"Fly Silent, Fly Cheap," Feature Article, February 2007

fly silent,
fly cheap

When a team of engineers set out to design the world's quietest airliner, they discovered that a silent airplane would be an efficient one.

by Jeffrey Winters, Associate Editor

some design problems are so obvious, they hit you right between the eyes. But in 2003, when engineers at the Massachusetts Institute of Technology in Cambridge and the University of Cambridge in England were looking for a project they could collaborate on, they found one that lands squarely between the ears.

"Airplane noise has been a serious limiting factor to the growth of aviation," said Ed Greitzer, a professor of mechanical and aeronautical engineering at MIT. "The noise level has been dropping, but it's been harder and harder to make improvements. We wanted to see what would happen if we took reducing noise as the primary design goal. What would the airplane look like? And what noise level could we reach?"

Three years and dozens of design iterations later, the engineers and researchers unveiled a design for an otherworldly looking aircraft that could glide in for a landing while creating a small fraction of the racket of conventional airliners. The design concept, code-named the SAX-40, calls for a wide fuselage that provides lift, reducing the speed necessary to keep the plane aloft as it approaches the landing strip. The slower the speed, the less noise that's generated.

But another, unintended benefit of the design is fuel efficiency. The SAX-40 would carry a passenger 20 percent farther on a gallon of jet fuel than the Boeing 747.

Noise may not seem to be a major aviation issue, but for those who live near an airport, it is an enormous quality-of-life issue. That's one reason why cities are building new airports on the fringes of their metropolitan areas. But while facilities such as Denver International Airport and Austin-Bergstrom International Airport in Texas may have fewer problems with their neighbors than Chicago's Midway or New York's LaGuardia, they have become so far removed from the central business district that they are less useful at serving the existing community.

To reduce the noise generated at takeoff and landing, engineers developing the SAX-40 designed a radical new airplane shape.

A common jet engine creates around 150 decibels at a distance of 100 feet. For planes taking off and landing, this means that noise on the order of 100 decibels or more escapes the confines of the airport. This noise level—equivalent to a chainsaw at 10 feet—can drown out virtually all other sounds. The sports complex near the approach into New York's LaGuardia Airport can be so affected that one mayor actually ordered planes to be diverted in order not to disturb the tennis matches during the U.S. Open.

The current noise level is a marked improvement over that of decades past. Jet engines have become quieter since the 1980s, thanks to advancements in turbine technology and a push from the federal government, which passed the Airport Noise and Capacity Act in 1990, restricting airplane noise that can be heard in the general community.

One of the first design aspects the team looked at was the shape of the craft. The airliner has had the same basic configuration for decades: a long, cylindrical fuselage supporting thin wings and a tail. But that familiar design has some drawbacks from the noise abatement standpoint. For one, the wings provide so little lift at low speeds that planes must keep their engines revved even when approaching the runway.

The solution that the Cambridge-MIT team hit upon was to increase the overall lift by widening and flattening the fuselage, turning the entire aircraft into a lifting body. In this way, less power would be needed to keep the plane in the air as it approaches the runway. In turn, that means the jet turbines need not work as hard and would thus run quieter. Also, since airplane noise scales to velocity raised to a factor of five or greater, every mile per hour reduction in approach speed means less noise being thrown off by the aircraft.

In addition to its squat appearance, the SAX-40 lacks the customary tail. Instead, lateral stability would be maintained via small winglets at the tip of each wing. "This makes it quieter, since there's less exposed structure," said MIT mechanical engineer Zoltan Spa- kovszky, who worked on the project.

The power plant was also reconfigured. The engines of the SAX-40 are tucked into the fuselage rather than hanging from the wings or tail section as is found in modern airliners. While that has an obvious advantage—the cowling helps muffle sound from the engines—it also adds complexity.

"We wanted to take advantage of the shielding effect of the aircraft," Greitzer said. "But there are challenges of integrating the aircraft and the engine."

Spakovszky added, "If you have an engine hanging off the wing, the air that is coming to it is fairly uniform, but when it's integrated into the body, the engine sees the air that's close to the body. That air isn't of uniform velocity, so it creates a forcing on the fan blades. That's a really hard design problem."

In the Details

Interestingly, there are lots of small details in the SAX-40 design that seem minor, but have an important effect in reducing noise. For example, a small fairing would be placed around the landing gear to cut turbulence created as the wheels drag through the air. Also, the team dispensed with the deployable drag-generating devices used to slow planes on approach, opting instead for a combination of elevons and thrust vectoring. That adds a great deal of complexity to the system to be sure, but the noise generated by the drag flaps is tremendous.

In all, the design changes would amount to a startling reduction in the amount of sound produced on takeoff and landing. The Cambridge-MIT team estimates that a landing SAX-40 would create less than 65 decibels of noise at the perimeter of the airport—about the same level as background noise. Transformed in this way, airplanes would be relatively neutral parts of the urban environment, rather than nuisances.

But there's a side benefit: fuel efficiency. The same design elements that cut back on noise also reduce fuel consumption. With less energy lost to creating turbulence, more power would be devoted to moving passengers.

"We started with the goal to design an aircraft that was quiet," Spakovszky said. "We chose an unconventional configuration that helps shield the noise. But what we didn't know in advance is that if we optimize the shape, we could also improve the fuel burn."

The difference in fuel efficiency is remarkable. Compared to a Boeing 777, which can fly about 100 passenger miles per gallon, the SAX-40 is calculated to fly about 120 passenger miles per gallon. That makes the silent aircraft something of an airborne Prius.

But don't book any flights just yet. The Silent Aircraft Initiative was just a design project, and there are still many outstanding questions about whether such a plane would be practical. "Before you think about launching such a craft, you really need to go through it and be very sure it's going to work," Greitzer said.

Even so, the SAX-40 ought to give the designers at Boeing and Airbus something to think about when they start drawing up the airliners for the middle part of the 21st century.