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TECHNICAL NOTE

D-995

NASA TN D-995

AERODYNAMIC LOADS ON AN

ISOLATED SHROUDED-PROPELLER CONFIGURATION

FOR ANGLES OF ATTACK FROM -10 ° TO 110 °

By Kalman J. Grunwald and Kenneth W. Goodson

Langley Research Center

Langley Air Force Base, Va.

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

WASHINGTON January 1962

https://ntrs.nasa.gov/search.jsp?R=19620000022 2020-04-01T18:51:08+00:00Z

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IX

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

TECHNICAL NOTE D-995

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AERODYNAMIC LOADS ON AN

ISOLATED SHROUDED-PROPELLER CONFIGURATION

FOR ANGLES OF ATTACK FROM -lO ° TO llO °

By Kalman J. Grunwald and Kenneth W. Goodson

SUMMARY

An investigation of the division of loads between the propeller

and the shroud of a shrouded-propeller configuration was conducted in

the 17-foot test section of the Langley 300-MPH 7- by lO-foot tunnel.

This investigation covered an angle-of-attack range from -i0 ° to ii0 °

and a range of advance ratios (0 to 0.595) typical of the transition

speed range of a tilt-duct VTOL aircraft.

For the configurations with fixed blade angle (24 ° at the three-

quarter radius), the normal force and pitching moment developed by the

propeller and spinner were only a relatively small part of the overall

normal force and pitching moment of either a stalled or an unstalled

shroud. The ratio of the thrust of the propeller and spinner to total

thrust was found to vary from about 0.4 in hovering to about 0.7 at the

highest advance ratio of the tests, 0.595, with the shroud at zero angle

of attack.

INTRODUCTION

The ducted- or shrouded-propeller configuration has been proposed

as one means of attaining VTOL flight. At the present time information

is available on the aerodynamic performance of wing-tip-mounted tiltable

shrouded-propeller configurations. (For example, see refs. 1 to 3.)

Information on the division of loads between the shroud and the propeller

is useful in analyzing aerodynamic performance _id structural require-

ments. Because of the scarcity of such information, the present program

has been conducted to obtain experimental data on the division of loads

for an isolated shrouded-propeller configuration.

The basic shrouded propeller of this investigation was previously

used (mounted on a wing tip) in the investigation reported in reference i.

The shroud had 5/16 the dimensions of the shrouded-propeller configuration

2

of reference 2 but the propeller design was different. The results inreference i showedthat, because of the low Reynolds numbersof the tests,the stalling characteristics of the smaller shroud differed from thoseof the larger shroud of reference 2. Therefore the basic configurationwas modified by the addition of shroud leading-edge fairings to alleviatethis shortcoming. Inasmuch as the range of Reynolds numbersof the pres-ent tests is the sameas that in reference l, both the basic model andthe modified model (which incorporated the smaller of the leading-edgefairings used in ref. l) were employed in the present investigation.The models were tested through an angle-of-attack range from -lO° to ll0 °.The advance ratio was varied from 0 to 0. 595. The propeller-blade angleat the three-quarter radius was held constant at 24°. Tests were con-ducted in the 17-foot test section of the Langley 300-MPH7- by 10-foottunnel (ref. 4).

Presented herein, for both the basic and modified configurations,are measuredforce and momentcoefficients of the shroud without a pro-pellet, the shroud with a windmilllng propeller, and the shroud with athrusting propeller. Included in the last category are measurementsofforce and momentcoefficients of the propeller and spinner operatingwithin the shroud.

