/N-O'7

NASA Technical Memorandu_m 105.9_79

_--_ .................... - - 2.J3 AIAA-93-0596 / -

, Model .....

_.............CounterrotationPr0pe!lerwith _a....... ..........

Forward-Swept Upstream Rotor ....

Richard P, Woodward .....

Lewis Research Center _-.... :_-. _ ._= ....... _ .......... ._.,_................... _

.-.... -.-.. :: _ieve!an-di_O_io " _ .....

David G. Hall

Sverdrup Technology, hlc. _

Lewis_ Research_Center Group

Brook Park, Ohio

and

Gary G. Podb0yand Robert J. Jer_cki

Lewis Research Center

Cleveland, Ohio

..... Prepared for the

31st Aerospace Sciences Meeting and Exhibit

sponsored by the American Institute of Aeronautics and Astronautics _

Reno, Nevada, January 11-14, 1993

° (NASA-TM-105979) TAKEOFF/APPROACH

_ NOISE FOR A MODEL COUNTERROTATION

_A_A PROPELLER WITH A FORWARD-S_IFPT

UPSTREAM ROTOR (NASA) 23 p

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J

q

N93-16715

Unclas

G3/07 0160772

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TAKEOFF/APPROACH NOISE FOR A MODEL COUNTERROTATION PROPELLER WITH A

FORWARD-SWEPT UPSTREAM ROTOR

Richard P. Woodward

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

David G. Hall

Sverdrup Technology, Inc.

Lewis Research Center Group

Brook Park, Ohio 44142

Gary G. Podboy and Robert J. Jeracki

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44] 35

Abstract j

A scale model of a counterrotating propeller with

forward-swept blades in the forward rotor and aftswept blades in the aft rotor (designated F39/A31) has

been tested in the NASA Lewis 9- by 15-Ft Anechoic

Wind Tunnel. This paper presents aeroacoustic results

at a takeoff/approach condition of Mach 0.20. Laser

Doppler velocimeter results taken in a plane between

the two rotors are also included to quantify the interaction flow field. The intention of the forward-swept

design is to reduce the magnitude of the forward rotor

tip vortex and/or wakes which impinge on the aft

rotor, thus lowering the interaction tone levels. A

reference model propeller (designated F3 l/A31), having aft-swept blades in both rotors, was also tested.

Aeroelastic performance of the F39/A31 propeller was

disappointing. The forward rotor tip region tended to

untwist toward higher effective blade angles under

load. The forward rotor also exhibited steady state

blade flutter at speeds and loadings well below the

design condition. The noise results, based on sideline

acoustic data, show that the interaction tone levels

were up to 8 dB higher with the forward-swept design

compared to those for th# reference propeller at similar

operating conditions, with these tone level differences

extending down to lower propeller speeds where flutter

did not occur. These acoustic results are for a poorlyperforming forward-swept propeller. It is quite possible that a properly-designed forward-swept propeller

_"would exhibit

reductions.

Copyright © 1993 by the American Institute of Aeronautics

and Astronautics, Inc. No copyright is asserted in the

United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under

the copyright claimed herein for Governmental purposes.

All other tights are reserved by the copyright owner.

substantial interaction tone level

Introduction

Modem high-performance turboprop aircraft offer

the promise of considerable fuel savings while still

allowing for a cruise speed similar to that of current

turbofan aircraft. Advanced counterrotation propellers

may offer from 8 to 10 percent additional fuel savings

over similar single rotation propellers at cruise conditions. 1 However, there is considerable concern about

the potential noise generated by such an aircraft,

which includes both cruise noise and community noise

during takeoff and landing.

The noise signature of a counterrotation propeller

includes rotor-alone tones, generated by forward and

aft rotor interaction with the flow field, and interaction

tones which arise from aeroacoustic interaction between the rotor flow fields. Figure 1 shows a typical

sound pressure level spectrum from a counterrotating

propeller. At takeoff/approach conditions (Moo = 0.2)

interaction tones dominate the spectrum above the

fundamental blade passage frequency. Interaction tone

levels have been well correlated with the aft rotor

interacting with the forward rotor tip vortex and

viscous wake. In the past, the interaction tone levels

2

have been somewhat reduced by increasing the rotorrotor spacing, or by reducing the aft rotor diameter so
thatit doesnotsignificantlyinteractwiththeupstream

rotortip vortex.

Anothermethodproposedforreducingtheinteractiontonelevelsis to employa "forward-swept"

forwardrotorin aneffortto reducethemagnitudeof

theforwardrotortip vortex,andprovidemorewake

decaythroughincreasedseparationat outboardlocations.Thisnoisereductionconceptisillustratedinthe

sketchof Fig. 2, and can be explained by discussing

how the wake flow of a forward-swept rotor might be

different from that of a conventional aft-swept rotor.

The tip vortex which results from an aft-swept rotor is

really a combination of two separate vortices -- the

vortex which forms along the tip of the blade and a

vortex which forms along the swept leading edge. On

an aft-swept blade, this leading edge vortex begins to

form inboard, follows the leading edge outward, and

is convected downstream at a location near the blade

tip. This leading edge vortex is expected to merge

with the blade tip vortex into a single vortex. On a

forward-swept blade, if a leading edge vortex forms,

it starts to do so near the tip, and would be expected

to follow the leading edge radially inward, and be

convected downstream at a radial location inboard of

the tip vortex. Therefore, the tip and leading edge

vortices would exist independently downstream of the

rotor. At a given operating condition, each of these

two separate vortices would be expected to be weaker

than the combined vortex which exists downstream of

the aft-swept blade, and consequently would be

expected to generate less interaction noise. Additionally, the forward-swept geometry results in a greater

rotor-rotor axial separation in the tip region which

would allow for more dissipation of the tip region

vortex before it interacts with the aft rotor.

This paper presents results for a model forwardswept counterrotation turboprop, designated F39/A31,

which was tested in the NASA Lewis 9- by 15-Ft

Anechoic Wind Tunnel. Data were taken at Mach 0.2,

which is representative of takeoff/approach conditions.

