If you've ever lived near an airport, or visited someone who does, you'll be familiar with the window-shaking rumble that can keep many a household up at night. NASA has taken it upon itself to address the problem, testing three new noise reduction technologies on a series of Acoustic Research Measurement (ARM) flights, and managing to cut airframe noise during landing by more than 70 percent.
For this project, NASA focused on reducing airframe noise, which is created not by the engines but by wind rushing past the frame of the aircraft. To test out the new technologies, three experimental designs were mounted on a Gulfstream III research aircraft, which was flown at an altitude of 350 ft (107 m) over an array of 185 microphones laid out on the Rogers Dry Lake at Edwards Air Force Base.
"The number one public complaint the Federal Aviation Administration receives is about aircraft noise," says Mehdi Khorrami, principal investigator for Acoustic Research Measurement. "NASA's goal here was to reduce aircraft noise substantially in order to improve the quality of life for communities near airports. We are very confident that with the tested technologies we can substantially reduce total aircraft noise, and that could really make a lot of flights much quieter."
The landing gear as a plane is approaching an airport was identified as one of the main culprits of airframe noise, so two of the new pieces of tech were designed to improve that area. To reduce the noise of air rushing past the fairings, NASA designed a version full of tiny pores on the front face, that allows some of the air to flow through it while pushing the rest around the landing gear. That idea has been done before, but NASA says this design is based on detailed computer simulations that maximize the balance between noise reduction and drag.
Landing gear cavities, the openings in the underside of the aircraft body that the landing gear retracts into, are also notoriously noisy. While they're open, air rushes in and loudly bounces off the inner walls. To reduce that, the team added sawtooth patterns called chevrons to the area just in front of the cavity, and stretched a net across the opening. That helps the air flow past the gap more efficiently, while a soft sound-absorbing foam on the back inner wall reduces the noise from the air that does get in.
The final piece of the puzzle targets the wing flap, the part that raises and lowers from the trailing edge of the wing to give more lift during takeoff or help slow the plane during landing. But these mechanical parts leave gaps between them and the rest of the wing body, which can get noisy and reduce fuel efficiency.
NASA's answer to that is the Adaptive Compliant Trailing Edge (ACTE) wing flap, a technology it's been testing for a number of years. This new version is flexible and seamless, bending to do the job without leaving gaps in the wing surface.
After the ARM test flights wrapped up in May, NASA reports that the three new technologies together managed to reduce airframe noise during landing by over 70 percent.
"This airframe noise reduction produced by NASA technology is definitely momentous, and the best part is that it directly benefits the public," says Kevin Weinert, ARM Project Manager. "While there are obvious potential economic gains for the industry, this benefits the people who live near major airports, and have to deal with the noise of aircraft coming in to land. This could greatly reduce the noise impact on these communities."
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