Few sounds are louder than a jet aircraft at takeoff.

The decibel level of a climbing jet engine at full power can be higher (140 dB) than that of a chain saw (110 dB) or ambulance siren (120 dB), according to the National Institute for Occupational Safety and Health.

But relief may be a toggle away, if recent university research finds its way into aircraft cockpits.

Engineers at Ohio State University (OSU) are testing a silencer technology that creates electrical arcs at the flip of a switch to control turbulence in engine exhaust gas flow. Air turbulence in exhaust streams is known to be the chief cause of engine noise.

The electric current from the OSU silencers–called plasma actuators–heats air passing near the engine nozzle in a flash, creating a pressure pulse that modifies turbulence in the exhaust.

Mohammad Samimy, director of OSU’s Gas Dynamics and Turbulence Laboratory, and colleague Igor Adamovich, OSU associate professor of mechanical engineering, demonstrated the technology in a series of laboratory tests using laser light to illuminate a simulated engine exhaust stream, then studied how different arrangements of actuators affected the exhaust flow.

Silence of the Cams
Researchers tested the plasma actuators using two types of gas stream, one simulating the exhaust from a commercial aircraft and another simulating that of a high-speed military aircraft. Both test regimes showed that the plasma actuators succeeded in manipulating turbulence structures in the airflow.

With this technology, Samimy believes pilots could turn the actuators on and off at will–particularly while taking off and landing–and reduce noise around airports.
“The way we have addressed noise reduction in the past 50 years has been by going to ever higher bypass-ratio engines, reducing the exhaust velocity of jets by migrating from turbojets to large turbofans,” Samimy said.

The only recent major development in noise reduction technology has been the introduction of structural modifications made to the trailing edge of the exhaust system called tabs or chevrons that are beginning to appear on some new aircraft.
Chevrons, which are modified serrated, or saw-tooth, exhaust nozzle features, are an effective method of modifying the jet mixing structure and reducing noise, as successfully demonstrated on the Rolls-Royce/ Boeing Quiet Technology Demonstrator in 2001. The technology, the result of 10 years of effort between NASA and the aviation industry, is also now emerging on GE CF34 engines powering new regional jets entering passenger service.

Chevrons cause increased mixing of engine exhaust with ambient air, thus reducing noise-causing turbulence. But while chevrons might reduce noise, they also tend to lower fuel efficiency and cause some performance (thrust) loss.

“Thrust loss is something we need to minimize,” said Joseph Grady, engine noise reduction project manager in NASA’s Quiet Aircraft Technology (QAT) project. Noise-reduction systems are generally needed only during takeoff and landing, but chevrons are permanent fixtures on the nozzle housing and cannot be disengaged at cruise altitude to increase fuel efficiency.

One way to address this is through the use of shape memory metals, such as those being developed by Rolls-Royce. When the aircraft is on the ground or flying
at low altitude, the temperature is warmer and the shape memory alloy used in the chevron curves inward. This creates turbulence and reduces the noise of the engines because the sound waves are dispersed.

Once the aircraft reaches cruise altitude, the temperature drops and the shape memory alloy is bent out of shape by another layer of metal that has been trained to lie flat.

When the aircraft descends into an approach pattern in warmer air, the shape memory alloy reverts to its original shape, once more reducing the noise of the gas stream.

Noise reduction at the flip of a toggle switch would be a novel way to cut noise. The ability to control nozzle turbulence from the cockpit is one principle feature that separates Samimy’s plasma actuators from chevron technology.

“The plasma actuators would accomplish the same thing as chevrons but could be turned off when not needed during the longer cruise part of the flight,” Grady said.

Now Hear This
Aircraft noise has been a quality-of-life issue near airports since the emergence of commercial jet aircraft in the late 1950s. Even though absolute single-event noise levels have decreased with the introduction of newer and quieter aircraft, fleet noise impact continues to retard the growth of air travel as the public demands more be done to lower community noise levels.

This concern is particularly acute in Europe, where individual airports have introduced noise-related fees or prevent operations entirely for louder airplanes.
A number of airports in the U.S. are also facing strong public resistance to new construction, runway expansion or flight paths that route air traffic over populated areas.

Airport noise abatement restrictions and associated penalties are becoming increasingly severe. At John Wayne-Orange County Airport in Southern California, fines against airlines for exceeding noise limits can run as high as $500,000. To avoid penalties, airlines now use manual procedures for noise-abatement departure profiles, sometimes reducing takeoff weight at the expense of revenue to ensure the aircraft does not exceed community noise limits.

Thus, any emerging technology aimed at muffling jet engine noise is particularly appealing.

“A number of active techniques for modifying the jet structure on takeoff are being investigated, including deployable chevrons and aerodynamic modulation of the jet,” said a Rolls-Royce spokesman. “In this category are plasma actuators, although much research is required before any such technique is shown to be viable for airline operation.”

Traditional noise reduction methods have consisted of passive hush kits employing muffling and baffling techniques. Some older aircraft are still flying because these kits are used to achieve regulatory compliance. Installing a hush kit on a 737 costs about $2 million. In the same price range are hush kits for the Gulfstream II and III developed by Quiet Technology Aerospace, Really Quiet and Stage III Technologies.
More active methods employ mixing inlet air with the exhaust stream to reduce exhaust velocity. Although these ideas do cut noise, the reduction is achieved at the expense of added weight and diminished engine efficiency.

Newer efforts have arrived at special avionics to address noise abatement challenges, such as those beginning to appear on some Boeing airliners already in service. The Quiet Climb System (QCS) an advanced flight management feature currently offered on the 737-900, is one of those.

QCS automatically reduces engine thrust during takeoff to meet the noise-abatement departure profiles outlined in Advisory Circular AC 91-53A, which include a minimum thrust cutback altitude.

QCS is designed to maintain a safe climb and airspeed, eliminating the need for flight crews to manually reduce thrust several times to reach the proper thrust and climb angle. QCS may therefore allow for increases in passenger and cargo weight as airlines discover they are staying below airport noise limits.

Sound Check
Samimy’s plasma actuator work is funded by NASA, which has been in the noise abatement chase for more than a decade. NASA provides enabling technologies that could make aircraft quieter without sacrificing performance, emissions, safety or cost.
Anticipating the need for quieter aircraft, in 1994 NASA formed the Advanced Subsonic Technology (AST) noise reduction program to develop new technologies for cutting aircraft noise.

AST, a 1994-2000 cooperative effort among the FAA, U.S. aerospace companies, universities, small businesses and three NASA research centers (Langley, Glenn and Ames), addressed all aspects of subsonic, fixed-wing aircraft noise issues, including engine noise, nacelle aero-acoustics, airframe noise, interior (cabin) noise and community noise impact. The goal was to lower community noise impact of commercial subsonic airplanes by 10 dB relative to 1992 production technology.

That program ended in 2001, to be succeeded by the current Quiet Aircraft Technology program, whose near-term goal is to establish a means of reducing community noise by 50 percent by 2008, then by 2020 to demonstrate technology to reduce community noise by 75 percent, according to Grady.

Samimy’s plasma actuator is one of a series of technologies being evaluated for long-term noise reduction.

Fundamental proof-of-concept work was accomplished when OSU demonstrated that plasma actuators modify turbulence. The next step is to evaluate the concept on a simulated jet exhaust test rig to measure noise reduction benefit, then test on a full-scale engine.