NASA Researchers Work to Reduce Noise in Future Aircraft Design

It’s a noisy world out there, especially around the Nation’s busiest air­ports, so NASA is pioneering 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, which is expected to get worse as officials predict that air traffic will double in the next decade or so.

"It’s always 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 always have to stay a step ahead to fulfill the needs and demands of the next generation of air travel.”[1366]

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 airports with much shorter runways than exist today. There are no clear solutions yet to these tough challenges, neither is there a shortage of ideas from NASA researchers who are confident positive results eventually will come.[1367]

"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 organized its research to look three generations into the future, with conceptual aircraft designs that could be introduced 10, 20, or 30 years from now. The generations are called N+1, N+2, and N+3. Each generation represents a design intended to be flown a decade or so later than the one before it and is to feature increasingly sophisticated meth­ods for delivering quieter aircraft and jet engines.[1368]

"Think of the Boeing 787 Dreamliner as N and the N+1 as the next generation aircraft after that,” Envia said.

The N+1 is an aircraft with familiar parts, including a conventional tube-shaped body, wings, and a tail. Its jet engines still are attached to the wings, as with an N aircraft, but those engines might be on top of the wings, not underneath. Conceptual N+2 designs throw out con­vention and basically begin with a blank computer screen, with design engineers blending the line between the body, wing, and engines into a more seamless, hybrid look. What an N+3 aircraft might look like is anyone’s guess right now. But with its debut still 30 years away, NASA is sponsoring research that will produce a host of ideas for consid­eration. The Federal Aviation Administration’s current guidelines for overall aircraft noise footprints constitute the design baseline for all of NASA’s N aircraft concepts. That footprint summarizes in a single number, expressed as a decibel, the noise heard on the ground as an airplane lands, takes off, and then cuts back on power for noise abate­ment. The noise footprint extends ahead and behind the aircraft and to a certain distance on either side. NASA’s design goal is to make each new aircraft generation quieter than today’s airplanes by a set number of decibels. 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. So far, the decibel goal for the N+1 aircraft has been elusive.[1369]

"What makes our job very hard is that we are asked to reduce noise but in ways that do not adversely impact how high, far or fast an air­plane is capable of flying,” Envia said.

NASA researchers have studied changes in the operation, shape, or materials from which key noise contributors are made. The known suspects include the airframe, wing flaps, and slats, along with components of the jet engine, such as the fan, turbine, and exhaust noz­zle. While some reductions in noise can be realized with some design changes in these components, the overall impact still falls short of the N+1 goal by about 6 decibels. Envia said that additional work with design and operation of the jet engine’s core may make up the difference, but that a lot more work needs to be done in the years to come. Meanwhile, reaching the N+2 goals may or may not prove easier to achieve.[1370]

"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 Langley Research Center. "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. You may have to investigate new areas that have not been researched heavily in the past.”[1371]

Efforts to reduce noise in the N+2 aircraft have focused on the air­frame, which blends the wing and fuselage together, greatly reducing the number of parts that extend into the airflow to cause noise. Also, according to Thomas, 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.

As for the N+3 aircraft, it remains too early to say how NASA researchers will use technology not yet invented to reduce noise levels to their lowest ever.

"Clearly significant progress has been made over the years and air­planes are much quieter than they were 20 years ago,” Envia said, not­ing that further reductions in noise will require whole new approaches to aircraft design. "It is a complicated problem and so it is a worthy challenge to rise up to.”