A new nozzle design for F-18 fighter planes has been designed that dampen the deafening roar of the engines without hindering performance, engineers at the University of Cincinnati (UC) have said.
The nozzles, which have been shown to work on 1/28th-scale jet engines, feature triangular fins like rows of shark teeth which significantly reduced jet engine noise in lab tests.
“They’re simple attachments that change the behaviour of the flow coming out of the engine with minimal effect on its performance,” said professor Ephraim Gutmark, a researcher on the project.
UC’s lab tests showed the new nozzle could reduce engine noise by five to eight decibels, a significant decrease considering decibels are measured logarithmically.
“Typically, engine companies are happy even getting a half-decibel improvement,” Gutmark said.
Hearing loss and tinnitus are the leading causes of military disability claims in the US, affecting more than 2.6 million former service members, according to the US Department of Veterans Affairs. It spends more than $1bn per year on hearing loss cases, which represent about 15 per cent of new disability claims that are filed with it each year.
Jet noise in particular represents a serious health risk in military and commercial aviation. According to the Naval Research Advisory Committee, Navy personnel on flight decks are exposed to noise in excess of 150 decibels.
“On aircraft carriers, the crew that works with pilots on the flight deck needs to be very close to the plane when it takes off. Because of the short runway on the carrier, they must operate the engine with afterburners, so it’s very loud,” Gutmark said.
Jets are so loud that the noise and vibrations can affect even the aircraft itself – a phenomenon called acoustic loading. “By suppressing the noise, you are helping the crew but also helping the longevity of the airplane itself,” Gutmark added.
The UC test engines are mounted to the ground inside an anechoic chamber — a room designed to completely absorb reflections of acoustic waves. Students can turn on jet engines remotely outside the chamber and use a variety of sensors to measure and analyse the noise from the exhaust plumes.
An array of sensitive microphones surrounds the jet in the chamber, which are distributed in an arc so that the noise can be detected going downstream, upstream or sideways.
The engineers have tested their nozzle designs on both cold and heated jets with an exhaust that burns as hot as nearly 600°C.
Doctoral student Mohammad Saleem said aircraft are quieter today than they were just 20 years ago. Both commercial and military aviation have a strong public and financial interest in reducing noise. Even incremental improvements can have a profound impact, he said.
“These noise reduction technologies are extremely beneficial to the communities living around air bases and personnel working on aircraft carriers,” Saleem explained.
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