Figure 3 shows the F39/A31 propeller. Figure 4 shows

the propeller installed in the 9- by 15-Ft tunnel with

associated acoustic instrumentation. Acoustic data were

taken with fixed microphones which were attached to

the tunnel wall, with an axially-translating microphone

probe which was mounted to the tunnel floor, and with

a polar microphone probe, attached to the aft propeller

housing, which could take both sideline and circumferential acoustic surveys. A reference aft-swept propeller, designated F31/A31, was also tested at similar

operating conditions to provide an aeroacoustic comparison. Data were taken for several blade setting

angles, at propeller axis angles-of-attack up to 16° , and

at two rotor-rotor spacings. Acoustic results presented

in this report are for the propeller operating at 0°

angle-of-attack and were taken with the polar probe.

Additionally, laser Doppler velocimeter measurements

were made between the blade rows for selected propeller operating conditions to further understand the

inter-blade flow structure.

Acoustic performance of the forward-swept

propeller did not meet expectations. In general, the

tone levels (both rotor-alone and interaction) were

higher than those observed for the reference propeller.

This was due, in part, to problems with the aeromechanical forward-swept blade design which resulted

in significant tip untwist (increased pitch) under load,

and a tendency for the blade to go into incipient

flutter, which limited the test matrix to below-design

rotative speeds. It is quite possible that the adverse

acoustic effects observed for the F39/A31 propeller are

not typical of a properly designed forward-swept

propeller.

Apparatus

Anechoic Wind Tunnel

The NASA Lewis 9- by 15-Ft Anechoic Wind

Tunnel is located in the low-speed return leg of the

8- by 6-Ft Supersonic Wind Tunnel. The maximum

airflow velocity in the test section is slightly over

Mach 0.20, which provides a takeoff/approach environment. The tunnel acoustic treatment provides

anechoic conditions down to a frequency of 250 Hz,

which is lower than the range of test propeller acoustic

tones. 3,4

Acoustic Instrumentation

The acoustic data presented in this paper were

acquired with the polar microphone probe, which may

be seen in the photograph of Fig. 4. Figure 5 shows a

cross-sectional sketch of the installed model and polar

probe. The probe was instrumented with a 0.64 cm

(0.25 in.) diameter condenser microphone with a

"bullet nose" flow protector. The polar probe was
mountedto the downstreampropellerhousingand

surveyeda cylindricalsurfaceat a radiusof 61cm

(24in.)fromthepropelleraxis.Resultsfromthepolar

proberepresentfree-fielddatafor anuninstalledpropeller.Theprobecouldtakesidelinesurveysof +45 °

relative to the aft propeller plane (45 ° < 0 < 135°).

The polar probe sideline surveys were taken at a constant angular velocity relative to the aft propeller

plane, and required approximately 180 sec to

complete.

A computer-controlled FFT analyzer was used to

acquire 62 representative sound pressure level spectra

(0 to 10 kHz, 32 Hz bandwidth). A computer analysis

program then isolated desired tone orders (BPF, F+A

interaction, etc.) to generate tone sideline directivities.

Acoustic comparisons presented in this paper are for

polar sideline surveys which were taken in the horizontal plane toward the far tunnel wall (right side in

Fig. 4). These surveys are representative of the propeller noise field since there is essentially no circumferential variation in the noise field for 0 ° propeller

axis angle-of-attack. 2

Test Propellers

Figure 3 is a photograph of the forward-swept

F39/A31 propeller. A reference aft-swept propeller

(F31/A31) used the same aft rotor. Table 1 presents

selected design parameters for the two propellers. The

propellers were tested in the 12 + 10 blade configuration. All data were acquired with an 18.24 cm

(7.22 in.) spacing between rotor-rotor pitch axes, with

the exception of one F39/A31 test which was performed at a closer 14.99 cm (5.90 in.) spacing

(Table 2). The propeller installation in the 9- by 15- Ft

Wind Tunnel was powered by two independent air

turbine drives, allowing operation of the forward and

aft rotors at different speeds. The two rotors were

operated at about 50 rpm difference to avoid the

higher test rig vibration experienced with nearly equal

speeds. The unequal blade numbers facilitated spectral

separation of the various propeller tone orders.

Aeromechanical performance of the forwardswept F39 rotor was disappointing. The rotor tended

to go into flutter at rotative speeds above 75 percent

design -- thus limiting the test matrix. Also, excessive

tip untwist tended to increase the effective blade angle

in the tip region during operation. Various attempts

were made to improve the rotor aeroelastic stability

(see Fig. 6). The forward-extending leading edge was

clipped at first 10 percent, then 15 percent of the tip

chord. The tip-region airfoil was sufficiently thin that

this clipping had little effect on the leading edge

radius. Additionally, the leading edge was rounded to

a 4.06 cm (1.6 in.) diameter. These efforts produced a

marginal improvement in the blade aeroelastic

stability.

It was determined from these early F39 tests that

the blade aeromechanical performance could be improved with changes in the method of graphite fiber

layup. (The rotors were of titanium spar/graphite fiber

layup composite construction.) A second set of F39

blades were built with a modified layup procedure to

enhance stiffness. Testing with these blades showed a

marginal improvement in aeromechanical performance.

After initial testing showed little aeroacoustic improvement, the tip regions of these blades were also modified as shown in Fig. 6.

Table 2 lists the propeller test configurations

which were used in this study. The aft-swept F3 I/A31

propeller showed no aeromechanical anomalies, and

was tested, as planned, at two blade setting angles.

(Data were also taken for a higher-loading condition,

but corresponding F39/A31 runs were not possible.)

Aerodynamic Results

Data for the forward-swept F39/A31 and the

reference aft-swept F3 l/A31 propellers were taken on

two propeller operating lines, with the F39 rotor

setting angles at 13= 32.0 ° and 35 °, and F31 rotor at

13= 34.1 ° and 38.0 °. This corresponds to low and

intermediate blade loadings for an actual aircraft

application. Higher F39 blade setting angles could not

be tested due to the flutter problem. Table 3 is a

listing of selected aerodynamic results for the F39/A3 I

and F31/A31 propellers. Figure 7 shows the total

power density (PQAT) based on the forward rotor

annulus area as a function of the forward rotor advance ratio, Jl" Data points for similar operating

conditions for the F39/A31 and F3 I/A31 propeller are

displaced slightly due to differences in rotor diameter,

and consequently, advance ratio for the same rotative

speed. The propeller operating map for the forward

rotor (Fig. 8) shows similar performance on each

operating line for F39 and F31. The common aft rotor,
A31,for whichthebladesettingangleremainedconstantat13= 38.2°, showedessentiallythesameaerodynamicperformancefor all test conditions(Fig. 9).

