United States Patent (19) 11 Patent Number: 4,836,469 Wagenfeld 45 Date of Patent: Jun. 6, 1989

(54) SYSTEM FOR REDUCING AIRCRAFT NOISE 4,723,626 2/1988 Carr et al...... 181/213 AND HUSHKT OTHER PUBLICATIONS (75 Inventor: Robert E. Wagenfeld, New York, "Silencing an Executive ', Noise Control N.Y. Engineering, vol. 5, No. 2, Sep.-Oct. 1975, J. R. Brooks, 73) Assignee: Walsan Partners Limited Partnership, et al. New York, N.Y. "Quieting the JT3D Powered 707", Sound and Vibra (21) Appl. No.: 55,635 tion, vol. 8, No. 2, Feb. 1974, J. Woodall. Primary Examiner-Galen Barefoot (22 Filed: May 29, 1987 Assistant Examiner-Rodney Corl 51) Int. Cl...... B64D 29/00; B64D 33/04 Attorney, Agent, or Firm-Cooper & Dunham 52) U.S. Cl...... 244/1 N; 181/213; [57] ABSTRACT 181/220; 181/222; 181/296 58) Field of Search ...... 244/1 N, 55; 181/213, A system is provided for existing three-engine jet air 181/220, 222, 296 craft to reduce the noise levels at take-off and landing in order to meet governmental noise regulations. The (56) References Cited system is particularly suitable for 727-200 aircraft hav U.S. PATENT DOCUMENTS ing three JT8D engines. A hush kit is formed of the 3,002,341 10/1961 Muzzy et al...... 181/220 modified components for installation in existing air 3,237,891 3/1966 Wotton ...... 244/55 planes. 4,379,191 4/1983 Beggs et al...... 52/806 4,456,204 6/1984 Hapke ...... 244/55 4 Claims, 4 Drawing Sheets

