NASA

ZI--,-

Z

TECHNICAL NOTE NASA TN D-4142

A WIND-TUNNEL INVESTIGATION OF A

7-FOOT-DIAMETER DUCTED PROPELLER

by Kenneth IV. Mort and Berl Ga,-,o _'1_8 "_Ames Research Center ' _ _ _ " 8 (THRU)

i(ACCESSION NUMBER)

_ ,Moffett Field, Calif. /-_,_.%_/ /,ooo_,// ,oCL

(NASA CR OR TMX OR AD NtJMBE;R)

NATIONAl. AERONAUTICS AND SPACEADMINISTRATION • WASHINGTON, D. C. ° AUGUST 1967

https://ntrs.nasa.gov/search.jsp?R=19670025554 2020-04-01T19:18:14+00:00Z

/

NASA TN D-4142

A WIND-TUNNEL INVESTIGATION OF A 7-FOOT-DIAMETER

DUCTED PROPELLER

By Kenneth W. Mort and Berl Gamse

Ames Research Center

Moffett Field, Calif.

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

For sale by the Clearinghouse for Federal Scientific and Technical Information

Springfield, Virginia 22151 - CFSTI price $3.00

A WIND-TUNNEL INVESTIGATION OF A 7-FOOT-DIAMETER

DUCTED PROPELLER

By Kenneth W. Mort and Berl Gamse

Ames Research Center

SUMMARY

Power, free-stream velocity, blade angle, and duct angle of attack were

varied to define the aerodynamic characteristics of a full-scale ducted pro-

peller. Dynamic pressure ranged from 0 to 106 psf, rotational velocity from

1200 to 2590 rpm, blade angle from 14° to 49° , and power from 0 to 1250 hp.

The results indicate that the ducted propeller achieved a maximum figure

of merit of 81 percent and a maximum propulsive efficiency of 74 percent.

Stall boundaries for both the upstream and downstream duct lips were

based on pressure distributions. It was found that for a representative

V/STOL configuration employing this ducted fan the upstream lip could stall

at low power and at high duct angles; however, stall of the downstream lip

did not appear to be likely.

INTRODUCTION

Experimental information concerning the performance of large-scale

ducted propellers suitable for lifting and propelling VTOL aircraft has been

very meager. Therefore, an investigation of a 7-foot-diameter ducted propel-

ler was made in the Ames 40- by 80-Foot Wind Tunnel.

The test objectives were (i) to define the aerodynamic characteristics

of the ducted propeller; (2) to evaluate the propulsive performance; (3) to

define the onset of upstream (or lower) and downstream (or upper) duct lip

stall; and (4) to define the aerodynamic characteristics of the duct exit

vane •

NOTAT ION

A e net exit area, (shroud exit area) minus (exit vane blockage area),ft2

fan blade chord, in.

c shroud chord, ft

czi

CD

CDo

Cf

CL

Cm

Cp

%

d

de

FM

h

J

m

n

N

P

PC

Po

q

r

R

S

blade-section design lift coefficients,

drag

drag coefficients, qS

section design lift

qb

drag coefficient at zero lift coefficient

friction drag

flat-plate skin-friction drag coefficient, (wetted area) q

li ftlift coefficient,

qS

m

pitching-moment coefficient,qSc

power coefficient_P

pnSd 5

thrust coefficient,T

pn2d 4

propeller diameter, ft

duct exit diameter, ft

figure of merit, 50CTs/2

Cp, percent

fan-blade thickness, in.

