It's a noisy world out there, especially around the nation's busiest airports. NASA is looking at new technologies and aircraft designs that could help quiet things down a bit.
Every source of aircraft noise from takeoff to touchdown is being studied for ways to reduce the racket. Without innovations such as lower fan speeds, improved acoustic treatment surrounding engines, and chevrons to improve exhaust jet mixing, noise annoyances will only get worse as air traffic doubles in the next decade or so.
"It's too noisy. You have to always work on making it quieter," said Edmane Envia, an aerospace engineer at NASA's Glenn Research Center in Cleveland. "You have to stay a step ahead to fulfill the needs and demands of the next generation of air travel."
Noise reduction research is part of a broader effort by NASA's Aeronautics Research Mission Directorate in Washington to lay a technological foundation for a new generation of airplanes that are not as noisy, fly farther on less fuel, and may operate out of smaller airports with shorter-than-average runways.
NASA and its industry partners have met the same tough challenges in the past, enabling the introduction of quieter and more efficient airliners. Now the bar is set higher, but researchers are confident they can clear the hurdle.
"Our goal is to have the technologies researched and ready, but ultimately it's the aircraft industry, driven by the market, that makes the decision when to introduce a particular generation of aircraft," Envia said.
NASA's research focuses on future aircraft design concepts that could be introduced 10, 20 or 30 years from now. The first "next" generation—10 years from now—is referred to as "N+1." The second generation is "N+2." The third and farthest out generation is "N+3." Each design generation offers quieting features more sophisticated than those of the generation before.
"Think of the Boeing 787 Dreamliner as N and the N+1 as the next generation aircraft after that," Envia said.
An N+1 aircraft actually would look quite familiar with a conventional tube-shaped body, wings and a tail. But its landing gear could be more streamlined; its flaps and slats might not have sharp edges that create noise; its jet engines might push slower, cooler air around the hot engine exhaust; and its big fan blades might turn at slower speeds—all of which will contribute to lower noise.
An N+2 aircraft could look quite different. It could appear to be more of a seamless hybrid shape where the wing blends into the body and the engines are mounted on top of this "blended wing."
NASA's design goal is to make each new aircraft generation quieter than today's airplanes by a set number of decibels, which is a measure of loudness. The noise level of a busy urban street is approximately 70 decibels. The modern Boeing 737 creates 85 to 100 decibels of noise, depending on whether it is taking off, in flight, or landing. The N+1 goal is 32 decibels quieter than a fully noise compliant Boeing 737, while the N+2 goal is 42 decibels quieter than a Boeing 777.
Achieving the decibel goal for the N+1 aircraft has been elusive.
"What makes our job very hard is that we are asked to reduce the noise but limit as much as possible any negative impacts on the overall performance characteristics of the airplane, including fuel burn, aircraft weight and range," Envia said.
Options for reducing noise that NASA researchers have studied include changes in airframe, landing gear, wing flaps and slats as well as the jet engine internal components—anything that disturbs the smooth flow of air and creates noise.
Slight changes in geometry of the airframe, construction of specific parts from new materials, and adjustments to the operation of the jet engines all can bring the noise levels down, but the experts still come up an average of six decibels short of the N+1 goal.
Envia said that additional work with the core of the jet engines where many parts rotate at high speeds, and the high-pressure hot combustor where fuel is burned, may make up the difference, but a lot more work needs to be done.
Meanwhile, reaching the N+2 goals may prove easier to achieve.
"We're starting from a different aircraft configuration, from a clean sheet, that gives you the promise of achieving even more aggressive goals," said Russell Thomas, an aerospace engineer at NASA's Langley Research Center in Hampton, Va.
"But it also means that a lot of your prior experience is not directly applicable, so the problem gets a lot harder from that point of view," Thomas said. "You may have to investigate new areas that have not been researched heavily in the past."
Efforts to reduce noise in the N+2 aircraft have focused on the airframe design, which blends the wing and fuselage together, greatly reducing the number of parts that extend into the airflow to cause noise.
Thomas added that the early thinking on the N+2 aircraft is that the jet engines will be on top of the vehicle, using the airplane body to shield most of the noise from reaching the ground.
"We're on course to do much more thorough research to get higher quality numbers, better experiments and better prediction methods so we can really understand the acoustics of this new aircraft configuration," Thomas said.
What an N+3 aircraft will look like and how it will be even quieter is anyone's guess. NASA is sponsoring research that will produce a host of ideas for consideration.
If NASA is successful over the next two decades in achieving its most aggressive goals, then a day may come—maybe as soon as 2035—when a commercial airplane taking off from or landing at a busy airport cannot be heard above normal background urban noise outside the airport boundary. That is when the dream of “silent” aircraft will have been realized.
NASA Aeronautics Research Mission Directorate