NASA Technical Memorandum 105369AIAA-92-0371

Far-Field Noise and Internal Modes Froma Ducted Propeller at SimulatedAircraft Takeoff Conditions

Richard P. WoodwardNational Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio

Lawrence A. BockPratt & WhitneyEast Hartford, Connecticut

Laurence J. HeidelbergNational Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio

and

David G. HallSverdrup Technology, Inc.Lewis Research Center GroupBrook Park, Ohio

Prepared for the30th Aerospace Sciences Meeting and Exhibitsponsored by the American Institute of Aeronautics and AstronauticsReno, Nevada, January 6-9, 1992

NASA

https://ntrs.nasa.gov/search.jsp?R=19920007484 2018-04-20T16:05:50+00:00Z

FAR-FIELD NOISE AND INTERNAL MODES FROM A DUCTED PROPELLER AT

SIMULATED AIRCRAFT TAKEOFF CONDITIONS

Richard P. WoodwardNational Aeronautics and Space Administration

Lewis Research CenterCleveland, Ohio 44135

Lawrence A. BockPratt & Whitney

East Hartford, Connecticut 06108

Laurence J. HeidelbergNational Aeronautics and Space Administration

Lewis Research CenterCleveland, Ohio 44135

David G. HallSverdrup Technology, Inc.

Lewis Research Center GroupBrook Park, Ohio 44142

Abstract

The ducted propeller offers structural andacoustic benefits typical of conventional tur-bofan engines while retaining much of theaero-acoustic benefits of the unducted propel-ler. A model Advanced Ducted Propeller(ADP) was tested in the NASA Lewis Low-Speed Anechoic Wind Tunnel at a simulatedtakeoff velocity of Mach 0.2. The ADP modelwas designed and manufactured by thePratt & Whitney Division of United Technol-ogies. The 16-blade rotor ADP was testedwith 22- and 40-vane stators to achieve cut-onand cut-off criterion with respect to propaga-tion of the fundamental rotor-stator inter-action tone. Additional test parametersincluded three inlet lengths, three nozzle sizes,two spinner configurations, and two rotor rubstrip configurations. The model was testedover a range of rotor blade setting angles andpropeller axis angles-of-attack. Acoustic datawere taken with a sideline translating micro-phone probe and with a unique inlet micro-phone probe which identified inlet rotatingacoustic modes. The beneficial acoustic effectsof cut-off were clearly demonstrated. A 5 dBfundamental tone reduction was associatedwith the long inlet and 40-vane stator, whichmay relate to inlet duct geometry. The fun-damental tone level was essentially unaffectedby propeller axis angle-of-attack at rotorspeeds of at least 96 percent design.

In f.rnA 11 rf'.l nn

The advanced propeller program"' hadsuccessfully demonstrated significant perfor-mance improvements for single and counter-rotating propellers relative to that of currentturbofan engines at typical cruise conditions ofMach 0.8 and 10 688 m (35 000 ft) altitude.However, uncertainties over new propellertechnologies and inherent structural andacoustic benefits associated with propellershrouds have directed current research towardthe advanced ducted propeller, which is amarriage of the turbofan and propeller tech-nologies. The advanced ducted propeller willtypically feature a low number of rotor bladesand a stator designed to satisfy the acousticcut-off criterion. 3 Bypass ratios of 15 orgreater will be characteristic of thesepowerplants.

The objective of the present study was toquantify the aero-acoustic effects of flow pa-rameters such as inlet length and stator vanenumber for a model ducted propeller. Thispaper will present acoustic results for a modelAdvanced Ducted Propeller (ADP) designedand built by Pratt & Whitney Division ofUnited Technologies, which was tested in theNASA Lewis 9- by 15-Foot Anechoic WindTunnel (Fig. 1(a)). The rotor diameter was43.8 cm (17.25 in.). The model was testedwith two different stator vane numbers (for

cut-on and cut-off conditions), three inlets,two spinners, three nozzles, and with groovedand smooth rotor rub strips. A range of rotorblade setting angles was investigated. Limit-ed diagnostic data are included from a uniquerotating inlet microphone probe which allowedfor separation of individual acoustic modes.Tests were conducted at Mach 0.2, which istypical of takeoff conditions. Figure 1(a) is aphotograph of the model installed in the windtunnel. Acoustic data were obtained from anaxially-translating microphone probe. "Fly-over" directivities could be obtained fromthese acoustic probe surveys. Fixed micro-phones were also located near the tunnel wallin the same approximate plane as the trans-lating probe; however, data from the fixedmicrophones are not included in this report.

