1、.: .,nam.elyjthe problem Of foreca,siingthe ini+nsityof icing conditions andthe problem of defining thephiccd ch,araCterZsticsof themaximum icing conditions for which ico-p:revcntionequipaent willbe wcpocted tO zOVidCd.q,?,te protection.Apyrocifi.tion,is extended.to the Army AiiaXorcesjthe U.S.TJe.,
2、therBWeaIIJ r.ndUnltud A2iALinus, Inc. fez-their activecooperation in tho research program. In pa.iticularjthe servicesof United Air L.nosC2ptain Carl foC0 to 6.o grans per square centimeterhourModorato ico . . 6.o to.12.O gmms pesquare centimeterhourHecvy ice . ., . 12.0 and over grans per square w
3、ntineterhmrThe values cf collecticnlefficiency f cyl = (3.0-1.5) 0.72= 1.08 grams per clnbicmeterThis close agz”eementwith the measured value, though partlyfortuitous, is a faiz-lyfloodindication that the water content ofthat portion of the cloud had not been a.pprecie.blydepletedbymixing or precipi
4、tation although the convection extended to a,highaltitude and very low temperatx.rre.Themethod just described for calculatin the liquid Watercontent of cu,mulusclouds may be applied,alsoto stratus and strato-cumulus clouds when those are formed by active mixing:within thesurface turbulence layers si
5、nce such mixing tends to ;moduce a.condition of constant total we,termixing ratio and adiabatic lapserate. Clouds of this kind have been discussedinreference 6 whichincludes curves giving the liquid wa,iercontent in terms of thecloud-base tempe”a.tureand the heiglta,bovethe cloud base foincloudsform
6、ed adiaba,tica,llyby convection or turbulence. Observationsmadeduring the 1945-46 season indicate that the observed values of watercontent reported in reference 6 were sonewhalttoo hih$ sce theywere obtainedby the dew-pointmethod and are su.bJectto sllingProvided by IHSNot for ResaleNo reproduction
7、or networking permitted without license from IHS-,-,-. .-.-.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-.-The second casementionedpreviously, the situatcm,in whichmany snowilakesare present, has been treated in :rwfw?ence“i,inwhich it in Ehcmn th
8、at stabls precipitating, warm-front”typa cloudsystomsdo not, in enmal, contain liquid walxm dropb at below-freezint:temperatures uxcept in tho imwdiate vicinity of thefreezing level., erie.gce ding tho .5-4 season j.nd,ica;that the discussion given therein apylies in genera.i;othealtostatus-ti.nlbos
9、uatlscloud eystem asociated with cyclonic stormsProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-14 NACA NO. 1393.=. .even thGugh tl-m,j?rcmta.1structx?ramay be confused and indefi.nite.In the jird C.SC,rasaindensity of the cloud mass, iTraceLi+t!Mode
10、rateHeavy,LYGi iclouds o- 0.11 TrZMm ,; 37; I o oassured 0.I.2-o.68 qit Ii olo i () I ooTrace 14 0clouds O.oai.kg Light 135115i2oCWsuncddianeter, O.0-1.00 ModerateI I317 nicrons.1over 1.OO1 Heavy 1 0 I 2 ; 2. The data.in the table show that out ef 232 cases the two scalesagzzeefor 18o case (78 perce
11、nt), and that the alto.rma.tedefinitionindicates e higher iciv.gi.tnsityin 40 cases (17 percent) andlower intensity in 12 cases (5 percent),It should be noted ttmt the data,of figure which were used tochoose the assumed values cf drop size weie also used to verify thevalidity of theresults. If these
12、 data,do not constitute a,representa-tive sanplc, the degree cf eqyeenent indicatedw,illnot lo attainedin general.It is fully realized that more accurate and dependable estinateof the intienaityoficing conditions could bo nade if the.?drop sizecould be predicted, end therefore that tiledcvelcpientof
13、 methodsof predicting drop sizes is desirable. Untl such nethds arediscovered, however, the nethod proposed herein for estirm.tingicingintensity on the basis of an inensity scale”based on arbitrarilyassuned values of drop size offers to netooralcgists a p-r.ctic,landfairly eliciblomans of dealing wi
14、th the problm of foi-ecastingtheilltf3TWitj- of icing conditions.The foregoing discussion has beenba,sedon the scale M icingintensity defined by the Weather Bureau in roference12. This typeof definition is pi-dbablm good m c.zmbe devised as a scalo to beProvided by IHSNot for ResaleNo reproduction o
15、r networking permitted without license from IHS-,-,-18 NACA NO. 1393umd for gmeral puiiposesin the dispatching Gf aircrfit and rePrtinhea.vyicing conditionsare not likely tobe encountered continuouslyfo more Jan 2 to 3 minutes.TheProblem of Defining the Physical Characteristics of IcingConditions fu
16、r -thePurpose of the Desj.gnof Ice-ProtectionEquipmentTheseeond major problem in this investigationis that ofdefining the sigm.ificantproperties of the most severe icing condi-tions likely to be encountered in the coti-seof all-weather transportoperations in a given area during a.particular season.
