Document
MR Jan. 1943 NATIONAL ADVISORY COMMIT TEE FOR AERONAUTICS
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ORIGINALLY I SSUE D January 1943 as Memorandum Repor t DRAG MEASUREMEN TS OF A PROTRl.Jl)lNG .50-CALIBER MACHlNE GUN "'lITH BARREL JACKET R»10VED By Arvo A. Luo ma Langley Memorial Ae ronautica l V l boratory Langley Field, Va.
WASHIN GTON NACA WARTIME R EPO RTS are reprints of papers ori~nally issued to provide rapid distribution of advance research results to an auth o rized gr oup requiring them for the war effort. They were pre- viously held under a security status but are now unclassified. Some of these reports were not tech- hically edite d. All have been repr o duced without change in order to expedite general distribution.
for Bureau of Aeronautics, Navy Department DRAG fm:"SUHETlIEN'rS OF A PROTRT.JDIHG .50-CALIB3R ;','L CHInE GUN vnTH 3AJ.c{REL JAcr'::3T RE,IOVED By Arvo A. Luoma SUMr.~ARY Tests were made in the NA CA 8 -f oot high-speed tunnel to determj ne the drag reduction pO!.J8ible by eliminating the barrel j a c ke t of 8 Dr otrurl in g . 50 - caliber aircraft gun .
It ViC.',:;' found t:lat the d r ag of a standard aircraft gun pr ~t ruding; into the airstream at right anGles to the flow c an be r educed by 23 percen t by discarding the barrel jacket .
At 35 0 nilGs pe~' hour and sea-level condl tion3 this 9..'TI0U.i"1ts
to a drop in cl1~ac from 83 to 64 pounds and a decrease in
horsepo-'.f6r absorbed by drag fro !a 78 to 60 hon?Gpov:er.
A r ouGh su:>face finish on the burrel 'das found to 118.ve no adverso effects on the drag of the barrel, the drag be~ng actUl .. ,J ly less a t high Mach nurn.bers.
'Ytw SignifIcance of V'us is t ha t, as far as aerodynami c considerations nre involved, a barrel finish ~roduced by a rough machining o~eratlon is no wors e - bu t p r obc;.i)ly so~newhat bet tel' - than onE' produced by a fine machining o: )ereti on.
At the request of the B ur eau of :'\.eronautic;s, Navy T;epa.rt:r.1C"1t ,
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- 2 ., tests were made jn t he N .. CA 8- f oo t high - speed t unnel t o deter - J~~i .. 1e thf.' (~.r' ttg re,ductlou p038ible by eliminating the barre l j ack et of 8 protrudin : . 50 - c a libe r aircraft gun . ~ . ccording t thf:' ' Ntlv, IJ ,pI1 rt me- nt , r irir ~f1: tests 11a, .-e shown t hat t hf., dJ s - pe r sian pa ttprns of a . 5 0 - c ~ li be r aircr af t gu n ~W.ch had been : nocH f'j cd Yy l' r.mo i:-lg the j G.c} ;:et and sU !) ::Jt i tu ti ng a sh o rt of tl13 trun ni on equa l ly as g ood as those of the Atandard g un .
In re f e r 0 nce 1 t he a ll" d r a~ of a ctanda r d . 50-c H 110 e r 8 i r - cr a ft gun '. 8.S cl eterr. . .' ned , t9 .~ {iell as the basic da t u ne e ., ssary to pe r m it th e C l: lcu l3.l ion of t'16 pCMe r t o drive : :. ;uch ;.n 'otrtJ. d ing g uns when used In P O\';" f=.: -r · -op(:rated turr e t s .
similar d a t E we r o oht,lned for a . 50 - c a li be r eir c raf t Gun wi t h th e 0 i:,r rcl. j n ck e t r e mo v e d .
'1;he effec t on dr eg of rou g:'1 :1e s s on t he b arre.l supface ( e. g., ro ughn es s d ue to c oarse ma c hinl.n < .
Ol)e r ~. tionG) WEtS Dl so de t ermI n ed .
