Quiet nacelle system and hush kit

A quiet nacelle system for jet aircraft engines includes a nose cowl and nose dome having a sound-attenuating liner, a concentric ring between the nose cowl and nose dome, and fan duct components having a sound-attenuating liner. The system is particularly suitable for JT3D-3B and JT3D-7 engines for use with DC-8 aircraft. The components may be combined as a hush kit for retrofit in existing nacelle systems.

BACKGROUND OF THE INVENTION 
Prior to 1969, the aircraft industry paid little concern to commercial jet 
aircraft engine noise. In December 1969, the U.S. Federal Aviation 
Administration ("FAA") promulgated specific noise level regulations for 
aircraft under authority of Public Law 90-411. Existing airplanes were 
required to be certificated for compliance with Federal Air Regulation No. 
36 ("FAR 36"). Similar noise standards were prescribed by international 
civil aviation organizations (for example, "ICAO Annex 16"). Several 
states and municipalities also established airport noise levels. Thus, it 
became imperative for airframe and engine manufacturers and owners to take 
noise considerations into account in designing, building and using jet 
aircraft. 
By about 1970, Douglas Aircraft Co., on behalf of the National Aeronautics 
and Space Administration, had undertaken studies concerning fan-compressor 
noise from the Pratt & Whitney JT3D-3B engines used with DC-8-50/61 
airplanes made by Douglas. The studies showed that noise levels might be 
reduced in a short duct nacelle design having revised fan inlet and 
exhaust ducts containing acoustically absorptive linings. Various 
configurations were tested. Although it was shown that typically up to 
10.5 EPNdB ("effective perceived noise") reduction in the noise level 
could be achieved (at maximum certified landing weight and at a point on 
the ground beneath a 3.degree. landing-approach path one nautical mile 
from the runway threshold), static take-off-rated gross thrust was reduced 
by 2.5% and fuel consumption was increased by 3%. Moreover, direct 
operating costs were estimated to increase 4.4% and return on investment 
would decline about 6%. Douglas also tested the use of a splitter ring 
between the nose cowl and the nose dome. 
At the same time, The Boeing Co. was conducting similar studies of noise 
reduction for JT3D engines used with Boeing 707-320C airplanes. The 
studies showed that noise reductions up to 15 EPNdB could be achieved on 
landing approach. It was planned to accomplish this reduced noise level by 
installing one or two acoustically treated rings in the engine inlet and 
by acoustically treating an extended 3/4 length fan duct configuration. 
That modification resulted in a range reduction of 200 nautical miles and 
a direct operating cost increase of more than 9%. Further, the use of 
rings was thought to interfere with de-icing of the engines. 
Thereafter, the major airframe manufacturers undertook substantial research 
effort toward developing retrofit kits for existing airplanes to attempt 
to meet FAR 36 noise level requirements. However, they were unable to 
develop kits which would meet the noise requirements without, at the same 
time, degrading performance, increasing fuel consumption and unreasonably 
increasing costs. As a result of the unavailability of retrofit hush kits, 
the value of existing airplanes fell significantly as their useful lives 
neared an end. As of Oct. 1, 1979, there were about 155 DC-8 airplanes 
with JT3D engines in service by U.S. airlines and about 218 in service by 
foreign airlines. Although the effective date of the FAR 36 noise 
requirements was extend on several occasions, and the requirements were 
modified, the FAR 36, Stage 2, regulation finally became effective as of 
Jan. 1, 1985. On that date, the existing DC-8 airplanes became essentially 
obsolete for use in the United States. 
The studies which had been conducted showed that noise radiates from a low 
by-pass, fan jet engine in several directions. High-frequency fan noise 
radiates both forward through the air inlet cowl and aftward through the 
exhaust ducts. Low-frequency jet noise generally radiated rearwardly. At 
low engine thrust, the high-pitch whine of the fan is more pronounced. At 
high engine thrust, the low-pitch jet rumble is more noticeable. 
Each noise component must be dealt with separately, as well as in 
combination. Generally, some studies showed that noise attentuation 
material was useful for reducing some noise components. One type of 
material frequently used consists of honeycomb core cells bonded to a 
porous sheet on the airflow surface and an impervious sheet on the 
rearward surface. Such noise attenuation material has been used in several 
different nacelle configurations for different airplanes. 
Other studies showed that nacelle modifications could suppress engine 
noise. However, those modifications which, it appeared, could succeed in 
suppressing noise would also severely degrade airplane performance or 
substantially increase fuel consumption. 
