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Engineering a Quieter Future for Jets

Engineering a Quieter Future for Jets

Whether it’s the buzz of a single-engine plane or the roar of a fighter jet, pilot Charlie Pyles loves the sound of aircraft.

But Pyles, curator of the Cincinnati Aviation Heritage Society & Museum at Lunken Airport, knows he’s in the minority.
“Airports are closing all across the country for the same reason: noise,” he said.

The University of Cincinnati is working with the U.S. military and General Electric to find ways to reduce jet noise, both to keep the peace with airport neighbors and to protect the health of ground crews and service members such as Pyles, who served in the U.S. Air Force.

Ephraim Gutmark, an Ohio Eminent Scholar and UC distinguished professor of aerospace engineering, presented several papers with his students on UC’s novel efforts to reduce jet noise at the annual American Institute of Aeronautics and Astronautics SciTech Forum in Florida.

“The main purpose is to reduce the impact on neighboring communities. The Navy had a big lawsuit in Norfolk, Virginia, from people who lived near the base and complained about hearing problems, sleeping problems and the diminished value of their properties,” said Gutmark, a professor in the College of Engineering and Applied Science. “So the military has to limit its operations and stop operating at certain times of day. It’s a severe problem.”

Cincinnati/Northern Kentucky International Airport was the subject of 395 complaints about aircraft noise in 2016, according to the airport’s latest annual report. The airport has taken steps to address noise by helping to soundproof nearby homes and schools and directing flights over less populated areas.

A major air hub like Chicago’s O’Hare International Airport fields as many as 2 million noise complaints in a single year.

Jet noise is also a concern whenever the military proposes expanding aviation programs or introducing new aircraft. Residents in Madison, Wisconsin, last year complained about the loud engines of the U.S. Air Force’s new F-35 Lightning II fighter planes that will be deployed to a local military base.

But Gutmark said the military and commercial aviation also want to reduce health risks for people who work around jets.

“Even with the best ear protection, the noise is still beyond the safe range,” Gutmark said. “So they have to limit exposure. Air crews can only work so many hours under those high-noise conditions.”

UC’s research promises real-world benefits to aviation workers such as the aviation museum’s Pyles, a retired commercial freight specialist from Cold Spring, Kentucky.

Pyles spent his career around jet aircraft, making sure cargo was properly balanced. He ran the annual air show at Lunken for several years before joining its aviation museum.

Even now he can tell a DC-10 from a Boeing 747 just from the sound of its engines.

“I was always conscientious about ear protection. The military taught us that, too,” he said. “My wife says I can’t hear anything, but I’m 73 and I hear just fine.”

The U.S. Air Force updated its noise regulations in 2016 with a requirement that new personnel receive baseline hearing tests. Members of the Air Force will undergo additional hearing tests upon discharge or retirement so the military can track hearing loss across their military career.

Hearing loss and tinnitus are the two most common causes of military disability claims, affecting 2.6 million former service members, according to the U.S. Department of Veterans Affairs. Hearing-related medical conditions are responsible for about 15 percent of new disability claims filed with the VA every year.

“NASA has very ambitious goals of reducing aircraft noise,” Gutmark said.

Today’s aircraft have a noise “footprint” that is 60 percent smaller than previous generations. But Gutmark said NASA wants to reduce aircraft noise by as much as 30 decibels within 15 years, which would make new airplanes far quieter than the quietest commercial planes today.

“The engines of the new F-35 are even stronger,” Gutmark said. “The stronger the engine, the more noise they make. So there is a greater need for this technology, which is getting a big investment from the Navy and Air Force and companies that build the engines.”

Modern fighter planes generate between 143 to 146 decibels of sound at a range of 50 feet, according to a 2009 study by Lockheed Martin and the military’s F-35 Lightning Joint Strike Fighter program. Fighter planes generate as many as 150 decibels when using their afterburners, which provide extra thrust for takeoff or high-speed pursuit.

And one challenge for proposed supersonic business flights is limiting the impact of sonic booms, which occur when planes break the sound barrier. NASA is developing standards for acceptable noise limits to accommodate renewed supersonic flights.

Gutmark and his students are exploring multiple solutions through novel projects in his aeroacoustic lab in Rhodes Hall, where students have built two “anechoic chambers,” rooms plastered on all six sides with fireproof, insulating and sound-absorbing foam. There is no echo at all here so the human voice sounds quiet and weirdly flat.