Apparatus

Anechoic Wind Tunnel

The NASA Lewis 9- by 15-Foot AnechoicWind Tunnel is located in the low-speedreturn leg of the supersonic 8- by 6-Foot WindTunnel. The maximum airflow velocity in thetest section is slightly over Mach 0.20, whichprovides a takeoff/approach test environment.The tunnel acoustic treatment providesanechoic conditions down to a frequency of250 Hz, which is lower than the range of testpropeller acoustic tones.4

Acoustic Instrumentation

The acoustic data presented in this paperwere acquired with the translating microphoneprobe, which is shown in Fig. 1(a). Thisprobe was instrumented with two 0.64 cm(0.25 in.) diameter condenser microphones.Data in this report are for the outer micro-phone, which was located 167 cm (66 in.)from the propeller axis of rotation at 0°angle-of-attack. The probe could survey asideline of approximately 20° to 140° relativeto the propeller plane. The translating micro-phone probe allowed measurement of "flyover

directivities." The translating probe wasprogrammed to travel at an approximatelyconstant sideline angular velocity. Acousticsurveys required 180 sec to complete. A com-puter-controlled FFT analyzer was used toacquire 52 representative sound pressure level(SPL) spectra (0 to 20 KHz frequency range,32 Hz bandwidth). A computer analysis pro-gram then isolated desired tone orders (BPF,2BPF, etc.) to generate tone sidelinedirectivities.

A unique rotating microphone probe wasused with some configurations of the ADPmodel to perform a detailed investigation ofinlet acoustic modes. Figure 1(b) is a photo-graph showing this probe installed at theADP inlet.

Model Ducted Propeller

Table I presents design parameters for themodel duct propeller. The ADP model fea-tured a 16-blade rotor (43.81 cm (17.25 in.)diameter) which was tested at four blade pitchangles. The rotor blade setting angle is refer-enced to the design cruise angle, 0°. A bladesetting angle of "-11°" is considered to repre-sent the takeoff condition. These blade set-ting angles are relative to the design cruiseblade setting angle. Tests were conductedwith both 40 and 22 vane stators. The bladepassage tone for the 40 vane stator was pre-dicted to be cut-off at all test rotor speedsaccording to the blade and vane number crite-rion of Ref. 3. The ducted propeller was pow-ered by an air turbine drive. Exhaust airfrom the turbine was ducted well aft of themodel and was of no aero-acoustic significanceto the test results. The ADP model could berotated in the horizontal plane to achievepropeller axis angle-of-attack. This modelrotation axis was 157 cm (62 in.) aft of thepropeller plane. Table II is a listing of theADP test configurations. Figure 2 shows asketch of the forward part of the model withthree test inlets and two spinners.

2

Results and Discussion

Cut-Off Effect

Modern turbofan engines are oftendesigned to take advantage of cut-off wherebythe fundamental blade passage tone fromrotor-stator interaction is substantiallyreduced through appropriate selection ofblade/vane numbers. 3 This theory predictsthat the fundamental blade passage tone willbe cut-off when the number of stator vanes isslightly greater than twice the number ofrotor blades. The 40-vane stator satisfies thiscriterion for the ADP model.

Figure 3 shows representative SPL spectrafor the cut-on (22-vane stator) and cut-off(40-vane stator) configurations. These spectraare for a sideline angle, 0, of 83°. The funda-mental blade passage tone (BPF) is essentiallyin the broadband for the cut-off configuration(Fig. 3(a)), or about 20 dB lower than thatfor the 22-vane stator cut-on spectrum(Fig. 3(b)). Other features of the spectra,such as higher-order tone levels and broad-band, are similar for the two statorconfigurations.

Figure 4 shows corresponding BPF tonesideline directivities for the two stator config-urations. The higher BPF tone levels for the22-vane stator tend to be aft-dominant. Thistendency for the fundamental BPF tone to behigher in the aft quadrant has been frequentlyobserved for turbofans. Reference 5 suggeststhat acoustic blockage of the rotor-statorinteraction noise attempting to propagateagainst the high-velocity rotor airflow isresponsible for lower forward quadrant noise.Reflection of rotor-stator noise from the rotorpressure surface to the aft-quadrant is proba-bly insignificant for the BPF tone.6

The BPF tone level for the 40-vane cut-offstator configuration still shows some residualtone level at most sideline angles. The broad-band directivity in the region of the BPF isshown for reference on Fig. 4. The broadbandlevel for both stator configurations was essen-

tially the same. Close inspection of the ADPmodel did not reveal any likely sources ofinflow disturbances which might generateinflow-rotor interaction noise. Reference 7suggests that small manufacturing irregulari-ties in the stator can result in some rotor-stator interaction tone in an otherwise cut-offfan.

Modal Analysis

Modal analysis may be a useful diagnostictool in understanding the tone generationmechanisms in turbomachinery. Reference 3introduced the concept of rotating interactionmodes. These modes are defined in terms ofcircumferential order, m, and radial order, µ(m,µ), where the (0,0) mode would define aplane wave propagating axially down the inlet(or exit) duct. Possible existing circumferen-tial modes are defined by the relationship:

in nB + kV (1)

where B and V are the blade and vanenumbers, respectively, and n is the toneorder, n x BPF. k has the value of any± integer, including 0. However, predictedpropagation of a particular mode is dependenton having a "cut-off ratio" of 1 or greater.

