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TOTE KKUTION BY STRUT INAMACTION

Richard F. Woodward and JosNh A. lalembin

National Aeronautics and Space AdministrationLewis

Research Center

Cleveland, Ohio 44135

W

Abstract

A JT15o fan stage was acoustically tested inthe NASA Lewi!, anechoic chamber as part of thejoint Lewis-L,mgley Research Center investigationof flight simulation techniques and flight effectsusing the JT1SD engine as a common test vehicle.Suspected rotor-downstream support strut interac-tion was confirmed through the use of simulatedsupport struts which were tested at three axialrotor-strut spacings. Tests were also performedwith the struts removed. Inlet boundary layersuction in conjunction with an inflow controldevice was also explored. The removal of theboundary layer reduced the fan fundamental tonelevels suggesting that the mounting and mating ofsuch a device to the nacelle requires carefulattention. With the same inflow control deviceinstalled ;mod acoustic agreement was shown be-tween the engine on an outdoor test stand and thefan in the anechoic chamber.

Introduction

The development of effective inflow controldevices (ICO's) makes it possible to study noisegeneration mechanisms, such as rotor-statorinteraction, with reduced masking effects of in-flow disturbance. Modern Virbofan engines areofter designed with bladelvane numbers selectedto prevent propagation of the fundamental rotor-stator interaction tone. However, less considera-tion has been given to possible rotor interactionswith engine support struts. These struts areeither located downstream of the stator row or areintegr1ted into the stator as large cross-sectionvanes.

This pacer presents for a JT15D fan stagewhich was acoustically tested in the NASA LewisResearch Center anechoic chamber 2 as part of ajoint NASA Lewis-Langley investigation of flightsimulation techniques and flight effects u ing tJT15D-1 engine as a common test vehicle. Theengines used in these studies were instrumentedwith blade and vane pressure transducers to assiin isolating noise generation mechanisms.Although the primary goal of this study wa-. toevaluate inflow control techniques, the resultsrevealed that, for the JT15D-1 engine, in particlar speed ranges the fundamental tone was con-trolled by the presence of six engine supportstruts located downstream of the stator. Bladepressure results showing a strong six per revolution disturbance pointed to these struts as theprobable noise source. The interaction betweenthe 28-blade rotor of the JT150 and the six sup-port struts would result in a m . 22 acousticspinning mode having 22 circumferential lobes.This mode was shown to exist in the inlet ductof a JT150 engine using the results from twopressure sensors located in the duct so as toallow spinning mode identification by signal pharelationship. However, it was not possible toalter the support struts in the engine to estab-lish the behavior of this apparent noise source.

Downstream support strwts'rere .net "Miredfor the mso fan installation in the one to Ricchamber. Six simulated support struts were fab-ricated and installed in the test fan stake tosimulate the actual engine support strut iastalla-tion. These simulated struts were located atthree axial spacings from the stator trailingedge. Thus, in the present study results wereobtained for the spacing effect of downstream sup-port struts as well as for fan stage alone with nodownstream struts.

Two possible mechanisms could produce rotor-strut noise: the rotor wake could impinge on thestruts, or the strut potential field could extendupstream to influence the rotor. Resvlts fromreference 8 suggest that this second aschanism maybe the case. In this reference, tests with a twostage fan with downstream struts showed that thepotential field of these struts could Induce sig-nificant blade vibration in the upstream rotor.In fact, the data analysis in this referencesuggests that the strut potential field is trans-mitted through the stator row with little loss inmagnitude, resulting in an effective closer rotor-strut spacing.

The inflow control study of the JT15O fanstage in the anechoic chamber made use of twoinflow control devices which were prev4oyslytested at Lewis on the JT15D-1 engine. , Inaddition, the anechoic chamber installation hadprovisions for inlet boundary layer suction. Thefan stage could be run with either a hard inletduct surface, or with a porous metal section inthe outer duct wall which was connected to a suc-tion system to allow removal of about 10 percentof the inlet airflow. Removing the boundary layercould possibly eliminate irregularities containedin streamlines near the wall before they reachedthe rotor, thus reducing this possible noisesource. The inlet was run with hard walls exceptfor the boundary layer suction tests. Results foranother fan tested in an anechoic chamber sug-gested that boundary layer suction may lnwer thefan blade passage tone level through the removalof such flow irregularities.9

Apparatus and Procedure

Anechoic Chamber

F igure 1 is a photograph of the Lewisanechoic chamber. The research fan shown in thechamber does not have an inflow control deviceinstalled. Also seen in this photograph aresome of the fixed-position microphones and thetraversing boom microphone. Calibration of thechamber showed it to be anechoic to within 2 dBfor frequencies above 200 Hz.