Vadvance ratio, n--d

pitching moment about duct rotation axis, ft-lb

propeller rotational speed, rps

propeller rotational speed, rpm

power, (ft-lb)/sec

power coefficient,P

qVS

power at _ = 5v = 0O, (ft-lb)/sec

free-stream dynamic pressure, psf

radial distance from duct center line, in.

propeller radius, in.

reference area, Cde, ft2

SHP

T

Tc

To

V

Ve

V_

X

_v

q

P

input power, hp

thrust, ib

To

thrust coefficient_ q-_

thrust at a = 5v = 0°, ib

free-stream velocity, fps

computed velocity at duct exit, fps

free-stream velocity, knots

chordwise distance from duct leading edge, in.

angle of attack, deg

propeller blade angle at 3/4 radius station, deg

exit vane deflection (see fig. 2(a)), deg

propulsive efficiency, _J × i00, percent

air density, slugs/ft 3

MODEL AND APPARATUS

General Model Description

The ducted propeller model was essentially a full-scale duplicate of

those used on the Bell Aerosystems Co. X-22A airplane. The general arrange-

ment of the model in the wind tunnel is shown in figure i. Model dimensions

are given in figure 2 and in table I, and propeller blade characteristics in

figure 3.

Propeller Drive System

The propeller was driven by a 1500-hp electric motor through a right-

angle gear box. The motor speed could be varied continuously from 0 to

3000 revolutions per minute.

Instrument at ion

Forces and moments on the ducted propeller were measured by the

wind-tunnel six-component balance. The forces and moments on the wing

fairing and support structure were not transmitted to the balance and hencewere not measured. Press_tre orifices were located on the shroud in thepositions shownin figure 2(b).

The power input to the ducted propeller was determined from the motoroutput torque and rotational speed. The torque _s measuredby a strain-gagebalance.

TESTS

The propeller blade angle, rotational velocity, and free-streamdynamic pressure were set and the duct angle of attack wasvaried during thetests. The blade angles ranged from 14° to 49° , the dynamic pressures rangedfrom 0 to 106 psf, and the rotational velocities ranged from 1200 to 2590 rpm.

During most of the tests the exit vane was in place, but a few testswere madewith the vane off. Exit vane deflections from -17° to +20° wereexamined.

REDUCTIONOFDATA

Correct ions

No corrections for wind-tunnel wall effects were applied to the databecause they were not significant. No corrections were madefor gear boxlosses because they were estimated to be on the order of 1/2 percent of theinput power.

Accuracy of Measuring Devices

The various measuring devices used were accurate within the followinglimits which include errors in reading and reducing the data as well as theerrors of the device itself.

LiftDragPitching momentMotor input torqueRotational speedFree-stream dynamic pressure

Angle of attackPropeller blade angleExit vane deflection

±3 ib±3 lb±i00 ft-lb

±15 ft-lb

±0.5 rps

±0.i psf for values < 20 psf

±1/2 percent for val_es > 20 psf

±0.5 °

±0.5 °i2 o

4

RESULTS AND DISCUSSION

Basic Aerodynamic Characteristics

Thrust and power coefficient results at _ = 0 °.- The ducted fan thrust

and power coefficients (CT and Cp) in figure 4 are for the exit vane installed

at zero deflection (_ = 0°). The results in figure 5 were obtained with and

without the exit vane at Sv = 0°" The configuration with the exit vane isconsidered the basic configuration because the amount of duct diffusion

selected was based on the presence of the vane. The propulsive efficiency

and static efficiency were better with the vane. (The static efficiency was

better because at J = O, Cp was the same with and without the vane, whileCT was lower without the exit vane.)

From the results of figure 4 the variations in thrust coefficient appear

to be regular for blade angles from 14° to about 39 ° , but at 44 ° the value at

zero advance ratio is low and remained low until the advance ratio exceeded

about 0.7. This was probably because significant portions of the blades were

stalled. At 49 ° the blades appeared to remain stalled until the advance

ratio exceeded about i.

The coefficients in figures 4 and 5 are referenced to rotational veloc-

ity. Thrust coefficient referenced to free-stream velocity was determined

from the results of figure 4, and is shown in figure 6. This thrust coeffi-

cient, Tc, which was obtained at zero angle of attack and zero exit vane

deflection, is used in subsequent figures as a correlation parameter instead

of advance ratio and blade angle.