Thus, although existing DC-8 airplanes could not be used, at least in the 
United States, after implementation of the FAR 36, Stage 2, noise level 
requirements, no one prior to Aeronautic Development Corporation Limited 
had developed and certified with the FAA a system to reduce the noise 
levels and thereby make the obsolete DC-8 airplanes economically viable. 
Applicants' nacelle system was found effective in reducing the noise levels 
at take-off and landing to compliance with FAR 36, Stage 2, noise level 
requirements. On June 28, 1985, the FAA issued a Supplemental Type 
Certificate ("STC") approving use of the invention in connection with 
Pratt & Whitney turbofan JT3D-3B engines in Douglas DC-8-62 and DC-8-62F 
airplanes. The following Table 1 shows the reduced noise levels achieved 
with the quiet nacelles at take-off, sideline and landing in a DC-8-62 
airplane with JT3D-3B engines at various maximum take-off and landing 
gross weights, while Table 2 shows comparable information for unmodified 
nacelles: 
TABLE 1 
______________________________________ 
Max. Max. 
Take-off 
Landing Quiet Nacelles 
Gross Wt 
Gross Wt Take-off Sideline Landing 
(lbs) (lbs) (EPN dB) (EPN dB) 
(EPN dB) 
______________________________________ 
350,000 250,000 104.3 98.1 108.3 
350,000 240,000 104.3 98.1 108.3 
335,000 250,000 102.5 98.2 108.3 
335,000 240,000 102.5 98.2 108.3 
______________________________________ 
TABLE 2 
______________________________________ 
Max. Max. 
Take-Off 
Landing Unmodified Nacelles 
Gross Wt 
Gross Wt Take-Off Sideline Landing 
(lbs) (lbs) (EPN dB) (EPN dB) 
(EPN dB) 
______________________________________ 
350,000 250,000 111.0 103.0 114.0 
335,000 250,000 110.0 103.0 114.0 
______________________________________ 
Similarly, on July 15, 1985, the FAA issued an STC approving use of 
applicants' nacelle system in connection with Pratt & Whitney turbofan 
JT3D-7 engines in Douglas DC-8-63 and DC-8-63F airplanes. The following 
Table 3 shows the reduced noise levels achieved with quiet nacelles, while 
Table 4 shows generally comparable information for unmodified nacelles for 
that aircraft and engines: 
TABLE 3 
______________________________________ 
Max. Max. 
Take-Off 
Landing Quiet Nacelles 
Gross Wt 
Gross Wt Take-Off Sideline Landing 
(lbs) (lbs) (EPN dB) (EPN dB) 
(EPN dB) 
______________________________________ 
355,000 262,000 104.1 98.2 108.5 
355,000 258,000 104.1 98.2 108.5 
355,000 245,000 104.1 98.2 108.4 
______________________________________ 
TABLE 4 
______________________________________ 
Max. Max. 
Take-off 
Landing Unmodified Nacelles 
Gross Wt 
Gross Wt Take-Off Sideline Landing 
(lbs) (lbs) (EPN dB) (EPN dB) 
(EPN dB) 
______________________________________ 
355,000 275,000 113.9 102.8 114.3 
______________________________________ 
The reduced noise levels shown in Table 1 and 3 meet both FAR 36, Stage 2, 
noise level requirements and ICAO Annex 16 requirements. However, they do 
not meet the more stringent FAR 36, Stage 3, requirements. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide an aircraft jet engine 
nacelle adapted to permit engine operation at low noise levels at take-off 
and landing, and thus meet FAR 36, Stage 3, noise level requirements. 
Another object of the invention is to provide a nacelle system for a jet 
engine which reduces engine noise to a quiet level conforming with more 
stringent governmental airplane noise regulations, while maintaining 
airworthiness and flight performance of the airplane. 
Still another object of the invention is to provide a retrofit kit for 
existing jet airplanes which will permit continued operation of airplanes 
which otherwise would not comply with more stringent governmental noise 
regulations and could not be flown. 
The present invention contemplates achieving these objects through use of a 
combination of nacelle structural modifications and properly positioned 
sound attenuating material. To that end, the nose cowl of the nacelle is 
elongated and an acoustic liner is used on the airflow surface. The nose 
dome is entended to conform to the nose cowl elongation and an acoustic 
liner is used on the airflow surface. The nose dome is entended to conform 
to the nose cowl elongation and both the nose dome extension and the nose 
dome have acoustic liners on the airflow surfaces. A concentric ring 
provided with acoustic liners is disposed within the cowl surrounding the 
nose dome. The fan ducts are provided with acoustic liners on the airflow 
surfaces, and the splitters also are provided with acoustic liners on the 
airflow surfaces. The preferred acoustic liner is a single-layer honeycomb 
sandwich material. 
