NASA FactsNational Aeronautics andSpace AdministrationLewis Research CenterCleveland, Ohio 44135–3191 FS–1997–07–003–LeRC

Lewis Effort will Reduce Engine Noise

Making Future Commercial Aircraft Quieter

The U.S. aviation industry is a significant contribu-tor to the nation’s economy, boasting annual sales inexcess of $36 billion and providing nearly 1 millionjobs. With the recent boom in air travel brought onby the global marketplace, the industry stands togain an even greater share of the nation’s and theworld’s economy. Continued competitiveness isimportant to the industry and to the U.S. economy.Quieter airplanes have a competitive advantage.

Just as significant, aircraft noise continues to be anuisance. Increases in air traffic and growth inpopulations that surround airports are resulting in anoise impact on a larger percentage of the commu-nity and a stronger desire to reduce the noise aroundairports. The figure to the right illustrates the noiseannoyance “footprint” surrounding an airport andhow that might be reduced so that airplane noiseaffects fewer people. Commercial technologynecessary to significantly reduce the footprint is notyet available and may be 20 years away.

In response to this, NASA is working to develop therequired technologies by the year 2000. Thesetechnologies are being developed under the Ad-vanced Subsonic Technology (AST) program, whichwas initiated in 1992 as a partnership betweenNASA, the U.S. aviation industry, and the FederalAviation Administration. The goal of the ASTprogram is to develop high-payoff technologies thatenable a safe, highly productive global air transpor-tation system. This system will include a newgeneration of aircraft and engines that are bothenvironmentally compatible and economical.

The NASA effort to make quieter aircraft is dividedinto three parts: engine, nacelle (the cowling thathouses the engine), and airframe. As the LeadCenter for Aeropropulsion and the Center of

Excellence in Turbomachinery, Lewis Research Center isthe NASA center responsible for the Engine Noise Reduc-tion Element of the AST program. The goal of this elementof the program is to develop technology that would reduceengine noise by 6 decibels (dB) by the completion of theprogram in the year 2000. To put this in perspective, a 10-dB reduction is perceived as reducing the noise level by

The fade from black to light gray in this figure,illustrates how reducing the noise foot print around a

major airport reduces the number of peopleannoyed by airplane noise.

50 percent. This AST noise reduction program is makinggreat strides towards accomplishing this goal.

Turbofan Engine Noise Generation

There are many sources of noise from current aircraft.Refer to the illustration above of the engine cut-away tovisualize the workings of an engine. Turbofan engineswork on the principle of sucking air into the front of thenacelle duct and pushing that same air out the back at ahigher velocity. This change in momentum provides thethrust. The diameter of the engine is determined by the fan,which pulls air into the duct. This fan is a source of noise,similar to the noise caused by a propeller. The fan blades,by pushing through the air, cause noise by themselves.Once past the fan, the air is split down two different paths,the fan duct and the core duct.

First consider the flow in the fan duct. Downstream of thefan, the flow is swirling because of the spinning fan. Thisswirl causes loss of momentum before the air exits thenozzle so it is straightened out with a set of vanes calledstators. These stators are a large source of noise as thewakes of air from fan flow slap against the stators likewaves on a beach. This regular slapping takes place at therate of blades passing by and generates a tone at what iscalled the blade passage frequency, or BPF. Nonuniform-ities and nonlinearities result in many higher frequencytones being produced at 2 times BPF, 3 times BPF, and so

on. These tones are often associated with thepiercing sound generated by some engines. Fan/Stator interaction creates more than specific tones.The unsteadiness in the fan flow (often in the form ofturbulence) interacts with the stators to createbroadband noise. This is often heard as a rumblingsound.

In the core duct, the air taking this path is furthercompressed through a series of smaller fans calledrotors. Each of these rotor stages is separated by aset of stators to straighten the flow. This is anothersource of rotor/stator interaction noise. The com-pressed air is then mixed with fuel and burned. Thiscombustion is another source of noise. The hot, high-pressure combusted air is sent downstream into aturbine which drives the fan and the compressorrotors. Since the turbine tends to look and act like aset of stators, this is another source of noise.

