REPORT No. 98.

THE OESIC:\ OF WI. 0 TC:\NELS A:'<D WI:\D TU:\NEL PROPELLERS, II.

B~' F. lL X'lH.TOS ;\Ilr! EIJI'.'AIW P. W.\J(:->ER. X':otiQn:ll .hl\'i~ury Commiuee iot Aeronautics.

SUl'rfl\L\HY.

This report is 11. continuation of \l"lltiOl1l1l A(h-isor~'" Committee for Aeronautics Report:\0. 7:!, llnd WtlS undcrlnken fi(, the Lllnglcy )[crnorial Acrol1llutil'l1l Lahoratory for ihc purposeof supplying further c1IlUl to the de..:;i~ner of wind tunl1f'I... P:trtirulnr (,l11phl\si~ WItS plal'cdon the '-llId.\- llf dirf't:tiulll11 \':lrilltion in the wind .;:tl"(';1Ill. Fur thi, purpo,;;;c ;1. recording Yll\\-'IlIl'lcf. whil..'h could <11.'"Iu lll~ u...ed :1:":Hl air _~p('cd meter. Wa.... tk·n:lllped. and ga\'(~ '"cry slliisfaclor...·fc:'"uHs. It is rcgrclwhlc thnt the \"OltllgC supplied to the driving molor was not \~cry constnnt.due to nll'ying loads on the line, hut as this motor W:1S of .\ lightly londclI induction type, the....lll'il\tioll in -:-pced was not n..<:. largo :15 the nlrintion in "oltnge. Tho work was cnrried on hothin f\ I-foot model /lnd the .j-foot full-si7.ed tunnel, find wherc"er possihle f\ compll.rison WitS

made he tween them. [t was found Hwt p1l1cing radilll '-nnes directly before the propellerl1ctunl1y increl1.Sed the effu:iellc.," of the tunnel til :1 con;;idemhle extent: The pi/Icing of ahoneycomh I1t the mouth of the experimentl11 portion was of the greatest llid in improving theflow, but. of courso. somewh:lI rcdueed the cflieicnl'Y. :::'e\'Cfal t:-'pes of diffusers were triedin the return air. hut onl:-' slight impro\'emelll resulted in the stendiness of flow, the:-' not heingnearly as effect i,e as the honeycomb.

APP:\HATUS.

The efflcicncy of the tUllllel l1nd the slip of the propeller were determined by the samemethod us described in Report Xo. 7:l, hut to bOllCI' rccord tbe f1uctuntions in velocit~.. anddirection fl recording instrument WIts constructed. This instrument. as shown in Figs. I and2. consists of n thin mien dillphrnglll whose mOn"lllenl- rotn.tcs It very light spindle contniningn small sil"crecl mirror. Light rr'om nil illuminated slit is lr'lnsmiltcd by :l lens to this mirrorn.nd Lhe reflected benm i:5 thcn fOl"used on 11 mo,ing photogr:\phic film so lh:ll nn:-" mo\"'"elllentof the mien dinphragm is recorded ns :t continuous curn!. By this method lln~' slllllil lindmpid Yilriation in the Itil' flo,.... of the tunnel is indicntc<lllud recorded by means of n Pilot-slatittube which is connected to the two compart·monts scpnl"11.ted by til(' di:lphrllgm. :LlId llny('!lange in direction is rccorded in the same way by connecting the sides of l1. ynwhend totho COmplll"lmonts on opposite sides of the micn dil\.phl":\gm. The Pitot and tlie connectingtubes arc mIllie comparati'-ely large so that nny rapid f1uctufltioll in velocity ('1m he illlmccliatelytmnsmitted to the dil1phl"llgm withollt 'hmping 01' lag. 0,-('1' .50 r{'cord:;; were token hut onlylL fe\" t:-rpiClll ones arc reproduccd hcre. Xumcrou5 expt'rimellls on the cflit-ielley of tunnelsanti art speed Jluctu:1tion h:1'\e prC',-iou:ily been n1l1.de in Eng-blll!.l, :,3

EF'FICIE:"lCY A:\"D SLIP WITH :\"EW PROPEL LEn.

