,” Mr. Smith said. Hydraulic fluid is flammable, and the lines that carry it are big and heavy, among other disadvantages. He added, “The technology curve is such that electric systems fundamentally have a better efficiency associated with them.” In other words, all the energy must eventually come from a jet engine, but using the engine to make electricity to apply the brakes or move a flight-control service requires less fuel than using the engine to compress air or fluid to do the same job.

The 777, a mid-1990s design, can generate up to 270 kilowatts of electricity, enough to run a small neighborhood of houses. The 787, he said, would make five times as much, 1.35 megawatts.

The long-term change, though, may come in the fuel itself. For example, the Institute for Air Science at Baylor University ran a 60-hour test of a turboprop airplane, a twin-engine King Air, with one engine flying on a mixture of 80 percent jet fuel and 20 percent biodiesel, and the other on straight jet fuel. Whenever the plane landed, it was obvious which engine was using the mixture because it had much less soot on its cowl, said Grazia Zanin, the director of renewable aviation fuels development at Baylor.

Biodiesel can be made from soybeans or other biological sources, with much less carbon dioxide output per gallon. But “we don’t think biodiesel is the answer,” she said, partly because it does not function well at the extremely cold temperatures that jets must endure at high altitudes.

Her institute has had better luck flying piston-driven planes on ethanol. That can save money on fuel and reduces the need for engine overhauls, she said. But relatively little flying is done in piston-driven planes.

The industry will have to find some solution, Ms. Zanin said, because as carbon output becomes a major concern, “commercial aviation as we know it today is not going to survive.”


3 Responses to “NYTimes: A Cleaner, Leaner Jet Age Has Arrived”

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