The nacelle system of the present invention is particularly adapted for use 
with Pratt & Whitney JT3D-3B and JT3D-7 engines, for use with Douglas 
DC-8-62 and DC-8-63 aircraft, including those which previously have been 
modified to comply with FAR 36, Stage 2, noise level requirements, but is 
also useful with similar aircraft and engines.

DETAILED DESCRIPTION OF THE INVENTION 
Generally, as shown in FIG. 1 of the drawings, the present invention 
relates to a jet airplane 10 having a plurality of nacelles 12 suspended 
from the airplane wings 14 by pylons 16. Each nacelle 12 contains a jet 
engine adapted to propel the plane. The nacelles 12 are modified in 
accordance with the present invention so as to operate at noise levels 
which meet governmental noise regulations, including particularly the 
various stages of FAR 36, and still maintain airworthiness and structural 
integrity under other parts of FAA regulations. 
The nacelles 12, in accordance with a first embodiment of the invention, 
are shown in greater detail in FIG. 3. Each nacelle 12 includes a housing 
comprising a nose cowl 20, a forward cowl 22 trailing the nose cowl 20, 
and an aft cowl 24 trailing the forward cowl 22. The nose cowl 20 defines 
the inlet 26 to the nacelle. The forward cowl 22 and aft cowl 24 
circumscribe the fan, combustion and turbine sections of the engine 28. 
The exhaust 30 of the engine is defined by the thrust reverser 32 and 
nozzle 34. Other than as described herein, each of those elements is a 
standard component in existing nacelle systems known to those skilled in 
the art. 
The nose cowl 20 of the nacelle 12 is modified in accordance with the first 
embodiment of the invention to have an extension or elongation beyond the 
nose cowl length conventionally employed in existing nacelle systems for 
DC-8 airplanes in use at the present time. It has been found that an 
extension of approximately 12 inches provides, in combination with the 
other components of the system, particularly effective reduction in noise 
levels. A longer extension is unnecessary and adds weight to the system 
without improving performance. The term "elongated nose cowl", as used 
herein, means a nose cowl which has been extended to about that optimum 
length. 
As best seen in FIG. 4, the nose cowl 20 comprises an outer barrel 36, 
which forms the exterior surface, an inner barrel 38 which forms the 
interior airflow surface and a lip assembly 40 connecting the inner and 
outer barrels and which includes the anti-icing system for the engine. The 
inner barrel 38 is lined or bonded with sound-attenuating material 42 on 
the airflow surface. However, the sound-attenuating material 42 is not 
applied to the lip assembly 40, so as to avoid interference with the 
anti-icing system. It has been found that the forward edge of the 
sound-attenuating material 42 should be located about 6 inches behind the 
lip assembly for that reason. Preferably, the material 42 forms a liner 
having two different thicknesses. It has been found that the material 
desirably has a thickness of about 3/4 inch for most of the length of the 
inner barrel 38, but that the thickness should be reduced to less than 1/2 
inch for the rearwardmost portion of the material adjacent the engine 
attachment flange at the aft bulkhead 44. This facilitates attachment of 
the engine 28. 
The nose dome 50 entends from the fan 52 of the engine 28, as shown in FIG. 
3. The nose dome 50 is bullet shaped and conforms to the shape of the 
inner barrel 38 of the nose cowl 20, so as to provide smooth airflow 
through the inlet 26. The nose dome 50 of the nacelle 12 is modified in 
accordance with the first embodiment of the invention to have an extension 
or elongation 54 adjacent the fan 52. The extension 54 preferably should 
be about 12 inches in length to cooperate and provide effective sound 
attentuation in combination with the extended nose cowl 20. The 
pitotstatic PT 2 probe and anti-icing system incorporated in the nose dome 
are not affected by the extension. 
As best seen in FIG. 5, in the first embodiment of the invention, the 
existing nose dome 50 is not structurally changed, but the extension 54 is 
attached between the rearward portion of the dome 50 and the fan 52. The 
entire extension 54 is lined or bonded with sound attenuating material 56 
on the airflow surface. However, the sound-attenuating material 56 is not 
applied to the unmodified portion of the nose dome 52, so as to avoid 
changing the airflow characteristics in the inlet. It is desirable to 
avoid changing the airflow liner and aerodynamic loft lines, if not 
necessary to meet noise regulations, because any such changes may affect 
the performance and airworthiness of the plane. 