Comparisons should be valid between F39/A31 and

F3 l/A31 for a particular propeller operating line with

similar PQAT and J values.

Acoustic Results

Acoustic results will be presented from data taken

along a 61 cm (24 in.) sideline by the polar microphone probe. Results will be presented in terms of

sound pressure level (SPL) spectra and sideline tone

directivities.

Figures 10 and 11 compare SPL spectra for the

two propellers operating on the intermediate operating

line. The F39/A31 propeller spectrum is for the

remanufactured F39 blade shape without tip modifications (see Table 2). Figure 10 compares spectra at

75 percent design propeller speed, while Fig, 11 shows

results at the maximum safe test speed for the

F39/A31 propeller (83 percent design) versus 85 percent design speed for the reference, F3 l/A31 propeller. Maximum levels for selected tone orders are

indicated on these spectra. Tone levels for F39/A31

are consistently higher than corresponding levels for

F31/A31 for both rotor speeds, with differences

typically being on the order of 8 dB. If the poor

acoustic performance of F39/A31 were due to its tip

leading edge flexibility and subsequent higher loading

and/or the flutter problem, one would expect that

differences in tone level would be negligible, or even

reversed at the lower operating speed. However, significantly higher tone level differences were observed

throughout the range of propeller test speeds.

Figures 12 and 13 show sideline directivities for

the F+A and 2F+A interaction tone orders for the two

propellers on the intermediate operating line (same

conditions as for Figs. 10 and 11). The forward-swept

concept was designed to minimize the forward rotor

tip vortex and thereby significantly reduce the interaction tone levels. In these results the forward-swept

propeller is clearly seen to have higher tone level

directivities for both tone orders at 75 percent speed

(Fig. 12) and at 83/85 percent speed (Fig. 13).

Broadband levels were determined from examination of representative SPL spectra for the F39/A31 and

F31/A31 propellers at the F+A and 2F÷A tone frequencies. Broadband curves are also presented in

Figs. 12 and 13 which likewise show higher levels

associated with the forward-swept F39/A31 propeller.

Windmill data from earlier propeller tests conducted in

the NASA Lewis 9- x 15-Ft Tunnel suggest that tunnel

background levels are typically about 86 dB in the

frequency region of the present propeller F+A tone,

and 84 dB in the region of the 2F+A tone, indicating

that tunnel background levels are not influencing these

results.

Figures 14 to 16 show maximum tone levels

along a 61 cm (24 in.) sideline as a function of

percent propeller design speed. Results for the two

propellers for the low blade loading operating line are

shown in part (a) of each figure, while part (b) of each

figure is for the intermediate operating line. The F39

blade configuration for the low loading operating line

(15= 32.0 °) was with the originally-manufactured blade

with 15 percent of the leading edge clipped (see

Fig. 6). Two F39 blade configurations are represented

for the intermediate operating line results. The remanufactured blades had the design tip shape and were

run with 15 = 35.0 °. The originally-manufactured

blades had 15 percent of the leading edge clipped and

the tip rounded to a 4.06 cm (1.60 in.) diameter, and

were operated with 15= 35.3 °.

Figure 14 shows the maximum forward rotor

BPF tone as a function of propeller speed. There is

little tone level difference between F39 and F31 at

speeds below 85 percent for the low blade loading

(Fig. 14(a)), but both F39 modes show considerably

higher tone levels relative to the F31 rotor at the

intermediate blade loading (Fig. 14(b)) at speeds above

75 percent. This may be related to the flutter condition

experienced by these blades at higher rotative speeds,

and/or the tendency for the forward-swept rotor tip to

flex under load.

Corresponding results for the F+A and 2F+A

interaction tones are shown on Figs. 15 and 16. Tone

levels for the F39 rotor are consistently higher than

those for the reference F31 rotor. Also, the difference

in tone levels appears to be just as large (sometimes

larger) at the lower propeller speeds as for the higher

speeds. One would expect that if the poor aeromechanical performance of the F39 rotor were adversely

affecting the interaction tone levels, this difference
betweenF39andF31 wouldbemostpronouncedat

higherpropellerspeedswhere the F39 rotor experienced the most tip flex and flutter. However, these

results suggest that the forward-swept F39/A31 propeller may be fundamentally noisier than the reference

F31/A31 propeller regardless of propeller speed,

although the forward-swept noise reduction concept

may be viable for a properly-designed propeller.

Laser Doppler Velocimeter Measurements

Laser Doppler velocimeter measurements were

made to define the wake flow generated by the

forward-swept and aft-swept forward rotors. All three

velocity components were measured in selected axial

planes downstream of the rotors. When each velocity

measurement was made the instantaneous angular position of each rotor was recorded, allowing detailed

blade-to-blade variations in the flow to be determined.

Figure 17 shows axial velocities measured in the wake

of each of the forward rotors, F39 and F31. Both sets

of data were obtained in the same constant axial plane,

just upstream of the aft rotor. This plane is depicted in

the sketch above each data plot. The view here is from

behind the measurement plane looking upstream. Here

the axial velocities are shown plotted relative to the

circumferential position of the front rotor, which

would be rotating counter-clockwise in this view. Any

influence of the aft rotor would be smeared out circumferentially. In generating these plots, the data of

of the F31 blades. It was first thought that this region

of increased wake thickness downstream of the F39

blades might be due to flow separation from the blade

surfaces. LDV measurements within the F39 blade

passages could indicate if separation occurred; t, nfortunately, intrablade measurements were not made at this

operating condition. It should be noted, however, that

similar F39 blade wakes were measured at each of the

other operating conditions at which LDV data were

obtained, and that intrablade measurements made at

other operating conditions did not show any flow

separation. This increased blade wake thickness might

simply be a consequence of wake spreading. Wider

wakes might be expected at these outer measurement

locations since they are further downstream from the

blade due to the forward sweep of the trailing edge.

