.— ..—

REPORT No. 137

POINT DRAG AND TOTAL DRAG OFNo. 1 MODIFIED

NAVY STRUTS

By A, F. ZAHh~ R. H. SMITH, and G. C. m .

Bureau of Construction and RepairNavy Department

125

t

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.-.

REPORT No. 137—.

POINT DRAG AND TOTAL DRAG OF NAVY STRUTS No. 1 MODIFIED.

By A. F. Z-, R. H. fhm, andG.C.HIM.

INTRODUCYITON.

ThB report prtxenta the results of tests on struts conducted at the Washington NavyYard for the lkeau of Construction and Repair of the Navy Depmhnent. Two models ofthe moddied Navy strut, No. 1, were tested in the 8 by 8 foot wind tunnel. The ksts were

r~

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...—

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---

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~G. L-Kwystmts Nw. m. 4.-Eavisd appei-atmfa Ineasmringrdstance on Elm2 md & Iahuce.

made to determine the total resistance, end eftectj and the pressure distribution at mriouswind tunnel speeds with the hmgth of the strut trsmverse to the current. Only the measure-ments made at zero pitch and yaw are given in this report submitted to the National AdvisoryCommittee for Aeronautics for pubkation.

DESCRIPTION OF MODELS.

The two struts were normally 5 feet long by 3 inches thick and of the cross-sectiomd shapeshown in iigure 1, the Iavger strut being 12 inches wide and called No. la, the smaller 10.5inches and called No. lb. The offsets are given in Table I, and are derived from the original

_._=—

127

128 REPORT NATIONAL ADVISORY COMMITTFIE FOR AERONAUTICS

.

Navy 1 strut 3 inchw thick by uniformly stretching it along stream so aa to change the originalabscissas to the present ones. Both struts were made of pine wood, varnished, and satisfactorilyverified by application of their SW templates; both were hollow for the admission of a centralsteel shaft; both had detachable erid segments to fill up all or a portion of the space betweentheir ends and the floor and ceiling.

As shown in figure 2, the central steel shaft rested in a conical socket on the floor of thetunnel and extended vertically through the strut and its d~y end piem, thence up throughthe ceiling of the tunnel. By altering the lengths of. the seg-gmntaldummy struts at the topand bottom, any length of gap at either strut end cogld be provided, ranging from one-@irty-second of an inch to. 18 inches, and by rotating the “shut system about the shaft any dmiredangle of incidence could be maintained during the measurements of pressure distribution ovor -a median section.

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Fm. 2.–Arrange.rnentforvaryinggapand FIG.8.–Bin prWIIM oolkatorfor Navy sttnt No. 2.awla of yaw for Navy stint No. 2.

The point pressure on the larger strut was not measured. For the smaller of the two strutsthe a.rrangmmnt of the 14 pressure collectors, to which 14 tubes from a multipl~tube manom-eter were attached, is shown in f3gure 3. An arched bronze side plate, flush with the strutat its middIe, has 14fine holes on its outer surface and as many tipples inside serving as pressureleads, from which rubber tubes run up through-the hollow strut to the manometer.

METHOD OF MEASURING PRESSURE DISTRIBUTION. \

The pressures at these 14 holes, referred to that in the undisturbed stream well to one sideof the strut, were measured at 20, 30, 40, 60, and 60 miles an hour. They were simultaneouslyindicated on the inclined multiple-tube manometer, whose reservoir was joined to the staticlead of the referimce speed nozzle in the undisturbed air stream. The pressrqe could be readaccurately to ~ per cent at spei%lsabove 40 miles an hour. For the lower speeds of the tmt itcould be read thus accurately at all the holes where it equaled or exceeded 40 per cent of thenose pressure, as given in Table III. The usual pressure drop correction was made by multi-plying the volume of the strut by the static pressure gjradient aIong the tunnel.

The angle of yaw was set accurately to about one-fiftieth of a degree by means of atemplate whose extremities served as a reference line, Slight lateral and angular displace-ments of the strut werg prevented by fine stay wires anchoring its leading and trailing edges

~OINT DMG AND TOTAL DRAG OF NAVY STBUTS NO. 1 MODIFIED. 129

to the walls of the tunnel. The end dummies were kept accut,atdy in line and orientationwith the strut.

