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NASA TMX-68131
N A S A T E C H N I C A L
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JET AIRCRAR ENGINE NOISE REDUCTION
by E. William Conrad and Carl C. Ciepluch
Lewis Research Center
Cleveland, Ohio
TECHNICAL PAPER proposed for presentation at
EASCON '72
Washington, B.C., October 16-17, 1972
JET AIRCRAFT ENGINE NOISE REDUCTION E. WILLIAM CONRAD and CARL C. CIEPLUCH NASA - Lewis Research Center, Cleveland, Ohio between these high velocity streams and the surround- Abstract ing air. A second important source of noise is called machinery noise and it.is radiated from the interior of Aircraft flyover noise is a serious problem to liter- the engine, as shown in the figure by the fan (which pro- ally millions of people, and the federal government is pagates out the fan inlet and discharge ducts), the com- . doing something about it. Through NASA and the FAA, pressor, the turbine and possibly the combustor.
advanced technology is being aggressively developed, as described herein, to identify and minimize noise Jet noise. - Jet noise is a major and frequently domi- sources in aircraft engines, and to absorb noises nant contributor to overall engine noise. The amount which cannot otherwise be eliminated. The economic of noise generated is strongly related to the Jet veloc- impact resulting from reducing engine noise levels ity, as shown in Fig. 2 (from ref. 1) for several kinds will also be presented and the challenge posed to re- of data. The jet noise is presented in terms of the ducing this economic impact by further improvement overall sound pressure level (OASPL) which has been in engine noise reduction technology will be discussed.
corrected for density and area differences by the term 10 log (pj A). • Superimposed is an SAE correlation t- curve which is commonly used for hot jets at velocities Introduction w above 1000 feet per second. Obviously a reduction in jet velocity yields a major reduction in jet noise. By The problem of aircraft noise needs no introduction exploiting this relationship (low jet velocity produces ; because most people have experienced it at one time low jet noise), major noise reductions have already or another in the vicinity of major airports. In con- been achieved in the new engines powering the DC-10, cert with the FAA, the NASA is deeply involved in B-747, and L-1011 wide -body jets. Further Jet noise solving this problem by developing new noise reduc- reductions are possible, particularly for the core jet, tion technology that will quiet future airplanes and have but there is a practical limit. As jet velocity is re- a minimum impact on airplane economics.
duced, more air must be pumped in order to maintain constant thrust. This requires a larger fan and results The material that is to be reviewed in this paper can in a heavier engine as well as an increase in nacelle provide only an overview of the complex matrix of drag. Accordingly, one must make a judgment of the problems involved in aircraft noise reduction. First jet noise reduction in light of the attendant penalties.
will be presented a discussion of the major noise sources, the noise generation mechanisms involved, One "fix" which has been used to reduce jet noise is the and what can be done to reduce or suppress the noise concept of breaking up a large single jet into many generated. Following this, an indication of the current small ones. Sound pressure level (SPL) is shown in performance and economic penalties involved in achiev- Fig. 3 (from ref. 1) as a function of frequency for a ing aircraft noise reduction will be discussed. And J-75 engine operating statically with a large single ex- finally an indication of the importance of advanced tech- haust nozzle and also one formed of 37 round tubes.
nology in reducing the cost of aircraft noise reduction The noise reduction is impressive, particularly below will be shown.
1200 Hz, but such noise improvement is achieved at the cost of reductions in thrust on the order of 5 percent, Noise Sources and significant increases in nozzle weight. Also, the effectiveness of such multi-tube jet noise suppressors A cross section of a typical turbofan engine, the type diminishes as jet velocity decreases, and as a result which has been commonly used in transport aircraft they are less effective for use on engines of conven- since the introduction of the B-707 and DC-8, is shown tional take-off-and-landing type aircraft. It is likely in Fig. 1. The engine thrust results from the high ex- that further reductions in jet noise without commen- haust velocities developed in both the core and fan Jet surate performance penalties will bo difficult to strwims. These high Jot velocities are nmonc the accomplish.
primary sourcf.s of engine noisu. The Jet nolso is cmiscrl by the highly turbulent mixing which occurs Fan noise. - Fan noise competes with jet noise for the One method of decreasing fan noise, which has been used dominant role in the overall engine noise picture. Data successfully in newer engines, is illustrated In Fig. 6 from fans and engines (shaded symbols) have been com- •• (from ref. 1). In this figure the noise level is presented bined to produce the shaded noise bands in Fig. 4.
