Design and performance of acoustically lined fan discharge duct for noise level reduction during aircraft landin

Mc Cormick, R. B.

English

NASA Technical Reports Server

15. FAN-DUCT DEVELOPMENT 
By Ralph B. McCormick 
The Boeing Company 
SUMMARY 
The fan noise radiated from the fan discharge duct during the landing approach of 
current fan jet airplanes is the largest single contribution to the composite engine noise. 
The Boeing Company has designed and ground tested an acoustically treated long fan duct 
to reduce this component of engine noise. Data obtained during the tests show a reduction 
in the peak noise level of the treated duct over the existing duct of 9 to 16 PNdB for 
approach power settings at a distance of 370 feet. Engine thrust measurements indicate 
an increase in thrust of approximately 2 percent over the existing short duct. Flight-test 
fan ducts are being constructed for evaluation in mid-1969. It is estimated that the long 
treated fan ducts will increase the weight of each nacelle by approximately 750 pounds. 
INTRODUCTION 
The Boeing Company has been involved in the development and evaluation of a turbo 
fan nacelle modification to reduce fan noise since the fan whine emitted from the fan duct 
is the largest single contribution to the composite engine noise. The basic program goal 
is a 15-PNdB noise reduction of a Boeing 707-320B airplane during landing approach and 
is to be accomplished by attenuating the fan-duct component of noise by this amount. 
The program for development of an acoustically treated fan duct was divided into 
three phases: (1) a trade study of various concepts was made to aid in the selection of 
a configuration; (2) a ground-test duct was  designed and tested for performance evalua- 
tion; and (3) four flight-test ducts are being constructed for flight evaluation in mid-1969. 
OBJECTIVES 
An attenuation goal for the fan duct is shown in figure 1. The goal was selected to 
give the peak attenuation at the fan frequency during the approach condition. A peak atten- 
uation of 30 dB is required and a suitable bandwidth was selected to obtain the required 
attenuation over the range of power settings associated with a normal approach. Fig- 
ure 2 shows a typical sound level spectrum of the peak noise attributed to the fan duct at 
an engine speed of 5000 rpm. The data were obtained from ground tests of a production 
untreated duct and were adjusted to a 370-ft distance. The effects of the selected 
227 
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attenuation goal a re  also shown and indicate the sound level required to achieve a peak 
noise reduction of 15 PNdB. 
The noise reduction must be accomplished without imposing a severe penalty on 
the performance of the airplane. The current production fan duct on the 707 airplane 
shown in figure 3 is very short in length. It was designed to minimize the weight and 
external drag of the nacelle. The limited length and cross-sectional area are not suffi- 
cient to install a significant amount of acoustic material. It is necessary to design a 
substantially longer duct to contain the required amount of acoustic material. This will 
mean an increase in the nacelle size and weight and will  result in an increase in the inter- 
nal and external drag of the nacelle. The short duct, however, discharges high-velocity 
air over the nacelle afterbody causing a significant amount of scrubbing drag. This drag 
can be reduced with a longer duct. A long duct design will require that the duct be 
bifurcated to clear the support strut at the top of the nacelle and the engine accessories 
at the bottom to maintain sufficient ground clearance. 
TRADE STUDY 
To obtain an insight into the most reasonable compromise of fan-duct length, cross- 
sectional area, and shape, a trade study was  made. A number of duct design concepts 
were studied to determine the amount of acoustic treatment required and the effect on 
airplane performance. Three of the concepts studied are illustrated in figure 4. This 
study led to the selection of concept 3 for further development. Concept 1 extended to 
about seven-eighths of the length of the nacelle and had a kidney-shaped cross  section and 
nozzle. Concept 2 was  a full-length duct with a kidney-shaped cross  section and nozzle. 
Concept 3 was also a full-length duct. The cross  section was changed to an elongated 
kidney shape to reduce the internal turning losses and to decrease the separation distance 
of the acoustic material from approximately 9 to 7 inches. A coplanar annular nozzle 
was used. All the concepts had the duct divided into five channels on each side to con- 
trol the turning losses and provide for the structural design. The trade between duct 
internal flow area and nacelle maximum diameter was studied by considering three duct 
flow areas at the nacelle maximum diameter. The areas provided for (1) a contraction 
from 828 sq in. at the fan exit to a flow area of 660 sq in., (2) a constant flow area for 
much of the duct length, and (3) an expansion from 828 sq  in. at the fan exit to a maxi- 
mum flow area of 900 sq in. 