Reference 8 develops the application ofmodal theory to include effects of free-streamand duct Mach numbers. From this reference,the mode cut-off ratio is defined as:

2D f roC (2)

E 1 - MD

where f is the tone frequency, c is the localspeed of sound, r o is the "mode release"radius, and MD is the duct Mach number.The eigenvalue, E, is a function of the modeorder, (m,p).

Reference 8 gives an expression for thesideline angular location for the maximumlevel of a particular mode's principal lobe as:

M_ - MD + (1 - MDM JF^2cos B =

1/2

1 -MD 1 72 1 + M^(M. - 2MD) + (2M.- MD - MDMDF

jj

(3)

where M is the free stream Mach. The freestream and duct Mach numbers have a nega-tive sign for modes propagating from the inletduct, while both are positive for aft-propagating modes.

Tone noise is usually associated withinflow-rotor interaction and rotor-stator inter-action. The fundamental BPF tone is cut-offwith respect to rotor-stator interaction for the40-vane stator, but higher tone orders arecut-on with either stator number. There aresome practical concerns in applying the modalanalysis of Ref. 8. Calculation of the modecut-off ratio (Eq. (2)) is affected by selectionof duct radius ro and duct Mach, MD . Inthe following analysis the inlet highlight radi-us (see Table I) was chosen as the moderelease radius (r o ), while the duct Mach num-ber near the fan face was used for MD . It isconceivable that the mode release radius maybe somewhat greater than the inlet highlightradius, and that selection of a duct Machnumber in another portion of the duct may bemore appropriate.

Likewise, Eq. (2) was derived for a cylin-drical duct—the existence of an annulusformed by the inlet/nozzle and spinner/centerbody could also affect the selection ofro . Proper selection of these inputs will affectthe mode cut-off ratio calculation. The side-line angular location of the acoustic modeprincipal lobe (Eq. (3)) is similarly affected bythe input cut-off ratio and MD.

The rotating microphone probe was usedwith the ADP model to perform a detailedinvestigation of inlet acoustic modes(Fig. 1(b)). Data were taken with the rotat-ing probe and concurrent far field microphoneswith the 40-vane stator, medium inlet, shortspinner and nozzle ADP configuration,although the rotating probe was also testedwith the other two ADP inlets without far-field acoustic instrumentation. The modelwas at 0° angle-of-attack for these tests.Figure 5 is a sketch of the rotating probeinstalled with the short ADP inlet. Therotating probe featured a radial rake with five0.25 cm (0.10 in.) diameter pressure transduc-ers. The rake rotated at a precise fraction ofthe rotor speed and was synchronized with therotor, which allowed for acoustic separation ofall "m" order rotating circumferential modes.The five radial microphones are to providesome indication of the corresponding radial"y" mode order.3

The ADP model with the rotating probewas run in both a clean inlet and a four-inletrod configuration. These four 0.48 cm(0.19 in.) diameter rods protruded about25 percent into the flow passage and wereintended to generate specific rod-rotor interac-tion modes. These rods were positioned6.79 cm (2.67 in.) from the rotor stacking lineplane. According to Eq. (1), circumferentialmode orders, m, of 0, ±4, f8, ±12, etc. maybe expected from this interaction. Figure 6 isa representative spectrum of the mode sound

pressure levels as a function of circumferentialmode number. These data are for 102 percentdesign rotor speed. The microphone measur-ing point was 5.00 cm (1.97 in.) axially im-mersed from the inlet highlight, and radially1.10 cm (0.43 in.) from the inlet flow surface.The BPF tone for the 40-vane stator configu-ration is predicted to be cut-off. However,there is still evidence of considerable modalactivity for the clean inlet configuration. (At213PF the in -8 circumferential mode isexpected to be cut-on.) The sideline SPLdirectivity of Fig. 4 likewise showed someresidual BPF tone for the 40-vane stator.Introduction of the four inlet rods shows theexpected increase for the rod-rotor interactiontone levels.

Figure 7 shows BPF tone sideline directiv-ities for the clean inlet and four-rod configura-tions at 102 percent design speed. The SPLfor the inlet rod configuration is typically10 dB or more greater than that for the cleaninlet. Also, note that the maximum tone levelfor the inlet rod configuration occurs in theforward quadrant rather than in the rearquadrant as was the case for the 22-vanerotor-stator interaction tone of Fig. 4. This isconsistent with the concept that rod-rotorinteraction noise is generated at the rotor faceand is not attenuated by traversing the rotorflow field before leaving the fan inlet. Themodal theory of Ref. 8 was used to predictmode maximum level angular locations, andthese are noted for the four-rod directivity inFig. 7. The predicted peak sideline angles arein reasonable agreement with the data. How-ever, some significant peaks, such as that atO = 118° for the four-rod configuration arenot predicted by theory. As previously dis-cussed, this may be due to somewhat incorrectselection of input parameter values forEqs. (2) and (3).