Basic aerodynamic characteristics at _ _ 0°. - The basic aerodynamic

characteristics are presented in figure 7 for several values of Tc ._ Lift

coefficient is plotted as a function of duct angle of attack, drag coeffi-

cient, and pitching moment, and power coefficient Pc is plotted as a func-

tion of _. The results in figure 7 are for several exit vane deflections

and in figure 8 for the exit vane out.

Figure 9 shows the aerodynamic characteristics with the propeller removed

and with the exit vane at _ = 0 °. As shown, the variation of CD with CLCL2

is consistent with the expression CD = CDo + to maximum CL as (delc)was found for the small-scale ring wings of reference i. This suggests that

the presence of the exit vane did not affect the induced drag.

iThe rotational velocity and blade angle at which the test was conducted

are also included to allow determination of the actual test conditions if

desired.

Evaluation of the Static and Propulsive Efficiency

Efficiency.- The static efficiency and propulsive efficiency computed

from the faired curves of figure 4 are presented in figure I0. The static

efficiency is shown in figure iO(a) as the figure of merit (based on net exit

area) plotted against blade angle. The maximum figure of merit achieved was

about 81 percent. The propulsive efficiency is presented in figure lO(b);

the maximum achieved was 74 percent.

These values of efficiency were compared with those achieved by the

ducted propellers of references 2 and 3. The ducted propeller of reference 2

and the "static" ducted propeller configuration of reference 3 were designed

to produce good static efficiency. (The "static" configuration of ref. 3 had

a large bellmouth inlet.l The "cruise" ducted fan configuration of refer-

ence 3 was designed to produce good high-speed efficiency. Comparison with

these models indicated that the 7-foot model achieved very good static effi-

ciency (the model of ref. 2 achieved a figure of merit of 7 8 percent and

the "static" model of ref. 3, 80 percent) and fair propulsive efficiency

(the propulsive efficiency of the "cruise" configuration of ref. 3 was

80 percent).

These results suggest that the design of the 7-foot model was a good

compromise in satisfying both the low-speed and high-speed design require-

ments. However, it appears that some improvement in propulsive efficiency

should be possible. In the next section the experimental results will be

compared with theoretical considerations, and it will be shown that improve-

ments in the propulsive efficiency would necessitate a reduction in the drag

of the duct, centerbody, struts, etc.

Comparison with theory.- From the results of figure 4, the maximum thrust

which could be achieved was determined as a function of free-stream velocity

for a constant input power equal to the design value of 1250 hp. The results

are presented in figure ii with the theoretical thrust for comparison. This

theoretical thrust was computed according to "ideal momentum ducted fan

theory" using zero duct drag and a propeller efficiency of 90 percent based

on the air velocity at the propeller. (This efficiency is an average, over

the velocity range shown_ of the values estimated using ref. 4. The estimated

variation over the velocity range was about ±3 percent.) For free-stream

velocities up to about i00 kmots the thrust obtained experimentally follows

the ideal curve reasonably well. However, as velocity is increased further

the difference between the experimental curve and ideal curve increases

because of the drag of the duct, centerbody, strut, etc. This is shown in

figure ii by a curve which was obtained by subtracting the propeller-out drag

data of figure 9 from the ideal curve. 2 The resulting curve compares rather

well with the experimental thrust curve. Hence, it may be concluded that the

drag of the duct, centerbody, struts_ etc., causes the reduction in thrust

shown at high forward speeds. It may also be concluded that the propeller

2The increase in drag due to the higher intermal velocity when the pro-

peller is present is negligible compared to the total duct drag. Hence, the

propeller-out drag is representative of the total duct drag _ith the propeller

present.

efficiency is approximately 90 percent as wasassumedsince the measuredthrust agrees well with that predicted using a propeller efficiency of90 percent and propeller-out duct drag.

To estimate how muchof the drag in figure ii was due to shroud friction,the friction drag of the shroud was estimated and an increment subtractedfrom the ideal curve. The resulting curve, also shownin figure ii, representsthe best possible thrust v_ich could be approaehed if the ducted propellerwere designed exclusively for high speed. Because of the large differemce

between this curve and the experimental curve, it is evident that the major

portion of the experimentally determined drag is not due to shroud friction

drag, and suggests the possibility for making large reductions in drag and

hence increasing the propulsive efficiency.