An analysis of the airplane performance was made using a constant payload and 
field length to determine the effect on range. Consideration was given to the internal flow 
losses, weight, and external drag of the various concepts. The changes in range as com- 
pared with the range of the production airplane are shown in figure 5 for the various flow 
228 
areas considered. Concept 3 with a maximum flow area of 900 sq in. has the least effect 
on airplane range. 
The acoustic analysis of these concepts (fig. 6) determined the peak noise reduc- 
tion as a function of the treatment length. The elongated kidney shape with the smaller 
treatment separation distance required approximately 67 inches of treatment length to 
meet the 30-dB peak attenuation goal; whereas, the kidney shape required an additional 
15 inches. The analysis was based on experimental data obtained in duct flow tests. 
Other experimental and theoretical data indicate that an additional noise reduction of about 
4 dB can be achieved with coplanar concentric nozzles. These results along with the 
performance analysis indicated the selection of concept 3 for further development. 
The configuration selected (fig. 7) has an internal contour developed from the geom- 
etry indicated by the trade study. It was designed in four sections: A bifurcated section 
turns the flow from the annular fan exit to the bifurcated elongated kidney shape and also 
expands the flow from 828 sq  in. at the fan exit to 900 sq in. A linear flow-area dis- 
tribution was chosen to preclude a large local diffusion. The constant-area section will 
contain most of the acoustic material. The flow area  is a maximum through this section 
to give a low velocity over the rough acoustic material and reduce the internal losses. 
The transition section provides the change from the elongated kidney shape to the annular 
nozzle, The flow area  through this section was determined by the space available between 
the engine and the required external nacelle contour, The nozzle is annular and approxi- 
mately coplanar with the primary nozzle. 
A 1/5-scale mo"de1 of the internal configuration of the duct was tested. Measure- 
ments of thrust, airflow, and pressure loss confirmed the estimated internal losses used 
in the trade study. 
GROUND-TEST DUCT 
A full-scale ground-test duct was constructed to verify the estimated acoustic and 
propulsion performance. The selected internal configuration was used and the duct was 
designed so that only minimum changes to the engine systems would be required. Each 
half of the duct was constructed in four sections as shown in figure 8. The bifurcated 
section was constructed of epoxy-fiber glass. The side cowl is constructed of polyimide- 
fiber glass sandwich honeycomb that is designed to provide for both the acoustic and 
structural requirements. A cost and weight study indicated the use of polyimide-fiber 
glass construction in this area. A thrust reverser  was not developed for this configura- 
tion,. but a space provision was provided. The transition and nozzle section is con- 
structed of aluminum sheet and frames. It has a 30-inch-long polyimide-fiber glass 
acoustic insert panel at the forward end. The panel is removable and will be used to 
determine the effect of extending the treatment length. 
229 
The acoustic treatment used in the side cowl consisted of double-layer treatment in 
the outside wall and single-layer treatment on the splitters and inside wall. The treat- 
ment depth selected to provide noise reduction over the selected bandwidth was 1.0 inch 
for the double layer and 0.5 inch for the single layer. The flow resistance of the material 
varies from 5 to 35 rayls for the double layer and from 6 to 30 rayls for the single layer. 
The value at any location is a function of the local sound pressure level and flow Mach 
number. 
The acoustic treatment for the aft insert panel is the same as for the side cowl, 
except that the inside wall is not treated. Including the panel there is a total of 267 sq f t  
of acoustic treatment. This is in excess of the estimated amount required to account for 
manufacturing problems, accuracy of predictions, and possible deterioration in acoustic 
effectiveness due to contamination. 