Research Fan

The JT150 fan stage was installed in theanechoic chamber with an ICO attached to the inletas shown in figure 2. The fan stage used the same

he

st

u-

se

bypass flow passage contours as did the actualJT15D-1 engine. The core drive of the engine wasreplaced by an external electric drive, with thefan airflow exhausting through a collector assem-bly. Table I presents selected fan stage designparameters. The production JT150-1 engine had33 core stator vanes. For acoustic cut-off con-siderations, an aerodynamically similar 71-vanecore stator was used for this as ustic program.Simulated engine support struts could be installedin the fan bypass duct as shown in figure 3 ataxial spacings of 23, 5.1 (engine design spacing),and 8.9 re from the stator trailing edge. Thesestruts had an axial length of 14.5 cm. The faninstallation also had eight thin sheet metal turn-ing vanes located in the core flow passage tostraighten to axial the 25' flow swirl exitingthe core stator. These thin cross-section vaneswere not expect;,d to affect the fan acousticperformance.

The boundary layer suction assembly shown infigure 2 allowed removal of a portion of the inletflow near the outer wall to reduce flow irregulari-ties in this region and hence reduce this possiblenoise source. Outer wall airflow was removedthrough a 5.4 cm (2.5 in.) length of porous treat-ment located 15.2 cm (6 in.) upstream of the rotorface. The inlet was run with hard walls exceptfor the boundary layer suction tests.

Figure 2 shows the fan stage installed in theanechoic chamber with inflow control device Ito. 12attached in the fan inlet. Construction detailsof this ICD are shown in figure 4. This ICD wasshaped so that the honeycomb cells are alignedwith the flow streamlines calculated from a poten-tial flow program. A second, ICD, designatedNo. 5 was dimensionally similar, except that ithad six rather than nine support ribs, an innersupport wire mesh, and mounted on the fan inletslightly ahead of where ICD 12 mounted. Furtherconstruction details of ICD 12 may be found inreference 7; details of ICD 5 are in reference 3.Except where noted, all results in this paper arefor ICD 12.

Dynrmic Instrumentation Data Reduction

In this test program, both rotor and statorwere instrumented with high response pressuretransducers. )Signals from the stator werebrought out directly, while those from the rotorwere FM transmitted from an antenna mounted inthe spinner to another mounted along the insidecasing. Results for the 83 transducer, which waslocated near the rotor tip (fig. 5), are presentedin this paper. Installation of these small (1.2 mmdiameter sensing area, 0.8 mm thickness) devicesinvolved cementing them to shims at the blade sur-face, and then fairing over them.

Data reduction of the pressure signals con-sisted primarily of computing the average pressureover a revolution and the corres ponding spectra.Spectra were computed with a nominal 20 Mz resolu-tion, for 512 revolutions of the fan. A tacho-meter pulse occurring once/revolution was used tosynchornize these analyses with the fan speed, sothat pressure variations not synchronized to thefan speed would be discounted.

Acoustic Instrumentation

Far field acoustic date were acquired on a7.b a (25 ft) radius from 0 0 to 90o from the faninlet :xis in 10* increments. Signals from the0.64 cm (0.75 in.) diameter microphones wererecorded on magnetic tape for later narrow band-width spectral analysis. The output of thisnarrow bandwidth sound pressure level analysiswas digitized and transmitted to a computer forfurther analysis. Using a computer reduction pro-gram, narrow bandwidth sound power level spectrawere generated for the forward hemisphere (0^ to90' from the fan inlet axis).

The boom micrOwm (seen in fig, 2) was usedto obtain continuous directivity results at a6.1 m (20 ft) radius centered in the plane of thefan inlet highlight. A narrow bandwidth spectralanalyzer was used to determine the fundamental andovertone levels for these boom traverses.

Results and Dicussion

Aerodynamic Results

The fan operating map (fig. 6) shows theaerodynamic performance of the JT15D fan as testedin the anechoic chamber. The flow restrictions inthe exhaust ducts were adjusted for fan operationon the same operating line as was measured for theJT15D-1 engine at the Lewis vertical lift facility(VLF). Overlaid results from the staticallytested VLF engine tests show good agreement withthe fan results.