A scaled-down supersonic jet engine is bolted to the ground in the middle of each chamber so Gutmark and his students can conduct their sound experiments. Students study sound produced by the jets and adjust metal plates near the engine nozzle to mimic an aircraft's rudder or tail. This changes the direction of sound coming from the engine.

Students use an array of 16 microphones to record noise immediately surrounding the engine and a semicircle of microphones farther away to study far-field noise.

One engine is one-eighth the size of that found on a real fighter plane. In the adjacent chamber is a smaller jet (just 1/20th scale) that can heat the engine to 1,000 degrees, mimicking the temperature that actual jet engines reach during supersonic flight.

“This impact of heat on acoustics and flow was neglected for many years. People didn’t recognize how important it was to look at the effect of real temperature from the exhaust of an engine,” Gutmark said.

One of Gutmark’s doctoral students, Aatresh Karnam, can identify where noise from jet engines originates and in what direction it travels.

“We can then create a sound map,” Karnam said. “Depending on where a person is standing, we can tell the intensity of the sound they’ll experience.”

Gutmark has more than 60 registered patents on jet flow, combustion and acoustics, along with his myriad other research interests. In recent years, he brought his expertise in air flow to doctors at Cincinnati Children’s Hospital Medical Center and UC’s College of Medicine to find better treatments for patients with obstructive sleep apnea, vocal impairments and vascular disease.

The U.S. military already adopted some of Gutmark’s noise-dampening ideas. In particular, UC and the U.S. Naval Research Laboratory optimized the shape of serrated edges called chevrons found on the engine nozzles of F/A-18 Super Hornets. Chevrons alone helped the military reduce jet noise by 2 decibels, Gutmark said. That might not sound like much but since decibels are measured in a logarithmic scale, it’s significant.

“Anything a jet engine company can do to achieve even a half-decibel reduction is something they’re interested in,” he said.

UC also is exploring nozzle shapes to reduce noise. Instead of a round engine port, some aircraft use rectangular nozzles like those found on the supersonic Concorde, which was retired in 2003. Gutmark said he always stops to admire the Concorde on public display at Charles de Gaulle Airport when he visits Paris.

Gutmark, a fellow of the American Physical Society and the American Institute of Aeronautics and Astronautics, also wants to know more about the types of sounds produced by jet engines. Anyone who has been to an air show knows that jets produce not merely a continuous roar but also shrieks, booms and crackles, the sound produced by the solid-rocket boosters during a space shuttle launch.

“One of the new components of sound we identified is called ‘crackle.’ That happens only when you have high temperatures. It’s like the sound of tearing paper. It’s a very irritating noise,” he said.

Supersonic jets create shock and expansion waves that interact with turbulence inside their supersonic flow. Gutmark’s lab captured images showing how these waves form a diamond pattern in the flow. Shock waves produce the sharp screech and shock-associated rumble associated with fighter planes.

Gutmark’s lab has to consider how every refinement affects performance and fuel efficiency. Since jet noise has the biggest impact near runways, Gutmark is exploring ways to adjust the nozzles and the flow stream to dampen sound on takeoff and landing while reverting to a louder but more efficient free flow when the plane is cruising at a higher altitude.

UC is studying injecting small jets of air around the stream coming from the jet nozzle, which has the same noise-dampening effect as the serrated chevrons on the nozzles.

“These microjets are virtual aerodynamic serrations and produce the same effect. You can deploy them when you take off and stop the stream when airborne,” he said. “The engine is unaffected.”

Doctoral student Florian Baier said he came to UC because it promised the opportunity to examine real-world engineering problems. Baier had a chance at UC to work with Sweden’s military and its Gripen fighter plane.

“You can design all these parts on a computer. But here we build them and mount them on an experimental rig and conduct our own experiments,” he said. “That kind of hands-on work is very rewarding.”

Now Gutmark is developing a new flameless-combustion aircraft engine that is cleaner and generates more consistent heat. It has attracted the interest of the U.S. Navy, which is studying his concept.

“You have combustion that is very hot but you don’t see the flame. It’s technology that initially was developed in Germany for home-heating furnaces,” Gutmark said.

This combustion is both efficient and extremely stable, making it ideal for aviation, he said.

In the meantime, Gutmark and his students will continue looking for ways to reduce the noise of jet engines. He is optimistic he can reach NASA’s goals for next-generation aircraft.

“I think we can do it. It’s great to have a challenge like that,” Gutmark said. “And it promises us many years of research.”

By Michael Miller
Photos by Joseph Fuqua II/UC Creative Services