Ground tests were run on a test stand at a remote location. Baseline tests of the 
existing production short duct and the full-length treated ground- test duct have been com- 
pleted. The tests were run both with and without suppressed inlet noise. The results 
obtained indicate that the inlet radiated noise interferes with the measurements of duct 
radiated noise when the inlet noise is not suppressed. All data shown were recorded with 
the inlet noise suppressed. Acoustic data were obtained from 24 far-field microphones 
(fig. 9) located around the engine in both horizontal and vertical planes. 
Analysis of the data was  made in preferred 1/3-octave bandwidths with a signal 
integration time of about 6 seconds. Perceived noise levels were computed for both 
horizontal and vertical measurements using sound-pressure-level corrections for dis- 
tance, atmospheric absorption, and ground absorption when applicable. Noise levels 
measured from one engine at a distance of 200 f t  were considered approximately equal to 
values for four engines at 370 f t  (inverse square relationship and correction for atmo- 
spheric absorption). 
Preliminary results for the existing duct and the treated duct configuration for three 
test conditions are shown in figures IO, 11, and 12. The data from both the horizontal and 
vertical measurements were adjusted to a distance of 370 ft.  The engine conditions shown 
correspond to static thrust settings that approximate in-flight thrust at approach, cutback, 
and take-off. Values of peak noise reduction between the existing duct and the treated 
duct configuration for both the horizontal and vertical planes are shown as a function of 
static thrust in figure 13. Estimated reductions in peak perceived noise level take into 
account the data scatter between runs and give both maximum and minimum values as 
shown in figure 14. Although the perceived noise levels computed from the horizontal 
measurements do not compare directly with the values computed from the vertical mea- 
surements, the reductions agree well. 
230 
Typical spectral data of the peak noise attributed to the fan duct are shown in fig- 
ure 15. The data were recorded at 5000 rpm at 1100 in the horizontal plane. The data 
are adjusted to a 370-ft distance. Data accumulated during the test indicate a reduction 
in peak noise level of 9 to 16 PNdB. The data also show a reduction of 2 to 6 dB in sound 
pressure level at the frequencies associated with the jet noise. This reduction is attri- 
buted to the coplanar annular nozzle configuration. The magnitudes of the peak frequen- 
cies have been reduced substantially more than the broadband level. This reduction indi- 
cates the presence of a broadband background noise in the 2 to 6 kHz range that limited 
the noise attenuation measured. Further testing of the treated long duct is planned to 
provide better definition of the broadband background noise. A more accuracte evalua- 
tion of the treated duct will be made when a fully treated nacelle is tested. 
Measurements of thrust, fuel flow, and duct pressure losses were made to evaluate 
the internal performance. The propulsion performance of the engine with the treated duct 
is shown in figure 16. Gross thrust measurements indicate approximately a 2-percent 
increase in thrust over the standard production duct. The increase is attributed to the 
relatively small internal loss and the absence of a scrubbing drag. 
FLIGHT-TEST DUCT 
A flight-test configuration of the duct (fig. 17) has been designed and is currently 
under construction. The design is similar to that for the ground test and uses much of 
the same tooling for manufacture. The internal contour has only minor changes and the 
external contour was modified to conform to flight requirements. Flight tests are planned 
for mid- 1969. 
The amount and location of the acoustic treatment are the same as for the ground 
test but treatment has been changed. Recent experimental data indicate that a suitable 
combination of single-layer treatment will  meet the target attenuation. The single-layer 
treatment is much simpler to manufacture and less costly. The treatment depths are 
0.75 inch on the outer walls, 0.50 inch in the splitters, and 0.30 inch on the inner wall. 
The aft acoustic panel has been modified to be part of the nacelle structure. The flow 
resistance of the acoustic treatment will vary from 18 to 30 rayls. It is estimated that 
the flight-test treated long duct will increase the weight of each nacelle by approximately 
750 pounds. The design will also reduce ground clearance by approximately 3 inches. 
CONCLUDING REMARKS 
The noise reduction goal of 15 PNdB for a Boeing 707-320B airplane during landing 
approach appears feasible. Ground tests indicate that the acoustically treated long fan 
duct will result in a gain in thrust of approximately 2 percent at take-off conditions. The 
fan-duct modification will result in a weight increase of approximately 750 pounds for 
each nacelle. 
