Quieter Aircraft to Take Cues From Birds, NASA Expert Says

John Roach
for National Geographic News
August 22, 2006
Bird-inspired technologies may be the key to dampening aircraft noise around airports, according to a NASA scientist.

"We are learning to make aircraft more like Mother Nature," said Dennis Huff, chief of the acoustics branch at the NASA Glenn Research Center in Cleveland, Ohio.

For example, Huff's team developed the chevron nozzle, a serrated nozzle that fits on the backside of jet engines.

The design is modeled after the sawtooth pattern of the feathers on the wings of some birds, including owls (read "Owl's Silent Flight May Inspire Quiet Aircraft Tech" [December 2004]).

The jagged edge mixes the airflow coming out of the jet engine in a way that reduces turbulence. Less turbulence translates to less noise, Huff explains.

Current chevron technology reduces noise by about three decibels, Huff adds.

"An analogy I like to use is two lawn mowers running at the same noise level and [then] turning one off. That would be perceived as a three-decibel reduction," he said.

Little Compromise

Terrance Scott is a spokesperson for Boeing Company, an airplane manufacturer based in Seattle, Washington.

Scott says that engineering quiet technology for aircraft requires a host of considerations beyond noise.

"First and foremost, safety is the key consideration," he said in an email.

In addition, engineers must look at how any solution affects emissions, flight performance, maintenance, and cost.

Finding successful noise reduction solutions also requires engineers to consider all the sound sources on an aircraft from takeoff to landing and then tweak several of them at once.

(Explore 3-D models of past and present aircraft with this online feature.)

Huff likens the task to tuning down each instrument in an orchestra to get an overall noise reduction, not asking just the violinist to play more quietly.

In the case of the chevron nozzle, the researchers considered the jet engine itself and what happens to the air when it exits the engine.

"The trick is to do something to the engine to change the flow field downstream of the engine in a way that reduces jet noise," he said.

According to Huff, the chevron nozzle is a winning technology, because it significantly reduces noise while reducing jet performance by less than 0.25 percent.

Once the chevron nozzle concept was proven in the lab, NASA researchers shared their results with the airline industry.

From there, Huff says, several manufacturers funded the development of nozzles for their engines.

"We ended up with a couple of those; those are the real winners," he said. "For each of those, we have a dozen that did impact performance or cost, and they came off the drawing board."

Future Technologies

Scott, the Boeing spokesperson, says that in addition to trying the chevrons, the manufacturing firm is currently working with engine companies to further reduce fan and jet noise.

The company is also exploring ways to reduce noise created as air flows over the airframe, especially around the flaps and landing gear.

Ultimately, Scott said, Boeing aims to keep all "objectionable noise" within "noise compatible" areas around airports.

"Achieving such large noise reduction will require revolutionary changes, new technologies, and a significant expenditure of capital," he said.

Huff said future aircraft will likely look more and more like birds to achieve desired noise levels.

"I see a morphing of the structure, a smart structure approach to different components," he said.

Such systems would be less rigid than today's aircraft.

Already, Huff says, Boeing is working on a "smart" chevron nozzle design that can essentially turn on and off.

That way aircraft noise can be reduced near the ground, but once airborne the nozzle changes position so the airplane can regain any loss in performance.

"That's more the way birds are constructed, and that's a good thing," Huff said.

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