The fundamental rotor-stator interactiontone for the 22-vane stator is cut-on. Figure 8shows the sideline BPF tone directivity for the22-vane stator ADP configuration. Except forvane number, the cut-on ADP configuration ofFig. 8 is the same as the cut-off, clean inlet

configuration of Fig. 7. Peak locations forpredicted acoustic modes are indicated, andare in good agreement with observed peaks inthe data. However, there are significant addi-tional data peaks which are not predicted bytheory.

Effect of Blade Setting A

The ADP is designed to "takeoff' with arotor blade setting angle of -11° from thereference cruise setting angle. The model wastested over a range of blade setting anglesfrom -7° to -18°, with -7° being the mosthighly-loaded case with a correspondinglyhighest mass flow.

Figure 9 shows the change in BPF SPL asa function of blade setting angle for the40- and 22-vane stator configuration. At86 percent design speed the cut-off 40-vanestator data shows little sensitivity to bladeloading (Fig. 9(a)), while the cut-on 22-vanestator shows about a 5 dB tone level increasefrom -14° to -7° blade setting angle. Thistone level is significantly greater than wouldbe predicted by changes in thrust level (ndB= 10 log (thrust ratio)). Thrust changesalone account for only 1.5 dB when the bladesetting angle is changed from -7° to -18°.Although not predicted to be cut-on, the40-vane stator data begins to show trendssimilar to (but significantly lower in levelthose of the 22-vane stator data at 107 per-cent speed (Fig. 9(b)).

Effect of inlet length

Three inlet lengths were tested with theADP model. Aerodynamic performance wasessentially identical for these three inlets.Figure 10 shows sideline BPF tone directivi-ties for the three inlet lengths with the modeloperating at 107 percent design speed. Thesedata are for the 40-vane stator, short spinnerand nozzle, and -11° blade setting angle. Thedirectivity for the long inlet is up to 10 dBlower than the directivities for the other twoinlets in the forward quadrant. The occur-rence of the highest levels in the forward

5

quadrant are typical for rotor-inflowinteraction, suggesting that at this highesttest speed there may be some unidentifiedinflow irregularities which are interacting withthe rotor. Inspection of the overlay of thethree inlet contours in Fig. 2 shows that onlythe long inlet extends upstream beyond theshort spinner highlight, as opposed to theother two inlets which form an annulus withthe spinner. Therefore the mode release plane(assumed to be the inlet highlight plane) isdifferent for the long inlet, and therefore mayhave a somewhat different acoustic attenua-tion characteristic.

Figure 11 extends the results of the previ-ous figure to show the maximum upstreamBPF tone level for each inlet as a function ofrotor speed. Again, there is about a 5 dBdifference between levels for the long inlet andthose for the other two inlets. The BPF tonelevels for all inlets—and especially for theshort inlet—take a sharp upward turn as thefan speed is increased to 107 percent design.

BPF tone level differences between inletconfigurations were not as significant for the22-vane cut-on data. Figure 12 shows themaximum upstream BPF tone level as a func-tion of rotor speed, while Fig. 13 shows simi-lar results covering the entire sidelinedirectivity. There is evidence that acousticattenuation of rotor-stator interaction noiseattempting to propagate upstream throughthe rotor flow becomes more significant at thehigher rotor speeds (Fig. 12). The cut-onresults of Fig. 13 show an expected increase intone level with rotor speed as would beexpected for rotor-stator interaction tones (incontrast with the relatively small tone levelchanges with speed shown for the cut-offresults of Fig. 11).

Effect of Ducted Propeller AxisAngle-of-Attack

Tone level variations in the circumferen-tial acoustic field of a free propeller operatingat rotational axis angle-of-attack has beenwell-documented in the literature. 9 Cyclical

blade loading resulting from non-axial inflowproduces a corresponding asymmetrical cir-cumferential noise field. In concept, a ductedpropeller should exhibit an increasing acousticresponse to non-zero angle-of-attack as theouter shroud length decreases. At 86 percentdesign speed (Fig. 14) the ADP appears toexhibit this acoustic response, showing thegreatest BPF tone level sensitivity to propel-ler axis angle-of-attack for the short inlet, andprogressively less acoustic response associatedwith longer inlet length.

However, at rotor speeds above 86 percentdesign the ADP shows little acoustic sensitiv-ity to angle-of-attack regardless of test inletlength. For example, at 96 percent designspeed the BPF tone level is essentially thesame for all inlets and all test angles-of-attackat corresponding sideline angles. This resultshows that the rotor inflow is conditioned byeven a short inlet length to negate angle-of-attack effects on inflow-rotor acoustic inter-action. The results in Fig. 14 are normalizedat 0° angle-of-attack.