The other noticeable difference between the two

flow fields results from the tip vortices shed from the

blades. The vortices shed from the forward-swept

blades are shown to produce larger axial velocity

perturbations than those of the aft-swept rotor. Mean

axial velocities as low as 46 m/sec (150 ft/sec) and as

high as 137 m/sec (450 ft/sec) were measured within

the tip vortices shed from the forward-swept blades;

while mean axial velocities measured within the aftswept blade tip vortices varied from 52 to 119 m/sec

(170 to 390 ft/sec). The axial velocity perturbations

induced by the tip vortices are also shown to occur

over a larger area downstream of the forward-swept

the twelve individual blade passages were averaged blades. These data suggest that the tip vortices shed

into one "composite" passage. The composite passage

flows of each rotor are plotted here, and they are

shown repeated to illustrate any transitions which

might occur across the boundaries of the passage. This

composite passage is shown a total of three times for

each rotor. These data are for propeller operation on

the low blade loading operating line at 85 percent

propeller design speed. At this condition the forwardswept F39 blades did not flutter. The F31 blade setting

angle was 35.0 ° for the laser data, slightly different

from the 34.1 ° for which acoustic data have been

presented.

Two significant differences can be seen in the

velocity fields of the two propellers. One is a result of

differences in the blade wakes shed from the two sets

of forward rotor blades. While at inboard radial locations the blade wakes are similar, at outer locations the

F39 blade wakes appear to be much wider than those

from the forward-swept blades are stronger than those

produced by the aft-swept blades.

The data plotted in Fig. 18 provide some insight

as to why the forward-swept blades could be expected

to generate stronger tip vortices than the aft-swept

blades. In this figure the axial and tangential velocities

of Fig. 17 have been replotted; but here they are

shown after the data obtained at each of the different

radial locations have been circumferentially averaged.

Hence, Fig. 18 shows radial distributions of circumferentially averaged axial and tangential velocity

similar to those that might be determined by radially

traversing a low frequency-response pitot-static pressure probe at this axial location. In order to show both

velocity components on the same graph, the freestream velocity of 67 rrdsec (220 ft/sec) was subtracted

from the axial velocities. The data measured downstream of the aft-swept rotor shows both the axial and

5
tangentialvelocitiesincreasingraduallywithdecreasing radius from the tip until each distribution peaks

inboard of 50 percent span. These data indicate that

the loading of the F31 blades peaked at an inboard

location and that the tips of the blades were relatively

lightly loaded. This is the sort of radial loading

distribution that one would expect for a properly

designed rotor of a counterrotating configuration. In

contrast, the velocity distributions measured downstream of the F39 blades indicate much higher loadings in the tip region. Since tip vortex strength is

expected to increase with tip loading, the F39 blades

would be expected to generate stronger tip vortices.

The high tip loading on the F39 blades is probably

related to the high flexibility of these blades. The tip

regions may be twisting under load to higher than

expected effective blade angles. Similar velocity profiles taken at 75 percent design speed also showed evidence of high tip loading (blade untwist), suggesting

that noise generation from higher tip loading may be

typical of the entire range (70 to 85 percent design) of

propeller test speeds.

It is important to note that the axial velocity

contours presented in Fig. 17 for the two front rotors

shown evidence of only a single vortex in the flow

downstream of each blade. As discussed previously,

when a leading edge vortex forms on an aft-swept

blade it migrates radially outward and merges with the

tip vortex. Therefore, in this case only a single vortex

per blade would be expected. In contrast, if a leading

edge vortex were to form on a forward-swept blade it

would move inward and be convected downstream at

a radial location inboard of the tip vortex. Therefore,

if leading edge vortices were forming on these

forward-swept blades, they would appear in the velocity contours as separate regions of very steep axial

velocity gradient. These are not shown to occur in the

wake flow of the forward-swept blades. This probably

means that the leading edges of these blades were not

swept forward enough to allow the formation of leading edge vortices at this operating condition. This is

significant since this noise reduction concept relies, at

least in part, on the ability of the forward-swept blades

to generate separate leading edge and tip vortices;

individually these vortices would be weaker than a

single tip vortex, and therefore, would be expected to

generate less interaction noise. Since no leading edge

vortex is shown to occur, little if any noise reduction

would be expected. As discussed above, at a similar

operating condition, the forward-swept blades are

thought to have actually generated stronger tip vortices

than the aft-swept blades. This supports the acoustic

findings presented earlier which indicated that the aftswept F3 l/A3 l propeller performed better acoustically

than did the forward-swept F39/A31 configuration.

Effect of Rotor-Rotor Spacing

Reference 2 presented results for two counterrotation propellers which were previously tested in the

same facility. These propellers shared a common aftswept forward rotor (designated F7), but had different

aft rotors. The A7 rotor had essentially the same

diameter as did the F7 rotor; however, the A3 rotor

was designed with a smaller diameter in an effort to

avoid interaction with the forward rotor tip vortex, and

thereby result in lower interaction tone levels. The A3

rotor was otherwise designed with a larger chord and

increased blade setting angle to achieve aerodynamic

performance similar to that of the A7 rotor. Maximum

interaction tone levels as a function of rotor-rotor

spacing for these two propellers showed the tone

levels to decrease more rapidly with spacing for the

F7/A3 propeller. The A3 rotor was apparently reacting

with the more rapidly dissipating upstream rotor

viscous wake rather than the tip vortex which was

"missing" it.

The following discussion compares acoustic

results for the F7/A7 and F7/A3 propellers with those

of the current study in an effort to gain a further

insight into the F39/A31 interaction tone mechanism.

The F39/A31 propeller was run at two rotor-rotor

spacings w the closer spacing corresponds to the

maximum spacing run with the propellers of reference 2. Unfortunately, the F39 rotor was slightly

modified when it was operated at the closer rotor-rotor

spacing (see Table 2) by having 15 percent of the

leading edge chord clipped.