U.S. Patent Jun. 6, 1989 Sheet 1 of 4 4,836,469

U.S. Patent Jun. 6, 1989 Sheet 2 of 4 4,836,469

U.S. Patent Jun. 6, 1989 Sheet 3 of 4 4,836,469

U.S. Patent Jun. 6, 1989 Sheet 4 of 4 4,836,469

4,836,469 1. 2 ments. Those regulations affect the usefulness and value SYSTEM FOR REDUCING AIRCRAFT NOISE AND of a much greater number of aircraft, including particu HUSH KIT larly -200 aircraft having Pratt & Whitney JT8D engines. About 1,700 of those airplanes currently BACKGROUND OF THE INVENTION are in operation. Heretofore, no one has successfully Prior to 1969, the aircraft industry paid little concern developed a system and hush kit to extend the useful life to commercial jet aircraft engine noise. In Dec. 1969, of those airplanes. the U.S. Federal Aviation Administration (“FAA') It is a difficult task to make those existing airplanes promulgated specific noise level regulations for aircraft meet the stage 3 noise level requirements. Studies which under authority of public Law 90-411. Existing air 10 have been conducted show that noise radiates from a planes were required to be certificated for compliance low bypass, fan in several directions. High with Federal Air Regulation No. 36 (“FAR 36'). Simi frequency fan noise radiates both forward though the lar noise standards were prescribed by international air inlet cowl and afterward through the exhaust ducts. civil aviation organizations (for example, "ICAO Low-frequency jet noise generally radiates rearwardly. Annex 16”). Several states and municipalities also estab 15 At low engine thrust, the high-pitch whine of the fan is lished maximum airport noise levels. Thus, it became more pronounced. At high engine thrust, the low-pitch imperative for airframe and engine manufactures and jet rumble is more noticeable. Each of those noise com owners to take noise considerations into account in ponents must be dealt with separately, as well as in designing, building and using jet aircraft. combination. But, making a modification to suppress In the 1970's, The Boeing Co. conducted research to noise may also severely degrade airplane performance, determine how noise levels could be reduced upon the substantially increase final consumption, require longer take-off and landing of its various models of commercial runways for take-offs or landings, mandate lesser pay airplanes. Boeing, of course, was concerned with design loads or otherwise greatly increase costs or require of future airplanes to meet more stringent noise stan operational limitations. dards, although it was also interested in modifications to 25 currently operated aircraft to meet current noise stan SUMMARY OF THE INVENTION dards. With respect to its Model 727 aircraft with pratt & One object of the present invention is to provide a Whitney JT8D engines, Boeing proposed to treat the system which will permit three-engine jet aircraft to nacelles for the jet engines with sound-attenuating ma 30 take-off and land at noise levels which meet FAR 36, terial. Boeing suggested adding the material to the inlet Stage 3, and ICAO Annex 16, Chapter 3, requirements. cowl, on some of the engine ducts and to the tailpipe. It Another object of the invention is to provide a system was suggested that the treatment would reduce high for three-engine jet aircraft which permits operation at frequency fan noise, but would have virtually no effect a quiet level conforming with more stringent govern on low-frequency jet rumble during take-off. Other 35 mental airplane noise regulations, while maintaining modifications attempted by Boeing were reported to cost-effective performance and providing at least no severely degrade airplane performance. greater fuel consumption. With respect to its Model 737 aircraft with Pratt & Still another object of the invention is to provide a Whitney JT8D engines, Boeing proposed similar sound retrofit or hush kit for existing three-engine jet aircraft attenuating treatment. Boeing further suggested using a which will permit continued operation of airplanes mixer device to mix the air flow from the fan with the which otherwise would not comply with governmental air flow through the engine to reduce the peakjet ve noise regulations and thus could not be flown, at least locity and thus the jet noise from the engine. Boeing on a cost-effective basis. reported its test results in its "Summary In accordance with the system of the present inven 707-727-737-747 Noise and Emission Reduction Activi 45 tion, an airplane is modified so as to comply with FAR ties', April 1980, Document D6-40613-K. 36, Stage 3, and ICAO Annex 16, Chapter 3, noise Thereafter, the major airplane manufacturers and regulations. The system comprises replacement of the others continued substantial research effort toward two existing outboard jet engines with new engines, developing retrofit kits for existing airplanes to attempt including the associated nacelles, thrust reversers, struts p meet FAR 36 noise level requirements. However, it 50 and other structural support. The existing center engine was difficult to develop a kit which would meet FAR is retained, but is fitted with an internal forced air mixer 36, Stage 2, requirements without, at the same time, and acoustically treated nozzle, in lieu of a center en degrading aircraft performance, increasing fuel con gine cascade thrust reverser, which is removed. sumption and unreasonably increasing costs. Although The system is particularly useful in the modification the effective date of the FAR 36 noise requirement was 55 of existing Boeing Model 727-200 aircraft having Pratt extended on several occasions, and the requirement was & Whitney JT8D-7/9/15/17 engines. However, the modified, FAR 36, Stage 2, went into effect on JUN. 1 system also may have applicability to other aircraft and , 1985. Implementation of that regulation affected the other jet engines. The components of the system may be usefulness and value of many existing aircraft, including combined in a hush kit for retrofitting existing airplanes. particularly Douglas DC-8 airplanes with Pratt & Whit ney JT3D engines. Aeronautic Development Corp. DESCRIPTION OF THE DRAWINGS Ltd. was successful in devising a quiet nacelle system FIG. 1 is a perspective view of a Boeing 727-200 and hush kit to make those airplanes economically via aircraft as modified in accordance with the system of ble. That system and hush kit is shown in allowed appli the present invention. cation Ser. No. 06/769,141, now U.S. Pat. No. 65 FIG. 2 is a graphic depiction of the reduced noise 4,723,626. level contour achieved by the present invention, as Many municipalities and airports have now adopted compared to the noise level contour of an unmodified the more stringent FAR 36, Stage 3, noise level require aircraft. 