Figure 15 shows corresponding results at102 percent speed. Peak BPF tone levels forto single-rotation advanced turboprop (SR-7A,Ref. 9) are superimposed on this figure, show-ing the unducted propeller's sensitivity toangle-of-attack effects.

The translating microphone probe wasfixed to the tunnel floor and aligned with thetunnel flow. Consequently, acoustic modeswhich propagate at a particular angle relativeto the ADP axis-of-rotation will appear atdifferent locations along the sideline directiv-ities as the model angle-of-attack is changed,moving forward with increasing propeller axisangle-of-attack. Figure 16 shows sidelinedirectivities for the model at angles-of-attackfrom 0° to 30°. Data are for the cut-on22-vane stator with the rotor operating at96 percent speed. The modal theory of Ref. 8was used to designate the sideline angularlocation of a representative forward-radiatingmode (-12,5), designated mode "F," and twooverlapping aft-radiating modes (-6,2) and

6

(-12,3) designated mode "A." With geomet-ric correction, peaks in the sideline directiv-ities corresponding to modes "F" and "A"are evident in the sideline directivities at alltest propeller axis angles-of-attack.

Effect of Spinner and Nozzle Length

In general, no appreciable acoustic changewas associated with spinner length for the cut-off stator. (The cut-on 22-vane stator wasonly tested with the short spinner.) The oneminor exception to this observation was forthe 40-vane stator at 96 percent rotor speed.At this test condition there is some change inthe directivity shape and level for the longer,plug spinner (Fig. 17). The reason for thisdifference is unknown, although a plausibleexplanation may be that BPF tone noise isreflected from the longer spinner and in somemanner generates a measurable interferencewith the directly radiating tone noise at thisparticular rotor speed and tone frequency.

Three nozzle configurations (see Table I)were tested with the 40-vane stator. Asidefrom aerodynamic loading changes, no meas-urable acoustic changes were associated withnozzle type, although changes in fan loadingcaused by nozzle restriction have been associ-ated with changes in broadband noise levels asnoted in the literature.

Effect of Rotor Rub Stri

The ADP model was tested with bothsmooth and grooved rotor rub strips. No fun-damental BPF tone level changes were associ-ated with type of rub strip; however, changesin rub strip configuration may yield aerody-namic benefits.

Summary of Results

Aero-acoustic tests on a model AdvancedDucted Propeller (ADP) were performed inthe NASA Lewis 9- by 15-Foot Low SpeedAnechoic Wind Tunnel at a simulated takeoffflight speed of Mach 0.2. The ADP modelwas designed and manufactured by the

Pratt & Whitney Division of United Technol-ogies. The model was tested with three inletlengths, three nozzle sizes, two spinner types,and two types of rotor rub strip. The modelhad a 16-blade rotor. Tests were made withtwo stator vane numbers (22 and 40) toachieve cut-on and cut-off conditions withrespect to the fundamental rotor-stator inter-action tone. The model was tested over arange of rotor blade setting angles and propel-ler axis angles-of-attack. Acoustic data wereacquired with a translating microphone probewhich was attached to the tunnel floor. Aunique rotating microphone probe was used toidentify and measure rotating acoustic inter-action modes propagating in the fan inlet.The model with the rotating probe was testedwith both a clean inlet and with four rods inthe inlet to generate known interaction modes.

The following significant results wereobserved in this study:

1. The fundamental rotor-stator inter-action tone for the cut-on 22-vane stator wasup to 20 dB higher than that for the cut-off40-vane stator, with the maximum tone leveloccurring in the aft quadrant. This observedhigher tone level in the aft quadrant mayrelate to tone attenuation associated withpropagation upstream through the rotor flowfield as suggested in an earlier, referencedstudy.

2. There was still some residual fundamen-tal tone content observed for the cut-off40-vane stator. This residual tone was some-what higher in the forward quadrant, suggest-ing that it may arise from some unidentifiedinflow-rotor interaction.

3. Data from the rotating inlet acousticprobe effectively identified acoustic modecircumferential order and magnitude, includ-ing residual tones for the cut-off stator.

4. The BPF tone level for the long inletand 40-vane stator was consistently about5 dB lower than corresponding tones for themedium and short inlets at all rotor test

7

speeds. This may relate to the highlight of

Acoustic Characteristics of NASA 9- bythe long inlet being upstream of the spinner 15-Foot Low-Speed Wind Tunnel Acoustichighlight, while the highlights for the other

Treatment," NASA TP-2996, 1990.

two inlets were in a region which formed anexit annulus with the spinner. 5 Philpot, M.G., "The Role of Rotor Blade

Blockage in the Propagation of Fan Noise5. The BPF tone level was essentially Interaction Tones," AIAA Paper 75-447,

unaffected by propeller axis angle-of-attack for Mar. 1975.rotor speeds of 96 percent design and higher,indicating that all inlet lengths effectively 6. Topol, D.A., Holhubner, S.C., andconditioned the rotor inflow. However, the Mathews, D.C., "A Reflection Mechanismmaximum sideline tone level "below the for Aft Fan Tone Noise from Turbofanmodel" did increase with angle-of-attack at

Engines," AIAA Paper 87-2699, Oct.

