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NATIONAL ADVISORY cOMMIT~E FOR AERONAUTICS

WAlrmm IUWORTORIGINALLY ISSUED

N oveniber 1942 asAdvance Restricted Report

FULL-SCALE-TUNNEL INVESTIGATION OF A MULTIEX?GINE

PUSHER-PROPELLER INSTAIUTICli

By Herbert A. Wilson, Jr.

Langley Memorial Aeronautical LaboratoryLangley Field, Va.

N’AC:A“ “’”’‘

INACA WARTIME REPORTS are reprints of papers originally issued to provide rapid d.istributicm of’advance research results to an authorized group requiring them for the war effort. They were pre-viously held under a security status but zwe now unclassified. Some of these reports were not tech- .

nically edited. All have been reproduced without change in order to expedite general distribution.

+

L- 246

NATIONAL ADVISORY COMMITTEE YOR AJ9ROMAUTICS

ADVANOE RESTRICTIllD REPORT

YULL-SOAL&!NJITNEL IHVrnSTIGATION OF A MULTIZ!IU31NE

PUSHER PROPELLZR IESTALLATIOlfl .

By Herbert A. Wilson, Jr.

SUMMLRY

AU part of the Inveetlgatlon in the RACA full-eealetunnel of the charactorlstlcs of propeller inetallatlonefor multienglne alrplanee, propeller designed epeclfl-oally for pusher operation behind fixed contravenes havebeen teeted on a large-ecale model of a four-engine a3r-plane . In this installation, the wing trailing edge waetwieted to serve ae a contravene and to produce the ro-tating Inflow required for optimum propulsive efflcleney.Teste of this propeller without wing twiet” and a conven-tional propeller were made for comparison.

Propulsive efflcienciee of 88A percent wore obtainedfor the pusher propeller with the contravan~e, a valuewhich was about 3$ nercent higher than that for the push-er propellers alone. The efficiency of the conventionalpusher-propeller Installation was about the came as thatof the special propeller without contravanee and of thegame order ae that obtalnod with tractor inetallatlone.

INTRODUCTION

Ae part of the investigation In the YACA full-~caletunnel of the characterletlcs of prop~ller installationson a large-scale model of a four-engine airplane (refer-ence 1), propeller designed partluularly for pueh%roperation behind fixed contravenes have been teeted. Thepropeller were deelgned to operate in a rotating Inflowestablished hy twisting the wing trailing edge. Airfoilehank eectlone were employed on these” propellers andtheir pitch and blade-width distributions were chosen tominimize the axial and rotational momentum losses. Theeffect on the propeller performance of pretwletlngthe stream was determined by testing the propeller attwo contravene angles and without the contravenes.

●

●

I —..

...—.

.3 -

●

Comparative tee”ts were also made with propellers of con-ventional round-shank design.

The tests Inoluded meaeurem~nts of the propulsiveoharacteriatica of the different inetallatione and surveyeof tho velocity and the angularity In the slipstrr!am,

SYM30LS

P

n

v“”

D

.,. T-

AD

T -All

P.

CT.

CP

9

n

P

V/nD

R

Do .

*

ma06 density of Eir

propoller rctatlcnal Fpeea

airspred

bladr angle at 0.763

pnopellcr diameter

propeller thrust (tension in propeller shaft)

Increase in drag of model due to proneller

effective thrust

power Input per propeller

thrust coefficient(s)

power coefficient(*.)

propulsive efficiency. .

advanae-diameter ratio

f

T-AD)VP )

of propeller

resultant drag force on propeller-model combination

propeller-removed drag of model

yaw angle of eir etream “

●

3

q. free-stream dynamle pressure._ . . ..-.*-- .--.,---- ------ .. . ..... .

q 100al dynamlo pressure

ia wlrig ohord

i1

MODEL AHD TEST EQUIPMlllllT

The four-engine midwing-airplane model on whioh thepusher propellers Were installed had a span of 37.25 feet.!Che wtng eeotions were symmstrioal and tapered in.thiok-ness from 0.180 at the root to O.1OO at the tip. Theoriginal wing had a plan form tapered 4:1 with a rootohord of 7.28 feet and an area of 172 square feet (refer-enoe 1) but, for these teets, the chord wag extended 20peroent at the tralllng edge by means of a thin sheet- Imetal flap that oould be differentially defle~ted toserve as a oontravane (figs. 1 and 2). l?he horizontaltall eurfaoee were r~moved to avoid Interference with theapparatus used for the slipstr~am survpys.