I"n order to gi,'c a. more' C\"l'1l flow of nil' in the exit ('nne :1 new propeller Wl\.-; de~igllcfi for themodel tunnel hnsing :1 In,rger pilch Itt the lip so thn.t thc nil' in this portion would he drnwnthrough wit.h fl. reln.ti'el~" grcater ,·elocity. Tn e,c,y other rcspect this prop('ller is "CI)' ~imilnrto the propeller used in the t~t. dos('rihcd in Report Xo. 7:~. which, o,\'ing to 11 piece of woodbeing dropped in the running tUlllICI. \"IIS complet('I~' de5tl"l)~·c.1. Tn Fig. 3 is shown the effi­ciency of this propeller when working in 11 pnmholi/' ('on(' !'tnd in It straight conc. [t will benoted thllt in the Si~mc W.lY liS wilh the lirst propeller the straight cone is considCfi\.hl.\" more

1 .\n In\'l~3~1~"liun11110 Ih.. ::;t.."dl1\,-"""r Win. I 0-11.111111'1.<, h~' I. n.,irsl"'''' mli JI,ln5 Il',,'th: nriti~h .\d·.. isurr('oml1lill'·O for .'~rO)m\lllil'S.

It. ,<.: ~t. 6;, :-icIHemher, 1912.• Experlmenls on .\trldel, 0/ II "'Duple.,,"' W1l1.Il"h'll1npI, hr .... E. :5t:1111Ql1 '1l,1 I. II. Ilyde: Bril..\. C. .\" Ie .\'-.\1. '.n. SOn"ember, 191~ .• Reports on TUl5 or:t. .\10.11'1 of the I'ml''',ll'<! :'/Ollt Wm<l Cham:..1 .It Ih,' R. \ E. hr c. I"'; ."':'nui,nll .,nU W. K .\lIard: Brit..\. C..\., B.

& M. ,;~I, Decemlwr. HilS.

·\;\"Xl".\L HI':I'OI:T X.\TIOX.\1. .\!l\·[:-:OI:Y L'O~Dr[ITEE FOr, A!':r:OX.\l"TICS"

etJi('icnt :1[, high speed thrill the paraholic calle. fn Fig. -l is shown the slip of this propellerin the paraholic cone anti in the :itl'llight cone :lnt! it is Iloted thrlt the slip i.:; less at high speedsfor the ;;trflight COliC. It is then e\~id('nt thllt the straight cone i.s l1erodynnmically superior tothe p'lraholic COile, in addition to being el\:;if'r LO huild.

40/0 20 30A/r speed /n h1.p. s.

I0H, SlIPWIFHPROPl'"l££R N!'Zf-In slro/?/hl&porobol/c: cones

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4000C,\!;;fJOO~o~ 2000ct(

500

40/0 20 30A/r speed.'n m.? s.

rr ,- - .~~ '-~""PC££CRN9~~~ .•.•._ •. _. .•. ".1'......

HIn slraight &parobohc cones.

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EFFECT 01;' SIZE Or THE ROOM.

All tbe test I'lIn~ de3erihcd ill Report Xo. i3 were conducted in a.lal'ge rOOIll, approximl1tingto fl'ee-flit' condition:;. fn tlte lc-;ts described ill this rcport it tempoHlI'y wom W:1S built ftroundthe tunnel. I'cpre5cnting to s/;:de tho huilding p/'o\'jdei.! fol' t.he 5-foot N. A. C. A. wind tunnel:l\nd illl /'lins except those shown in Figs. :3 nnd -, ".."cre made in this model room. The CI'OSSsect,ion of the model room and the wind tUllllel Ill'e shown in Fig. 5. For the same power thisroom denc:lscd the nil' speed from 60 Lo .j!) miles per hour 01' 11 decrease of 14.5 pCI' cent. [II

the small room the IllI\ximull1 \':ll'ialinn of specd W:)5 =7 per cent and the maximum \~llI'intion

in direction was == 10°. The nil' speed recol'fL.; :::how thnt for the first 20 second.s a(cerstnrting,in the Ltrge room. and for Lhe j'il"SL LO seconds ill the smnll room. the nir speed is ve":\' stellll~",

llnrl that the ftucwations suddl'nly i1ppel\1' ilt /1 (Iefinite time ancI ,,-iU be indicated on the record.This llppe:1r:lIlce of sudden f1ucllll1linl1~ ~eellls Lo indic:1te that the large part of the speed r-tuc­LU:\lion'S :lre due to lhe disturhcd nit' from the propeHer ilS it returns through the room to thcen t1'1\I1l'e cOile.