4,836,469 3 4. FIG. 3 is a side elevational view of a nacelle for one mesh 46 covers the perforated sheet 44 and forms a outboard engine of the modified aircraft, as compared smooth surface for laminar airflow. The honeycomb to a nacelle of an unmodified aircraft. core cells 40 communicate with the atmosphere FIG. 4 is a cross-sectional elevational view of a modi through the perforated sheet 44 and the woven wire fied outboard nacelle containing a JT8D-217C/219 jet 5 mesh 46. As is known to those skilled in the art, material engine. of this type may be tuned to particular noise frequencies FIG. 5 is a perspective view, partially cut-away of to provide effective acoustic performance. The particu the sound-attenuating material used in the nacelle. lar preferred material also provides aerodynamic FIG. 6 is a perspective view, partially cut-away, of smoothness and structural integrity. The smooth sur the nose dome within the nacelle. 10 face avoids drag, which impedes performance and in FIG. 7 is a cross-sectional elevational view of the creases fuel consumption. The acoustic material gives inlet cowl of the nacelle. linear performance and is a broadband noise absorber. FIG. 8 is an exploded, perspective view of a modified The material is sometimes referred to as a "liner', but in outboard nacelle showing the engine. fact is a structural substitute for a solid duct wall and is FIG. 9 is a perspective view, partially cut-away, of 15 structurally integrated with the other nacelle compo the mixer and acoustic nozzle within the centernacelle. nents. The nose dome 50 of the modified nacelle 22 is shown DETAILED DESCRIPTION OF THE in FIG. 6. The nose dome contains an anti-icing system INVENTION 52 to prevent the build-up of ice during flight. The nose Generally, as shown in FIG. 1 of the drawings, the 20 dome is bullet shaped and is positioned in front of the present invention relates to a jet airplane 10 having two engine fan 54, as shown in FIG. 4. outboard nacelles 12, supported on the sides of the fuse The inlet cowl 34 of the modified nacelle 22 is shown lage 14 by support struts 16, and a center nacelle 18 in FIG. 7. An anti-icing system 58 also is installed in the mounted on the empannage 20. Each nacelle contains a inlet cowl. The inlet cowl 34 is configured to cooperate jet engine adapted to propel the airplane 10. As modi 25 with the nose dome 50 to provide a smooth flow of air fied in accordance with the system of the present inven through the inlet to the fan 54 and thence through and tion, the airplane should operate at noise levels which around the engine 30. meet stringent governmental noise regulations, includ An exploded view of each modified outboard nacelle ing FAR 36, Stage 3, and ICAO Annex 16, Chapter 3, 22 is shown in FIG. 8. The jet engine 30 is mounted without losing airworthiness and structural integrity, 30 within the nacelle. As is conventional with such engines and without cost-reducing restrictions regarding run and nacelles, the inlet cowl 34 containing the deicing way length, take-off thrust, or payload reduction. system 58 is at the front of the nacelle. Upper and lower The invention contemplates replacement of the exist cowls 62,64 surround the engine 30. The lower cowl 64 ing outboard engines and nacells 12. Thus, the nacelles is comprised of forward and aft sections 66,68. The nose 12 are removed and new, larger nacells 22 are installed. 35 dome 50 containing the deicing system 52 is fitted in The difference in respective sizes is shown in FIG. 3. front of the engine fan 54 to provide for smooth air However, the larger nacelles 22 must be positioned so flow. A thrust reverser 70 and exhaust nozzle 72 are as to leave clearance for the aft cabin door 24. A JT8D mounted at the rear of the engine. 217/219 engine 30 is mounted in each outboard nacelle Because the outboard nacelles 22 and engines 30 are 22 to replace the existing JT8D-7/17 engines, as shown larger and heavier in the modified system than those in in FIG. 4. The newer engines have a larger fan, and the existing airplane, it is necessary to use struts 16 to different high and low-pressure compressors. The connect the necelles to the fuselage 14 which can carry newer engines are quieter than the older engines, but more weight. Portions of the aft fuselage also must be still not to an extent sufficient to meet the Stage 3 noise strengthened. The replacement jet engines 30 develop level requirements. Other than as described herein, the 45 about 10,000 pounds more thrust than the original en nacelle elements are standard components known to gines. The overall thrust produced by the two outboard those skilled in the art. engines is of the same magnitude as the thrust produced As best seem in FIG. 4, each outboard nacelle 22 is by the three original engines. modified to include sound-attenuation material 32 in It has been found to be impractical to replace the selected portions of the nacelle. It has been found that it 50 centernacelle 18 and center engine of the airplane. The is most efficient used to use the sound-attenuation mate modified outboard nacelle and engines are large and rial 32 on the nose cowl 34 defining the inlet to the heavier and would not fit into the space left by the nacelle, the forward and aft cowls 36 circumscribing removed centernacelle 18. Accordingly, it is a feature the engine and the exhaust nozzle 38, as well as on some of the invention that the center nacelle and engine are of the bulkheads. 