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.,.P PA R.! 'llTJ 0 )J rD ~, 1El 1 EOJ) S rrhesE; t c:.::ts WE re l'w.cJe 1n t:,8 NAC:~-\ 8 - foot hie;h - sDeed tunnel.
1"l~i s is 11 sIn g l · - return, circu _.H r-section, c losed -t hroat tl lnne.L and ha s an airs'tJecd c on tinuously control lab le from appro) : Lna t ely 75 to more than 500 Mi les per ho ur.
A . 50 - c L-- IL )c r aircre .. ft [;ur ~ whicb th o lUrcr",f t .i.rr. laJ. en t Unl t , Uava l Air St &t ion, No rf ol k , Va. , modifIed by removin g the bar rel j acke t , was used in t~ e t e sts.
However , t he s h ort ':Jea r1 .n g J vihie .1 wai-i subs ti tu te c f or the barre l j He ke t and whi ch
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was loc ated f o rward of the trunnion, was not inclu ded on the wind· · tun~el model . ,Ji t;:n the gu n pi vot ed as shown in figure 1 and. "hen ~lerpend :Lcu la r to th e air flo w, the bearJ.ng v:ou1::1 have projec"Ged e .. oprox imatel y 1 1nch j.nto the str 0.am boundetry laye r
~Jh Lch 1. s about 5 inc:hes t hi ck - on th e tunn el wall, and the
increl'nent of drag due t o the pro trud ing Da rt of the bearing i~1 th e l ou - v el o c l ty a 1r of the boundary layer \Iould bE;; ne o lL .. ibl e .
Mo reov e r, when the gu n barr el is swun g through an an,;le r8.n~e, th e bearinr r.lO v e s c o: - !ple tf : l:r ou t of the nlr st~ea.l'1 . It was decid <.:.d , tllcre:t'or e , no t to pepresent the bearin~ on tho mode l.
'1'he model s c;tu':l and. the:. lr ,e thoci of tcst:i.nf:" were th.? ; Slll:1G as in r e i' e r : :o.ncc 1 .
speeds wer e includ6d .
R,SSU JTS AHL D IS :;(.:...SH)U The f 011 ovling ~'ym b ols are US Gd ( 80e fi g . 1): a ang l E; mad e by tb..e b arrbl of the I:18.chinc. Gun wi th the pe r pendicular to the ai r f l ow; the o.n 3 1e of the gun is po si U V8 :: hen the gun muzzle olnts Into the air .strr)!lr.l L l ength of gun p rotru di n B in t o o.~r stream, measur6d along gun ax is Do. aver age outside d iara e ter of len::;th of br;.rrel L ;:: , ro - tru d in g into ai r stream iI. axial cross - section area of b a rr e l i.n ail' st.rCHr.l: this nrea i s equal to L x Da -- - - - .- -~ - 4 - CD dr ag c oef fic ient based on ar ea A be or ss - wind force c oefficient based on a r aa A; (see ' j fig . l fo r def initi on of posit ive d:Lre ct ion ) DY'O j .:. ct ion of ler g th L on plane porpe ndicular to
a ir flow (L = L cos a)
d 41st a n c~ from t c p of t l mne l wall t o c enter of r essu r e of r e su I tant air f Qrc e on e;un axi s, measu r e d p ar' al l el.
t a L Op c en t er -o f- p r essu r e c.oeff i c;:lent (d/L) V 0 v E . lo c i t y i!1 t he undisturbod stream a s peed ~ f so un d III Mach numb e r (V ~/ a) The dl'ag , cr oss ·· wl n d force , 8.nd center - Of - ~ ) r3SSUr e co e f- fi ci en t s fo r t~e . 50 - calibe r ma c hine gun wi t hou t barrel j acket are sho " m : 1 t te d ae;a in s t .Vla c h nur 1ber in figure 2 f or severccl angle s a. . As Vi 8. 8 t he c as~ in r ef e r ence 1, th e for e e c oef ·· !i ci en t s a r e bn~ e d o. t he axial cros s - se ct io n area of the gun in t he a:i. .r st r eam . The a r ea of t h e p lain borrel is abou t
58 ~)er c en t l ess than tho c o rr e s p o nding a r ea of the stand ct-rd
g un .
