Airline of the future

Dec. 13, 2006

In spite of all the talk and alarm about greenhouse gasses and global warming, there’s almost no mention of the impact of high flying jet aircraft on the atmosphere. As we saw so vividly during the 9-11 hijackings, at any given moment there are hundreds, even thousands of planes in the air over the United States. Their exhausts are spewed out at high altitudes where they do the most damage to the ozone layer. If those emissions could be substantially reduced, it would make a big difference.
Making that reduction demands a wholly new type of aircraft. Such a plane, the SAX-40 (Silent Aircraft eXperimental), is being designed at the Cambridge-MIT institute. It resembles the tailless space ship doodles I once drew as a kid in high school. Yet, in spite of its innovations, the SAX-40 may never be built. To understand why, it’s helpful to examine what’s currently being flown and how we got to where we are today.
The first commercial jetliner was the DeHavilland Comet. Unlike today’s planes the Comet had the engines integrated into the wings. Some readers may remember it. The Comet went into service in 1952 and was a transportation breakthrough until one crashed mysteriously into the Mediterranean. When a second Comet went down over the Irish sea, a huge effort was made to scrape up all the pieces off the sea bottom. A fleet of trawlers was hired and the shocking result was the realization that the Comet, whose air frame depended on the strength of the aluminum skin, had simply disintegrated.
James Stewart played the role of an engineer in the 1951 movie, “No Highway in the Sky,” that may have predicted the failure of the Comet. In the film, it is high altitude chill and metal fatigue that causes a plane crash.
For good reason, jet aircraft have been made for years on the same basic model: an aluminum tube for the body with engines slung under the swept back wings so they can be serviced easily and upgraded when necessary. The use of standard parts makes it possible to lengthen the useful life of an airliner through various modifications. That’s why planes you see at airports basically look the same.
Currently, the next generation of passenger jets is Boeing’s 787 Dreamliner and the Airbus super jumbo monster. It won’t be ready for two more years. Airbus was the maker of the supersonic Concorde which never made a profit, but provided prestige until a fatal accident on the Paris runway took it out of service. The giant airbus doesn’t look any different from current jets, only bigger. The Dreamliner is also a logical improvement on existing technology. These are tweaking the basic design with no innovative breakthroughs in concept.
The proposed SAX-40 is entirely different. The purpose of the design is to reduce noise and improve fuel efficiency. If built as conceived, the SAX-40 will make almost no perceptible noise. What a blessing for people who live near airports.
The noise of current aircraft comes from turbulence over the landing gear, the passage of air over flaps, the leading edge of the wing, engine noise, and the turbulence of flowing air over the surfaces to the stationary air behind the plane. The roar of the typical low-slung jet engines is reduced in the SAX-40 by placing the engines on top of the wing so the sound is directed upward, not down toward the ground.
Besides noise, turbulence causes drag, which reduces fuel efficiency. That’s why, to save gas, you keep the top of your convertible up when tearing down the highway at 70 miles an hour.
The result of the redesign is a plane that resembles a manta ray without the stinger. The SAX-40 doesn’t have a conventional tail. A flexible cover reduces the noise of the leading edge wing slots used to improve lift during landings and takeoffs. The body and the wings are blended together, with the curved body providing some of the lift. This isn’t an entirely new idea, for the old Lockheed Constellation also had a curved body to gain some lift.
The proposed engines are different, too. Instead of one huge, noisy fan, the jet engines -- integrated into the air intakes -- have three smaller fans whose noise is more easily muffled. The exhaust is also controlled with a variable exhaust nozzle. The thrust can be directed to provide maximum lift during takeoff and adjusted for level flight.
To reduce the noise of air passing over the lowered landing gear the engineers propose fairings to cover the wheels and brakes, though these have the disadvantage of making it more difficult to cool the brakes.
Will the SAX-40 ever be built? Building a new aircraft is a huge investment, and no one can anticipate what modifications need to be made during a plane’s 30-year life expectancy. Unlike today’s conventional designs, it’s not so easy to extrapolate the SAX-40 to double the passenger capacity or change to more powerful engines when they are integrated in the wings. When Boeing made its first 747, the design was so radical that the plane wasn’t immediately accepted and thousands of workers had to be laid off. Since then, it has undergone many modifications, but still looks the same.
Technological change on the scale of the SAX-40 is a gamble. No one wants a repeat of the Comet failure: a fleet of trawlers scooping up the pieces to find out what happened. It may take a greenhouse penalty for fuel guzzlers to force the production of the economical SAX-40. Production would be in 2030 at the earliest.

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