23 1 
FAN-DUCT ATTENUATION GOAL 
5000 rpm, 370 ft 
40 r 
100 
90 
80 
70 
60 
SPL, 
dB 
I I I 1 
500 1000 5000 10OOO 
OL I 
100 
FREQUENCY, Hz (cps) 
- 
' 
Figure I 
FAN-DUCT PEAK NOISE REDUCTION 
5000 rpm, 370 ft 
-\ 1" 
REDUCTION 
I I GOAL , 50 L L I  i 
100 500 1000 5000 10000 
FREQUENCY, Hz (cps) 
Figure 2 
23 2 
EXISTING NACELLE 
KIDNEY 
Figure 3 L-68-8573 
TRADE STUDY CONCEPTS 
9 in. 
ELONGATED 
\ I  LONG 
NOZZLE 
Figure 4 
233 
DUCT PERFORMANCE ANALYSIS 
-501  
A RANG E, SELECTED- c n. mi. -100 
-150 1 
L L  
600 
I 
700 800 900 
MAX.FLOW AREA, sq in. 
Figure 5 
ACOUSTIC ANAL Y SI S 
,-ELONGATED 
PEAK 
ATTENUATION, 
dB 
- 
1-67 i n . 4  130in. I I 
TREATMENT LENGTH 
Figure 6 
234 
SELECTED DUCT CONFIGURATION 
FLOW AREA, sq in. 700 
TRANSITION 
BIFURCATION 
Figure 7 
GROUND-TEST CON FIGURATION 
BIFURCATIO TRANSITION 
ACOUSTIC PANEL 
FAN THR UST-R EVERSER 
ACOUSTIC TREATMENT 
DOUBLE LAYER 
Figure 8 
23 5 
3 
FAR-FI ELD MICROPHONES 
DATA SAMPLE 
Figure 9 
FAN-DUCT NOISE 
5000 rpm; 370ft 
0 EXISTING DUCT 
ATREATED DUCT 
HORIZONTAL VERTICAL 
MICROPHONE ANGULAR LOCATION, deg 
236 
Figure 10 
FAN-DUCT NOISE 
5950 rpm; 370 f t 
oEXlSTlNG DUCT 
ATREATED DUCT 
HORIZONTAL VERTICAL i 30 
8 8 0  
0 8 0 8 8  
PNL, 4 4 A  
LL 
'0080 100 120 140 80 100 120 140 
MICROPHONE ANGULAR LOCATION,deg 
Figure 11 
FAN-DUCT NOISE 
6700rpm; 370ft 
0 EXISTING DUCT 
A TREATED DUCT 
HORIZONTAL VERTICAL 
r 
PNL, 
PNdB 
110 ' 
100 - - 
80 100 120 140 80 100 120 140 
MICROPHONE ANGULAR LOCATlON,deg 
Figure 12 
23 7 
EFFECT OF THRUST ON NOISE REDUCTION 
370 ft 
15. 
Eib lo 
5 
2oY 
1301- 
120 
PNL, 
PNdB 
110- 
-- 
O 8, 
0 
0 
0 - 0 
MIN MAX APNdB 
fl g 4 - T  
I I I 1 
o HORIZONTAL 
0 VERTICAL 
I 1 1 
10 000 15 000 20 000 
0 '  
5000 
THRUST, Ib 
Figure 13 
NOISE REDUCTION MEASUREMENT 
Figure 14 
238 
ACOUSTIC RESULTS GROUND-TEST DUCT 
5000 rpm, 370 ft 
6o t GROUND TEST DUCT 
50 
Ll I I ,  I I 
100 500 1000 5000 10000 
FREQUENCY, Hz (cps) 
Figure 15 
G ROU N D-TEST ENG I N E PE R FO RMANCE 
GROUNDTEST 
EXISTING SHORT DUCT 
TAKE0 F F 
1.2 1.4 1.6 1.8 
16000 
GROSS 12000 
THRUST, 
Ib 
8000 
4000 
1 .o 
ENGINE PRESSURE RATIO 
Figure 16 
23 9 
TBI FURCATlOl 
FLIGHT-TEST FAN DUCT 
TRANSITION 
SIDE COWL--’ SINGLE-LAYER f 
Figure 17 
240 


Fan-duct development

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