E’our 25-horsepower eleotric motors installed inthe wings were used to drive the propellers and torqueswere obtained from an electrical calibration. Propellerspeeds were measured with an elrctrlcal tachometer.

Blade oharaoterleticm for the two 42-lnoh-diameterpropeller are given In figure 3. The oonventlonal pro-peller had Clark Y blade sections and tho special pusherdesign had NAOA 16-eeriem blade seotlons, The differen-tial defleotione of the trailing edge for the contraraneteats are given in figure 4.

TESTS

At an angle of attack corresponding to the high-epeed flight condition, propulsive characteristics weredetermined for a blade-angle range appropriate to thedesign conditions of the propellers for each installation.In this way, the peak of the envelope .of the propulslve-efflclency curvee was determined. The special pusherpropeller had a design blade angle of 400 and, with thebasic contravene twlet (fig. 4), tegte were made at$ = 40° and 45°. With 83 percent of the baBlc twist, It

. . . .. .. . . —- --. ——--—-- -. —-—-- -

4

I

was thought poseible that the maximum efficiency might .occur at a lower blade angle and aocordlngly tests weremade at $ = 35° and 40°. For the tests of this propel-ler without flap twist, the blade-angle range van ex-tended to inalude valuee of from 25° to 40°. A rslmilarrange of blade angles from 25° to 40° WELfIused for theteats of the conventional propeller.

In order to cover the range of V/nD for each pro-peller, the torque was held constant, the tunnel airspeedwas Increased in steps from 30 to 100 mllee per hour, andthe propeller speed was then reduced until zero thruetwaO reached. I’cr each combination of tunnel speed andpropeller speed, the motor torque and the aerodynamicforceta on the model were recorded. Propeller-removedlift and drag tegt~ for the determination of the effec-tive thrusts were made at all tunnel epeeds.

The eurveye of the slipstream dynamla preesure andangularity were made along a vi!rticcl llne through thepropeller axis in order that :Jze measured 8trenm anglescould be separmted reedily into yew angles due to pro-peller rotation and pitch angle~ due to wing downwasn.

RESULTS AFD DISCUSS1ON

‘i’hecheractcristfcs of the propeller Install.atlonsnre given Re values of the pro>ulsivc effic~ency q andthe tbrunt and power coefficie~te CT and Cp. The ef-fective thrust of the propeller combinations wae deter-mined from the relation.

T -AD=DO-R

In which T - AD Is the effective thrust of the propel-ler installation, Do is the dr~g of the model with thepropellers removed, and R Is the drr.g force measuredwith the propellers operating. Values of Do obtainedwith the treiling-edge flaps unreflected were used in thecomput~tlon of all the effective thrusts In order tocharge the drag of the twisted flaps for the contravenetests against the pr~peller th~ust.

Z!he special pusher-propeller Installation with .thebasic flap twist (fig. 4) gbve a maximum propulsive effi-ciency of i38A percent at a blade angle of 40° and a V/nDOf l.a (fig.25). Reducing the flap twiet to 83 percentof the basic value decreased tne maximum efficiency to

5

“..

. . .

“ 87 percent (fig.- 6) and Kid n~t appreciably change the@lade angle. or the..-V/~.D, at w&lq@_$he.. peak. stecur.red,.. . .Withqut the gontravanee, the maximum propulsive effl-

3clency was 85 percent (fig. 7) at a blade angle of 40°and occurred ~t the ellghtly higher V/nD Of 1,84. The

Aconventional propeller also gare a maximum effiatencyof 85 percent, but at a blade angla of 350 and a V/nDof 1.45 (fig. 8). The propulsive oharacteristlcs forall inatallatione are summarized in table I, which ln-cludea for oomparleon valuee obtained from the teet~ of

.reference 2 with a tractor installation of the specialpusher propeller.

The reaeonta for the variation in efficiency of thepusher propeller are shown by the slipstream 0urvey8(fig. 9). With the basic twist, the slipstream velocityas shown by the curves of slipstream dynamic pressure IB

fihiform and the angularity is almost negligible except Inthe wake of the spinner and wing. This type of slip-stream Batisfles the requirements for low axial and rota-tional energy lenses. !Vith 93 percent of the baBic twist,the angulnrlty Increheee in the direction to account forthe l+ percent decreaOe in the efficiency. With no twlet,the angularity is about 80 at the edge of the spinner andthe s llpstream veloclty Is much les~ uniform substantiat-ing the lower measured propulsive efficiency. The lowairepeed In the center of the allpatream 3s the wake ofthe wing, the flaps, the spinner, and the blade-epinner ““Junctures.