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SYMBOLS

b

C

CL

Cm

CQ

CT

CX

D

FX

h

L

propeller-blade section chord, ft

duct chord, 0.859 ft

lift coefficient, L/qS

pitching-moment coefficient, M/qSc

torque coefficient, Q/pn2D 5

thrust coefficient, Tp/pn2D 4

longitudlnal-force coefficient, Fx/qS

propeller diameter, 1.25 ft

longitudinal force, lb

propeller blade-section thickness, ft

lift, lb

L16

1

7

M

Mp

n

N

Np

q

Q

r

R

S

T

To

Tp

V

x

8.79

n

P

total pitching moment, ft-lb

propeller pitching moment, ft-lb

propeller rotational speed, rps

total normal force, ib

propeller normal force, Ib

dynamic pressure, 21--pV2

motor shaft torque, ft-lb

radius to propeller blade element, ft

propeller-tip radius, ft

projected shroud area, (Shroud chord) X (Exit diameter),

i. 21 sq ft

total thrust, ib

hovering thrust, 49 lb

propeller thrust (including spinner forces), ib

free-stream velocity, ft/sec

transfer distance from propeller-moment reference point to

duct-moment reference point, 0.129 ft

angle of attack, deg

blade-sectlon angle, deg

blade angle at three-quarter radius

net efficiency, TV/2_Qn

density, slugs/cu ft

4

MODELS AND APPARATUS

Photographs of the basic symmetrical model are shown in figure i.

A side-view sketch giving the principal dimensions and showing the major

features of the model support system is presented in figure 2. Basic

shroud coordinates are given in table I. The leading-edge fairing

employed to adjust the shroud-stalling characteristics of the model is

outlined in figure 3. Because of its purpose, no great attention is

given to details of the fairing.

The three-blade propeller was constructed of plastic reinforced with

glass fiber over a balsa-wood core. Geometric characteristics of the

propeller blade are given in figure 4. Blade sections consisted of

NACA 6412 airfoil sections. The blade-angle setting at the three-quarter

radius was 24 ° . The shrouded propeller had a tip clearance of 0.04 inch

and a root clearance of 0. i0 inch.

The model was supported in the tunnel by a shielded sting-support

system attached at the rear of the motor nacelle as indicated in fig-

ure 2. Total forces and moments were measured with a three-component

straln-gage balance located at the foot of the strut. Forces and moments

of the combined propeller and spinner were measured with strain-gage

beams mounted within the motor nacelle. The thrust of the combined pro-

peller and spinner Tp was measured as shaft tension without corrections

for pressure differences within the nacelle. Such corrections in the

present case are believed to be very small.

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TESTS

Measurements of the forces and moments of the shrouds with the pro-

peller removed and with a windmilling propeller were made at a tunnel

speed of i00 feet per second. Powered tests, with a constant propeller

rotational speed of 8,000 rpm, were made at the various tunnel speeds

required to cover the desired range of advance ratio V/nD. At each

selected advance ratio, the angle of attack was varied over a range which

would include anticipated conditions of steady level flight, climb, and

descent for tilt-duct VTOL configurations. Measurement of the thrust

defined herein as hovering thrust To was made at _ = 900; in view of

the large ratio of the volume of the surrounding enclosure to the volume

of flow through the shroud, the effect of the surrounding enclosure on

the measured hovering thrust is believed to be extremely small.

Since power-off tests were made at a wind speed of i00 feet per

second, the Reynolds number based on shroud chord was 550,000. For the

power-on tests, the Reynolds numbervaried from 0 at V/nD = 0 to 550,000at V/nD = 0.595 (the maximum V/nD of the tests).

DATAREDUCTION

The results are referred to the system of wind axes shownin fig-ure 5. locations of the shroud pitching-moment axis and propellerpitching-moment axis are given in figure 2.

The free-stream dynamic pressure was used in nondimensionalizingthe data for the power-off conditions and the data in the relativelyhigh-speed flight regime. However, in the transition speed range, coef-ficients based on the free-stream velocity would approach infinite valuesnear hovering flight; to avoid this problem, transition speed-range datahave been nondimensionalized by dividing by the static thrust in hovering.

RESULTSAND DISCUSSION

The basic data for the shroud configuration with the lip modifica-

tion are presented in figures 6 and 7. Figures 8 and 9 contain the

basic data for the unmodified configuration. Fi_Ires 6 and 8, which

contain the data for the highest speed condition tested, show the results

for conditions of power off with propellers off, power off with pro-

pellers windmilling, and power on with propellers operating at

V/riD = 0.595.