Support Strut Effect

Since engine support struts are not needed inthis anechoic chamber fan installation, an oppor-tunity was available to perform tests both withoutstruts and with simulated struts to better under-stand the apparent rotor-strut interaction ob-served in the engine tests. Simulated enginesupport struts were installed in the downstreamb;;..^: flow passage at three axial rotor-strutspacings as shown in figure 3.

Far field acoustic effects. Figure 7 showshow the sound power eve to g0' from thefan inlet axis) at the blade passage tone (BPT)changes with fan speed for the three strut posi-tions and also for no struts. The struts clearlyincrease t: ,e tone level at the 2.5 and 5.1 cmspacings. fhe closest spacing results in an in-crease of the tone level of 10 dB at 11 300 rpmfan speed. With the struts at the engine designspacing of 5.1 cm there is still a significantstrut-induced tone, with the greatest effect seenfor fan operation at 11 300 rpm. There was nosignificant strut effect on sound power of thisBPT at the 8.9 :m spacing.

The sound power level spectra for no strutsand for the struts at the two closer spacings areshown in figure 8 at 11 300 rpm. Again, thestrong effect of strut location on the fundamentaltore can be seen. The presence of the struts haslittle influence on the overtone (2 x BPF and3 x BPF) or on broadband noise levels. The spec-tral spike located at about 800 Hz is only seenwhen the struts are in place. This spike is most

ORIGINAL PAGE 1SOF POOR QUALMY

inlet radiation (ref. 14) show that the effect ofthickening the inlet li-i (i.e., increasing the lipradius) is to move the directivity pattern for-ward. A thick-lipped inlet results in acousticshielding, with less noise propagating to the aftangles. The thick-lipped inlet used for theengine tests of reference 3 is compared to thatused for the fan in the anechoic chamber in fig-ure 11. These lip curvature effects account for adirectivity shift in the engine results and theapparent agreement with the lobe peak predictionof equation (2). Equation (3) in which the groupvelocity remains unchanged, appears to be the pre-ferred radiation prediction, but lip shape mustalso be considered.

Figure 12 presents the first and second over-tone directivities at 11 300 rpm. There is asmall level increase at forward angles for thefirst overtone (fig. 10(a)) at the closest strutspacing, but no significant strut effects on thesecond overtone directivity (fig. 10(b)).

Blade pressure effects. The blade pressuretransducer resu is for the fan tests also showedevidence of strut interaction. Figure 13 snowsthe average blade pressure at 12 000 rpm as afunction of angular position for the three strutspacings and for the struts removed. With nostruts and at the farthest strut spacing (figs.13(a) and (b)) there is no observable struteffect, although there is evidence of interactionwith the 66 bypass stator vanes. However, at theengine strut spacing (5.1 cm, fig. 13(c)) there isclear evidence of six strut-induced pressure dis-turbances. The disturbances become quite strongat the closest spacing (f i g. 13(d)).

Figure 14 shows the blade pressure amplitudespectra corresponding to the average pressures infigure 13. In these spectra the fundamental strutinteraction (6/rev.) and its overtones (121rev.and 181rev.) are clearly seen at the closer strutspacings. Although weak, there is snore evidenceof strut interaction at the farthest strut spacing(fig. 14(b)). As expected, with the struts re-moved (fig. 14(a)) there is no pronounced distur-bance at the strut interaction frequencies.

Inflow Control

A major concern of the current study was toevaluate inflow control techniques for simulatingflight acoustic performance to determine that thesame structures were equally as effective in theanechoic chamber as in the outdoor engine stand.Acoustics tests were performed -ith the JT150-1engine flown on an aircraft, it ,imulated flightin the NASA Ames 40 x 80 foot mind tunnel, andstatically with several inflow control devices.The JT15D fan was likewise tested with two ICD'susing th_ same hardware as was used for the Lewisstatic engine tests. In the anechoic chamber itwas also possible to remove a portion of the inletboundary layer upstream of the rotor (see fig. 2)in an effort to identify noise associated withboundary layer disturbances or other disturbancesnear the wall.

Figure 15 compares the sound pressure levelspectra obtained with the struts removed for ICD's5 and 12 at 10 500 rpm fan speed. With the hardduct configuration, figure 15(a), ICD 12 is shownto be somewhat more effective in reducing the BPT

level. However, with the 10 percent bounderlayer suction in the inlet duct, figure _S(b^,both ICD's are shown to essentially reduce the OPTlevel to that of the surrounding broadband. Thus,it appears that a significant portion of the OPTlevel generated in static testing originates inthe rotor tip region. ICD inlet mating considera-tions must be a critical element of the overallICD design, since disturbances originating fromthis region would tend to enter the rotor in thetip region.