Examination of the Exit Vane Performance

The exit vane aerodynamic characteristics at zero free-stream velocity

are presented in figure 12 by showing lift, thrust, pitching moment, and power

as functions of 5v. The exit vane performance for free-stream velocities

greater than zero was determined from the results of figure 7 and is presented

in figure 13 for four values of T c. Here ACL, Z_CD, ACm, and Z_Pc are shown

as functions of %v. These results may be analyzed assuming the vane causes alinear variation in the force normal to the duct axis. This force can then be

resolved in the lift and drag directions. If this is done it is found that at

negative values of %v the AC L and Z_CD variations with _ are greater

than would be expected. However, at positive values of _v the _C L and Z_CD

variations with _v are less than would be expected as a is increased. For

example, at Tc = 5 and _ = 50 ° , AC L is essentially zero for positive values

of _¢. The variation in lhCm with _r in the results of' figure 13 is small

and hence less important than that for AC L and hCD, particularly if the

moment reference is very far from the ducted fan as for the four ducted

propeller V/STOL configuration of references 5 and 6.

Duct Lip Stall

Stall of both the upstream (or lower) duct lip and downstream (or upper)

duct lip was investigated. Stall of the upstream lip is of primary concern

because it is more heavily loaded and would result in a larger reduction in

lift when stalled. In addition, stall of this lip affects the propeller load-

ing asymmetrically. Stall of this lip will be considered first cud im moredetail.

Upstream or lower lip stall.- Flow separation was established on thebasis of pressure distributions. A sample is shown in figure 14. As can be

seen, separation occurred initially on the inside surface very close to the

propeller. As angle of attack was increased the separated area increased

forward until finally the flow separated over the entire upstream lip. In

figure 15(a) the angle of attack at which separation initially occurred and

the angle of attack at which the flow separated over the entire upstream lip

are shown as functions of the reciprocal of T c. This correlation was

obtained for blade angles of 19° and 29° and is considered valid for allblade angles between these values. On examination, the data in figure 7 showthat no large changes in forces, moments,and power accompaniedthe onset oflocal flow separation, but did accompanyseparation of the entire lip.

The propeller blade stresses were monitored during the investigation oflip stall. With the onset of flow separation there was only a small rise instress level. As the angle of attack was increased, the stresses increasedgradually. With the entire upstream lip stalled the stresses were still wellbelow critical.

The angle of attack at which separation occurred on the entire upstreamlip is shownin figure 15(b) for the present tests and for the small-scaleresults of references 6 and 7. (The 4-foot model of reference 7 was not ascaled model of the 7-foot model, but the contour of this duct is very simi-lar as the drawings show.) The flow over the entire lip of the 7-foot modeland the 4-foot model separated at very nearly the sameangle of attack, butthere is a large difference between the results shownfor the large-scalemodels and those for the i/5-scale model. The difference indicates a large-scale effect and suggests that there is a critical lip radius above whichincreases in radius do not delay separation significantly and below whichseparation will occur at muchlower duct angles of attack.

Downstream or upper lip stall.- Occurrence of separation on the down-

stream duct lip was also determined from pressure distributions. The angle

of attack at which separation occurred over the entire downstream lip is

shown in figure 16. In addition, the small-scale results of reference 6 are

presented. These results also indicate a large-scale effect. (A stall

boundary for the downstream lip was not established for the model of ref. 7.)

The data in figure 7 indicate that separation on this lip was not accompanied

by any detectable changes in forces, moments, and power nor was buffeting

evident during these tests. Consequently, separation on the downstream lip

was not considered nearly as critical as separation on the upstream duct lip.