A co ~nDa ri~ : o n of t h e c }r ;:'; G c oeff ici ent v a rIation vii t h Ma ch numbs r ( fi g . 2) for equal po s iti vs a nd n n gat i v e v a l u e s of ang l e 8 ho "\ '\7s quit e unexpected cliff e.r e nc es . EX C6? t fo r an f1 angle of 6o , t he c u rv es for an gle s with tl1e bar r el s un po inting a ft s h ow th e sharp r i s e in drag coefficient
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associated wi th Mach ntm ,b e r effects for speeds beyond t he i:;'or th e; for r.'ard ( positiv0 ) anc;les, however; crit~c a1 s peed .
this ri,so 3.11 drag coeffIcient is much 'more gradual or entirely 1 8ckirf!" .r.n explanati on for thi s diff e renc e I s not poss i'o1e 1'1'01.1 the da ta obtained . It may be suggestecl, however , t~1at, in ar:1cHt lon to t h e complication of three- dimensional flow, the t upe r in th e g un bar r el an d the air leakage through the s ma ll clear a nce g ap between the g un barrel and the tunnel vlal l m ay hewe produced - or aided in ;:)rodu c ing - sufficient c hanrs8 in the t y;::>e of air flo 'I , and hence se;Hl.ration phenomena , about the gun to account for t he diff erence in drag behavi or for posit i ve and negative an g l es . One effect of ta~er in the barr e l is that sections of the barrel exposed to the air flow arB more st r ear.'11inedwhen t he gun points aft than v:hen it 'r olnts forw ar'c1. A lso , when the gun '0oints forHard th':Jre is 8. cross ·· f1 0iI~ t ende ncy t ov·; a rd th e breech end and, \'/hen i t points aft J toward the muzzle end .
From th o data of f ig ur e 2 it is evident that critical Reynolds nur.lber o ffects , c'haracterized by an aD"reciabJ.e de- cr ea se in drag co effl ci en t it th increase in Reyno l ds nUI!1bsr 8.f} exe r~l~ lifie '~ by the dr ag data for the unslotted replica of r efe ren c e 1 , d id not deve lo p for the plain barrel bec~use of the onset of co mpre3sibi lity );1enOmena at those speeds at which the dr[ 'q;s deere ase e ould be exp e e ted.
Fo r a given size of cylinder , cri tical Reyno lds numb er effects can be I~lade to - 6 ..
occ ur a t lower vel oci tit'l s by increa ~d.ng the initial tu), .... bulence oT :' :1F:. str0Bl:l or by int r od uc ir\g rou iSh ness on t he surf a ce of t~l . ; 0 :11 ir..dc r (refer en c e 2) . R0 cour S3 was made to the second of these methods in an effort to de cr ease the b s rrel drag . Roughness vIas p roduced by she· UC'.cking the b a rr e l sur ·- fa '" :~ i. ~ld the n dust i ng no . 60 c~r;; OI' Un c.ur.l g_' ains unIforMly on th o Vie t shellac . v vhen dry , th'i3 shell ac f J. rmly lJondGd triG grail s to the barrel 2urf& 0~. N. ::" ; fIgure 2 ilL1.str a tes , the o.i 8 turbanco to the air flO'.\' int:r.' oduc ed by th e c arbo run c: um par ticles ~ a s not sufficiently gre a t co decre a se the c ritical Reyno lds numb e r . A lar'geI' size of c 8. rhorundum g r.5t in may have shown mo re fa vo r abl e results . Ho w ev e r, the t est does br ing out the f a ct that the drag of the b arI'el is n o t adv E lr se ly a ffect ed by rou ghne ss, betr1£; act ' ally less at h igh M ach num- ber s . ' fhi s me a ns that , as fay ' as af;rodynruni c consideratIons are involv e d , a 8<: 1'1'01 fInis h "', r od uc ed: by a rou:;11 machining op e r a tion is no v/orse - b ut n r oba"') l T somev /h at bette r .. t ha n one produ ced by a 1'1ne rnach lni ng o perat i on .