Power-Off Characteristics

In the plots of figures 6 and 8 for lift coefficient as a function

of longitudinal force coefficient, it can be observed that the maximum

lift coefficient of the shroud with a windmilling propeller is greater

than that of the shroud with the propeller removed. This higher lift

is attributed to the action of the propeller in redistributing flow

energy through the duct, to the lift component of the propeller, and to

a delay of flow separation within the duct due to the turbulent action

of the propeller-blade wake on the duct boundary layer; and of course

there may be other causes. Redistribution of flow energy in the duct

arises from the action of the propeller in removing energy from regions

of high velocity and adding energy to regions of low velocity in the

duct.

Power-0nCharacteristics

The basic data obtained through the angle-of-attack and advance-ratio ranges of the tests are presented in figures 7 and 9 for the modi-fied and unmodified configurations, respectively. As mentioned earlier,the low Reynolds numbersof the tests gave rise to a region of stalledoperation in the case of the unmodified shroud. Tuft studies madeduringthe tests gave evidence that the stall was due to flow separation fromthe shroud lip; the occurrence of stall wa_ marked by a distinct changein the character of the noise emitted by the model. In figure 9 thestall is marked by rapid reductions in the lift and pitching-momentcurves and by sharp increases in thrust and torque. For the modifiedconfiguration (fig. 7) the rapid changes do not occur. Up to the pointof llp stall, the characteristics for the unmodified configuration arepractically the sameas those for the modified configuration.

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7

Division of Loads

The primary purpose of this investigation was to determine the

division of aerodynamic loads between the shroud and the propeller.

The results in figure 7(c) (modified configuration) and figure 9(c)

(unmodified configuration) show that the propeller contribution to the

overall normal force and pitching moment of either configuration is

relatively small. Relative magnitudes are shown more directly in fig-

ures l0 and ll. Although stalled operation (fig. 9(c)) causes appreciable

changes in propeller normal-force and pitchlng-moment characteristics,

these quantities remain small in comparison with the overall values.

The ratio of propeller thrust to total thrust as a function of angle

of attack for a number of advance ratios is shown in both configurations

in figure 12. In hovering (V/nD = 0), the propeller carries approximately

40 percent of the total thrust. At the highest V/nD value of the tests,

0.595, the propeller carries about 70 percent when _ is near 0 °.

For the unstalled operation, figure 12(a), the ratio of propeller

thrust to total thrust generally decreases as _ increases at a constant

value of V/nD. Beyond the stall, on the other hand, the ratio increases

as _ increases (fig. 12(b)). This increase in thrust ratio is due

primarily to a reduction in shroud thrust caused by the lip stall

(fig. 9(c)), and is due partly to an increase in propeller thrust which

is also caused by the lip stall (fig. 9(b) or 9(c)).

In figure 13, the propulsive efficiency and thrust ratio obtained

for the modified configuration at _ = 0° and V/nD = 0.595 are shown,

together with curves obtained from reference 5 (a sharp-radius inlet

configuration). From the position of the propulsive-efficlency point

of the present configuration in relation to the efficiency curves ofthe reference configuration, it can be judged that peak efficiency wasnot reached with the present configuration; it occurs at a value ofV/nD higher than 0.595. For the reference configuration the thrustratio lies in the neighborhood of 0.9 at V/nD = 0.595, and the resultfor the present investigation is 0.7. Someof this difference is dueto the difference in the ratio of shroud-exit diameter to propellerdiameter for the two configurations.

SUMMARYOFRESULTS

For the shrouded-propeller configuration of the present investiga-tion the normal force and pitching momentdeveloped by the propeller andspinner are a relatively small part of the overall normal force andpitching momentof either a stalled or an unstalled shroud; therefore,the duct is the primary source of pitching momentand normal force loads.

The ratio of the thrust of the propeller and the spinner to thetotal thrust varies from about 0.4 in hovering to about 0.7 at the high-est advance ratio of the tests, 0.595, with the shroud at zero angle ofattack. For the unstalled operation this thrust ratio in generaldecreases as angle of attack increases at constant advance ratio. Beyondthe stall and at constant advance ratio, on the other hand, the thrustratio increases when the angle of attack increases.

Langley Research Center,National Aeronautics and Space Administration,

Langley Air Force Base, Va., September28, 1961.