Finally, the core duct and the fan duct flows areexhausted into the air outside the back of the aircraft.The interaction of these jet exhausts with thesurrounding air generates broadband noise called jetnoise.

The following graph is representative of the noisedistribution components for typical aircraft. Theimportance of engine noise, in particular the fan andjet exhaust noises, is clearly depicted. These two

Modern turbofan jet engine (courtesy of General Electric).

Nacelle

Fan

Stators

Fan duct

Core duct

main areas are the primary focus of the research beingdone in the NASA Lewis AST engine noise reductionprogram.

Jet Exhaust Noise Reduction

Jet exhaust consists of the fan stream and the core/combustion stream. The core flow stream is typicallyat a higher speed than the fan stream. As the two flowstreams mix with each other, noise is created in thesurrounding air. Of particular difficulty, the jet exhaustnoise is actually created after the exhaust leaves theengine. This means that jet noise cannot be reducedwhere it is created, but must be addressed before theexhaust leaves the engine.

The theory of noise generation is being studied andcomputer codes that can simulate the theory are beingdeveloped. The final goal of this effort is to have acomputer model for jet noise that will predict thesource of the noise and how it is sent into the sur-rounding air.

Theoretical understanding of jet noise is used todevelop ideas for noise reduction concepts that aretested in model scale. Ideas that have already been

tested or will be tested include mixer devices to combinethe flows quickly, which reduce the noise generationarea.

Recently, test data have shown that a 3-dB reduction injet noise can be achieved. The final goal is to demon-strate a 6-dB reduction.

Fan Noise Reduction

In order to make progress on fan noise reduction, it isnecessary to understand and be able to predict that noise.Therefore, as with jet exhaust noise, effort is being putinto learning the theory of fan noise generation anddeveloping computer codes that simulate that theory. Thefinal goal of this effort is to have a computer code for fannoise prediction that can be verified.

A second approach uses the theoretical understanding offan noise to develop a succession of ideas for testing,with each test providing both data upon which thecomputer codes are verified and results upon which thenext test might be built. Fortunately, the fan thrustprovides many options to explore and there are manycomponents to vary. Besides basic geometry, there are

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Engine noise

Takeoff noise

A breakdown of the noise components of a typical engine with 1992-level technology during takeoff and approachto landing indicates the large part played by engine noise. Note: because decibels are logarithmic values, thegraph's total bar is the sum of the individual bars.

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blade-wake tailoring, boundary-layer (a thin layer of airalong the duct wall that moves slower than the rest of theflow) effects, fan speed, number of blades and stators, andmany more. Recently, model test data showed that a 3-dBreduction in fan noise can be achieved. As with jet exhaustnoise, the final goal is to demonstrate a 6-dB reduction.

Active Noise Control

A new approach to noise reduction is the active noisecontrol effort. The primary principle of active noise controlis to sense the noise disturbances in the engine and cancelthem before they leave the engine. In effect, negative noiseis made to cancel out the engine’s sound waves so that nonoise is heard. This is a multidisciplinary effort involvingduct acoustics, controls, and actuator/sensor design.

NASA Lewis has a unique facility for this testing. TheActive Noise Control Fan is a 4-ft-diameter low-speed fandesigned specifically for active noise control testing(shown in the figure on this page). To date, several con-cepts have shown successful cancellation of selectedacoustic modes. Because noise is the sum of all possibleacoustic modes, this effort is still in its infancy, but it haspotentially high payoffs. Active noise control will contrib-ute to the 6 dB noise reduction goal of the AST program.

Full Scale Validation

The final validation of the noise reduction technol-ogy will be performed on full scale turbofanengines. The most promising concepts from themodel scale testing will be selected for real environ-ment demonstration. The validated technology willthen be available for use by all U.S. engine manu-facturers to incorporate into a new generation ofenvironmentally compatible and economical aircraftand engines.

The Active Noise Control Fan at NASA Lewis showing two circumferential arrays of acoustic actuators.

Two arraysof actuators