An Rdditlonal effect of the contravanea Is to iIICI?t?4LStI

the thrust and power coefficients of the propeller by vary-ing 8m0unts up to 40 percent at the 40° blade angle withthe basic flap twiet (table I). This increaae Ie equiva-lent to an Increase In solidity and results from thehigher anglea of attack and relative velocltleg of thepropeller blade sections oaused by the rotating inflow,

The lower blade angle for the maximum efficiency ofthe conventional propeller Is due to Its pitch dlstqibu-tion (fig. 3) and to the increased detrimental effecto ofthe round blade shanks at high blade angles. In theseret3Fecte , the conventional propeller is eimilar to mostof the propellers in use at present. Tlitithrust andpower coefficient for this propeller were from 15 to 30percent less than for the pusher propeller, owing to itsconsiderably lower solidity,

The propulsive charaeteristloe of the tractor lnetal-latlon given in table I have about the same values as for

I

,.,.,,,,,,.,,,-—-.. — . _.. ——..——

6

the pusher propeller without contravenes. The efficienciesshown are from 1 to 2 percent lower, but the tests lacksufficient experimental accuracy and similarity to justifya comparison of the relative merits of pusher and tractorinstallations. The results do serve to show, however,that no large difference is to be expected between aerody-namically clean pusher- a.nd trzctor—propeller installationsin the blade—angle range of these tests.

For high-sneed airplanes in which propeller-[bladeangles in the range of 50° to 600 are required, the gainsdue to the use of contravenes with pusher-propeller instal—lations may be somewhat larger than those measured in thesetests.

The results of this investigation show that gains in

efficiency @f a-Dout S$ percent at a %iade a~gl~ of 40° canbe obt::ined by t};e LISP of Cortr:.v?ne? with specially c?.e–signed pusher propellers. T}-,ecor,tr;.vz.ne: zlSJ give anj.ncrease in the power absor~tiou of the propellers equiva—lent to an increase +.r~tho sol:.cl.ity, !iithcut the con.tra-vanes , the spec;zl nroFel;er gLTre a maxi;lui; projjulsiveefficiency of E5 pe”rCF?rit at a ‘ol.a(len.ngle of 40° , which

was a.bcut tt.e same as thz peak ei”fic!,ercy obtained with aconvcntion:32 p-~sler -nropel”l’er i::stall.a,tion. The effi--cienci-$s obtaine~ wit~, the spec~.al aY16.the convclltion~lpropel lers witilcut ch.e contr;’v”-!r~es‘tt:re.3-Dout the sane aswere obta,il;ed wittl the tract;r i~LSt:Llla.tioIi of these pro—pell~rs.

Lang~ey Itiemoriai Aeronautical Laboratory,Nation.r~l.Advisorj- Comlilittee for Aeronautics,

Langley Field, Va. .

— . ...—. —.I.I . .. —-m- ■,m-m-mm-mmmm n,m—mmm, mnn,mmmmmml,mnmm II II ,-, ml Imll 1 111111

7

REI?ERENCES

1. Silverstein, Abe, and WilSon, Herbert A. , Jr. : Aero-

~“dynamic Characteristics of a 4-Engine MonoplaneShoving Effects of Enclosing the Erlgines in the

~ Wing and Comparisons of Tractor- and l?usher-Propeller Arrangements, NACA A.C.R., April 1938.

2. Harmon, Hub~rt N. : Win~-TUnnel Tests of Several ModelTractor-Propell~r and, Pllsher-Propeller Wing

Extension-Shaft Arrangements. NACA A,C.F., , June1941.

k’

8

TABLE I&.

SUMMARY OF THE PROPULS IVE CHARACTERISTICS-— ————.. --. —————-----—--

2---‘?I-J Propeller installation

——-————-_——--——— -- —-- —--

Special pusher prcpellerwith basic contravenetwist

Special pusher propellerwith 83 percent ofbasic contravafie twist

Special pusher propellerwitho~lt cop.tr.av3neS

Conventional propellerin a pusher installa-tion

Speci,n.1 pusher Fropellerused in a tr~ct,orinstalla.tier. (fromtests Gf reference 2)

—--—— .————__________ ____ ___

-—---——.

Bladeangle ,P

(deg).--———-.

4045

35

40

25G(j7540

2530354C

~5

4045

—— --—-

--—--—--—-. ——-—-—-—--——

Characteristics at maximumefficiency

------- -— ———— _-—.__.

TTV/nD(per~ent )

CP

4- -L--——-——-- —-—— —-—___

89.585.5

8587

89848485

8484,585FJ2

!.!