The speed of the air was held constant to within one-haIf of 1 per cent. It was meas-ured with a Standard pitot static tube placed about 18 inches to one side of the strut centerand nearly the same distance above and before it.

METHOD OF MEASURING TOTAL RESISTANCE OF THE S’ITWTS.

The tital resistance of the struts was measured at zero pitch and yaw and at speeds of20, 30, 40, 50, 60, and 70 mikx an hour and with various end gaps from one-thirty-second ofan inch to 18 inches. The manner of doing this is ilhstrated in figure 4. Two heavy prongsextending upstream from the shielded shank of the Ei&l balance supported the strut in anupright position and held it securely without causing material air disturbance, while alIowingit to e-wingfreely along stream with the smalI oscillations of the balance. Moment measure-

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Dhfunce fmm Nose (Inches)

FIG.6.-Obs8rwd presmmv%aftdhtancaoflmlesatzeroph?h snd yswfm Navystmt f’fa 2.

ments about one Me-edge -weremade, fit of the strut and prongs, then of the pronga alonewith the strut detached but not removed. The dii7erence was taken as the net moment ofthe strut, which divided by the arm gave the strut drag, escept for the pressure drop correc-tion already mentioned.

RESULTS OF PRESSURE MEASUREMENT.

Figure 5, plotted from Table II, delineates the pressure distribution at the 14 holes forzero pitch and yaw and for the five wind speeds used. For each speed there is a point of fullimpact pressure + pP at the nose, and two points of zero pressure at the side, the flrat at adistance of 3.’3 per cent of the strut width from the front, the second at a distance of 87.1 percent. At about one-fifth of the width from the leac?jng edge occurs the maximum suction,

——

.-

.—--

—-.*,

.

—,

-.

..—

—

180 REPORT NATIONAL ADVISORY 00 MMITJXE FOR AERONAUTICS.

which equals about three-fourths of the nose pressure. At the last hole the pressure is aboutone-sixth of the nose. pressure. These various characteristic points shift little if any withthe speeds used. This fact may be contrasted with those given in Report No. 191 of theBritish Advisory Committee for Aercnmuticsj d~cribing the sudden change in the characterof the air flow about a 6-inch sphere and a 6-inch cylinder at-25 to 30 miles & hour. Figure5 shows that at all the holes and at all the speeds us~d p is an increasing function of V..

As indicated bj the diagrams in figure 6, p]otted from Table III, for zero pitch and ya~v,the pressure at each hole .varks nearly M the square of the velocity, but with a degree of ‘-approximation slightly diminishing aft of the thickest part of the strut and more pronouncedlyat the lower speeds. The amount and direction of departure from the V law is clearlydisclosed in figure 6 and Table III.

The graphs of the faired values of the point-pressure p at 60 miles an hour, and at otherspeeds multip~ied by ‘(60/ V)2 to make the pressures comparablp, are shown in figure 7. ‘“ The

-.

integrals of the segments of each pressure graph, ~ving the elemtits of the pressural drag.

and the summation of these called the “form re&t~ce” or ‘{resultant pressural drag,*.,—

aregiven in Table IV and plotted in figure 16: With them are shown the total directly,measureddrag and the resultant friction, the latter being the difference between the whole drag- andthe whole pressural drag. The order of graphic integration here used to find the force J pdy,over the various portions of the surface of the l-foohlong center segment of the strut, isdetailed at the bottom of Table IV.

The lower half of Table IV is of especial int&t as show~ the relatio~ of the wholedrag to iis”parts. For this particular model the drag at 40 miles an hour is about one-fourthfriction and three-fourths pressure. The total upstream pr&su~e is 5.8 times the wholeresultut drag, 7,7 times the resultant pressWe, and 24 times the rewdtant friction; and thedownstream pressure is about 13 per cent greater t$an the upstream pressure. With such “thick streadine shapes, therefore, an error of 1 per cent in measuring the point pressure mayentail errors of the order of 8 per cent in the derived pressural drag, 25 per cent in the frbtional. No such difficulty is found in measuring the resistance of normal planea or thin planesplaced edgetie in the current, where the force is all pressure or all friction. .

The total drag and its elements, as seen in Fig. 161vary as V“ for the range of speeds used. -The velocity exponent n equals 1.99 for both the push and the suction before the major section.