Fan ducts 60 extend rearwardly from the engine fan 52. Compressed air is 
forced through the fan ducts 60 to provide increased thrust for the 
engine. The fan ducts 60 must be configured to fit in the narrow space 
between the outer housing provided by the forward cowl 22 and aft cowl 24 
and the jet engine 28. A nacelle which has fan ducts 60 extending from the 
fan 52 to the thrust reverser 32 is referred to herein as a "long fan 
duct" nacelle. 
As shown in FIG. 3, the fan ducts 60 comprise pairs of bifurcation fan 
ducts 62, constant section fan ducts 64 trailing the bifurcation fan ducts 
62, transition section fan ducts 66 trailing the constant section fan 
ducts 64 and aft section fan ducts 68 trailing the transition section fan 
ducts 66 and leading to the thrust reverser 32. It has been found that, in 
the combination of the present invention, no sound-attenuating material is 
necessary for the bifurcation fan ducts 62 and aft section fan ducts 68. 
If the transaction section fan ducts 64 and the aft section fan ducts 68 
are formed in one piece, then that entire structure may be provided with 
sound-attenuating material. 
The constant section fan duct assemblies 64 are shown in FIGS. 7 and 8. 
Each constant section fan duct 64 comprises an outer duct wall 70, an 
inner duct wall 72 and a plurality of separators or splitters 74 which 
hold the inner and outer walls together at a constant distance against the 
great thrust exerted by the fan 52. Each of the inner and outer walls 70, 
72 and each of the splitters 74 has a sound-attenuating liner 76 on the 
airflow surface thereof. It has been found particularly effective to form 
the splitters 74 of the acoustic liner material or to bond the material 
thereto. 
It has been found desirable to apply a high-temperature resistant coating 
78 to the innermost surface of the inner duct wall 72. That surface is 
spaced only a slight distance from the engine 28, which generates 
significant heat during operation. The coating 78 prevents fire damage to 
the sound-attenuating liner 76. A suitable material for this purpose is MA 
25 S silicon ablative coating manufactured by Martin-Marietta Co. The 
outermost surface of the outer duct wall 70 preferably is make of 
stainless steel to provide high strength within a tight space. 
The transition section fan duct assemblies 66 are shown in FIGS. 9 and 10. 
Each transition section fan duct 66 comprises an outer duct wall 80, and 
inner duct wall 82 and a plurality of separators or splitters 84 which 
hold the inner and outer walls together at specified distances against the 
fan thrust. Each of the inner and outer walls 80, 82 and each of the 
splitters 84 is lined or bonded with sound-attenuating material 86 on the 
airflow surface thereof. Similarly, it has been found desirable to apply a 
high-temperature resistant coating 88 to the innermost surface of the 
inner duct wall 82. The outermost surface of the outer duct wall 80 
preferably is made of stainless steel. 
The sound-attenuating liner material which has been found particularly 
effective for the purpose of the present invention is a single-layer 
"DynaRohr" liner material made by Rohr Industries, Inc. That material is 
shown and described, for example, in U.S. Pat. No. 4,379,191, the 
disclosure of which is incorporated herein by reference. The acoustic 
liners may be fabricated of many materials and in different sizes and 
strengths, depending upon specific operating conditions. As shown in FIG. 
6, the sound-attenuating liner material preferably comprises a plurality 
of honeycomb core cells 90, sandwiched between a solid aluminum sheet 92 
and a perforated aluminum sheet 94. A woven wire mesh 96 covers the 
perforated sheet 94 and forms a smooth surface for laminar airflow. The 
honeycomb core cells 90 communicate with the atmosphere through the 
perforated sheet 94 and the woven wire mesh 96. As is known to those 
skilled in the art, material of this type may be tuned to particular noise 
frequencies to provide effective acoustic performance. The particular 
preferred material also provides aerodynamic smoothness and structural 
integrity. The smooth surface avoids drag, which impedes performance and 
increases fuel consumption. The acoustic liner give linear performance and 
is a broad-band noise absorber. Although the material is sometimes 
referred to as a "liner", the material in fact is a structural substitute 
for a solid duct wall and is structurally integrated with the other 
nacelle components. 
The effect of the reduced noise levels which was achieved by the first 
embodiment of the invention is demonstrated graphically in FIG. 2. The 
drawing shows projected noise "footprints" or contours produced during 
landing and take-off by the quiet nacelle system of the invention and by 
an unmodified nacelle system. The runway 100 forms the base line. The 
noise level contour 102 is for the unmodified system and the much smaller 
noise level contour 104 is for the system in accordance with the first 
embodiment of the invention. It should be appreciated that a reduction of 
10 EPNdB corresponds to a reduction of 50% in the total perceived noise 
level. A similar drawing showing an even smaller noise level contour would 
describe the system in accordance with the second embodiment of the 
invention, to be explained below. 