Ef'f'ECf 0F RADJ.\L Y.\~ES.

Eight radinl \-,U1CS 3 mm. thick IInti ·1.)0 mm. rleep werc pbced symmetrie:1lly ill Ihe exit<'one immedi:1tely hefore Ihc pl'llpeller. Th~e \":lne~ joined in the ('l~ntcr in:l stiltioner.... spinnC'rwhich wa;o' of the fO:lIlle diameter :1' th£' pmpeller bll..'C. (Fig. 6.) These \-anes actu/llly incrc:1sedthe speed of the air in the tunnel for the Silme power b.... 5 per cent:but the fluctU:ltions in direc­tinn :lIld n·locit~" rcmnined UIlCh:1I1gcd. fn order to determine whnt part of the \nne gl1,e theincrensed eCTicienc~~. 2'5 mm. \\'a~ cut off of the outer end of e:1ch ,ane ilnd the run repe:1tedwhich ga\-e /1 3 pel- cent incrcase in speed for the S:lme power o,er the tunnel with no 'anes ..\gain the \-aneSWere CUt off on the elltl T.j I1lnl. r1nd in this ca.:;e lhe S:1me speer! wns obt:1ined /15with the tunnel \\-ithout \":Il1C~. This seems to ~how th:lL it is the \\'hole area of the 'llne which:\I'l,,; ;\:-0 a stmightener for the ail' now ;\Ild thal nl) p:lrtil'ul:tr part is especially \":lluahle in in­cre:!.;ing the eCTicielll.:y of the LlIlln~1. Eight ntlditiol1:11 ':llleS 3 mm. thick were then plal'edalfHlg the inne/' SUdr){'c of exit ,·OIW. e:tch \";1I1e hcing I.) mm. wide. This ili:,trihution of 'lineS

IlencllScd the speell hy 12 per ccnt (or the :;ame power :1I1d the ,:uiation in speed was ± 6 per cent

FIG Z.-r!E.CCROI'G AIR SPEED UE.TER I. TM :L_LMI';"- NG "'NO RE.C~ROING APPARATUS.

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r------------------------------L. 8'-O~ :I II ,I ,I ,• II II II I

I II II II 'I II ,

I II II ,I ,I ,I II ,I ,L_____ I J

I/"IODa I'WIID rUNNEL ANO ROOM

rig.S

RADIAL VANES IN EXIT CONEFig. 6.

VANES IN CNTR4NC{ CONErig. 7.

_\X='i"CAL HEPOI:T XATIOX_\L .\II\-I:-iOI:Y CO:-'UIITTEE 10'01( .n:nO~-.U-TIt·~.

;1n" the ''-llriatiun ill dircdiotl WII:'; ± tOo. The -;allle \"nne:-: were then placed in the cntnuH'Cconc, a........ hown ill Fig. 7, and ill thi:i case the SIlL'cd Wl\:-; dcc/'cll.~cd hy S pCI' cent and the V"arilltionin dil'cdinn W;l..'i ± So. 'Yith thi~ type of nll\C in lJulh the exit Ilnd entl'llnce ('one the speed wasdecl'cll':'~cd by 20 pCI' cent for the Sl~mc pOWCI' :1Jld the vllrilltioll in direction WilS ±So. It isevident from these tests that the llUITQW \"l1llC:'; ill eilher the exit or entrance COll~S ill'e of little"llluc in llny way.

EFFECT OF PLACIXG SCREE;"; ,4.CROSS THE T ~NEL

.\ section of chicken wire of 2.) mm. mesh WIts plated (l.cros::; the exil ('one -15 centimeters n.hendof Lhe propeller. The use of the chicken wire decret13ed the speed by only 3 per cent, so it docsnol seem thn.t this dislribution of scrcen "'ould be of nllY grcnt hnrm to lhe efficiency of the tunneland it ii; of great usc in pre\~enting small objects from being dl"fiwn into the propeller. A pieceof window screen placed n.t the beginning of the straight portion of the t.unnel decreased thespeed h.,' l-! per cent n.net the fluduation in "'pced W11:-: - 12 pCI' ('<'Ill and W:l.S - 10 pCI' tent indirectiun. showing thlLL the Scrcen in Ill) W:1.'- hplp..; thl' sll'adinf"·•.; of flo'" for the pllrticul;lrcundiLioll of thi:::; tcst. Scrcens ha'·e heen used to lId"llntage in other tunnels. \Tith windowscreen III the moulh of t.he entrance cone the speed W:lS decrel1secl by only 7 pel' cent.