.-

Aft of this section n = 1.895 for the suction, 1.985 for the push. For the total measured dragn= 1.88; for the total pressural drag 1.71; for their clifference, which is called the frictionaldrag, n= 2.35, which is doubtless too great.

Since the resultant premural drag and frictional drag are derived by taking the differencebetween much larger quantities, it is not believed that the V~USSso det~fied from tiepre9ent measurements are trustworthy. The friction on a plane equal to the side elevationof the strut seggntj computed directly by well-~own formulas, would be materially greaterthan the friction here found, and would vary approximately as the power n = 1.85, and if sub-tracted horn the total measured resistance would leave a smalk preissuralresistance varyingaccording to a greater value of n. No exact fo.rnmla.is available for computing the actual sur-face friction on the strut segment, even though the velocity at each point were known. Theother values plotted in Fig. 16 are much more trustworthy, and indicate the comparative effec-tiveness of the various elements of the strut surface.

—,=

It has just-been said that the downstream pressure excaeds the upstream by 18 per cent..“

In a frictionless fluid they would be equal. In N. P: L. Report 600 Jones and WiIbams, from “~.... _.their point-pressure meamrements on an ellipsoid, declare “they show that the form resistance

--

may be zero or even negative, or that the a&uracy of the experhuente is not sufficient to enable-—

it to be found.” A careful Italian reports the air pressure on a torpedo form greater upstreamthan down, A painstaking American physicist rece~tly tested a projectile which showed a

.POINT DRAG AKO TOTAL DRAG OF NAVY STBUTS NO. 1 MODIIKO%D. 181

negative drag in a high-speed air stream. II such things can be, one may ltik for an airshiphull competent to pull itself around the world without engke power.

The present data will aid in the study, soon to be mad~, of the lift and drag efTeck of sur-face finish and obstructions. It were better if such data could be checked by theory. But forlack of time the stream function was not determined for the present strut. The prassure overits forward part is theoretically approxbnated in the following paragraph.

I \l I I I

Fm. 7.-O&0rVed pm?sumvs. heIf thlckmsd Mskut at varims airqweds for Navy strut No. 2 at zero pitch and yaw.

PBESSIJRE ON AN ELUKL’IC CY13NDEE IN A PEEFEC1’ FLUID.

The point prassure on the front half of a long elliptic cylinder having the shape and size ofthe fore part of the smaller model was wdculated for zero pitch and yaw in a frictionless atmos-phere of standard dinsity flowing uniformly past it at 40 milee an hour. To do this the stream

.,.—.4

.-

—

—

..:

-.

—-

.-

——.

132 REPORT lTATIOiVAL ADVISORY COMMITTJN3 I?OR KERO?iAUTICS.—

velocity along the eUiptic section was found by the usual hydrodynamic process (Lamb, article71) and substituted in Bernouilli’s equation to find the point pressure along the section. Thevalues so computed are given in @e 8. The agreemant between the calculated and observedvalues is close for the foremost holes, but more or less-divergent for the holes farther aft in thepart of the surface before the major section. This discrepancy was caused, no doubt, by theshape of the after part of the strut as well as by the viscosity of the medium, The generalaggeement, as showm~ figure 8, maybe compared with a like diagram for the pressure distribu-

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‘FA” s Wn.P.% 50 m.

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4r qoeed= V(rn.ah)

Fm. 6.-Observed pressurem. velooftgat zaropitob andyaw for Navy strutNo.2.

FrG.9,-Rddrmcaandehapeccafdck.utsIN Nav atrutaNos.2and3.

tion over the spheroidal bow of a torpedo form, given in Report No. 600 of the British AdvisoryCqmmittee for Aeronautics for April, 1919.

.-

RESULTSOF TOTALRESIST~CE MEASUREMENT.

Tables V and VI give the measured and net resistance and derived coefficients for the twostrpta at various wind speeds and at zero pitch and yaw, for gaps of & inch and 18 inches.Figure 9 plotted on logarithmic paper from these tablea gives the net rmistanoe versus speed;

FOIN’I! DRAG KM) TOTAL DBAG OF NAVY STBUTS NO. 1 MODII?IED. 133

also the shape coefficient O and engineering coefficient Kplottad against-speed times thiclmwsfrom faired mduea tiken from the straightAine resistrumegraphs. At the test speeds above30 miles an hour the rmistmce graphs are properly straight lines; for the lower speeds theresistsmx values in each csse plot considerably above the straight line. Thwe diagrmns maybe compsred with similar ones given in earlier Navy aerodynamic reports on streamline forma,especially airship hulls, in which the data commonly lie W on a straight Iine.