Figure 19 shows that the F39/A31 and F31/A31

propellers operating on the "intermediate-loading"

operating line have a comparable aerodynamic performance to that of the F7/A7 and F7/A3 propellers of

reference 2. This was a chance result in that the

propellers of reference 2 had different blade numbers

(11/9) and rotor tip diameters. However, it is possible

to compare operating points with similar J and power

loading.
Figures20and21showtheeffectof rotor-rotor

spacingon the F+A interaction tone level. At a Jl

value of about 1. l I (Fig. 20) the tone level for the

reference F7/A3 propeller is seen to be much lower

than the corresponding level for the F7/A7 propeller at

the 14.99 cm (5.90 in.) spacing. As previously discussed, the higher level for the F7/A7 propeller is

thought to be caused by tip vortex interaction. Maximum F+A tone levels for the F39/A31 propeller with

spacing seem to be a continuation of the curve established by the F7/A7 propeller, suggesting that tip

vortex interaction may be the mechanism for the

F39/A31 interaction tone. A similar result is seen for

the higher propeller speed results of Fig. 21.

Concluding Discussion

A scale model of a counterrotating propeller with

a forward-swept forward rotor was aeroacoustically

tested in the NASA Lewis 9- by 15-Ft Anechoic Wind

Tunnel. A reference propeller with aft-swept blades in

both rotors was also tested in the same facility at the

same operating conditions. It was expected that the

forward-swept propeller concept would generate a

significantly-reduced forward rotor tip vortex and

wake, resulting in significantly lower interaction tone

levels for this propeller.

The aeroelastic performance of the forward-swept

propeller did not meet expectations. The forward rotor

was prone to enter flutter at modest propeller speeds

above 75 percent of design, depending on blade loading. Additionally, the tip region of the forward-swept

rotor proved to be excessively flexible, resulting in

additional effective blade pitch near the tip region.

Several modifications were made to the F39 rotor to

attempt to improve its aeroelastic performance. These

were to (a) clip the blade leading edge tip chord by

15 percent, (b) round the tip leading edge, and (c) remanufacture the forward-swept rotor to improve the

blade stiffness. These changes gave only a modest

improvement in the forward-swept rotor's aeroelastic

performance.

The aeroacoustic performance of the forwardswept propeller was also disappointing. Fundamental

rotor-alone tones for the forward-swept model tended

to be higher than those for the aft-swept reference

propeller. Comparison of the interaction tones showed

that levels for the forward-swept design were consistently higher by up to 8 dB. This difference was

observed at all propeller speeds tested from 70 to

85 percent design. Laser Doppler velocimeter data

gathered in the plane between the rotors for both

models indicated that the tip vortex induced by the

forward swept upstream rotor was larger and stronger

than that measured for the aft-swept forward rotor.

It is important to stress that the forward-swept

F39 rotor in this study did not perform well mechanically and aerodynamically. It is quite possible that the

adverse acoustic effects observed for the F39/A31

propeller are not typical of a properly designed

forward-swept propeller.

References

1. Mikkelson, D.C., Mitchell, G.A., and Bober, L.J.,

"Summary of recent NASA Propeller Research,"

Aerodynamics and Acoustics of Propellers, AGARD

CP- 366, AGARD. Neuilly-Sur-Seine, France, 1985,

pp. 12-1 to 12-24. (Also NASA TM-83733, 1984.)

2. Woodward, R.P., "Noise of Two High-Speed

Model Counterrotation Propellers at Takeoff/Approach

Conditions," Journal of Aircraft, Vol. 29, No. 4,

July-August 1992, pp. 679-685.

3. Dahl, M.D., and Woodward, R.P., "Comparison

Between Design and Installed Acoustic Characteristics

of NASA 9- by 15-Ft Low-Speed Wind Tunnel

Acoustic Treatment," NASA TP-2996, April 1990.

4. Dahl, M.D., and Woodward, R.P., "Acoustical

Evaluation of the NASA Lewis 9- by 15-Ft Low

Speed Wind Tunnel," NASA TP-3274, Nov. 1992.
TABLE I.--PROPELLER DESIGN CHARACTERISTICS

F31/A31, Reference all-swept forward rotor

Number of blade# ........................................................................................ 12/10

Design cntise Mach number ................................................................................. 0.80

Nominal diameter, cm (in.) .................................................................. 65.18/62.91, (25.66/24.77)

Nominal design cruise corrected tip speed, m/see, (ft/sec) .................................................. 256/251, (839/810)

Hub-to-tip ratio ....................................................................................... 0.41/0.39

Design power coefficient based on annulus area at takeoff ............................................................ 3.63

Takeoff advance ratio ........................................................................... 0.96 (at M_ = 0.20)

F39/A31, Forward-swept forward rotor

Number of blades ......................................................................................... 12110

Design cruise Mach number ................................................................................. 0.80

Nominal diameter, cm (in.) .................................................................. 66.05/62.91, (26.00/24.77)

Nominal design cruise corrected tip speed, m/sec, (ft/sec) .................................................. 259/2,51, (851/810)

Hub-to-tip ratio ....................................................................................... .0.40/0.39

Design power coefficient based on annulus area at takeoff ............................................................ 3.38

Takeoff advance ratio .......................................................................... 0.95 (at M,. = 0.20)

_' Forward propeller/aft propeller.

Propeller

F3 IIA31

r,,

F3 IIA31

F39/A31

F391A31

F39/A31

F39/A31

Table 2.--Propeller Test Configurations

Blade seiaing angle,

deg forward/aft

34.1/38.2

38.0/38.2

32/0/38.2

35.3/38.2

35.0/38.2

35.0/38.2

Forward rotor shape

Reference a_ swept

Reference aft swept

Original design, L.E. clipped

at 15 percent of chord

Original design, L.E. clipped at

15 percent chord and L.E. rounded

4.06 cm (0.60 in.) diameter

Rotor remanufactured for

additional stiffness

Rott_ remanufactured for

additional stiffness and L.E.

clipped at 15 percent chord

Rotor-rotor spacing,

cm (in.)