Comparison with JT150-1 Engine

A, was previously discussed, a basic goal ofthe current program was to validate inflow controlstructures on the JT150 fan/engine in several testenvironments. From prior engine tests conductedat Lewis, ICD No. 12 proved to be the most effec-tive in reducting the blade passage tone levelsamong several ICD designs thr.c were tested.Figure 16 compares results for the Lewis staticengine and fan tests using ICD 12. The anechoicchamber data were corrected for distance and band-width to the engine measurement conditions. Thereis reasonably good agreement for the baseline andICD :2 configu-etions between the engine and fan.The somewhat more lobed directivity for the fanwith inflow control may be caused by the anechoicchamber installation with its greater possibilityfor wall-induced turbulence (see fig. 2). Bound-ary layer suction essentially removes these lobesin the directivity results for the fan with ICD 12.

Summary of Results

1. The previously identified interactionbetween the rotor and the six downstream supportstruts of the JT15D engine was further investi-gated through the testing of simulated supportstruts at three axial spacings. The JT15D fanstage installation in the anechoic chamber did notnormally require downstream support struts, allow-ing the fan stage to also be tested with nostruts. The m . 22 spinning mode generated bythe rotor-strut .interaction was evident in theacoustic directivity results, where the soundpressure level was increased by as much as 10 d6at the closest strut spacing. A theoretical pre-diction for lobe maximum intensity angle based oninvariance of the group velocity from duct to farfield showed good agreement with the data fromthese static tests.

2. The quality of the mating region betweenan ICD and the fan inlet appears to be very impor-tant to the overall performance of the ICD inreducing the fan BPT level. By removing residualinlet wall disturbances with boundary layer suc-tion it was possible to further reduce the fan BPTlevel to that of the surrounding broadband whenthe support struts were also removed.

3. With the same inflow control device inplace, reasonably good agreement was shown betweenresults for the engine on the outdoor test standand the fan in the anechoic chamber.

References

1. Ho, P. Y., "The Effect of Vane-FrameDesign on Rotor-Stator Interaction Noise," AIAAPaper 81-2034, Oct. 1981.

2. Wazyniak, J. A., Shaw, L. M., and Essary,J. D., "Characteristics of an Anechoic Chamber forFan Noise Testing," NASA TM X-73555, Mar. 1977.

rE

i

3. McArdle, J. G., Jones, W. L., Heidelberg,L. J., and Nomyak, L., "Comparison of SeveralInflow Control Devices for Flight Simulation ofFan Tone Noise Us i ng a JT15D-1 Engine," AIM Paper80-1025, June 1980.

4. Schoenster, 1. A., "Fluctuating Pressureson Fan Blades of a Turuafan Engine." NASA TP-19769Mar. 1982.

S. Priesser, J. S., Schoenster, J. A.,Golub, R. A., and Horne, C., "Unsteady Fan BladePressure and Acoustic Radiation from a JT15D-1Turbofan Engine at Simulated Fo r ward Speed," AIAAPaper 81-0096, Jan. 1981.

6. Chestnutt, D., *Flight Effects of FanNoise," NASA CP-2242, Fept. 1982.

7. Homyak, L., McArdle, J. G., and4e+delberg, L. J., "A Compact Inflow ControlDevice for Simulating Flight Fan Noise," A rAPaper 83-0680, Apr. 1983.

8. Yokoi, S., Nagano, S., and Kakehi, Y.,"Reduction of Strut Induced Rotor Blade Vibrationwith the Modified Stator Setting Angles," Interna-tional Symposium on Airbreathin En ines, 3We e by F. A. Faranjpe and M. 5. Ramachandra,Bangalore, India, Feb. 1981, pp. 61-1 to 61-7.

9. Kantola, R. A., and Warren,.R. E.,"Reduction of Rotor-Turbulence Interaction Noisein Static Fan Noise Testing," AIM Paper 794656,Mar. 1979.

I.O. Englund, D. R., Grant, H. P., and Lanati,G. A., "Measuring Unsteady Pressure on RotatingCompressor BiaAes," NASA TM-79159, 1979.

11. Heidmann M. F., Saule, A. V., andMcArdle, J. G., " predicted and Observed ModalRadiation Patterns from JT1SO Engine with InletRods," Journal of Aircraft, Vol. 17, No. 7, July1980, PP•

12. Rice, E. J., Heidmann, M. F., and Sofrin,T. G., "Modal Propagation Angles in a CylindricalDuct with Flow and Their Relation to Sound Radia-tion," AIM Paper 79-0183 Jan. 1979.

