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TECHNICAL NOTE 3092

HYDRODYNAMIC DRAG OF 12- AND 21-PERCENT-THICK

SURFACE-PIERCING STRUTS

By Claude W. Coffee, Jr., and Robert E. McKann

Langley Aeronautical Laboratory Langley Field, Va.

NACA Washington

December 1953

BEST AVAILABLE COPY

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Q NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

TECHNICAL NOTE 3092

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HYDRODYNAMIC DRAG OF 12- AND 21-PERCENT-THICK

SURFACE-PIERCING STRUTS

By Claude W. Coffee, Jr., and Robert E. McKann

SUMMARY

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The hydrodynamic drag of three surface-piercing uirtapered struts at approximately 0° angle of yaw was determined at depths up to 6 chords for speeds up to 80 fps at various angles of rake. Tvo struts had NACA 66^-012 airfoil sections, one with a 4-inch chord and the other with an 8-inch chord. The third strut had an NACA 66^-021 airfoil section and a 4-inch chord.

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The section drag coefficient was determined from plots of drag u.gy.-Liisi, uc^iol Uvci one jail range of Reynolds number where wind-tunnel data were available for comparison, the tank data, at subcavitation speeds, were in good agreement with wind-tunnel results. Raking the struts changed the section drag coefficient as expected because of the change in effective thickness ratio that resulted from raking. The drag coefficient due to the drag at the surface intersection was approximately constant at Froude numbers above 8.0 and at subcavitation speeds. Within the speed range investigated, no surface ventilation was observed for any of the struts. The inception of cavxtation was noted at a speed higher than that predicted from two-dimensional-flow theory. This difference was probably due to the influence of the free-water surface on the pressure distribution.

INTRODUCTION

The present trend toward the use of underwater lifting surfaces on water-based aircraft and surface vessels has emphasized the need for drag data on supporting struts which pierce the water surface. Such data are limited; in addition, these data were generally obtained as tares during tests of lifting surfaces snii + he accuracy of th*> measurements for the strut alone is of dubious value because of the lack of sensitivity of the measuring instruments.

The present investigation conducted in the Langley tank no. 1 was made to determine the hydrodynamic drag of struts which pierce the water surface and Lhe effect of thickness ratio, size, and angle of rake on the

1

NACA TN 5092

drag. The speed range investigated is believed to be in the order of the actual speed range that would be encountered in practical applications.

c strut chord parallel to undisturbed water surface, ft

d depth of submersion of strut tip below undisturbed water surface, ft

t strut thickness, ft

DQ extrapolated drag for zero depth, lb

Dg section drag, lb

D^ tip drag, lb

D,p total strut drag, lb I

c^ section drag coefficient Ds

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V speed, fps

VQ speed of inception of cavitation, fps

F Froude number, V"/Vgc

R Reynolds number, Vc/v

g acceleration due to gravity, 52.2 ft/sec2

p mass density of water, varied from I.966 to I.969 slugs/cu ft

v kinematic viscosity of water, varied from 1.311 x 10"5

to 1.65^ x 10 '' si it/sec

DESCRIPTION OF MODELS

The NACA 66-series airfoil sections were chosen for the strut models ;heir high theoretical cavitation speeds, low drag, and small

NACA TN 5092

frontal angles. A small frontaJ angle is desirable to reduce the vater pileup at the intersection of the strut leading edge and the water surf^e;

Tvo struts had NACA 66^-012 airfoil sections with chords of * and 8 inches in order to investigate the effect of scale. The third strut had an NACA 6614.-O2I airfoil section and a 4-lnch chord. The cr-ii- nates of the strut sections are given m table I and the cross sections of the three struts are shown in figure 1. All the struts had tr.e sub- merged tip parallel to the undisturbed water surface. (See fig. 2.)

In the position with 30° angle of rake of the T-lnch-ehord struts, the chord parallel to the undisturbed water surface became -.e-2 inch**. The thickness ratios decreased from 12 and 21 percent to }.C- ar i lr.^ -c: cent, respectively.

The struts were made of stainless steel with a vie".* g'rer^tS. if approximately 115,000 lb/sq in. and were polished *.c a ssoc*.':. finish *..-.&• gave a measured surface roughness of 8 to 10 res nlcroir.r-r.es.