8

REFERENCF_

i. Goodson, Kenneth W., and Grunwald, Kalman J.: Aerodynamic Character-

istics of a Powered Semispan Tilting-Shrouded-Propeller VTOL Model

in Hovering and Transition Flight. NASA TN D-981, 1962.

2. Yaggy, Paul F., and Mort, Kenneth W.: A Wind-Tunnel Investigation of

a 4-Foot-Diameter Ducted Fan Mounted on the Tip of a Semispan Wing.

NASA TN D-776, 1961.

3. Tapscott, Robert J., and Kelley, Henry L.: A Flight Study of the

Conversion Maneuver of a Tilt-Duct VTOL Aircraft. NASA TN D-372,

1960.

4. Kuhn, Richard E., and Hayes, William C., Jr.: Wind-Tunnel Investiga-

tion of Longitudinal Aerodynamic Characteristics of Three Propeller-

Driven VTOL Configurations in the Transition Speed Range, Including

Effects of Ground Proximity. NASA TN D-55, 1960.

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

Numbers From 0 to 0.60. WADC Tech. Rep. 58-604, ASTIA Doc.

No. AD-205464, U.S. Air Force, Dec. 1958.

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9

TABLE I .- DUCT COORDINATES

kO

Yi

V =

X_

in.

0

•o78•125

•188

•25o•312.438.625

.938i.25oi.562i.8752.1882. 500

2.812

Straight line

t5. 375

5.9386.25o6.562

6.8757.188

7.5oo7.812

8.125

8.4388.750

9. 375

i0. 312

Yi,

in.

0.875

.663

.589513

4634o6

357263

163

i00

•050• 019•006

0

0

0

0

0

0

0

0

Straight line

.)50

YO_

in.

0. 875

1.144

l, 206

1. 256

1. 287

i. 325

i. 375

i .457

1.506

i.556i, 587

1.613i •625

1.637

i. 644

i .644

i. 644

1.644

i. 644

i. 644

i .644

1.625

i. 600

I. 575

1-537

]_.5OOi. 463

1.425

i. 375

I. 3511.281

1.163

.981

lO

,-I

LrXI

0LD

r--

1I--

o

t_Oo

4-_0

!

_D

Figure 1.- Concluded. L-60-_43

12

Shroud pitching-moment reference

3.5

15.5 -_

'rPitch I

axis

I

I

1690

,I

Strutshield

Strut

t-

t--

i-

Tunnelfloor

Balance

Figure 2.- Sketch of model and model support system. Dimensions are ininches.

13

_D

J

o

0

.-4

!

I

,,p

o

b_

A

!

°r..,I

14

_3

_ooqqb

_o°_

6ap "B" a/ 6uo

O/q,O!/O./ _a/ au.m!p -o4 - p./oq.3

I'-

-,'4

_3

o,-4

,--tt_O

-.o

I1)R

Q)

0

0

m

11)

-0°,-.IffJ0

°r-t

o

©ul

o

-o

!

°r..,l

16

cL

36

32

28

2.4

20

1.G

/.2

.8

.4

0

/>

/

/

//

0 Propeller off

m Pro oeller windmilling

0 Propeller rotational speed -

BOO0 rpm ,V/nD =.595

_"___ \

i

-.41.G 12 B .4 0 - .4 -8 - 12

Cx

Figure 6.- Aerodynamic characteristics of the modified-llp configuration

in the power-off and low-power condition.

2X

17

r__D

CL

/6

,8

Cm

0

-8

Cx

2.4

/.6

.8

0

-,8

-,8

0

.8

-15-20

z,J"

0

r'l

<>

iPropeller off

Propeller windmilling

Propeller rotationol speed

80C r V/nO =.595

<

/I/

/

//

/Y

//

s//

0 20 40 60 80

a ,deg

im

!II

/00 /2O

Figure 6.- Concluded.

18

l

14

12

.2

0 _

V/nD

D 0n .0320 O90

.164

.187

298

.506

_, 401

0 .595

_6

-4

-2

0

2

I_ I [ i I

I0 20 30 40 50 60 70 80 90 I00 IlO 120

a, de 9

(a) Lift, longitudinal force, and pitching moment (modified-lip

configuration).