18.24 (7.22)

18.24 (7.22)

18.24 (7.22)

18.24 (7.22)

18.24 (7.22)

14.99 (5.90)
Table 3.--Selected Aerodynamic Results

Part (a)--Forward-Swept F39/A31 Propeller

Speed,

percent

7O

75

80

85

Spacing, a Blade

cm (in.) angle,

deg

18.24 (7.22) 32.0/38.2

18.24 (7.22)

Original design, 15 percent clipped leading edge

PQATz b PQA l

1.353 0.558

1.552 0.678

1.722 0.779

1.859 0.853

PQA 2 Jl c J2 rPm 1 rpm 2

70I

75

80

85

[7oI

75

80

83

17oI

75

80

83

18.24 (7.22) 35.0/38.2

14.99(5.9o)

Original design, rounded and 15 percent

35.3/38.2 1.744 0.928 0.996 1.173

1.910 1.032 1.096 1.100

n/a rv'a rda n/a

2.257 1.226 1.270 0.973

Remanufactured blades, design shape

1.712 0.900 1.001 1.176

1.917 1.012 1.103 1.103

2.093 I 1.118 I 1.191 1.036

[ 2.210 1.185 1.265 0.995

0.976 1.159 1.205 5341 5392

1.082 1.082 1.129 5717 5755

1.163 1.014 1.056 6097 6145

1.246 0.952 0.993 6488 6529

elipped leadingedge

1.218 5361 5422

1.150 5716 5737

n/a n/a n,'a

1.013 6460 6514

1.225 5352 5394

1.144 5711 5780

1.075 6100 6169

1.036 6346 6394

1.228 5343

1.146 5725

1.071 6100

1.031 6340

70

75

80

85

90

70

75

80

85

9O

Remanu_ctured blades, 15 pe_ent clipped leading edge

35.0/38.2 1.692 0.872 1.006 1.181

1.915 1.006 1.123 1.099

2.100 1.104 1.218 1.030

2.207 1.168 I 1.272 0.991

Part 0a), Aft-Swept F3 l/A31 Pro 3eller

1.352 0.524 0.961

1.533 0.617 1.052

1.702 0.697 1.149

1.816 0.759 1.218

1.965 0.820 1.295

1.711 0.922 0.918

1.957 1.016 1.090

2.094 1.088 1.159

2.256 1.174 !.241

2.400 1.247 1.331

18.24 (7.22) 34.1/38.2

18.24 (7.22) 38.0/38.2

1.193

1.111

1.043

0.984

0.929

1.205

1.130

1.061

1.001

0.943

1.231

1.142

1.070

1.013

0.950

1.244

1.165

1.092

1.029

0.971

5332

5712

6118

6463

6865

5356

5720

6099

6459

6871

0-degree Angle-of-attack, M.. = 0.2.

Subscripts: I denotes forward rotor; 2 denotes aft rotor.

aSpacing is defined as axial distance between rotor pitch changes axes.

bpQAT is defined as power/[ (p) (rev/sec) 3 (13) 3 (annulus area)] where p is the freestream air density, and D is the propeller

diameter.

el is the propeller advance ratio, defined as V,/(rpm/D), where V. is the freestream velocity, and D is the propeller diameter.

5396

5766

6165

6405

5355

5756

6175

6508

6954

5376

5749

6140

6510

6913
m

"10

J

13.

co

130

BPF1 + BPF2 I

120 F 1

2BP| 2 _2BPF1 _l_

3BPF 1

110 , _ 1t | 3BPF2 ',4BPF2_

90 1 1 I

0 1 2 3 4

Frequency, kHz

Rgure 1.--Typical counterrotation turboprop sound pressure

level spectrum (subscripts I and 2 refer, respectively, to

forward and aft rotor).

_-2BPF 1 + BPF 2

BPF 1 + 2BPF21_ _ BPF. +

3BPF

Flow

Rounded ,/_

leading edge --/

Tip vortex

Aft-swept F31/A31

Increased tip-tip

separation

Vi.__ous wake /

Forward-swept F39/A31

Aft-swept

• Forward rotor tip vortex is strengthened

by leading edge vortex, and interacts

with aft rotor.

• Forward rotor viscous wake interacts

with aft rotor.

Forward-swept _ ::=:;

intens_ (_ffi_d/ngedg_e vortex

contribution), and increased dissipation

dist=_ce-to aft-i'oto-r. Tip_vo-dex may_

migrate toward hub pdor to relea_

Forward rotor viscous wake interaction

with aft rotor may be more significant at

mid-radius due to closer rotor-rotor speclng.

Figure 2.--Aeroacoustic comparison of forward- and aft-swept counterrotation propeller concepts.

10
Figure 3.--F39/A31 forward-swept counterrotation turboprop.

11
Fixed

microphone

array

C-92-05893

Figure 4.--Photograph of forward-swept counterrotation propeller installed in the 9x15 anechoic wind tunnel.

12
T

61 cm

(2 ft)

Polar microphone probe

Flow

Figure 5.--Sketch of the turboprop model and polar microphone probe.

/-- Leading edge rounded

/ to 4.06 cm (1,60 in.) diam

15°/=ch°_ _ _ /

_'ow I___ \ _

7

I

Figure 6.--Modifications to F39 rotor tip region (actual rotorrotor spacing was greater than indicated in this sketch).

13
3.0

D.

_2.0

1.5

1.0

-- • F39/A31,131/_2 = 32.0138.2 (Original tip shape)

• F391A31, _1/132= 35.3/38.2 (F39 rounded tip LE.)

• F39/A31,131/_2 = 35.0/38.2 (Remanufactured blades)

O F31/A31,131/132 = 34.1/38.2

_ A F31/A31, 1_1/_2= 38.0/38.2

I I I I I I I

.90 .95 1.00 1.05 1.1 0 1.1,5 1.20 1.25

Forward rotor advance ratio, J1

Figure 7.mForward - and aft-swept counterrotating propeller aerodynamic performance (o_= 0 °, Moo= 0.20).

2.0

'_ 1.5

1.0

• F39/A31, p1/132 = 32.0138.2 (Original tip shape)

• F39/A31, Pl/l_2 = 35.3/38.2 (F39 rounded tip L.E.)