The values of C and K, taken directly from Tables V and VI, sre plotted on plain sectionpaper in Figures 10 to 13 for convenience of comparison with previous strut reports. On

t 1 I I I I I 1 I I?heoreflcalundOhservedPomf-Fi-essu~ overSfruf ondOscu/oforyE/ flpfkof Qhhder

Number of Hole I 2 3 4 5 6 1 7]7a 1819 10 fr [ 12 f3 ~14?i%ore+ialpressffre [000 .699 .A74z136 -44f -.686@5*04 — I — — —1 — —)—

Observe&Pressure 1006.602 .072-546‘59+-69677~ — ~66L+.574386-J5C/~47f6.15L+70

I

i

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\-\\-.- Theorefko/ .

I 2 3 4 5 6 7 8 9 10Dkfonce +om Nose (hche~

Fra. S.-P~ dfstributfanatmmPitchand yaw, sfr speed40m p. h. fm Naqvstzut No. 2

transferring the shape-coefficient graphs for an 18-inch gap b the similsr graph for four NavyNo. 1 stru~, it app&rs that thepr&nt coefficients f~ ;bove the others-at high speeds andbelow them at low speeds and show that the original Navy No. 1 struts twe better for practicalflying speeds.

Table No. VII gives the effect of gap on the shape coeflbient for the two struts at variousspeeds and at zero incidence in pitch and yaw. Figures 14 and 15 exhibit these resultsgraphically. From these diagrams it appears that the effect of edmging any gap beyond oneetrut thickness is practically negligible. The running reaietrmceof both struk .is tien 8 b 12per cent greater than with zero gap, or for a strut infinitdy long. Reference may here bemade to N. P. L. Report T843 showing an increase of 8 per cent in resistance ss the strut gapincreases from zero to a hrge amount for two struts, also 5 feet long, exposed like the prwent

ones, but to a wind speed of 45 feet per second, the one messuring 3 by 7+ inches in crosssection, the other measuring 3 by 15 inches.’

DRAGTERSUSSTRUTlJ?NGTH.

It maybe assumed that the increment of drag due w end turbulence for these 5-foot strutshas the same absolute value as if they were somewhat shorter, or were indefinitely longer and

—

:-—-

-.

134. REPORT NATIONAII ADVISORY COM.MH’TEE FOR AJ3RONAUTICS.

placed in a like stream large enough to obviate wall blmketing. If this assumption be true, the

resistance plotted egainst strut length is a straight we of the form ~ = K1 + 11,where 1 is the

variable strut length and E the constant increment of drag due to end turbulence. Also it may

Fm. 10.-ShaW. mftldentin tam of thlo?am and apxlfor8° xIW NrnvyatrutNo.2 at zaropkwhandyaw.

I R=f?esistince of.w&utin Ibs. ]I I D= 7hi&ness of stiutin intiea I I

Fm.12.-’~K” lntsrms of t.hcknesaand speedfor lM” x 8“, N8wshut No. 2 atzsropk.ohml yaw.

Gap Inimhes. .-

mo.14.-sllapa MaaMarltfn terms of gap for 10V’ x w’, Navyatmt No. 2 at rem pftch and ymr.

. --. .

.Ohy.5J’.O4OJ+d+?.o%

4.qy” ~ ~

“/2 /6, 20 /34 28 32

Fw lL-ShaP codidantfmWma of thlokntwsand@for8“ x 10i”, Navy strut No. 8at rampitchandyaw.

mm13,—I(KM~ te~ of t~ and speedfOr 12~fx 8“, Navystrut No. 3 at zeropltoh and yaw.

Fm. IS+3hapo ooaiMent h tsmnnof gaP forB“ x 8“, Navy strutNa8atmropltchandyaw.

.—

-.

.-

. ..-

--

be assumed that in these various oases the pressure distribution at the strut middle would besubstantially the same at the same wind speed. But no experiment wss made to test theseAssumptions.