86 percent design rotor speed, with the short

1987.inlet being most sensitive to angle-of-attack.

7. Sofrin, T.G. and Mathews, D.C., "Asym-metric Stator Interaction Noise," AIAA

References Paper 79-0638, Mar. 1979.

1. Hager, R.D. and Vrabel, D., "AdvancedTurboprop Project," NASA SP-495, 1988

2. Groeneweg, J.F. and Bober, L.J., "NASAAdvanced Propeller Research," NASATM-101361, 1988.

3. Tyler, J.M. and Sofrin, T.G., "Axial FlowCompressor Noise Studies, SAE Transac-tions," Vol. 70, 1962, pp. 309-332.

4. Dahl, M.D. and Woodward, R.P., "Com-parison Between Design and Installed

8. Rice, E.J., Heidmann, M.F., andSofrin, T.G., 'Modal Propagation Anglesin a Cylindrical Duct with Flow and TheirRelation to Sound Radiation," AIAAPaper 79-0183, Jan. 1979. (Also, NASATM-79030.)

9. Woodward, R.P., "Measured Noise of aScale Model High Speed Propeller at Simu-lated Takeoff/ Approach Conditions,"AIAA Paper 87-0526, Jan. 1987. (Also,NASA TM-88920.)

8

TABLE I.—ADP DESIGN PARAMETERS

Rotor blades ............................................. 16Stator vanes ........................................ 22 and 40Rotor-stator spacing (cruise blade setting angle):a

22-vane stator, chords ..................................... 1.940-vane stator, chords ................. .................. 2.3

Rotor-stator spacing (takeoff blade setting angle):b22-vane stator, chords ..................................... 2.240-vane stator, chords ..................................... 2.6

Stage pressure ratio ...................................... 1.243Stage mass flow, kg/sec (lbm/sec) ....................... 119.9 (54.5)Rotor diameter, cm (in.) ............................. 43.81 (17.25)Rotor tip speed, m/sec (ft/sec) ........................... 257 (844)Rotor mid-span chord, cm (in.) .......................... 7.65 (3.01)22-vane stator mid-span chord, cm (in.) .................... 6.76 (2.66)40-vane stator mid-span chord, cm (in.) .................... 3.73 (1.47)Inlet dimensions:Short

Rotor stacking line to highlight, cm (in.) ................ 1209 (4.76)Highlight radius, cm (in.) ............................ 23.41 (9.22)Throat radius, cm (in.) .............................. 21.63 (8.52)

MediumRotor stacking line to highlight, cm (in.) ................. 21.03 (8.28)Highlight radius, cm (in.) ............................ 22.27 (8.77)Throat radius, cm (in.) .............................. 20.43 (8.04)

LongRotor stacking line to highlight, cm (in.) ................ 26.14 (10.29)Highlight radius, cm (in.) ............................ 21.35 (8.41)Throat radius, cm (in.) .............................. 19.66 (7.74)

Nozzle dimensions:Short

Rotor stacking line to exit plane, cm (in.) ................ 36.93 (14.54)Exit plane radius, cm (in.) ........................... 20.45 (8.05)

MediumRotor stacking line to exit plane, cm (in.) ................ 45.27 (17.82)Exit plane radius, cm (in.) ........................... 19.43 (7.65)

LongRotor stacking line to exit plane, cm (in.) ................ 52.26 (20.57)Exit plane radius, cm (in.) ........................... 18.90 (7.44)

'Rotor mid-span chord. Listed chord values are for aerodynamic chord.bCruise blade setting angle-11°.`Fan face to rotor stacking line at cruise blade setting angle is 3.07 cm (1.21 in.).

9

TABLE II.—ADP ACOUSTIC TEST MATRIX

Statorvane

number

Blade settingangle,deg'

Inlet Spinner Nozzle Rub strip Comments

40 -7 Long Short Short Grooved Cut-off-11 Long-7 Medium

-11-11 Medium

-11 Long

-14 Short

-18-11 Short-7 Short Plug

-11 Short Plug

-11 Long Short Smooth

-11 Medium Smooth Inlet rake,four rods

22 -7 Long Grooved Cut-on

-11 Long

-14 Long-11 Medium-11 1 Short

'Measured from design cruise blade setting angle.