The components of the invention are advantageously packaged together as a 
"hush kit". A hush kit is a group of components which are substituted for 
existing components in a nacelle, such that the nacelle may be retrofitted 
without replacement of the entire nacelle system. Thus, the hush kit of 
the first embodiment of the invention may comprise, within a single 
package, the elongated nose cowl 20 with the sound-attenuating liner 42; 
the nose dome extension 54 having the sound attenuating liner 56; a pair 
of constant section fan ducts 64 having the sound-attenuating liners 76; 
and a pair of transition section fan ducts 66 having the sound-attenuating 
liners 86. The hush kit contains all the necessary major components to 
retrofit an existing nacelle system. The total weight increase due to the 
modified components is only about 250 lbs. per nacelle. 
In accordance with a second embodiment of the invention, the nacelles 112 
are modified as shown in FIGS. 11 through 13. The forward portion of the 
nacelle 112 has an elongated nose cowl 120, similar to the nose cowl 20 of 
the first embodiment. The elongated nose dome 150 extends from the fan of 
the engine and is shaped to match the nose cowl. It should be appreciated 
that, in the second embodiment of the invention, it may not be necessary 
to extend the nose cowl and the nose dome. As shown in FIG. 11, the nose 
dome 150 is lined or bonded with sound-attenuating material 156 on the 
airflow surface, except at the forwardmost portion which contains the 
anti-icing system. 
To meet the requirements of FAR 36, Stage 3, it has been found necessary to 
make additional modifications, both structurally and acoustically, beyond 
those needed to meet the requirements of FAR 36, Stage 2. 
To that end, a cylindrical ring 158 is concentrically mounted between the 
nose cowl 120 and the nose dome 150. The ring is lined or bonded with 
sound-attenuating material on the airflow surface. As shown in FIG. 11, 
the ring 158 may be mounted to the nose cowl 120 by struts 160. 
Preferably, three or four struts are employed, spaced about the 
circumference of the ring. The struts 160, which may be formed of 
sound-attenuating material, are hollow so that heated air may pass to the 
ring 158 from the nose cowl's anti-icing system to prevent icing of the 
ring. The ring 158 should be contoured so as not to affect the aerodynamic 
characteristics of the nose cowl. The front of the ring should also be 
spaced about 12 inches from the front of the nose cowl 120 and spaced 
about 4 inches from the fan to avoid interference. It is believed that the 
cylindrical ring effectively reduces forward propagating fan nose or howl 
with minimal flight performance loss. 
FIGS. 12 and 13 show two further modifications in accordance with the 
second embodiment of the invention. In FIG. 12, the ring 158 is connected 
to the nose dome 150 by struts 162. The struts 162 are forwardly angled to 
achieve correct aerodynamic performance. The nose dome 150 is not formed 
entirely of sound-attenuating material, but only the extended portion 154 
is formed of the material. Again, the struts 162 are hollow so that the 
ring may be connected to the anti-icing system in the nose dome. In FIG. 
13, the ring 164 is connected to the nose dome 150 by struts 166. The 
struts 166 are rearwardly angled for proper airflow. In this modification, 
the nose dome, other than for the very forwardmost portion, is formed of 
sound-attenuating material, as are the struts 166 and ring 164. 
The components of the second embodiment of the invention also may comprise 
a hush kit. The hush kit desirably includes, within a single package, all 
the modified components. Alternatively, if the airplane already has been 
modified to meet FAR 36, Stage 2, requirements, only the further modified 
portions, including the nose dome and the ring, need form the hush kit to 
meet FAR 36, Stage 3, requirements. Still further, use of the hush kit may 
enable a modified airplane which does not presently fully meet FAR 36, 
Stage 2, to meet those requirements. 
The invention has been described with particularity for a DC-8 airplane, as 
shown in FIG. 1. It has been found that the invention is especially 
effective when used for Pratt & Whitney JT3D-3B and JT3D-7 low by-pass, 
fan jet engines used in nacelles for DC-8-62 and DC-8-63 airplanes. 
However, the invention is applicable to a variety of long duct nacelle 
systems used with somewhat different engines and airplanes including 
DC-8-55 and DC-8-61 airplanes. The detailed description is intended to be 
illustrative of quiet nacelle systems using the present invention in the 
preferred manner. Nevertheless, it should be appreciated that various 
modifications could be made in nacelle systems which remain within the 
spirit and scope of the invention. Many other uses of the invention should 
be apparent to those working in the industry who are skilled in the art.