Hushing The Roar Of Air Traffic Growth

Higher, faster, and farther have long been the goals of aircraft designers; less obvious has been the quest for quiet.

In the fall, Boeing wrapped up Quiet Technology Demonstrator II (QTD2), a program that included General Electric, Goodrich, NASA, and a brand-new 777 loaned by All Nippon Airways. In the tests, conducted in Glasgow, Mont., the widebody jet flew repeatedly over a sophisticated microphone array at only 100 ft. The demonstration focused on modifications to three areas: engine inlets, engine nozzles, and landing gear. The results were so good that Boeing plans to incorporate some of these noise improvements on production 787 and 747-8 airliners.

Boeing's interest in pursuing quiet technology is driven by a desire to provide passengers with a peaceful ride -- comfort is one of the many selling points of the new 787 -- and help its airline customers deal with noise restrictions.

"We can see that requirements are getting tighter and tighter around airports as far as community noise goes, so we're being proactive in reducing the noise of our aircraft so they'll meet the future requirements," says Larry Craig, chief engineer for noise and emissions at Boeing Commercial Aircraft. "The cabin noise [reductions are driven] more by ourselves from a passenger comfort standpoint."

During the demonstration, the inlet of one of the General Electric 90 engines on the 777 was modified, increasing the area that could be treated acoustically to soak up noise.

That arrangement wiped out the buzz that permeates the cabin as the aircraft climbs. Buzzsaw noise is a byproduct of the large number of tones at the rotational frequency of the engine as the tips of the fan approach transonic speeds, creating instabilities and shock waves. It does not happen during takeoff or cruise flight, but can be annoying during the climb, says Craig.

The new inlet design will be stock on the 787 and the 747-8, but the technology can be transferred to any engine. Craig says Boeing is considering the inlets for all future development programs.

Another change to the engine had to do with adding chevron nozzles to the exhaust. This reduces noise both in the cabin and outside the airplane. The chevrons reduced peak jet noise at low frequencies and aft angles from the standpoint of community noise and yielded a 5-6-dB reduction in the rear of the cabin.

"That's quite significant," says Craig. "We had passive weight treatment in the sidewall of the 787 to combat that noise source, and we've taken it out, and it's a savings of several hundred pounds."

Craig says noise levels in the aft cabin of a typical airliner hover at around 75-80 dB, a figure Boeing wants to lower on the 787. For comparison, a normal conversation is 60 dB, while the traffic you would hear standing on the curbside of a busy street is 80 dB. The scale is logarithmic, so a few points' difference has a large effect on how we perceive noise.

In basic terms, the chevrons work by accelerating the mixing process in the jet flow, says NASA's Whitfield. By speeding up the mixing process, you slow down the average velocity of the jet exhaust, which reduces noise.

Other concepts Boeing is looking into include chevrons that actually change shape during different parts of flight, Craig points out. Since the chevrons are sticking into the exhaust flow, they add some drag and incur a fuel-burn penalty. By using so-called shape-memory alloys that shrink and expand with changes in temperature, it is possible to make a chevron that retracts itself out of the jet flow in cruise flight when sound mitigation is not as important as when the aircraft is close to the ground. The shape-change alloy chevrons are still in the beginning phase and not yet slated to go on any aircraft, says Craig, but the technology is interesting because it is automatic and involves no moving parts.

Boeing also looked at the noise caused by the landing gear when the aircraft is approaching an airport at low power settings. When a four-engine 747 is on approach, the noise produced by the wings and landing gear is about equal to that made by the engines.

"But with the new 787 engines, we're going well below the 747 current model engine noise," Craig says. "So we definitely have to be looking at airframe noise in the landing gear."

In wind tunnel tests conducted in cooperation with NASA and Goodrich, researchers made an interesting discovery: The noise heard on the ground from the landing gear is caused more by smaller details such as hydraulic lines and wires than by the wheels and struts.

The tests, which involved a toboggan-like cover attached to the 777's main landing gear, were successful in reducing noise further.

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