Figure 7 .1 Effect of advance ratio and angle of attack on the aerodynamic

characteristics of the modlfied-lip configuration.

19

.024

i i

i [J, i ] - ;

[ J _ __

,-i_o,-i

co

.020--

.0/2

i......

V/nD

n 0o .032<> .090_, .164o .187

_. .298306

v .401

o .595

.004

C T

]

]0

2O

./6

L ,l 12:

08 i i:'! ---II i--i

! t

it

i

i

i[ i

J

r -: i

t.,0 I0 20 30 40 50 60 70 80 90 I00 1I0 120

a, deg

(b) Propeller thrust and torque coefficients (modified-lip configuration).

Figure 7.- Continued.

20 ""

21

32

0 Propeller off

n Propeller windmiHing

(> Propeller rotational speed

8000 rpm , V/nD :.595

CL

0

\

I

-Z2

Figure 8.- Aerodynamic characteristics of the original-lip configuration

in the power-off and low-power condition.

22

Cm

.8

0

-.8

7

%

1

CL

Cx

40

2#

16

.8

0

-8

-.8

0

.8

16-20

0 Propeller off

o Propeller windm q0 Propeller rotal_i speed

8000 rpm , V/nD =.595

,<

!i

0 20 40 60

a, deq

80 lOO 120

Figure 8.- Concluded.

23

2

6

BI0 10 20 30

y s//i ! I

I

40 50 60 70

a, de_

I

V/_D

o 0a 0450 /28

189

o 193268

o JOl371

o 595

i I

(a) Lift, longitudinal force, and pitching moment (original-lip

configuration).

Figure 9.- Effect of advance ratio and angle of attack on the aerodynamic

characteristics of the origlnal-lip configuration.

o32 j I

02B I

024 I

!

P

==:.L_ ¸ • _Iz_

t ,. .........

I

Il

CO

J

.020, r

OI6_--

L

i

I

,OO4

0

.24

V/nD

[] 0r_ .0450 .128a .189o .193_. .268O .301

_7 .5710 595

C T

201

.16

.12.

O8

i

Ji

IIIi

!

o4 ! II

00 lO 20 50 40 50 60 70

a jdeg

8O 90 IO0 IlO 120

(b) Propeller thrust and torque coefficients (original-lip configuration).

kO

Figure 9-- Continued.

2_

,.-4

!

o

o

o+_

+_

.

Co

_-_

_._

_o_

0

_._

o_v

0

o

o

o_D

o

f

26

o o

+ _.

I

+

k_r_.

931oj /Ou././ON

I

d,o

o

o

,.-t

.00

4._

.0

o

og-i

,-4

o

o,--t,-t

0

ao,_o_ /o_,_ON

t-

e--

I

27

b-

.0

!

J_

JuaLuOul _ulq31 i d

IUBUdO_ _U/_31/d

0 _.,-t •

o oo 4_

, _,--I -o

-o

,-4,-t

o o

0

•,4 __ 0_ m0 _o %

,--I

o !°r-I

o i1)

28

v,,.p

0-- ' 0 .090

A /64

_.29B

v 4010 595

..... + --

.... +. _

70 80 90 I00

i

l

(a) Modified-lip configuration.

T

p---....7._--_. +-

.6 ..... _

-- I3 ..... _

i

12....

I .-

i

I _ _ --- _ ......

_--_- __ __

_- +--+ .... _ -____+_

I0 20 50 _ 60 -8_

a, deg

l

i

o 0o ,128

.18.9

_. ,268

.3 71

0 .595

(b) Original-lip configuration.

Figure IP._ Variation of the ratio of propeller thrust to total thrust

wlth angle of attack.

I

29

8i

6

4

2

Reference 5

o Figure 12 (a)

TpT

0 19.75 =17 °

/f

LO If jJ

J

.8

.6

,4 j

_,75 =27°

/

.2

=

00 .2 4 .6 .8 10 12 14

V/nD

Figure 13.- Propulsive efficiency and ratio of propeller thrust to total

thrust for modified configuration and configuration tested in refer-

ence 5. _ = 0 °.

NASA-Langl.ey, 1962 L-1617