APPARATUS AND PROCEDURE

A description of the Langley tank no. 1 and towing carriage is i-T^" in reference 1. The single-component balance, supported frue "_;.e aain structural members of the towing carriage, is shown in figure 5. 2asL- cally, the balance consisted of a heavy floating fraar supr?r-.»d ty r'rm;a~ gage beams from a rigid frame attached to the carriage. irntrcnaur-^rir 1- jaws which conformed to the contours of the iadiv£&*&', '"."-"^ ».*re —»- in a clamping unit in the vertical plane. Adjurtaents r*oen*»ry it =«:=.- tain negligible angle of yaw were made by rotating the clvz.^.-^ uni*. ;n the horizontal plane. Wind-tunnel results indicated tr-At EZJE..- Rr-g.-*? vf yaw near 0° would have negligible effect on the intf, !-x-.tr*r, ^f»-"- -r the interaction of side force with drug on the strain-gac? ba.s^rr, ~ side-force gage was used to set the ar.s-Ie of ynv *.c give -. sin-asus ^: i« force. Corrections were made to *r:

p H-A<? roa.v»rs frr th* **rt*r*vt*iosus resulting from the remaining small si-Je forces.

A shield was provided to prevent wetting of the gear aci §*ir*L£ gages. (See fig. 2.)

The drag was measured during constant-spaed tests ev*»r a ra:x' "' speed f-rnm jo to GO Ips witn Lne struts at IT angj.e o: y«*v am u^pv&is «; submersion ranging from k to 2*4- inches. The three str.its were **»?*•«* a^t 0° angle of rake and bcth 4-inch-chord struto were tested u* *?-"° IU'*--?

of rake. The angle of rake was considered to be positive v.-er. •'-." sub- merged tip was rak&d forward and negative when the submerged t-p -as raked aft (fig. 2). Tie output fiom the strain gages .as reai free ?.

KACA at 5092

tcicrcasine'.er or recorded by a pen-type RT.Mp recorder. The r1 «£*•* «*JS data include the air drag of the length of ^trut exposed belov tr.s spr^;" shield.

3f>my photographs and underwater photographs were taken o* *r.e s* r~*« -7 ~s-=* f-««;i-pc*ition flash equipment similar to that described m refcr-

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_^ .„-.«oa drag coefficient, based on a stat ; 7- !ner^ ^ proJ«t*-s* \t Plotted against Reynolds nucr-er ir. figure h. Inci-d^i i- ?•«-:-

NACA TN 5092

is the Schoenherr line (ref. 5) thr.t represents average skin-friction coefficients for fully turbulent flow on smooth flat plates and the Blasius line (ref. -*) that represents iajtinar flow. In general, the secTion drag coefficients for the various struts lie in the transition region between the laclnar- ani turbulent-flow lines and decrease with increasing Reynolds number.

Ir. general, raxing the struts cipher furwarl or rearward reduced the section irag oefficie-^*. fret tnit for C° angle of rake as would be e5T.e,-'.^i Tr^c". *~e change ir. effect ire tr.icteess ratio that resulted froc, -akir-. The sc~tior. ;rr^ cr*-ff i r ier.t s ?*••.» inei for the two 12-percent- tnicfc struts were ir. r-;c: i^rtcw:.*. ir.^i r.r effect cf scale was indicated Ir. t*s# rang* cf Ft*yr»fel-ii --*_»£><-- infestit-i'.ri. •inJ-tunnel data for these a^r'cil sec* i.=r.s. a*. th* r^nge if Reyracl.ii r.^±tr cf these tests, are arA^r; *•_- *** wirjd-t _-U-J?. r*c£—."« Crrf. '-V tn.nt car. be conpared with t.\* t-inx ***-*•. art ir. ££»ci agr^-ear*-:-*. fig. t;. This agreeaent indicates •._-_it .•.:•,:-• j.^L *a*.a -aj r-= _;«=•* ". - *•-". La**.*- se.-ticr. drag coefficients far r_rf*c^-5i*rci=« _-•.*-as*-r«--i rir.J.i -grating at sut cavitatlor. speeds

.-...^ f-r *-«- •-«g ' • =• =* — -t t :j- J\ wrr* "'•is'.tfd by using -_.-.- ::•»£ - '-*f f ii i«-c" f r :;i--- "-J-" &-r~~ «- ref ei er.-ce t. By subtracting t:* tij ir~=g fr-a Oss- =«-.r«.j«.i*.««.5 i--ag i" itrs .-Urplh, a value for the tz^i. *' *-*•«• ^_rf*r<- :-*«---.r-~« ; „c «^s - ,-.*lrje-i a', raTxravitatioa speeds »rr? <•=-? esrpr^-j.fc-i i- :»fr;r;t*.'. fcrc as

? :^;--a-.« I?-**, frr al: it* rtr.'.i tejtt*, this coefficient was 5*-- .7 - r-r. •; t *_-1 a* F-:-_'* = r. i'«-r: al-r-re r. .; *rsi a*. * -bcaviiation