,L F39/A31, pl/_2 = 35.0/38.2 (Remanufacturad blades)

O F31/A31, I]1/[32 = 34.1/38.2

A F31/A31, _11_2 =38.0/38.2

I I I ! [ I ]

.90 .95 1.00 1.05 1.1 0 1.1 5 1.20

Forward rotor advance ratio, J1

Rgure 8.--Forward rotor power dansity as a function of advance ratio (a = 0 °, Moo= 0.20).

1.25

14
2.0

1.5

1.0

.5 m

0

.90

• F3g/A31, _1/132 = 32.0/38.2 (Original tip shape)

• F3g/A31, [_1/132 = 35.3/38.2 (F39 rounded tip L.E.)

• F39/A31,131/[32 = 35.0/38.2 (Remanufactured blades)

O F31/A31, _1/132 = 34.1/38.2

Z_ F31/A31,131/1132 = 38.0/38.2

I I 1 I 1 I

.95 1.00 1.05 1.10 1.15 1.20

Forward rotor advance ratio, J1

Figure 9.--Aft rotor power density as a function of advance ratio (oc= 0 °, M® = 0.20).

!,

"r

I

1.25

120 --

110

go

"o

100

9O

8O

F+A

BPF

-- | F+2A

._ It • _ll 2F+A

-- --II--

F39/A31,131/P2 = 35.0/38.2

F31/A31,131/p2 = 38.O/38.2

,I I I

0 1 2 3

mm m _1 Bm 7--

-- II /,', I L

1 l I I [ I I

4 5 6 7 8 9 10

Frequency, kHz

Figure lO.--Compadson of SPL spectra for forward- and aft-swept propellers (32 Hz bandwidth, 0 = 82 °, _ = 0°, 75% design speed,

M= = 0.20).

15
BPF

F+A

12oI- __ _F_

/ __ F+_I

° 1 =' '_

90 h • F39/A31,131/[_ 2 = 35.0/38.2

J . F31/A31, 1311132= 38.0/38.2

8oI I I t I I

0 1 2 3 4 5

Frequency, kHz

_m

il_

II

It

II

! i

I I I I i

6 7 8 9 10

Figure 11 .---Comparison of SPL spectra for forward- and aft-swept propellers (32 Hz bandwidth, O = 82 °, _ = 0 °, F39/A31 at 83%

design speed, F31/A31 at 85% design speed, M® = 0.20).

16
120 F39/A31 131/132= 35.0/38.2

_ooI_J "-'r'_'" _: _,',

"'

..... "Broadband level

(33

"o

_ 8o/ I I I I I I I I I

(a) F + A interaction tone.

•

110

1 O0

90 _ Broadband level

80 I I I I I I I I I

45 55 65 75 65 95 105 115 125 135

Angle from upstream propeller axis, deg

(b) 2F + A interaction tone.

Figure 12.--Sideline tone directivity (61 cm (24 in.) sideline,

a = 0 °, 75% design speed, M® = 0.20).

m

"o

._ 90

==

_130

120

130 --

F39/A31 _1/132 = 35.0138.2

F3t/A31 1311132= 38.0/38.2

120 m

110 _ .i "_ _..'_, I

I

_ ..... Broadband level

I I I I I I I I I

(a) F + A interaction tone.

100 _ • |

! _ Broadband level

90 ""[" _ I I l I I I I

45 55 65 75 85 95 105 115 125 135

Angle from upstream propeller axis, deg

(b) 2F + A interaction tone.

Figure 13.--Sideline tone directivity (61 cm (24 in.) sideline,

¢x= 0°, F39/A31 at 83% design speed, F31/A31 at 85% design

speed, M® = 0.20).

17
_" 120_0 F3F31/A31' _,/132=341/382=

(a) Low blade loading.

O F31/A31, 1311132= 38.0/38.2

[] F39/A31,1311132 = 35.0/38.2 (Remanufactured

blades)

F39/A31, 1311132= 35.3/38.2 (F39 rounded tip LE.)

._'_ 120 _

_o 110

0_ 100 [- J I I

70 75 80 85

Percent design speed

(b) Intermediate blade loading.

Figure 14.--Maximum forward rotor BPF tone level along a 61 cm

(24 in.) sideline as a function of propeller percent design speed.

(,_=0 °, M= = 0.20).

"_ 120 FO F31/A31, 1311132= 34.1/38.2

(a) Low blade loading.

C) F31/A31,131/132 = 38.0/38.2

[] F39/A31,131/[32 = 35.0/38.2 (Remanufactured

blades)

A F39/A31, 1_1/132 = 35.3/38.2 (1:39 rounded tip LE.)

70 75 80 85

Percent design speed

(b) Intermediate blade loading.

Figure 15._aximum F + A interaction tone level along a 61 cm

(24 in.) sideline as a function of propeller percent design speed.

(,_ = 0 °, M= = 0.20).

120 _ F31/A31, _1/132=34.1/38.2

"_ ii- 1 F39/A31, 13111_2=__._._

(a) Low blade loading.

O F31/A31,131/_2 = 38.0/38.2

[] F39/A31,131/_2 = 35.0/38.2 (Remanufactured

blades)

A F39/A31,131/P2 = 35.3/38.2 (F39 rounded tip LE.)

==

¢3.

_=110 o

10ol J ! I

70 75 80 85

Percent design speed

(b) Intermediate blade loading.

Figure 16.--Maximum 2F + A interaction tone level along a 61 cm

(24 in.) sideline as a function of propeller percent design speed.

(,_=0 °, M® =0.20).

18
Tip .i

_._. _',_.__ <_/.L,19o

i _ 310 F___210...

: "_ ".-"_J- /":,J"_lll_.J .I 230

"_/2 _" Lines of constant !_\_.'\

Hub .... /// .... = _',\_

"-:5 \ .... _\\\ ',

Forwardrotor .;::_-=_<_j?' "\_ll_ ',

rotation _ C L -_1 _.__

,al .............. _ ................. _...... -:--' _............ _................. ,_......... k.. ..

Measurement Measurement

plane plane

F Fow. 