THE EFFECf OF EXTEXUI:\"G THE EXIT CONE

BEYOXD TlfE PROPELLER.

SPhVNE"R

r-SI#l't"I. __Wlr~s

-~450. lS",m.

- ".' .....

EFFECf OF PLACI:\"G SPIXXERS BEFORE THE PROPELLER.

A spinner 75 mm. in din.meter and -150 mm. long was supported h~ steel wires before the pro­peller, :15 sl!o\\'n in Fig. S. The use of this spinnerseemed to hl1"e no m:lterial e(fect on the nil' flow_

l3y e:\tell1ting the e.'\iL cone as shown ill Fig. 9, thereWil..<; no chnnge of the air flow inside the tunnel, butthe lnngentilll flow, which hlld been not.iced heforewith lhe propeller, v..-as somcwh:lt st.l"nighteneci out, andthe nil' flow WlLS more direclly to the reflr through thec:\tcnsion of the cone_ A cylinder WflS then t1ttnchedto lhe propeller end of the tunnel I1S shown in Fig. 10,

which decrefl5ed the airspeed ahout 5 per cent, the nil" issuing from the tunnel at it considerably

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ClIN/C4l £xrCNs/On - FI!;l./O.

CYlINaR/C'Al CKTC/VS/ON

hig!lCl" \'e!ocity :\[I(l iu 11 more COlllpllcL stream, the uorders of Lhe strelllll still being sbl1fplyu.efincd at a distance of 20 fect. As exten~ions of Lhis kind m~:ln it larger ilnd longer buildingfOI" the wind tunnel there would certllinly he no ndnllilnge in using tltem.

EFFECT OF HONEYCO:\fIlS.

-\ honeycomb was COIl5LJ"u<:tod :IS :,;ho\\-n in Fig. II :lnd was plncel! at the enLl'llnce toLhe straight pOl'tion of Lho tuanel. O\\'iug' to Lhe difficulLy in oilL:lining thin-w:l!Ied metal tubingn.nd to the expense of consLrucling honC'ycombs of lhis type. only this one \\-il.'S .tried. It is

477

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---Ft9·1I. ~/..f.-nm_--i

seCTION OF IiONEYt:OHB

quito oddont, howo\-el", o\'on from this ono test that Lhe honeycomh io:; of the greatest importancein stTtlightening out the flow, The speed is reduced IS per cent !lIld the energy ralio 45 pel' cent..by this honoycomb, but tho mnximll1l'l speed Yal:iaLilJll was only =2pCI' cent find thr:ynril\Lion in direction Wl1S roduced to :::O ..jc. Inorder to show morc Cl01Lrly the greiLt incl'ol\5e in stel1diness of now. ft.

curTe takon with il recording p1wmcter is sho\\'n for tho open tunnelfind for the tunnel cont.aining tho honeycomb. (Figs. 12 :lnd J3,) Itis oyidcnt from theso how grcl1t ic; the 11d\"l1.lltngc of Lho honeycomb,..-\S tho length diameter rll.tio in the tubes of thi~ honeycomb arconly 2~ it is quite possible th:lt by using longer tubes the f10\\'

would be oyen botter n.ncl tho reduction in speed should not bo appreciable. There seems to bono doubt from theso tests that tho honoycomb is absolutely essentii\l in most wind. tunnels.

EFFECT OF DlrFl"SERS.

ThofirstdiO\15"l"tricd j..: ... h<l\\'llin Fig, 1·1 ,tnt! I..'illl..,j..,t,,; 0S:':L'lltially of a "ubical oox ofwltich bothsides nrc perforated with sllll\ll holus. whoso dillllleLer is equal to tho thickness of tho wcdl of thobox find ",.'hose spncing hf!t\\'oon conlors is about t\\'icc that of tho dilUlloter of the hole. This

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A" 60cm_B·1.8mm,C-.1.6mm.

cvelCAL OlrrvSfR

Experimentalch(N77tJ~r.