10

Long inlet

(a)Photograph of the advanced ducted propeller model in-stalled in the 9x15 ft anechoic wind tunnel.

(b) Photograph of installed fan mode measurement probe.

Figure 1.—Advanced Ducted Propeller acoustic installation

/—Medium inletShort inlet

16 blade rotor

Plug spinner ^

Short spinner

Figure 2.—ADP inlet configurations.

26PF

BPFI 36PF

48PF 5BPF- 66PFRef.

(a)40-vane stator, cut-off.

T

0 2 4 6 8 10 12 14 16 18 20

Frequency, kHz(b)22-vane stator, cut-on.

Figure 3.—Typical sound pressure level spectra for cut-on andcut-off configurations (102% design speed, long inlet, shortspinner and nozzle, a = 0°, 0 = -11 °, 0 = 83°, M, = 0.2).

iv

NN

aC7O

10 dB

/

III r \

\ \ /O\O O O \ ^/

Rotating rake

40 Vane stator 0--- 22 Vane stator— — Broadband level

c 10 dB ` r

)

0 20 40 60 80 100 120 140 160

Sideline angle, 0, deg

Figure 4.—Sideline BPF tone directivities for 40- and 22-vanestators (102% design speed, long inlet, short spinner andnozzle, a = 0 1 , p = -11 °, M m = 0.2).

rakeModal pattern

Figure 5.—Details of fan mode measurement rake.

12

T10

1d B

1dd

(nNdacOU)

(a) 86% Design speed.

+ Denotes mode level with4 inlet rods installed

+ +

+

N1odB + ++ ++++dd T^ T0U)

Modes rotate with rotor Modes rotateagainst rotor

24 20 16 12 8 4 0 -4 -8 -12 -16

Acoustic mode number, mFigure 6.—Inlet acoustic mode levels (40-vane stator, 102%

design speed, medium inlet, short spinner and nozzle,a = 0°, 0 = -11 °, M_ = 0.2).

A(--6

NNa 10 dB

^o s are aft-radiating

I I 1 1 1 1 1

0 20 40 60 80 100 120 140 160

Sideline angle, 0, degFigure 8.—ADP sideline directivity showing angular locations

of principal lobe maximum level (BPF tone, 22-vane stator,102% design speed, short inlet, spinner and nozzle, a = 0°,0 = -11 °, M_ = 0.2).

Stator

4 Inlet rods--- Clean inlet

(4,3) Primed modes are aft-radiating

(0,4) 1

(4 2)(12,1) (4,12),

(0,3) (0, 5) (8, 1)

(0 5)' 1 (0,3)'(S, 1)

(8,2)(4 3)' (0 ,4)

a (12, 1)'CO

I N \ ^^ Ic

10dB 1 J /^J j 1

4—­4

L

1 / VJ

0

0 20 40 60 80 100 120 140 160

Sideline angle, 0, deg

Figure 7.—Sideline directivities with and without inlet rods (40-vane stator, 102% design speed, medium inlet, short spinnerand nozzle, 0 = -11 °, M_ = 0.2).

Increased blade loading

-18 -14 -11 -7

Blade setting angle, 0, deg from cruise angle

(b) 107% Design speed.

Figure 9.—Effect of blade loading on maximum sideline BPFtone level (22-vane stator with long inlet, 40-vane stator withmedium inlet, short spinner and nozzle, a = 0°, M m = 0.2).

13

Inlet L/Dfan

Short 0.21--- Medium .41

— — Long .53a^d eN

1 O d B ` t,1 tI

o

U) J—'), A

1 1 1 1 1 1 1

0 20 40 60 80 100 120 140 160

Sideline angle, 0, deg

Figure 10.—Effect of inlet length on BPF tone directivity (40-vane stator, 107% design speed, short spinner and nozzle,a = 0°, 0 = -11', M_ = 0.2).

Inlet L/ Dfan

a)>N O Short 0.21

q Medium .41

J--N

Q Long 53Nd 1OdBa

c00Cn

I I

70 80 90 100 110

Design rotor speed, percent

Figure 11.—Effect of inlet length on maximum upstream BPFtone level (40-vane stator, short spinner and nozzle, a = 0°,0= -11°,Mm=0.2).

Inlet L/ Dfan

d

O Short 0.21q Medium .41

w 10 dBd O Long .53

a

D0

70 80 90 100 110

Design rotor speed, percent

Figure 13.—Effect of inlet length on maximum sideline BPFtone level (22-vane stator, short spinner and nozzle, a = 0°,0=-11°,Mm=0.2).

ddWNm 10 dB

a i

c

0U)

0 10 20 30

Angle of attack, a, deg

Figure 14.—Maximum sideline BPF tone level (22-vane stator,86% design speed, short spinner and nozzle, R = -11',M_ = 0.2).