POINT DRAG AITO TOTAL

&40 miles an hour, and including

DRAG OF NAVY STBU~ NO. 1 MODR?IED. 135

DISK RXC’10.

end effect, the ratio of drag of the major section of thestrut to the actuaI dr~~ on the strut ikelf is 21.87 for the smaller-model, 24.93 for the larger.The drag of the major section -wastaken as the resistance, aotmallymeasured, of a thin rectan-gular plate 3 by 60 inches held normal to the wind at 40 miles an hour.

F& the 3 by 60 inch Navy 1 strut the disk ratioat 40 miles an hour is 23.7. For the C(best form ofstrut as regards resistance” given in N. P. L. ReportR. and M. No. 416, the disk ratio for a 3 by 60 inchmodel at 40 rides an hour is 18.15, M estimated fromPlate II of that report.

The present I&h values of the disk ratio, basedon strut resistance measurements made with the oldE~el balance, have not yet been checked againstmeasurements made with the new balance. But theEiflel balance was usually found reliable for meas-uring the resistance of ~ strut held verticaI on twoprongs pointing upstream, as shown in &u.re 4.

NOTE.

On going to press it is discovewd that a too-high

gradient was used in computing the pressure-drop

correction, thus eut~u an error of about 4 per cent

in the values here given of the total drag at all

speeds. -CONCLUSIONS.

The total resistance and end effect found for thetwo struts herein described showed no new featuresbeyond those disclosed by well-lmown tests.

The point pressurealong thecontourof themiddIecross section of the 3 by 10.5 inch strut, when in-tegrated to give the pressural drag, showed this tobe about ~three-fourths of the total drag at 40 milesau hour, the remaining one-fourth being frictionaldrag aIong the section. The re&dtaut pressure(downstream) was 11.5 per cent of the integrateddownstream pressure, 13 per cent of the “integratedupstream pressure.

All along the section the prawure (p) inore~edcontinuously with the air’ speed ~; closely as VIbefore the thickest part of the strut.,and 1*s nearlYat P farther aft.

100-80.8o

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FrG. M.—To&d drag and i@aWts for Naw stmb N= 2

& usual the nose pressure is @P; also at all spe* one point of zero pressure occurs at3.3 per cent of the strut width aft of the nose, another at 87.1 per cant aft.

.-

The total ~C of the 5-foot strut neglecting end effect varied about M V’.” and was”8 to - ---12 per cent 1sss than for a free-ended strut 5 feet long. The whole drag, inchding end effect,was about one hmnty+econd that of a normal plate having the same front elevation.

From the kaited premises of this text it may not be well to draw very general conclusions.One might expect however that the chief observations would be fairly repeated on other well-shaped struts not too different from those here described.

59000-~lo.

136 RmoM! NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS.

Doubtless for all thick shapes of slight “resistance the resultant pressure is small compared

with the integrated upstream or downstre~m pressure. Hence the prwsural drag can not be

very accurately determined from poinhpressure measurements of ordinary accuracy nor the

friction by taking the pressural drag from the tot~ drag ss is sometim~ dons.

It is believed ,that the present method of analyzing the pressure elements may bc usefully

employed to under:tmd or improve the charactm of the drag on stream-line models genertdly.

The drag of this strut is less at low speeds, greater at high speeds, than that of the original

Navy J!lo. 1 strut of the same size, which latter is one of the best on record.

TABLE L-Spt%tj%u’offnetefor NWI drub No. 2“and No. S.

...=

Navg strut No. 8.!TavgstrutNo. 2.

Thick-- ofstrut

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—. . .

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.

,.. . . . .

.—.

.,. ,., ,\----. . . —

TABLE H.-(lbserwd preaaun “p” at tlu 1.$hoks at varioua air aped and mo pitch and garo.

—. i“””:”” . ‘ .-”,Mrspwd inmfle$-&rhour. . ,

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Im&t prezsurep‘k- ofwater. i.

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.188

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POINT DRAG AND TOTAL DRAG OF NAVY STRUTS NO. 1 MODIFIED. 137

TABLE HL-Poiti prm.w.rein tams of noee premure, pi p P, atW/

warimw m“ndspeed for .Va~y No. z etru.&Angle ofpitch a yaw ZHO. .

lillmbelof hole.