55 left in place and modified, rather than removed. The The sound-attenuating material 32 which has been modified centernacelle 18 and engine, together with the found particularly effective for the purpose of the pres replacement outboard nacelles 22 and engines, are suffi ent invention is a single-layer "DynaRohr' liner mate cient to reduce the noise levels to an extent meeting the rial made by Rohr Industries, Inc. That material is FAR 36, Stage 3, requirements. As the thrust produced shown and described, for example, in U.S. Pat. No. by the two new outboard engines is substantially greater 4,379,191, the disclosure of which is incorporated than that of the replaced engines, it is not necessary to herein by reference. The acoustic liners may be fabri have a thrust reverser on the center engine, and the cated of many materials and in different sizes and thrust reverser is removed. strengths, depending upon specific operating condi In the position vacated by the thrust reverser, as tions. As shown in FIG. 5, the sound-attenuating mate 65 shown in FIG. 9, there is mounted a forced-air mixer 80. rial preferably comprises a plurality of honeycomb core The scalloped-shaped mixer combines the hot air pass cells 40, sandwiched between a solid aluminum sheet 42 ing through the center engine with colder air passing and a perforated aluminum sheet 44. A woven wire around the engine. A mixer developed by Pratt & Whit 4,836,469 5 6 ney for JT8D engines installed on airplanes Boeing 727-200 airplanes, having Pratt & Whitney has been found suitable for this purpose. Behind the JT8D engines. However, those skilled in the art will mixer in the nacelle, a nozzle 82 is mounted. The nozzle understand that the system of the invention also is appli or tailpipe is lined with sound-attenuating material 84, cable to other aircraft and engines by modifications such as that shown in FIG. 5. which do not depart from the spirit and scope of the The system of the invention has been found to be invention, effective in reducing noise levels to the extent necessary I claim: to meet FAR 36, Stage 3, requirements. As shown in 1. A method for quieting jet engine noise produced FIG. 2, an unmodified Boeing 727-200 produces a noise by an airplane having three jet engines upon take-off contour or "footprint' about a runway 102. Use of the 10 and landing, comprising: system of the invention result in a far smaller footprint (a) replacing two outboard-mountedjet engines at the 104. The contours shown are with a take-off weight of rear of the fuselage with jet engines which produce 197,000 lb. lower noise and greater thrust; FAR 36, Stage 3, requires that the noise generated be (b) retaining a center fuselage-mounted jet engine, substantially less than in Stage 2. Applicant's system 15 and removing a thrust reverser from the aft posi will reduce the noise level on take-off from 102.5 tion thereof; EPNdB to 93.0 EPNdB (“effective perceived noise'); (c) mounting a forced-air mixer on the center engine sideline noise from 104.2 EPNdB to 97.4 EPNdB, with at the position from which the thrust reverser was center engine take-off thrust at 90% of normal power removed, the mixer being adapted to combine cold and outboard engines at 100% of normal power; and 20 air passing around the engine with hot air passing approach noise from 106.2 EPNdB to 100.7 EPNdB at through the engine, to reduce engine noise emitted 40 flaps, and to 97.4 EPNdB at 30 flaps. It should be by the center engine; and appreciated that a variance of less than 2 EPNdB at any (d) mounting an acoustic nozzle comprised of sound position will still meet the requirement. It should also be attenuating material aft of the forced-air mixer on appreciated that a noise reduction of 10 EPNdB corre 25 the center engine, sponds to a reduction of 50% in the total perceived whereby the noise level produced the airplane is re noise level. duced to a sufficiently quiet level to meet governmental As a bonus, it is believed that the system will improve noise regulations while remaining airworthy and per fuel economy by about 12 to 18%, and increase maxi mitting operation without reduced payloads or thrust. mum range by about 300 nautical miles for standard day 30 2. A method as in claim 1 further comprising replac operation. An airplane equipped with the system re ing two outboard-mounted nacelles with larger nacelles quires a runway at least about 17% shorter for take-off, to accommodate the two replaced outboard-mounted if the center engine is operated at full power. In one test, jet engines at the rear of the fuselage. the runway needed was about 5,650 ft., as opposed to 3. A method as in claim 2 further comprising fitting 8,200 ft. for a conventional airplane. Time to climb to 35 heavier struts for the two replacement outboard the desired flight altitude is reduced by about 23%. The mounted nacelles for the two outboard-mounted jet maximum take-off weight and landing weight are about engines at the rear of the fuselage. the same. 4. A hush kit for adapting a jet airplane having two jet The invention is advantageously packaged in the engines mounted on the rear of the fuselage and one jet form of a hushkit, which can be used to retrofit existing engine mounted on the empennage, so as to meet gov aircraft. A "hush kit' is a collection of replacement or ernmental noise level regulations, comprising: substitute components which, when combined and in (a) two nacelles adapted to be mounted to the rear stalled on an airplane, enable that airplane to operate at sides of the fuselage, each containing a jet engine lower noise levels. In the present invention, the hushkit having lower noise and greater thrust then the comprises the two outboard nacelles 22, each contain 45 outboard mounted jet engines on the airplane; ing a JT8D engine 30, the forced-air mixer 60 for re (b) means adapted for mounted the nacelles to the placing the thrust reverser on the center engine, and the fuselage; acoustic nozzle 82 for mounting aft of the mixer on the (c) a forced-air mixer adapted to be fitted to the em center engine. The hush kit also contains, of course, the pennage-mounted jet engine; and associated struts, engine mounts, structural reinforce 50 (d) a nozzle lined with sound-attenuating material on ments and other hardware necessary to install and as the airflow surfaces thereof and adapted to be semble the hush kit components. Whenever a system is - mounted aft of the forced-air mixer, changed, instrumentation, controls, electrical and hy whereby the components of the kit are adapted to be draulic system need to be modified, as will be apparent substituted for existing components of the airplane to to those skilled in the art. 55 facilitate reduction of engine noise during take-off and It should be appreciated that the invention is particu landing of the airplane. larly applicable to a three-engine jet aircraft, such as it is

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