in terms of perceived noise which has been tone cor- Here are shown maximum perceived noise level (PNdB) rected (PNLT). As the stator row is moved downstream for 90, 000 pounds take-off thrust plotted against fan away from the rotor blades, the noise level is signifi- pressure ratio. Fan noise level is seen to increase cantly reduced. This is as one would intuitively expect, with fan pressure ratio for the three classes of fan con- since the wakes from the rotor will mix with adjacent figuration indicated., Also shown in the figure are the undisturbed air and be reduced in intensity (velocity de- pressure ratio ranges found appropriate for STOL, fect) before striking the stators. As usual, there is a CTOL, and AST types of aircraft. These acronyms re- counterbalancing penalty, for as the spacing is increased, fer to "short take-off-andrlanding", "conventional take- the engine becomes longer and heavier.
off-and-Ian ding" and "advanced supersonic transport" aircraft, respectively. It can be seen that fan noise Another concept under study for reducing wake-produced _ : will be a problem for supersonic aircraft because of noise is the use of leaned stators. With this concept, the high fan pressure ratio required, but it will also be shortly to be demonstrated in several experiments, the a problem for STOL aircraft, even though the fan pres- stator blades are leaned away from the radial position.
sure ratio and therefore its noise will be lower, be- Thus each, rotor blade wake, which precedes toward the cause of planned operations from extremely small air- stator in the form of a more-or-less radial sheet, does ports in downtown areas where allowable noise levels not encounter the entire span of the stator blade at the will be extremely low. same instant. One can think of the leaned stator blade as progressively slicing through this wake sheet, first Two of the most significant mechanisms which generate at one end and then progressively to the opposite end,, fan noise are illustrated in Fig: 5. Looking at the tip producing only local small vortices and hopefully lower of a fan assembly (upper sketch), one sees a row of noise. If successful, this concept may allow the spac- rotor blades moving to the left past a set of stationary ing to again be reduced resulting in a still quiet but stator blades. As the incoming air flows over a rotor lighter engine.
blade the air near the blade surfaces is slowed down by • friction, producing a low velocity wake behind the blade Noise Suppression (shaded areas). These wakes, shown relative to the moving rotor, then intermittently pass over each of the The discussion thus far has dealt with a few of the many stator blades, changing momentarily the angle of attack. concepts for reducing noise generation. The noise Thus each stator blade will experience an oscillating: levels desired by society, however, are lower than can lift, and for each change in lift will create a vortex presently be achieved by reduction in the source noise downstream. The creation of such vortices is believed generation. Accordingly, intensive efforts are devoted at least partly responsible for the dominant fan-noise to learning how best to absorb some of the residual source in subsonic compressors and fans. The higher noise. This absorption is accomplished with the forms noise levels shown in Fig. 4 with two-stage fans are and arrangements of acoustic "treatment" shown in due to the additional wake-interaction noise of the sec- . Fig. 7. Although these .arrangements are intended pri- marily for fan noise suppression, the same principles ond stage.
can be employed for suppression of other internal noise The second important noise generation mechanism pro- sources such as the compressor, turbine, and com- duces a large number of discrete tone noises called bustor if they become significant contributors. All multiple pure tone (MPT) noise. To get more pressure available surfaces of the air flow passages within the out of a single stage fan and thus a lighter engine, de- engine and nacelle are formed of porous or perforated signers have increased the tip speed of the fan rotor materials with closed backing cavities underlying. The design of such system is based upon a highly sophisti- blades well into the supersonic regime. At supersonic speeds, shock waves are formed at the leading edge of cated-application of Helmholtz resonator theory. One each rotor blade. A family of such waves is shown in may envision a noise (pressure) wave having a momen- Fig. 5, propagating upstream toward the inlet of the tary pressure higher than the pressure in the backing nacelle. Because of slight blade variations due to cavity, causing flow through the porous face sheet Into the cavity. The face sheet orifice and cavity combina- manufacturing tolerances, these waves are not all par- allel; Some waves coalesce and reinforce one another tion will form jets which alternately flow into and out of the cavity in response to the rising and falling acoustic to produce strong pressure waves which, along with pressure in the duct. Acoustic energy is removed in harmonics, are heard as a family of pure tones. Such multiple pure tone noise (MPT) can be both intense and the process by. turbulent jet dissipation and viscous in- irritating and is often called buzz-saw noise. The in- teraction with the walls of the orifice and cavity.