Actualvalues.

20 =L--vdnes

nm fedem law.

‘TABLE I_V.-Along-8tream forcee per foot run @tit .Mo.2 erpremed in poumik and in tenna of total measured drag—Ze70 pitch and yam.

&(nLph.). I=FZJ++=%4Z‘“~l~:

POUNDS PEE FOOT RUN.i.,

CL(E324 U&r ~ 0.1101 0.Owz I 0.0318 0m40 O.olnl iLa&1

% .14050.01s7

.2436 .02W40: .2497 : .1%9 ; .4266

. 1s1 .!lm . mlo .03s5

w;~l .3254

.3m2 ~ .2i37 ; .5329.8775 . GM . 01% .0646

al,.5ml .oi37

.ss19 .3650 , .W3. an .09m

I I. IIM %% .242 t .Im .Mm,. .142 !

I

DUQEAM I.

6wm5?kecmi7pushao-&wubeLbwn.sfreom &ti a oi-eo ced j=#==~~!a-_e~

rotol oreu-@o +ce&(efo+bc&=dgef-gcg

essurep-(fzaes dwuted

.. _____ ., . --

. .,: --

.. —; - ...=-.-._..-

.__~= .—-.—..__

——.-.—--——-. ..—,.. —

,A --—.. . —

,’ ----

..=-_.——,.=— —_-——.—

.--—..——..-—

———

.. -.=_:—— .-_—

. . -—_...-

.—

138 REPOET NATIONAL ADVISORY 00 M.MITTEE FOR AERONAUTICS.

TABLE V.—Reai8tanw valua for Navy No. 1?and No. S, 6@ot a&ut8 with l/82-inchGap, atVa&m8 & 8pG80iiand ZUo

pikh and yaw.

& “ ‘ ‘k~~(hx:;i(fi:~=l”-’.:rl,p.h.). ~m-&).

@un%).prti=we (pOmlda). ~ddk

,.~..wn

: M&

60 .61$6w .722770 .9770

8 BY 10JlNCH NAVY NO. Z STRUT.+

1

.- . . . -;O.im$ O.w””

f

o.OIS7 ““ .- ‘1 0.WO.Ooso .1924 .Wsti .1 J% :Rg :=.0181 .ma .0648Xl& , .m .OXM :

g *g ‘:%?IMJ g1s.34

.7069 .1412.0246 .m20 . IWO

,> - ‘-’ ‘“

8 BY 12IXOH NAVY NO. 8 STRUT.

TABLE VL—R&tumX vak% for Navy No. 2 and No. 3 5jbot dt’ut8 m“th 18-inch Gap at varknu ati qwea% and zeropitch and yaw.

I 1 , ., I . ..,

~ BY lq INCH NAVY NO. 2 STRUT.

fil8040al6070

.—

I ‘“ “10.1009 0.ocw :$ aom : “$2# J&J .0421 I.Hi! I ‘:MW

.afsl .0716. E847 . 019s . b462 .1090 ‘ w w I :%:.8044 .= . ma .MM . amo144

L WI .W6 1.04s5 .m 2% %% I . m4a

m2086404sM66m0670

0.oa47. 1W8.2876.s292.41.23.W7.OIss.T2n.eMo.9%36

8 BY 12 INOH NAVY NO. S rRUT.

R- Rdetenra w fmt ler@ of strut, tnpounds.n-strut thlokmiwh fnohe9.

It

-“-0.0403

:%J:%!

--i

I I I .-

Uiut

0304

0202

0306

.,. - --. .... .::_ . :,=--

.— —

:—. “.i-:=----.-

.

~:.,—

..-

. .

--

. ----

--

-—.

POINT DRAG AND TOTAL DRAG OF NAVY STRUTS NO. 1 MODIFIED.

TABLE VIL—Efl& of gtzpon shapecoe$%iati at various WM #pt?aikand ma pitch and yaw.

(&&4).

Mrs-pedfn Dlne9perhonr.

[ao; .gl I

Im 6!2 I 70‘1

Shapecmlkimt-C-=lW/PLD1VI%

8 BY lM IKCH NA= NO. 2 STEUT.

- I i

,S BY 12 IKOH NAVY NO. 3 STRUT.

0.0297

...—

—.——.-.

.-.

_—

--.— -

.