Inlet

O Short

q Medium

Inlet L/Dfan

O Short 0.21q

::t

.41O 53

I I

dd

mN

10 dBa

c

0W

p Longp SR-7A turboprop

(design speed, R = 37.8°)^ n>v

NNda

10 dBc

0

70 80 90 100 110

Design rotor speed, percent

Figure 12.—Effect of inlet length on maximum upstream BPFtone level (22-vane stator, short spinner and nozzle, a = 0°,0=-11°,Mm=0.2).

0 10 20 30

Angle of attack, a, deg

Figure 15.—Maximum sideline BPF tone level (22-vane stator,102% design speed, short spinner and nozzle, (3 = -11',M- = 0.2).

14

(b)a=10`.

A

A

Aftmode,

Forward Amode,

F

Ref

(a) a = 0°.

A

Ref.

aN

aa

00

(c) a = 20°.

Ref.10 dB

Spinner

Short

>--- Plug

10 dB

^ Plugspinner—\,----

y S^spinner

0 20 40 60 80 100 120 140 160

Sideline angle, 0, deg

Figure 17.—Effect of plug spinner on sideline BPF tone direc-tivity (40-vane stator, 96% design speed, short nozzle, a = 0°,0 = -11 °, M m = 02).

0 20 40 60 80 100 120 140 160

Flyover sideline angle relative to rotor plane, 0, deg

(d) a = 30°.

Figure 16.—BPF tone sideline directivities at several propelleraxis angles-of-attack. Representative inlet mode, T", andaft mode, "A" are shown on each directivity (22-vane stator,96% design speed, short inlet, short spinner and nozzle,R =-11°,M_=02).

15

Form ApprovedREPORT DOCUMENTATION PAGE OMB No. 0704-0188Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of thiscollection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for information Operations and Reports, 1215 JeffersonDavis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget. Paperwork Reduction Project (0704-0188), Washington, DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE REPORT TYPE AND DATES COVERED

1992[3.

Technical Memorandum4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Far-Field Noise and Internal Modes From a Ducted Propellerat Simulated Aircraft Takeoff Conditions

WU-535-03-106. AUTHOR(S)

Richard P. Woodward, Lawrence A. Bock,Laurence J. Heidelberg, and David G. Hall

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATIONREPORT NUMBER

National Aeronautics and Space AdministrationLewis Research Center E -6747Cleveland, Ohio 44135 - 3191

9. SPONSORING/MONITORING AGENCY NAMES(S) AND ADDRESS(ES) 10. SPONSORING/MONITORINGAGENCY REPORT NUMBER

National Aeronautics and Space AdministrationWashington, D.C. 20546-0001 NASA 369

AIAA - 992 -2-037

0371

11. SUPPLEMENTARY NOTESPrepared for the 30th Aerospace Sciences Meeting and Exhibit sponsored by the American Institute of Aeronautics and Astronautics, Reno,Nevada, January 6-9, 1992. Richard P. Woodward and Laurence J. Heidelberg, NASA Lewis Research Center; Lawrence A. Bock, Pratt &Whitney, East Hartford, Connecticut; David G. Hall, Sverdrup Technology, Inc., Lewis Research Center Group, 2001 Aerospace Parkway, BrookPark, Ohio 44142. Responsible person, Richard P. Woodward, (216) 433-3923.

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Unclassified - UnlimitedSubject Categories 71 and 07

13. ABSTRACT (Maximum 200 words)

The ducted propeller offers structural and acoustic benefits typical of conventional turbofan engines while retainingmuch of the aeroacoustic benefits of the unducted propeller. A model Advanced Ducted Propeller (ADP) was testedin the NASA Lewis Low-Speed Anechoic Wind Tunnel at a simulated takeoff velocity of Mach 0.2. The ADP modelwas designed and manufactured by the Pratt & Whitney Division of United Technologies. The 16-blade rotor ADPwas tested with 22- and 40-vane stators to achieve cut-on and cut-off criterion with respect to propagation of thefundamental rotor-stator interaction tone. Additional test parameters included three inlet lengths, three nozzle sizes,two spinner configurations, and 2 rotor rub strip configurations. The model was tested over a range of rotor bladesetting angles and propeller axis angles-of-attack. Acoustic data were taken with a sideline translating microphoneprobe and with a unique inlet microphone pr-be which identified inlet rotating acoustic modes. The beneficialacoustic effects of cut-off were clearly demonstrated. A 5 dB fundamental tone reduction was associated with thelong inlet and 40-vane stator, which may relate to inlet duct geometry. The fundamental tone level was essentiallyunaffected by propeller axis angle-of-attack at rotor speeds of at least 96% design.

14. SUBJECT TERMS 15. NUMBER OF PAGES

Shrouded propellers; Aircraft noise 1616. PRICE CODE

A03

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACTOF REPORT OF THIS PAGE OF ABSTRACT

Unclassified Unclassified Unclassified

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18298-102