ARCHIVED - Reducing noise pollution near airports
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December 01, 2011— Ottawa, Ontario
For anyone living near an airport, the rumbling of airplanes coming in for a landing is all too familiar. But Canadian researchers are finding ways to reduce that noise in a facility designed to study the sounds that come from the interaction of certain airplane parts and wind.
Indeed, half the noise made by an approaching airplane comes from fast-moving air flowing over the parts of the aircraft that are used for landing, such as flaps, slats and landing gear, according to NRC researcher Jerry Syms.
Flaps and slats are flat pieces of metal that are extended out on the wing when a plane needs a lot of lift at slow speeds. Those, plus the landing gear, create drag and help an airplane slow down. They are also important sources of the noise heard as aircraft slow down for descent and landing. The engine roar is only half of the noise equation when a plane is near the ground, Syms says.
Greening the airline industry
There is a general move in the airline industry to make airplanes quieter. Together, the International Civil Aviation Organization (ICAO) and NASA’s Environmentally Responsible Aviation program have set regulations to reduce emissions and noise. In the next 10 years, regulations will aim to reduce noise generated by commercial aircraft by 32 decibels relative to the current standard.
So, how does one study wind, noise and airplane parts? In a modified wind tunnel, of course. And that’s what NRC researchers did, modifying one of NRC’s Ottawa wind tunnels so that it could detect every nuance of noise from aircraft landing gear. They took sections of acoustic foam (resembling grey cardboard egg cartons) and mounted them inside metal frames. The frames were then covered in a fine mesh to create a smooth surface and cover up the lumps. The foam-filled frames were then mounted on the floor, ceiling and walls of a wind tunnel that measures two metres high and three metres wide.
A total of 64 microphones were placed inside the aero-acoustic wind tunnel, recessed in cavities in the foam so wind doesn’t blow over the mikes. The result is a wind tunnel in which the sounds of air blowing over objects inside it can be accurately measured. The tunnel is unique in Canada.
“We made an aerodynamic tunnel into an acoustic facility,” says Syms. “We now have the capability to accurately measure the noise generated by air flowing around aircraft components. Conveniently, we can remove the whole assembly if we need to convert the tunnel back to its original form.
One project Syms recently completed was to study the sounds emitted by wind flowing over the landing gear from a business jet. The gear was mounted in the middle of the converted wind tunnel, then exposed to winds of 145 knots (about 270 kilometres per hour).
“That is a typical approach speed for a plane heading in for landing,” Syms says. People on the ground would hear a loud rumbling from a plane of that size travelling at that speed. He adds that it’s possible to generate almost twice that speed of wind if needed for other projects.
While the gear is being exposed to the wind, video and audio devices record what happens. Researchers then produce sound maps from the test data that show exactly how much noise is coming from each part of the gear assembly.
The plane’s manufacturer can then use the information provided by NRC’s researchers to redesign or alter parts so they create less noise when exposed to fast moving air, Syms says. The acoustically modified wind tunnel could play a big role in helping manufacturers develop future generations of quieter, greener aircraft.
- NRC helps aircraft manufacturers reduce airframe and landing gear noise
- NRC’s 2-metre by 3-metre wind tunnel
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