crease in noise in going from the low speed to the high Although there is still much to be learned, the concept speed single stage fans in Fig. 4 is primarily due to works quite well and has been used extensively in re- this mechanism.
cent commercial engines, as well as the NASA Quiet Engine. But again the gains are balanced by perform- . marketplace.
ance losses and increased weight as shown In Fig. 8 based on data from lief. 2. Here.it is seen that noise Cost of Noise Reduction may be reduced progressively by adding more and more treated area in the forms of duct Inlet rings and splitter The dramatic improvement illustrated by the Quiet rings in the exhaust ducts. But it is also seen that the Engine and nacelle made possible with the technology direct operating costs of the airplane are progressively existing three years ago was not achieved without en- 'increased at an ever steepening rate.
gine performance and therefore airplane economic pen- alties. Thrust was reduced by 4-5 percent, and each Quiet Engine Results flight nacelle built to incorporate these features would be perhaps 500 pounds heavier than an untreated nacelle.
The objective of the NASA Quiet Engine program (ref. 3) was to develop engine noise reduction technology and to In the preceding discussion, for each gain made in re- demonstrate the lowest engine noise levels that could be ducing noise, performance and/or weight penalties obtained when all the noise reduction technology avail- were incurred with an impact on economics. If we are able was incorporated into an engine design. The engine to proceed using current technology to the noise levels was designed and built for NASA by the General Electric represented by the Quiet Engine and even further as we Company. It was designed to produce 22, 000 pounds of would like, direct operating costs (DOC) will become thrust at sea level static conditions. A cross section of higher than the airline industry can absorb without sub- the engine is shown in Fig. 7. An acoustically-treated stantial increases in ticket price. Thus, most of the nacelle was built for the Quiet Engine by The Boeing burden must ultimately be borne by the traveling public Company and a photograph of the engine and nacelle in the form of increased ticket price.
combination is shown installed in the NASA Lewis engine acoustic test facility in Fig. 9. The acoustic treatment A trade then becomes obvious between the number of Included three inlet splitter-rings and one fan-duct people annoyed by aircraft noise and the annoyance splitter ring which had acoustic treatment on both sides. imposed on the traveling public by higher ticket price.
In addition, acoustic treatment was placed on both the This trade is illustrated in Fig. 11 which shows the inner and outer walls of the fan inlet duct, the fan dis- number of people exposed to 90 EPNdB per takeoff as charge duct, and the core exhaust duct. a function of ticket price. Here the reference point is the new wide body aircraft such as the B-747, DC-10, The noise reduction results achieved by this Quiet and L-1011, which already include significant noise Engine and nacelle are translated most meaningfully in reduction technology. In order to simplify the estima- Fig. 10 taken from Ref. 4. The figure shows the areas tion of change in ticket price, the assumption is made on the ground wherein the noise level would be greater here that increases in DOC are directly related to than 90 EPNdB during landing and takeoff of a DC-8 ticket price. It is evident that with current technology, type aircraft. (The term EPNdB is the noise,unit the large reductions in annoyance below that of current Federal Aviation Agency (FAA) uses to regulate air- aircraft, will result in significant increases in ticket craft noise and it is found by taking the perceived noise price for the air traveler. However, continued vigor- and modifying it for duration and tone content.) With ous technology development should produce large re- the engines currently installed in the DC-8, which were ductions in annoyance without any increase in ticket developed ten years ago without regard for noise, price, as indicated by the arrow in the figure. The 65, 800 acres would be exposed to noise greater than shaded advanced.technology line is the estimated im- 90 EPNdB. This is about the noise level beside a very provement in noise reduction technology that will be busy freeway. If the airport were surrounded by city available in the early 1980»s.