! :;83

.————————---

11,8 o*2f57

2,2 .388

i

1.5 .1801.8 .252

~“ 921 .104

,1:211 .130I 1.411 .1721.84! .193

I11.oo1 .07~

!1>251 .0901.4.51 .130

11.75 .160

1.56 ,1401,23 .2052.05\ .2e7

I.-__—L ______

---—- -

CT

-—- —

0.131.151

. 102

.122

.0905

.090

.103

.089

,059.OG1.076,075

.075095

‘.116

———-—-

./---”

NACA . Figs. 1.2

Figure l.- The four-engine airplane model installed in the full-scaletunnel with special pusher propellers and twisted flaps.

,,

.m

Figure 2.- The four-engine airplane model installed in the full-scaletunnel with conventional propellers and without flap twist.

NACA

... ,..

70

60

50

,.

20

/0

o

.

Fig. 3

t/c.68

.

.64

.60.

..56

.52P/l) c/D

.48 24 ./2

.44 22 ./1

.40 2.0 ./0

.36 /.8 .09

.32 /,6 .08

.28 /.4 .07

.24 /.2 .06

.20 /.0 ,05

./6 .8 .04

./2 .6 .03

.08 .4 .02

.04 .2 .0/

o ./ .2 .3 .4 .5 .6 .7 .8 .9 /000

r/R

Fi~re 3.- The blade characteristics for the test propellers.

Figs. 4,9

--H=Orighd +raihng edge

/

b

%‘ -20

&

&‘O 30 40 50 60 70 80 90 /00

hs +cmce from fuselffge cen +er he, percen+ semispan0 10 2

F~,-,Ir.. :.:. - The tra~l.in~;-eti-,edeflections for the basic flap twist..

Q1>

:T

20

c.. + 16 t +–––—— Without confro-vanes

.~4~lo

1?)

k.5 0

z“ propeller rofo +ion

$\ J

*A& * o-- -._& -+= ~ . . . --

1

E+“--+

‘1 if -

. ~+

0 ,4+

&44 +

$ /0 1+~ +

% ~t,.Q + +-

20..t’ ..+-- --

Plus yow -“—

?f Minus yow _$

J‘w

4.(a) (b)

-/6 -F? o 8 16 24 0 .2 .4 .6 .8 1.0 /.2‘Angle ofyowin shpsfrearn S@freom dynamic-pressure ratio q/q.

(a) Stream angularity (b) Dynamic -pressure ratio

Figure 9.- Survevs of the air flow in the slipstream.

NACA Fig.

.44

.42

.40

.38

.36

.34

.32

.30

.28

.26

.24

&& .22

20

.18

16

./4

12

,/0 /

.08

.06

.04

.02

0 .2 .4 .6 .8 1.0 1.2 1.4 1.6 /.8 2.0 2.2 24 2.6 28 3.GV/nD

Figure 5.- Prouulsive characteristics for the mecial pusher-design propeller with thebasic flap twist.

5

‘00

80

20

,0

NfiCA

36/ ‘ \

N

.38

.3

.34

.14

HI++ H/6

.12

./0 /

.08

.06

/7”w-,40 ;

.02 20

0 00

!+hD

I Y/l I I I I I I I I I I/

00

80

FiOmre 6.- Propulsive characteristics for the special pusher-ciesi~m m-opener with 0.83 ofthe basic flap tv~ist.

1: *

NACA Fig. 7

I I I I I I I I I I I I I I.18

.16

[4

.38 .(2

.36 ./0

<

.34 .08

.32 .06

.04

.02

.30

.28

.26 0

.24

.22

.20

b’

.18

./6

./4

.12

/./0

.08

.06

‘00

80

60

,04 40

.02 20

0 .2 .4 .6 .8 /.o 1.2 /.4 1.6 /.8 .20~nfl

2.2 2.4 2.6 2.8 3.0 0

(a) Propulsive efficiency (b) Thrust coefficient (c) Power coefficient.

Figure 7.- Propulsive characteristics for the special pusher-design propeller withoutthe twisted flap.

NACA Fig. 8

. /,12

.10

.08

06&

.24 .04

.22 .02

.20 0

.18

.16

,14

c,

.12

.10 I 00

.08 80

.06 60 ~

:

.04 40;

.02 20

0 .2 .4 .6 .8 1.0 /.2 /.4 1.6 1.8 2.0 2.2 2.4 26 .28~nD

30 0

(a) Propulsive efficiency (b) Thrust coefficient (c) Power coefficient

Figure 8.- Propulsive characteristics for the conventional propell~r.