33O

\

\

\

\

•_ Forward rotor

i rotation

\

\

(a) Forward-swept propeller, F39/A31. (b} Aft-swept propeller, F31/A31.

Figure 17.--Inter-blade row axial velocity profiles from laser Doppler veloclmetry measurements (viewing upstream, F39/A31

1_/132 = 32.00138.2 °, F31/A31 1_1/_2 = 35.0°/38.2 °, 85% design speed, ¢x = 0 °, M= = 0.2).

19
140 -- 45 r

P

4O

120 --

100 -- 30

80 -- _m 25 --

_20i--

¢4o_$ 15-

10 --

-- [] F31 axiaJ velocity

20 5 -- 0 F39 !angen!!_. ve!oc!.ty

0 F31 tangential velocity

0 -- 0 --

-20- ,5 I I 1 1 I

10 15 20 25 30 35

Radius, cm

I l I I I I

4 6 8 10 12 14

Radius, in.

Figure 18.--Cimumferentiaily-averaged axial and tangential velocity profiles measured

downstream of forward- and aft-swept rotors (F39 13= 32.0 °, F31 13= 35.0 °, 85% design

speed, a = 0 °, M= = 0.20).

• F39/A31, p1/{32 = 35.3/38.2 (F39 rounded tip LE.)

• F39/A31, 1311132= 35.0/38.2 (Remanufactured blades)

A F31/A31,131/132 = 38.0/38.2

O F7/AT, 131/_2 = 36.4/36.5

[] F'//.A_, _11_2 = 36.4/43.5

3.00 --

O=

2.00

_. 1.50 --

1.0o I I I I

.8 .9 1,0 1.1 1.2

Forward rotor advance Patio, J1

Figure 19._Propeller operating map showing forward-swept

F39/A31 propeller and reference aft-swept configurations.

(oc = 0°, M== = 0.20).

2O
O F31/A31,131/_2 = 38.0/38.2, 75% SPD

[] F39/A31, _1/_2 = 35.0/38.2, 75% SPD

A F39/A31, 1311132= 35.3/38.2, 75% SPD

F39 Rounded tip L.E.

• F7/A7, 131/132 = 36.4/36.5, 70% SPD

130 -- • FT/A3, _1/[_2 = 36.4/43.5, 70% SPD

cD

"o

-_ 120

==

_110 m

IO0

5

\

I I I

10 15 20

Rotor-rotor stacking line spacing, cm

I I I I I I I

2 3 4 5 6 7 8

Rotor-stator stacking line spacing, in.

Figure 20.--Maximum F + A interaction tone level as a function of rotor-rotor axial spacing along a 61 cm (24 in.) sideline. (a = 0°, M= = 0.20).

O F31/A31,131/132 = 38.0/38.2, 85% SPD

[] F39/A31, 1311132= 35.0/38.2, 83% SPD

A F39/A31, 1311132= 36.3/38.2, 85% SPD

F39 Rounded tip L.E.

• FT/AT, 131/{32 = 36.4136.5, 80% SPD

140 -- • F7/A3, _1/_2 = 36.4/43.5, 80% SPD

rn

"o

130

_120

110 I I

10 15 20

Rotor-rotor stacking line spacing, cm

I I 1 I I I I

2 3 4 5 6 7 8

Rotor-stator stacking line spacing, in.

Figure 21 .--Maximum F + A interaction tone level as a function of rotor-rotor axial spacing along a 61 cm (24 in.) sideline. (_ = 0 °, M= = 0.20).

21
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

January 1993 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Takeoff/Approach Noise for a Model Counterrotation Propeller with a

Forward-Swept Upstream Rotor

6. AUTHOR(S)

Richard P. Woodward, David G. Hall, Gary G. Podboy, and Robert J. Jeracki

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

WU- 535-03-10

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-7479

9. SPONSORING/MONITORING AGENCY NAMES(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM- 105979

11. SUPPLEMENTARY NOTES

Prepared for the 31st AIAA Aerospace Sciences Mccting and Exhibit sponsored by the American Institute of Aeronautics and Astronautics, Reno,

Nevada, January 11-14, 1993. Richard P. Woodward, Gary G. Podboy and Robert J. Jeracki, Lewis Research Center, Clcveland, Ohio. David G. ltall,

Sverdrup Technology, Inc., Lewis Research Ccntcr Group, Brook Park, Ohio 44142. Responsible person, Richard P. Woodward, (216) 433-3923.

12s. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category 7 and 71

i12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

A scale model of a counterrotating propeller with forward-swept blades in the forward rotor and aft-swept blades in the

aft rotor (designated F39/A31) has been tested in the NASA Lewis 9- by 15-Foot Anechoic Wind Tunnel. This paper

presents aeroacoustic results at a takeoff/approach condition of Mach 0.20. Laser Droppler velocimeter results taken in a

plane between the two rotors are also included to quantify the interaction flow field. The intention of the forward-swept

design is to reduce the magnitude of the forward rotor tip vortex and/or wakes which impinge on the aft rotor, thus

lowering the interaction tone levels. A reference model propeller (designated F31/A31), having aft-swcpt blades in both

rotors, was also tested. Aero-elastic performance of the F39/A31 propeller was disappointing. The forward rotor tip

region tended to untwist toward higher effective blade angles under load. The forward rotor also exhibited steady state

blade flutter at speeds and ioadings well below the design condition. The noise results, based on sideline acoustic data,

show that the interaction tone levels were up to 8 dB higher with the forward-swept design compared to those for the

reference propeller at similar operating conditions, with these tone level differences extending down to lower propeller

speeds where flutter did not occur. These acoustic results are for a poorly-performing forward-swept propeller. It is quite

possible that a properly-designed forward-swept propcller would exhibit substantial interaction tone level reductions.

14. SUBJECT TERMS

Turbofan noise; Counterrotation

17. SEC URITY CLASSIFICATION

OF REPORT

Unclassified

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

NSN 7540-01-280-5500

19. SECURITY CLASSIFICATION

OF ABSTRACT

Unclassified

15. NUMBER OF PAGES

22

16. PRICE CODE

A03

20. LIMITATION OF ABSTRACT

Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102