OIF/"USER.

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SECTIONor Olrrt.lSCR.

°9. 16.

box WllS connected rigidly to the I'DIU' of tho oxit. cone 50 thi\t nil the nil' passing through tho pro­pellor must oscape tlu'ough these small holes. It \\'as hoped in this way to hrellk up flny pulsa­tions which would originnte from the propeller. This fUTltng('Jllent decreased the speed of the

tun.nel by 7 per cenL nncl the Ill:\.ximum nuiittion of speed was±6 pCI' cent and tlte direction n'lriation was =5°, so that itwoule! seem th:lt thl~ flow is .slightly straightened. but nowherenCllr :l.S much [j'" with the hone.reomb, o-\. second diffuser \\OOS

tried as 5ho\\-n in Fig. 15, which consists of u latticework ncrossthe tunnel room fit the experimentol ch:unber consisting of 50mm. square cells hadng n 6 mm. wnll with n length 2t timestheir diiunctt'r. This diffuser only reduced tho speed of the tun­nel by 2 pOI' cent. and the maximum \'IU'il\Lion WfiS .::: 7 pOI" cent}nnd tho vl\l"il\tion in direction wns ±5°. o-\.lthough this dmusor

hlls\'t:'ry little errcct on tho cHil"iency of tho tunnol. at the smllo timo it docs not much improve thosteadiness of flo\\'. .\ third difflli'er Wl1S coniitructcd;l;; 5ho\\-n in Fig. 16 ;1nd pillcod in the Sllfll0

position fiS the la5L. This diffu:"('1" dccre:1SCt! tho :lir ::;pt'cd for thl~ 5111110 power about 5 pN cent. tho\·nrintion in '\elocity was ±.j per cent, :lnd Lhe ,a..rintion in direction WitS -= ... :1, showing only 11 slight

HS .-\XXL:.\L HEPOHT 'XATIOX.\L .\D\"ISOI:¥ CO:\l)UTTl::J:: 1'01l .\EI:UX.\t:Tj("~.

impro,emcnL oYer tho open room. IL seem;; stnlngo that these difl'users did not. impro\"() thenir flow morc. as the DriLish hl\\·o found that ditrusers grantl)" impro\'o th~ lIow in their tunnels.The results of these tests would not, howc\Tcr, jU5lif.'· the usc of 1\ diffusor in It full-sized tunnelbCCllU~C of tho rllthcl' largo oxponso of construction of such n pic<:o of appnnlLu5.

EFFECT Or PEUFOR,\TING TI-m STHAIGHT PORTION Ol~ THE 1'UNNEL.

Tn ordor' to dclcl'Inino Lha afTocL on nil' flow of opening the doors in tho cylindriclll portion ofthe tunnel and in using small holL'S for tho introduction of :1ppllmtu~. \"lll'iOliS tests were madeon the model in ordor to sec ho\\' this would efrecL the efficiency and sLoI\c1incss r.f flow. .Alsothe \elocity of tho air in the oxperim('ntnl chamber \nts dotermined by l\ 51111\11 l1nemomcter.A slot was first cut in tho cylinciCl' pnrnllcl to its fiX is and one-fifteonth of the diametel' \\'ide,running the whole length of the oxperimf'ntal chnmhrr. The ltir flow extended out aboutthe width of the slot from the walls of the cylinder, and bl"',\·ond this tlwro Wfl<:: no flow in the{'h:lmhef and the efficil'ncy of the tunnel \\'IlS not flpprrciflbly :lfrCCLI"1. This :-:Iot w:\s thenin('n'll'::f'(l in width to onc-3ixth of Lhe diallh-'tcr oi t.ilt:: tunnel. tl1U:-: dccron..sing Lho efficiencyof t.he tunnel YCI'Y slight.ly, and tho lIow of flir ext.ended ahollt Clt1c-:;ixth of the tunnel ditulloLorinto the experimental cham her nelu·cst. the exit cone. but. this :1ir flo\\' \\':IS le-:s mflrkcd as the dis­tnnce to tho entrance cone wn.s docretL;;cd. When the width of the slot WllS incrcl1sed to threo­eighths of the tunnel diameter tho officioncy was decreased about 1.5 pOl' cont nnd the nil' flO'\-"

extended two-thirds of the width of tho slot into the oxperimental chit.mhol', noil!' tho exit cono,but there WllS no flow elsowhero in tho experimental chamber_

TESTS 1~ FUlL,SIZED TliXNEJ,.