13. Groeneweg, John V, and Rice, Edward J.,"Aircraft Turbofan Noise," ASME Paper 83-GT-197,28th ASME International Gas Turbine Conference,Phoenix, AZ, Mar. 27-31, 1983.

14. Baumeister, K. J., "Utilizing NumericalTechniques in Turoofan Inlet Acoustic SuppressorDesign," NASA T14-82994, Oct. 1982.

TABLE I. - FAN STAGE PARAMETERS AT TAKE-OFF THRUST FOR JT15D-1 ENGINE

Rotor blades . . . . . . . . . . . . . . . . . . . . . . . . . . 28esBypass stator van . . . . . . . . . . . . . . . . . . . . . . . 66

Corc stator vanes* . . . . . . . . . . . . . . . . . . . . ..

71Speed, rpm .

. . . . . . . . . . . . . . . . . . . 15 740

Rotor diameter, cm (in.) . . . . . . . . . . . . . . . . . . . 53 (21)Bypass ratio . . . . . . . . . . . . . . . . . . . . . . . . 3.3Bypass pressure ratio . . . . . . . . . . . . . 1.5Total mass flow, kg/sec (lbm/sec).

. . . . . . . . . . . . . 34.5(76)

Bypass mass flow, kg/sec (1bm/sec) . . . . 26.3 (58)*1.83Bypass rotor-stator s pacing . . . . . . . . projected axial

Care rotor-stator spa,;ing . . . . . . . . . . . 1.42 rotor chordsRotor-bypass strut spacing . . . . . . . . . . 4.92 (5.1 cm)

*Modified from production engine.

5

ORIGINAL' PA'TBLACK AND WHITE ° „'D RAPH

Figurel. - Research tan installed in anechoic chamber.

EXHAUSTCOLLECTOR-

ACOUSTICWEDGES

FLOWBOUNDARY S PL;T-1 ERLAYER 0 STRUTS I-\

ICD 12 SUCTION , f—".

28 BLADE ROTOR

FLOW i \N 4r

71 VANE CORE STATOR-66

\`^66 VANE BYPASS STATOR8 CORE STRUTS6 SIMULATED ENGINE SUPPORT STRUTS'

Figure 2. - Sketch of Ji 15D fan stage installation in Lewis anechoic chamber.

T112 crr(44 in.

L

_ _ f

i' 1

28 BLADE ROTOR 66 VANE STATOR

FLOW ENGINE DESIGN SPACING

ALTERNATE SPACINGS6 SUPPORT STRUTS (EVENLY SPACED)

Figure 3. - Unwrapped bypass outside diameter flow passage of the JT15D failstage showing spacinr,s for the simulated engine support struts.

HONEYCOMB 5 cm (2 in., DEPTH0.0033 cm (0.0013 in.)WALL THICKNESS

0 2 4 cm

I I I0 1 2 in.

!,— NINE RIBS0.8 mm(. 031 in.)THICK

i. /1 Lill

(28 in. l

Figure 4. - ICD 12 construction details,

IOF POOR PAGE

Of

B3TRANSDUCER-- TANDEMBYPASSSTATOR

rFLOW

ROTORCORESTATOR

Figure 5. - Location of B3 pressure transducer ("pressure side of blade,0.38 cm from blade leading edgi, 1.90 cm from tip).

1.3TAKE-OFF POINT01AA 80-1025)

O VLF ENGINE RESULTS '10

im 0 1.2

13.500

H1z 000 CORRECTED

a v, 1.1 300 FAN rpmr W iQ 500

am8450

1.0116750 + I I

10 15 20 25 30 35kg/sec

L___ ! I I I

25 35 45 55 65 75CORRECTED INLET MASS FLOW, LBMlsec

Figure 6. - Fan operating map (VLF engine results shown forcomparison).

ORIGINAL PAGE ISOF POOR QUALM

r.

150O NO STRIRS

M O STRUTS AT 2.5 cm S PACINGb 140 O STRUTS AT 5.1 cm SPACING

A STRUTS AT 8.9 cm S PACING

m 130

120

m 110

6 7 8 9 10 11 12 13 13.5

FAN, rpm (( 1000)

Figure 7. - Strut spacing effect on fundamental blade pas-sage tone power as a fum ion of fan speed (0 - 9d°. 80 Hzbandwidth).