Jr**"-*- *--- •==*•= f'- «=---t. '*:.'.i-...r *•?.- *r.svr t<- exist were oaitted fr-*. fi^.r^ '; -.-v^rc? . t-.« u'-s >.-. iri a* a. Frowi* rr.jrr<er cf 5-..-., repre- a*^-^ t^^ i«5f.f .<?-- f_- -„c i_-r*.-ci-r.".-t-i-< t*-„« vith the ..-inch *~~^~- -'• *': f;j, VJ; i;,;*-.a-.j .-ig,**-?- •..•,*_-. iata p-cir.ts for coefficients r-^r--*«p^«ril5=sp '.- •. ;vtr ij*^t<. A", :• fps Vc.is it rut say hAve Just started -.? :«•: -a- = . •.-.* •••-H-;" -^_i :^- Twl-pei :c sucr. an extent that

• "^-^ -'-.; ---v.r_K .-.- -r-rical effect cf spray around the strut at .= --*t--:- ajvi ifpt.-.- ire present-i m figure 6. From these photo-

ag-.i.-.ot 5pe--*i m ; igur«» ."-. AJ -ouii "re exjiectei, the spray height is

NACA TN 3092

independent of the depth of suDmergence over the range of depth investi- gated. An increase in speed from 50 to 80 ips increased the spray height at the strut trailing edge approximately 30 percent.

The effect of strut size and thickness ratio on spray can be seen from a comparison of the photographs presented in figure 10. For struts having the same airfoil sect/on (figs. 10(a) and (b)) tested at the same speed, the spray height at the strut trailing edge appears to be in direct proportion to the size of the strut. For the struts having the same chord but. different thickness ratios (figs. 10(b) and (c)), the spray height at the trailing edge was greater for the thicker strut.

Photographs showing the typical effect of angle of rake on the spray height at the trailing edge of the 21-percent-thick strut are presented in figure 11. The effect of angle of rake on the height of the spray at the strut trailing edge was to reduce progressively the spray height as the strut was raked from 30° to 0° to -30°. The spray height at the strut trailing edge was measured perpendicular to the water surface.

Photographs showing the typical effect of thickness ratio on the two 4-inch-chord struts raked -30° are shown in figure 12. As was noticed in figures 10(b) and (c), the thick strut again has the higher spray at the

; strut trailing edge. I ! r

Cavitation and Ventilation

The 12-percent-tnick struts gave no visible evidence of cavitation in the speed range investigated. The computed cavitation speed for this strut is approximately 83 fps.

i Underwater photographs showing effects of speed on cavitation on the 2i-percent-thick strut at 0° angle of rake are presented in figure 13. Visual observation and the break in the drag curves (fig. k) indicate that the inception of cavitation in these tests occurred at speeds greater than the theoretical cavitation speed of 59 fps which was based on the pressure distribution that would be obtained in two-dimensional flow. The greatest delay in the inception of cavitation was at the 4-inch depth where visible cavitation did not start until a speed oi approximately 70 fps had been reached. Cavitation did not begin near the water surface as might be

i expected on the basis of static pressure since apparently tne proximity of the water surface resulted in «n alt,f»rnt.inn of +h» nveoeimo dictribu tion and reduction of the pressure peak with consequent increase in the

l speed required for cavitation. The inception of cavitation occurred I approximately at the 60-percent-chord station, the design location of

minimum pressure. Raking the strut forward or aft caused a small increase in the speed at which cavitation was first observed from that for the

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No surface ventilation was observed for any of the struts over the range or speea ana angle cr rake investigabed. The intersection of the strut with the water surface as seei. in the nderwater photographs of figure 1J was typical of all the struts tested.

CONCLUSIONS

An investigation made to determine the hydrodynamic drag of three surface-piercing untapered struts at approximately 0° angle of yaw at depths up to 6 chords for speeds up to 80 fps and at various angles of rake indicates the following conclusions:

1. The section drag coefficients at subcavitation speeds were in good agreement with wind-tunnel results over the small range of Reynolds UIUUUC1 nilUl C n j. iikj.— 1^ uiii i.'- 1- ^t-t—t. v> i_*. r* ^. J. \_ iv Y ui-.tuuxv^ _L v x WUI^/V-VJ. J. i_"_<,ii. •

2. Raking the struts reduced the section drag coefficient from that with an angle of rake of 0° as would be expected from the change in effective thickness ratio that resulted from raking*

3- The drag coefficient due to the drag at the surface intersection was approximately constant at Froude numbers above 8.0 and at subcavi- tation speeds.

k. The inception of cavitation was noted at a speed higher than that predicted from two-dimensional-flow theory, probably because of the influ- ence of the free-water surface on the pressure distribution.