Significance of lip stall boundaries.- The significance of the duct lip

stall boundaries is examined in figure 17 which presents duct angle of attack

as a function of forward velocity for the four ducted propeller V/STOL config-uration of references 5 and 6. The duct angle of attack for trimmed condi-

tions was obtained using a lift force of 3750 pounds (one-fourth of the

design gross weight of 15,000 ib). The lift and drag of the vehicle exclu-

sive of the ducted propellers were neglected, equal duct angles were assumed,

and trim drag was neglected. The effect of these assumptions is small for

this vehicle at the low velocities where the lip may stall. In addition to

the trim curve, the curves at which lip stall occurs are shown. From these

results it can be concluded that upstream duct lip separation may occur on

the simulated vehicle in level flight. However, complete separation of this

lip will not occur. (Of course, at low power and at high duct angles, as in

descending flight, complete separation of the upstream lip may occur.) Down-

stream lip stall probably would not be encountered by this type of vehicle.

8

CONCLUDING REMARKS

The ducted propeller had a maximum figure of merit of 81 percent and a

maximum propulsive efficiency of about 74 percent. If the shroud, centerbody,

struts, etc., had been designed primarily for high forward velocities, this

propulsive efficiency could have been significantly higher. This can be con-

cluded from the fact that the drag with the propeller out was significantly

higher than the estimated shroud friction drag. (The friction drag of the

other components is small compared to that of the shroud.)

Examination of the exit vane aerodynamic characteristics indicated that

at positive vane deflections the variations in CL and CD with vane deflec-

tion were significantly lower than would be expected as the angle of attackwas increased.

Stall of both the upstream and downstream duct lips was examined. The

significance of the stall was examined for a four ducted propeller V/STOL con-

figuration employing this ducted propeller. It was found that the onset of

flow separation on the upstream lip will be encountered; however, complete

separation on this lip will be encountered only during conditions of low power

and high duct angle of attack corresponding to high rates of descent. Flow

separation on the downstream lip would probably not be encountered by this

type of vehicle.

Ames Research Center

National Aeronautics and Space Administration

Moffett Field, Calif., 94035, June 6, 1967

721-03-00-05-00-21

9

REFERENCES

1,

21

o

o

.

.

7,

Fletcher, Herman S. : Experimental Investigation of Lift, Drag, and

Pitching Moment of Five Annular Airfoils. NACA TN 4117, 1957.

Mort, Kenneth W.: Performance Characteristics of a L-Foot-Diameter

Ducted Fan at Zero Angle of Attack for Several Fan Blade Angles.NASA TN D-3122, 1965.

Grose, Ronald M.: Wind Tunnel Tests of Shrouded Propellers at Mach

Numbers From 0 to 0.60. WADC TR 58-604, United Aircraft Corp., 1958.

Anon.: Generalized Method of Shrouded Propeller Performance Estimation.

Hamilton Standard Division of United Aircraft Corporation. Publicationnumber PDB 6220.

Giulianetti, Demo J.; Biggers, James C.; and Maki_ Ralph L.: Longitudinal

and Lateral-Directional Aerodynamic Characteristics of a Large-Scale,V/STOL Model With Four Tilting Ducted Fans Arranged in a Dual Tandem

Configttration. NASA TN D-3490, 1966.

Spreemann, Kenneth P.: Wind-Tunnel Investigation of Longitudinal Aerody-

namic Characteristics of a Powered Four-Duct-Propeller VTOL Model in

Transition. NASA TN D-3192, 1966.

Mort, Kenneth W.; and Yaggy_ Paul F.: Aerodynamic Characteristics of a

4-Foot-Diameter Ducted Fan Mounted on the Tip of a Semispan Wing.NASA TN D-1301, 1962.

i0

TABLEI.- BASICDIMENSIONSOFDUPEDPROPELLER

DuctImside diameter at propeller, in .................. $4.75Maximumexternal diameter, in ................... 101.56Chord, im............................. 49Exit diameter, in ......................... 93-3Net exit area, sq ft ....................... 44.08Net area at propeller, sq ft .................... 37.84Duct rotation station, percent of duct chord ............ 64

