Electric Arcs to Quiet Jets

Engineers look for new ways to dull the roar around airports

4 August 2004--Talk to people who live or work near an airport, and they will likely complain about the noise. Property prices in areas in the flight paths of busy airports like London's Heathrow reflect the fact that people don't like living near high-decibel noise. In recent years, civic authorities have clamped down on aircraft noise. British authorities, for example, have mandated that noise levels near Heathrow cannot increase from current levels. With more people flying every day, and more-powerful aircraft engines, this means that tomorrow's planes have to be quieter. Engineers are trying both radical airplane redesigns and innovative but less ambitious adjustments to jet engines to meet the demand.

"Noise is a prime design variable," when it comes to designing new engines and new aircraft, said Professor Edward Greitzer of the Massachusetts Institute of Technology, in Cambridge, who is an authority on engine noise. He and colleagues at MIT are collaborating with researchers Cambridge University, England, on the Silent Aircraft Initiative, an ambitious project launched last year to take a fresh look at aircraft design.

"The shape of aircraft hasn't changed in the past 40 years," said Greitzer. The goal of the project is to redesign aircraft completely to make them ultimately no louder than average background noise. What would a new aircraft look like? Greitzer says the concept design, which will be produced in two years, will probably be completely different from what we are accustomed to. In one of the geometries being considered the engines are placed on top, so that the frame of the aircraft shields the noise on takeoff.

A near-term approach is to focus on just the jet's engines, their exhaust in particular. Powerful fans and compressors suck air into the engines, causing a high-pitched whine. When the air is forced out of the engines to produce thrust, it causes turbulence as it mixes with lower-temperature, calmer air, generating a lot of noise.

"If you [want] to reduce the engine noise, you have to create a new flow structure for the exhaust," says Mohammad Samimy, a mechanical engineering professor who has been working on engine noise for more than a dozen years, first at NASA's Glenn Research Center in Cleveland, and then at Ohio State University in Columbus.

According to Samimy, NASA legend has it that about 15 years ago a NASA test engineer was pointing out features in a jet engine prototype with a screwdriver, when it accidentally strayed into the exhaust stream. The turbulence diminished and the engine noise promptly changed. Ever since, researchers have focused on altering the flow of exhaust by inserting metal tabs into the path of the exhaust. Often called chevrons, because of their inverted V shape, they cut down engine noise by 3 to 5 decibels. At takeoff, a jet engine registers at about 140 decibels at a distance of about 100 feet. (By comparison, normal conversation is about 60 decibels. Prolonged exposure to noise above 85 decibels can cause hearing loss.) The problem with chevrons, according to Samimy, is that they increase aircraft fuel consumption because of the added weight.

"You really want to use chevrons only at takeoff," he says, since that is when engine noise is most an issue. "You don't usually care about noise when the aircraft is cruising far above the ground." So once the plane is airborne, the chevrons are dead weight.

Building on an idea from Igor Adamovich, a colleague at Ohio State who is an expert on plasma science, Samimy set out to construct weightless chevrons, by making them out of ephemeral ionized gas.

He and colleagues ran an experiment where they inserted eight electrodes into an exhaust pipe in his laboratory, and sparked electricity between them when the engine was running. The sparks caused plasma--hot ionized gas--to form around the electrodes, in effect creating ridges of pressure that mimic the action of chevrons. By experimenting with the frequency of the sparks, they found that certain frequencies would greatly influence the turbulence in the exhaust plume, causing engine noise to drop significantly. Tests on actual engines are up next.

The beauty of this approach, says Samimy, is that you can turn the plasma actuators off when they are not needed. Thus, one could have the plasma actuators reducing the engine noise only during takeoff. Then once the airplane is cruising, you could turn them off, and save on power and fuel. "And this technique is not expensive," he adds.

There is another advantage to using plasma actuators: they can be adapted to local weather conditions. The turbulence produced in the aircraft exhaust depends on the temperature of the environment. Thus, if an aircraft is taking off when it is sultry outside, the exhaust turbulence will have different characteristics from when the aircraft is taking off when the temperature is below freezing.

"You can tune the plasma actuator frequency to get better performance," Samimy says, pointing out that there was no way to do this with fixed chevrons.

At MIT's Silent Aircraft Inititative, they have been following Samimy's work with interest. In fact, says a member of the initiative, Zoltan Spakovsky, MIT has hired one of Samimy's former post-doctoral researchers into the group.

"Samimy's technique is quite promising," he says, and would result in up to a 5 decibel drop in engine noise.

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