5. No surface ventilation was observed on any of the struts over the range of speed and angle of rake investigated.

Langley Aeronautical Laboratory, National Advisory Committee for Aeronautics,

Langley Field, Va., October 8, 1953-

_A_ — ---

8 NACA TW 5092

REFERENCES

1. Truscott, Starr: The Enlarged N.A.C.A. Tank, and. Some of Its Work. NACA TM 918, 1939-

2. Kapryan, Walter J., and Weinstein, Irving: The Planing Characteristics of a Surface Having a Basic Angle of Dead Rise of 20° and Horizontal Chine Flare. NACA TN 28C4, 1952.

3. Davidson, Kenneth S. M.: Resistance and Powering. Detailed Consider- ations — Skin Friction. Vol. II of Principles of Naval Architecture, ch. II, pt. 2, sec. 7, Henry E. Rossell and Lawrt-ace B. Chapman, eds., Soc. Naval Arch, and Marine Eng., 1939, PP- 76-83•

k. Blasius, H.: The Boundary Layers in Fluids With Little Friction. NACA TM 1256, 1950.

5. Abbott, Ira h., Von Doenhoff, Albert E., and Stivers, Louis S., Jr.: Summary of Airfoil Data. NACA Rep. 824, I9J+5. (Supersedes NACA T.TT3 T R^n \ Kit U-_/UV • j

6. Hoerner, Sighard F.: Aerodynamic Drag. Publ. by the author (148 Busteed, Midland Park, N. J.), 1951.

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Chord, in.

Rake, deg

.03

°.02 *>*

a A

O < >

66X-012 66i-012 66J-012 664-021 664-021 664-021 66x-012 664-021

S '4

0 0

30 aft 0

30 aft 30 forward

• 0

. Wind-tunnel data Wind-tunnel dnta

M v« 01 0 °.01 y •a.OOg c

,-—Schoenherr

0

£.006 e 55

D~ fc —- •"^—-ft- .j

\ --C c ^

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.003

002

*- t> -gr- ~ "" — • ---

__ 1

up b & 10w 2 Reynolds nunber, R

8 10 .7

/

Figure 6.- Variation of section drag coefficient with Reynolds num^pr

**fc **

NACA TK 5092 21

0) 60

05 TJ

0 0

30 aft

30 aft

30 forward

•>

^ • O C

O

1

00 Jr j± jt J- Jr

NACA

airfoil

section

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\0 \0 \D vO \0 vO vO NO \0 NO VO VO

0 a <] O v A

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53

-p

n°

-p 0

-ifo r-HOJ

4H o

o •H -p a!

<u

•H Pn

22 NACA TN 509 op

V = 30.5 ?ps V = 30.2 fps

V = 51.2 fps V » 51.0 fps

= ouo ips v

, a) i^-inch depth

L-81266

(b) 24-inch depth.

w " ' " ~ o* ~~r~ -~~ — - ~ it srT_iv. IIACA 061 —012 ^-'ri'oi U-inch chord; angle of rake, 0-".

• p"ti on vn f.h

1 "*

— —| m • 4

1? '

,,,.. -,..., p. „i

1 NACA TN 5092

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d) P en J3 bO

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2k NACA TN 3092

o o

•

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k NACA TN 3092 25

•

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n 0.

00 -P

ft vT a} t-t H

O' fc CO I

Hi

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g nJ > a

4 CO

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<n A! 1 a cd *

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I |x<

26 NACA TN j>uy2

1

I I

7 • U0.5 fps

V « 50.6 fps

V • 82.0 fps

/ \ *.. -~.. i- /- va; NACA 00^-012 airfoil section.

V = 1+0.8 fps

v = 50.5 fps

V = 81.2 fps

(b) NACA 66^-021 airfoil section.

L-812 69

Figure 12.- Photographs showing effect of thickness ratio on spray height. 4-inch chord; angle of rake, -30°.

NACA TN 3092 27

n R.

cc

II

>

UN IN.

-P ft (D

o a

•H I

O

O (3 o- O

a P. CO

•H +-> O cu v. 1 tn

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11 CH

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<L> 0 O j- Tj •H • -p (11

NO A-i efl rH 0 -p b0

11 5 •H C > •H

1 > cd

CO 0 TT -^—*• c LJ

^0 0 O ' XJ -d 0 0) cu .c ft 0 (0

•H OH 1 O J-

P 0

m P. 0

6-1 CM

O •*H SO vD '.

II K*\ > rH

0) H Bo •H

28 NACA TN 3092

V = 63.I fps V = 67.0 fp3

(c) 16-inch depth.

v = 65.3 fpj

(d) 2^-inch depth.

Figure 13.- Concluded.

NACA-Laiwley . 12-18-53 - 1000