dwellers, on the order of 800, 000 people could be an- noyed. With the new Quiet Engine and nacelle using Thus far the thrust of our efforts has been to learn how duct treatment only, the area decreases to 2670 acres. to reduce noise and thus community annoyance. Work in the future will be directed not only to further re- The further addition of treated inlet rings and exhaust splitters reduces the acreage to 930, of which most of duction in noise but also towards developing noise re- the 90 EPNdB "footprint" is on the unpopulated airport duction technology that will have less of an economic property. impact on airplane operations. Through the applica- tion of advanced research it is believed and expected that we can move into the region shown by the shaded In addition to the Quiet Engine program which is in- tended for guiding new engine developments, NASA in area in Fig. 11. Such progress should allow a much conjunction with the FAA, has just embarked upon a improved noise environment with prices that the trav- retrofit program intended to determine the most prac- eler can accept.
tical engine or nacelle modifications to quiet the ex- isting installations in the DC-8, 707, 727, 737, and Conclusions DC-9 airplanes. This program should produce major relief in community noise in the 1975-80 time period Significant progress has been made in the development of new engine noise reduction technology as evidenced before any completely new quiet engine can reach the by the Quiet Engine Program. The prospect for re- ducing the noise levels of future aircraft below that of the new wide body jets is good. However, a significant penalty In the form of Increased cost to the traveler will be expected as new airplane noise levels are gradually reduced. Continued efforts in the noise re- duction technology area should allow for not only much lower aircraft noise levels, but with correspondingly reduced cost to the traveler.
References 1. "Aircraft Engine Noise Reduction, " NASA SP-311, 1972.
2. D. D. Hufford, J. A. Ross, and K. W. Hoefs, "The Economics of Subsonic Transport Airplane Design, Evaluation and Operation, " in Society of Automotive Engineers, Atlanta, Ga., May 10-13, 1971 meeting, Paper No. 710423.
3: C. C. Ciepluch, "The NASA Quiet Engines, " NASA TMX-68121, 1972.
4. Statement of David Cochran, General Electric Co., to the Sub-committee on Advanced Research and Technology of the Committee on Science and Astro- nautics, U. S. House of Representatives, Jan. 19, 1972.
r FAN JET NOISE JET Jti J COMBUSTOR Figure 1. -Turbofan noise sources.
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CS-63310 Figure 2. - Effect of jet velocity on jet noise.
J-75 ENGINE WITH 37-TUBE SUPPRESSOR NOZZLE PRESSURE RATIO, 2.4 28 IN.
STANDARD NOZZLEA SPL, dB, AT 200 FT AND 140° ANGLE 100 SUPPRESSOR NOZZLE^' I I I I I L 50 100 200 500 1000 2000 5000 10 000 1/3-OCTAVE BAND CENTER FREQUENCY, Hz Figure 3. - Noise reduction with multi-tube nozzles.
ro FAN NOISE 90,000 LB TAKEOFF THRUST - 1000 FT FLYOVER ; TWO-STAGE D 120- SINGtf-STAGE ' HIGH-SPEED MAXIMUM NOISE 110 LEVEL, PNdB CLOSED SYMBOLS DENOTE TESTS OPEN SYMBOLS DENOTE PREDICTIONS 100- AST- 1.0 1.2 1.4 1.6 1.8 2.0 2.2 FAN PRESSURE RATIO Figure 4. - Fan noise levels.
GENERATION OF DISCRETE BLADE PASSAGE NOISE BY PERIODIC WAKE CUTTING INFLOW ROTOR BLADES- r BLADE ' WAKES STATOR VANES SHOCK GENERATED NOISE -- SHOCK WAVES ROTATION ROTOR / BLADES^' Figure 5. -Two mechanisms producing fan noise.
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BASELINE DUCT WALL FULLY SUPPRESSED E2^3 I Figure 7. - Sound suppression by acoustic treatment.
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TRADED NOISE, EPNdB Figure 8. - The cost of noise suppression.
CO r- i W CS-63315 Figure 9. - NASA quiet engine number 1 in acoustic nacelle.
FOR DC-8 TYPE AIRCRAFT i i JT3D ENGINE MENT DUCT TREAT BASE LINE* 10 - 90 EPNd \ ARE \ = 1 670 / CRES FEET »• — (OOO's) BASELINE PUTTERS 90 EPNd 1 ARE A = 9 30 AC IES -60 -40 -20 0 20 40 60 SO 100 120 140 160 FEET(OOO's) Figure 10. - Impact of quiet engine technology on community noise.
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Figure 11. - Community noise relief versus cost to the
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