.-\. few Lcsts were mltdo in tho largo tunnel in order to afford 11 compfll'ison with t.he modoi.In Fig. l"j is shown tho slip in tho hugo tunnel. In comparing this with :1 similar condition intho mouel tunnel (Fig. -I) it is seon thil.L for tho same nil' speed tho ro\'o!ut.iOllS pOl' minule is

I 1/0 SL IP OF FIVE roar TUNNEL

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oFt9·18.

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account for this difl'cronce.

/0 co .]0 40A,r speed In n'I.p.s.

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IFIG 12-VARIATION IN DIRECTION IN THE MODEL TUN"E!.. WITH NO HONEYCOMB.

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FIG. n._VARIATION IN DIRECTION IN THE MODEL TUNNEL WITM A HONEYCOMB.

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f 2.FIG. 19. VELOCITY "M~t"'TIONS IN LARGE TUNNE_ WHEN Tl-E D~IVING MOTOR W.\5 CONNEC-eO TO A 25

K. W. GASCLI'lE GENER ... TING SET.

:IG 20__VE~OCI-Y IA::ZI.\,TlONS 1'1 LARG:: TJ'I'IEL \'11TH O~~/i:-lG ~"'OTOR CCNNEC-EO TO A lOO -<. W.LIBEiHY G::NERAT,!'>C. 5;;7.

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lil'::j/G~ OF WI),"!) TlJNNELS ANn WIXD TUXXI~L pnOPELLERS. II. 479

..\5 the exact, cflic.:ienuy IJf t,he dri'"ing motor in t,he large tunnel is unknown, a CUT\'C of!tOI'Scpc)wcr supplied to tlie motor is ploUed agninst nil' speed} but to giye some idea of thepower supplied to the propeller n dllttcd ctUTe is drnwn from the cstimntecl motor efficiency.(Fig. IS.)

In compl1l'ing this cun"c wi!.h the one obtnined in t.he model) it is seen that the full-sizedtunnel is slightl~" morc efTicicnt) so thn,t 1"~sults mny he taken from modC'!s to safely predict thepcrfonn:lnce of the full-sized tunnels. It. is also interesting to notice that the power does notincreilsc liS rapidl.,· 11S the cube of the speed but more nearly as l-~·~) although, ns the eflici<:llcyof the motur is nOl eXllctly known l the nduc of the exponent cun not be determined "cry dosely.

Heconls were tnken in the full-sized Lunnel (If "ari:llions in velocity, flnd these arc repro­duced in Figs. If.) nnd 20. In the first figure the \\'ind-tuunel motor was connected to n. gilsoline­driven generator (Jf 25 kilowl1tls and records taken at se,'ertll speeds. In Fig. 20 the motor wosconnecled to a 300-kilowatt g('lwrnt,)]" dri\"C1I by ;\ Liberty moUn'. TIll' most importllllt chnJ':l(,­tf.'J'istic of thcst' rN'ol'ds is Ih1\1 the Illilg-niludps ()f the flllctuations tj,) not inf'["e:l.5(' as J'npirllyas the nil' speed, :;0 tltnt. IlL the higher speeds, quile clmtr:lry to expeetntions, the ,elocitr isrelnti\"(~ly steadier. The mllximum variation in nil' speed at 90 miles per hour wns about:::: 1.5per cent, wherens in t.he model it, was about::: :! pel' cent., so tbllL il waultl seem that the stCfllli­ness WIlS fl.bout the S:lITIe in any size of LUnne!.

Yllwmeler records we're nlso taken in the lllrgc tunnel, but were not reproduced, ns theyshow pr:1cticllll.'- 11 strilight line, inclicfl.ting thnt the honeycomb was slttisf:lctorily stl'l1ighteningout the flow.

NATURA!. J~ERIODS OF TUN~EL

.\. wind tunnel aels as <In open org:lI1 pipe llllt! its 11l\ lurll I period will be given by:

",h('['e l is the length of the tunnel in feeL,and I" is the ,~elocity of sound} or 1,0-10 ft./scc.

