Japan’s space agency exploring new ways to make jet planes quieter

People living near airports may be getting some relief as studies to reduce noise from screaming airliners are showing signs of progress.

Engines had been the main focus of noise-reduction efforts in the past. But as they have become quieter, developers are trying to find ways to suppress noise from wheels, wings and the fuselage during takeoff and landing.

The primary cause of aerodynamic noise is wind, proven to be a major noisemaker through tests like a recent one at Noto Airport in Wajima, a city in Ishikawa Prefecture on Japan’s west coast.

Some 200 microphones were placed along the runway from the middle of September till early October to measure noise from a small test jet plane flying over the airport. The test was aimed at checking the efficacy of noise reduction technology developed by the Japan Aerospace Exploration Agency, or JAXA.

The test plane was equipped with noise-reducing components on landing gear and the back of the main wings. The plane flew over the airport at a height of 30 to 60 meters for a total of 25 hours, gathering data for the agency.

Taming winds

The two main types of aerodynamic noise are generated by the engine and when a plane flies against a headwind.

Noise from jets has dropped significantly since the 1960s due to improvements in engines and fuselage design. But the planes still blast out 70 to 90 decibels as measured near Osaka International Airport and Fukuoka Airport, making them as loud as subway trains or manufacturing plants.

Planes are loudest when flying against the wind during takeoff and landing. “It is a real challenge for reducing noise,” said Kazuomi Yamamoto, manager of the JAXA project. This is especially true when landing, because the engine gets louder as it powers down.

Wind noise occurs when airflow becomes erratic after hitting a plane, causing air pressure to change. Air whirls also form when the wind hits wings, and during the resulting turbulence — dramatically increasing noise levels, according to experts.

Japan’s space agency exploring new ways to make jet planes quieter

RIMI INOMATA, Nikkei staff writer
Small parts on the main wings — slats and flaps mounted on the leading and trailing edges — are the main culprits. Both are indispensable, as they are used to increase the plane’s lift, but they also create strong air whirls to form near the edges of wings.

Another source of noise comes from landing gear, an intricate assembly of axles, tires, brakes, shock absorbers, hydraulic pipes and other parts. This complex structure tends to generate erratic air currents and cause strong whirls.

Vexing whirls

Air whirls differ depending on the parts that generate them. Hence, each noise-reduction measure must be tailored to the part it affects. This can be difficult with moving parts like slats, flaps and landing gear, due to design constraints.

JAXA got around this by reproducing the parts that generate air whirls on a computer and examining airflow where noise occurs. The project team then designed and produced noise-reduction devices for each part and tested their effectiveness in a wind tunnel.

They devised a flap with round, globular edges and protrusions on its leading edge, and mounted porous covers over landing gear. The devices were designed to reduce erratic airflow, and the air whirls they cause, directing whirls that did occur away from the fuselage.

Because the test plane used at Noto Airport did not have wing slats, noise reduction devices were mounted on the flaps and main landing gear only, resulting on average of a 2-decibel cut in noise.

The technology “may be able to reduce by half the locations [near airports] afflicted by aircraft noise,” JAXA President Naoki Okumura said. And by coupling this technology with even quieter engines, JAXA is targeting a noise reduction of 4 decibels.

Participants in the project include Mitsubishi Aircraft, Kawasaki Heavy Industries and Sumitomo Precision Products. Starting in 2019, the project will conduct tests on Mitsubishi Aircraft’s MRJ regional jet aircraft, using specific noise reduction devices for different plane parts, including slats.

JAXA hopes to market its noise control technology to Japanese aircraft manufacturers in 2030, Yamamoto said, with plans for eventual export.

Japan Aircraft Development Corp. and other sources have forecast the global number of air passengers to grow at an annual rate of nearly 5% until 2036. Meanwhile, the number of takeoffs and landings is expected double over the next two decades, raising noise at airports around 3 decibels on average.

As anti-noise regulations become stricter, developing quiet aircraft has become an imperative.

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