The drag coefficl e nt of the p l ain ba rrel is ah out 29 pe r - c e nt g r ea ter t han t hat of the s t andflrcl gtl.n , but , since the ex ~ r'
posed area j,n the al r ,.,trearfJ. is r edu ce d b: 3 ~) ne rcent wh en the
barrel j a cket is el i m:~nated , tl 1.l3re is 10<11 ap prf J ciable drop in po un ds of dr tt g for t~, gun v Hh0ut t Le jac k et.
The p ro p or comparison of th e d r ag of the machlne gun wi th a nd wi t ho ut b a rrel j a c ke t is br ~ )Jg h t o11t j .n f igure 3 i n whtch 8.ctu~.l OJ?ag
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in pounds i s ~lotted against speed for sea - l eve l conditions.
Figures 3 Bnd 4 arG based on the drag coefficient data of
o In converting to sea- figure 2 ['end rf'ference 1 for a = 0 .
level c onditions , the data were computed for tl'_8 correct Maeh numb er . T:-le ReynClids nuraber for the flignt sea -l evel eXEmp Ie diff e rs sllg:1.tly from the valuos obtained in the wind-tunnel test at tLe Sill.le Mach mElber, but thE: effect of this differe nce is inap9r ec i ab le o~ the v ~ lue of the dra~-force reduction due to the elimination of t~e jacket. By eliminating the barre l j acket it is se;en t1. 1.Fl.t t l1e drag of the gUT: when vertical to the air flow is r educed by 23 pe rcent. JI.t 350 miles iJer hour the
drop in drag Is fro ::n 83 Dounds to 64 - po'mds . jJ. so inc luded
in fi eu r e .3 is the drag of the barrel \I;~1en rouGhened - \:i th carborundum grains . Above 400 miles per hour there is a de- crease in drag due to rou ~bness on the surface. This improvem e llt may be due to less adverse separation chnr/: : cter -
istics when rou~):.ness is introduced. Fi p;ure 4 shows the
horsepower absClrbed in air dra~ by a machine gtm with aJ:~d.
without barrel jacket .
Eliminatin3 the barrel jacket de-
creases the horsepower absorbed in dr&g from 78 to 60 hor3e-
power at 35 0 miles per hour.
The c ente r- of - pressu re data shown in fi gure 2 are some- wha t less accurate because of smaller forces than the corre- sponding data for the standard gun of ref e r enco 1.
curve for 60 aft was not faired bec ause of the scatter of the
test points .
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C ON CLUSI on s By e li minat i nG the ba rre l jac ke t of a . 50-caliber o.lr - cr a ft gun, t he drag was reduced 23 per ce nt; or, at 350 mil es per lour a nd s e a -- level conc . it i ons , tbe d rag decreased fro m 83 to 64 p oun d s .
A r Ol.gh s ur f a ce finish on t he gun barrel he.d no adverse effects 011 t he d r ag .
Lan gley Me mori 8.1 . A.e r ona ut i c. al Labor ato ry, Nati on a l n.dvi s ory Oo)" :,,-'11 i t tee f or }. cronau tic s , L n g ley Field , Va ., J a nuary 26, 1943 .
1. L lOJ.:l8. , .I. rvo 1'. . ~ Drag 1.l e o.su re men t s o f 8. P rotr uding 0 . 50 - Cn l iber Machine Gun . UACA A. C . R ., .July 1 9 ·1 1.
2 . Fa Ge , :l,. • . 1 and l;"!fa rs ap , JO t H,,: The Effe c t~ of Turbulen c u a n Sm:-:facE) Rou.="hne ss on t h e DrQr; of a Circular Cy l inder .
R. & hl ~ ~o . 1 283 , B rit i sh A . ~ . C ., 1 9~O .
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