The model t.unnel would then hft,'e 11 period of 0.03 seconds ftnd the Inrgc tunnel n. periodof 0.15 seconds. \'ibl'fttions of Lhis nllturo ilrc very endent ftudibly in the tunnels at certainspeeds: hut do noL seem to he present un the records, tiS the pilOt t.ubc is. n'ry nenrly ilL the nudeof the vibration. The hOllcycomb hns n considcrnble inl!uenl'(, in dumping thesc dbrations ..,vhich lIrc more of l,L curiosity thiln of llll)' pr:1clienl interest.

AUTO:'iI:\.TIC HEGULATOHS.

o-\.s it is not pnlClicftl to supply iL consLflnt \"l.>lt:lgc to u. wind tunncl t llithough some teslSItll.,,-e heen madc witlt storngc hn.ttcries where nn extremely constant speed WflS required, it iseither ncccssnry to keep the \'oltllgc consulllt 11S nenrly as possible by hnnd rl'gullltions or IISC

some type of ilutunll\tic regulatur. J 11 sm:dl tunnels it is quite ensy to reguln.le lhe wind byhand, but in l:1rger tunnels thl.l inertia of the llloying pllrlS is so grent thnt lhere is considerableIlnlOunL of lag between the change in regulatioll and the response of the airspeed) milkingIWlld rcgulntion '"cry difficult.. _\. vcry complicfltl.'d rl'gullttor has b(,(,11 cunstructed at Gottingcn(S . ..-\.. C. A. File "Xo. 5346-10) ltI1d seems to hold the velocity quite CL)nstant. There nre11.1::;0 numerous electrical de,~iccs for m:tintaining :t constnnt motor speed .. and some of theseregllilltors will hold the speed within 0.1 pel' cent. It seems probable) ltl)WeHr, that even ift.he revolllt,ions pet' minute of the propeller is constllllt t.hnt there will slill be flucl.uations inthe air speed, so that 1'1. SUGccssfuJ regulator must he act.uated by the ail' How. There is n. greatdeil.! of work to be done on such regulll.tors, ilntl the S...\.. C. A. intends to cllrry on work ofthis kind in the near fULU1·e.

,:).1889-21--31

I. Const:lnt. direction of flow.2. Constant \"clocity of flo\\".3.· t.:n.ifoI'ITI velocity across scction.4. Emf·jeney.5. Elise of working around tunnels.6. Simplicity flnd choiLpnCS:i of cOIl::>LnwLil)11.

A good many of these qunlitics nrc cOlltl'i1dictory, llncl the b('st compromise Illllst be IDl1dcbetween them illlt! the type of work thllt is to be undertnken. For cxnmple, n tunnel for testingin.ilrurncnls should ha"c n high cflicicncy, but nerd not hnxe ll. ,cry stendy How. On the otherh:lnd: 11 lunnt'l fnr testing wings sh.luld h:w(' its C'HicicIlC'" 5o!l1ew!J:ll lowered in onlt.'r to obtnin11 ~lt':Hly fl,),,". It is quite !>os..:.ihle tn :01) :1rr:lIl~1' thl' hlmf''yI~omb :\1111 c1ifiu:::('l's th:\t they nlft.'"

he removed when it. is desired to oblilin the highest speed. It would olso be of vltlue to makeit possible to open the ends of the builtling, 11S there nrc mnny tln.rs when the wind would havelittle effect. 011 the steadiness, :lnti the efficiency ""ould nppnrently thus be considerably illcrensed.This arrangement would also mnke it possible to cool oIT the nil' in dle building in :l ,"ery shortlime, nn ::l.(h':1Dtllge t.hnt would be grently appreciated in hot wealher.

Tlus work secms to show conclusi\4ely thnl IL slrilight exit. cone is more cfTieient than acun"cd one, lIud it is certllinly cheaper to construct. DiITusers affect the nir fiow yery little,~lntl t.hey do not seem to wafmnt. the expense of construction. Honeycomus, howen~l', are ofthe greatest '"lllue nnd should be placed in ('\-cry tunnel.

480

nrc:

.\XX'L\L IlEPOr:T X.\T10XAL .\lJ\"ISOltY CO.\l.\lITTEE FIJI: .\EIWX.\l."TW~.

CO:\,CI.USIONS.

The qu/\Iitics th:lt should bo nimel! at in wind-t.unnel design in orLlt'l" of their importl1nce r

I