6 PF

140— h4 STRUTS

STRUTS AT 2. 5 cm SPACING

1 130 --- STRUTS AT 5.1 cm SPACING

6 130 ^_ 2xB PF

3xB PF

120

110 a I l I _1—^__I_—L.__^0 2 4 6 8 10 12 14 16 18 20

FREQUENCY, KHZ

Figure B. -Sound power level spectra as a function of strut loca-tion (0 - 90°. 11, 300 rpm, 80 H Z B. W. )

OPIC NAl ?AGE MOF POOR QUALITY

0 NO STRUTS0 STRUTS AT 2.5 cm SPACINGO STRUTS AT 5.1 :m SPACING

100 STRUTS AT 8.9 cm SPACING

90

BROADBAND

E 80Z

u^ \0chi

70

(a) 1a1a

NO STRUTS

---- STRUTS AT 2.5cm SPACING---- STRUTS AT 5.1 cm SPACING

N ---- STRUTS AT 8.9 cm SPACINGa 1 00

Jcc

90

Y, `M80

70 1 (b) I0 30 60 90

ANGLE FROM FAN INLET AXIS, deg(a) Fixed-position microphones,

(b) Boom microphone.

Figure 9. - Strut spacing effect on blade -3ssage tone directivity(11, 300 rpm).

QUA

O STRUTS AT 2.5 cm S PACING

1 O NO STRUTS (22,11 MODE

D 1 (c) I-_^

100 -

122,1) MODE

80 I (d) 1-- _-0 30 60 90

ANGLE FRCM INLET AXIS, deg

(a) 6750 rpm.

(b) 10, 500 rpm.

Ic) 11, 300 rpm.

0) 14000 rpm.

Figure 10. - Blade passage tone directivity at severalfan speeds.

85

75

65

95 --

Nz 85r0

m

75v

100

90KNNaCL

w0

g 80Co

ORIGINAL ^^^

OF. PANir- "THICK - LIP" INLET

"FLIGHT" INLET

f

"m►

FLOW

Figurf, 11. - Comparison of Inlet contours for "fiight" inlet usedin present study and "thick-lip" inlet used In reference 3.

100 F O NO STRUTS0 STRUTS AT 2.5(.m SPACING0 STRUTS AT 5.) cm SPACING

90

NE

oV

O.-r

70 (a)

^ 90a

80

700 30 60 90

ANGLE FROM INLET AXIS, deg

(a)2x BPF.

(b)3x BPF.

Figure 12. - Strut spacing effect on first and secondovertone directivity (11, 300 rpm).

5

-51 (a) I I I i I5 —

0

-5 (b) 15

0

(c)-5

10

5

0

-5

60 120 180 240 300

360BLADE ANGULAR POSIPON, deg

(a) No struts.

(b) Struts at 8.9 cm spacing.

(c) Struts at 5.1 cm spacing (engine location).

(d) Struts at 2.5 cm spacing.

Figure 13. - Average blade pressure as a function of angularposition (averaged over 500 revolutions. measured in direc-tion of fan rotation measured from vertical top (12.000 rpm)).

-100

3

160

140

120

100

80

160

140

120

Co 100WD

80CLd 160Q

140NN

120Wd

100

80

i60

140

120

100

800 2 4 6 8 10

FREQUENCY. KHZ

(a) No struts.

(b)Struts at 8.9 cm.

(c)Struts at 5.1 cm.

(d)Struts at 2.5 cm.

Figure 14. - Bade pressure amplitude spectra (12,000 rpm;pressure transducer 63).

I

!140t

0 o

omti

cE

ZLn

_vx

N

den

1CLLn

130 B PF

L2x6 PF 3x6 PF

120 - i

to _ -

110 L-1

120 r-

110— WITH ICD 12--- WfTF:ICD 5

100 b) L I I

0 2 4 6 8 10 12 14 16 18 20

FREQUENCY (X 1000) MZ

(a) Hard duct.

0b) With B. L. suction.

Figure 15. - Effect of B. L. suction on PWL spectra with supportstruts removed (10, 500 rpm).

• O NO ICDA v WITH ICD 12

SOLID SYMBOLS - FAN-ANECHOIC CHAMBER

90 OPEN SYMBOLS - ENGINE - LEWIS V. L. F.

80

70 Y

_ —ENGINE

-^tz:z:

60 FAN -1

,-BROADBAND

50 L0 30 60 90

ANGLE FROM INLET AXIS, deg

Figure 16. - Blade passage tone directivity (10.500corrected rpm, results adjusted for 30.5 m(100 ft) radius, 25 m, B. W., fan struts at enginespacing).