28.6Propeller station, percent of duct chord ..............Propeller

Diameter, ft ............................ 7Tip clesrance, in ......................... 0.4Numberof blades .......................... 3Integrated design lift coefficient ................ 0.43Total activity factor ....................... 504

Exit vaneArea, sq ft ............................ 31.8Thickness, in ........................... 5.3Hinge line, percent of duct chord ................. 85.2

ii

12

(a) 3/4 front view with duct at 90° angle of attack.A-32844

Figure i.- X-22A ducted propeller model mounted in the Ames 40- by 80-FootWind Tunnel.

13

(b) 3/4 rear view with duct at 0 ° angle of attack.

Figure i.- Concluded.

A-33771

14

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¢-

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b_.r-I

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4Z625

Propeller

14.00

17,750

19.60

21.520

Radius

42.375

t duct

4900

All dimensionsin inches.

D72

46.65

Outside surface ordinates

X r

0 47.625

.613 48.695

1.225 49.096

2.450 49.609

3.675 49.955

4.900 50.2057.350 50.5359.800 50.710

10.250

12.250 50.779

14.700 50.76317.75019.600 50.552

23.70024.500 50.164

29.400 49.64934.300 49.058

39.200 49.344

44.100 47.57646.550 47.16049.000 46.722

Pressure orifice location

Number

I

23

45

67

8

9I0II1213

14

151617

1819

Location, percent chordInside Outside

0

t2.5

5I0

1525

35

507090

9070

50

352515

51.8

(b) Shroud dimensions.

Figure 2.- Continued.

16

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Figure 4.- Ducted propeller thrust and power coefficient referenced to

rotational velocity with the exit vane; 5v = _ = 0°"

19

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Figure 5.- Ducted propeller thrust and power coefficient and propulsive

efficiency with and without the exit vane at _v = _ = 0°"

2O

34

32

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28

26

24

22

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18

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14

12

I0

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6

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2

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Figure 6.- Thrust coefficient referenced to free-stream velocity; _v = _ = 0°"

21

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0 I0 20 .50 40 50/9, deg

(a) Figure of merit as a function of blade angle.

Figure i0.- Figure of merit and propulsive efficiency computed from the

curves of figure 4.

31

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P

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8 v, deg

Figure 12.- Effects of exit vane deflection on the aerodynamic characteristics

at zero free-stream velocity; _ = 0 o, B = 19 ° .

34

4

AC L 0

-2

Tc a, deg

40 3050

60

0-30

50,40

0-30

40

50

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0-3040

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4

AP_ 0

0-405060

0-50

D-40 0-40

-4- 20 0 20 -20 0 20 - 20 0 20 -20 0 20

By, deg

Figure 13.- Summary of exit vane aerodynamic characteristics determined from

the results of figure 7 for several thrust coefficients; AC L defined as

CL less C L at zero vane deflectiom; ACD_ ACm_ and Z_Pc similarly

defined.

35

_DIV')

o_1 cO I_- 0 I.D od cO _1"le) od oJ oJ m D

0Jnss0Jd O!LUDU_p LUDOJIS-OOJ-i

(aJnssaJd O!J,OJ,S LuoeJ,_s-aaJ-i)-(aJnssaJd 0!$o_s 1000-1)

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a, deg

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Separation over entireupstream lip

k ,ocal eooro,'o 1

I I I I I

(a) Lip stall for the 7-foot-diameter model.

a, deg

I00-

8O

6O

4O

20-

\\

1

Leading edge Iradius, inches , Inside surface

__.__--t diametar d_

I 4 ft diameter ductedfan from ref.7

_--_Modified lip

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I I I I I0 .2 .4 .6 .8 1.0

I/Tc

(b) Comparison with other models of conditions at which

separation occurred on the entire upstream lip.

Figure 15.- Angle of attack at which stall of the lower or upstream duct lip

oc cD_rs,

37

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III

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NASA-Langley, 1967 -- i A-2511 39

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