17、The ftillowingremarks refer to conditions in the northern half of the United Statesduring wintei-except when other areas or seasons are specificallymentioned, as for oxe.mplein the disclwsiotiof tsunmercumulus clouds.Maximwn icing conditions in CUM,UIUSclouds.- It is seen byreference to fie 5 tha,tt
18、he heaviest icing conditions observedin cumulus clouds are much more severe than any experienced in layerclouds. It follows therefozethat the kwa.viestpossible icing condi-tion, chosen without regard to its extent or duration,may be expectedto occur in cuzmalusclouds. As mentioned in the preceding s
19、ectionthe m,aximwnliquid water content within the subfreezingcumulus cloud may be determined by ca.lcula.tin.gthe freeportion of a.water producedProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-20 NACATN No. 1393”.,by adiabatically lifting a.mass af a
20、ir from the cloud-base level.The observations of icing condition repwrted herein were all made during winter and spring trisituations in which +We temporaturw atthe base of tileclouds was near freezing or lowei?and in which thovertical extent of the cloud development did not often exceed 60CKlor 8i)
21、o0feet. If these conditionare taken as represcntativoiorthe icing season in northern UnitedStates, it would apyoar reason-a.hleto accc-pta cloud-base temporaturc of2 F and vertical extentof 8000 feet as rpzwsonting the maximum cumulus icing conditionlikely to occur witiiappreciable frequency in nort
22、hern United Statesduring winter. Under them conditions, the calculatedmaximun liquidwater concenix-a.timis 2.5 grams Dcr cubic meter and the ccrros,ponri-ing temperature is approximately fi” F. Tho corresponding value of?neaeffectivc diameter was est+aatcd frcm the data in ftgure . Itwas noted that
23、the seven observationsof mere than 1.2 gmms per c?bicmeter water content all had meameffective drGp diameters,inthorelatively namow range from 17 to 23 microns. Th5 avtizae of khcscobservations, 20 micronsj was chosen as the probable value of mean-effectiva diameter corrGspmding to a maximum water c
24、ent.mt of 2.5 .grams per cubic meter at a temperature of (ICF.It should be recognized that tiefaregoingmax-imumicing gcmdi- “ti.on was deried rom an assumxlcloud-basetempcratare of 32 F. lf “wazmeweather condlticms are consiered, the maximumicing conditionincreases considerably. As an example, the c
25、onditions in the upperpoiition of a tall summer cumulus cloud will be calculated. Eluri%ceconditions are a.ssweiias fellows: pressure altitude, 400 feet,tempera.twre,90 l?; dew point, 75 l; mixing ratio, 19.0 gm.ms ofwater vapor per kilogram of dry air. If ti sface air is l,iftd.adia.baticallyj cond
26、ensation occuzzs at 600 feet pressure altitude and71.5 F. Supyose the cloud extends. to 23JO00 feet pressure altitudejust %efore precipitationbegins. The ondition a.t22,000 feetwould be as follcws: termperature22.5 F; liquid m.tei+ content, 7.6grams per cubic meter. This representsan extremely sever
27、e Icingcondition; in ract, if the water contentwere reduced by 50 jmcentdue to precipitation and mixing, it would still be an extremely severecondition. There is reason to believe that such conditions occurfrequently in warm, mGist climates where convective showers are commonas along the C%lf Coast
28、in summer. Tfiesevere icin conditions, h5w-eve, are eharply limited hotlhin space and,the, and therefore wot.iLdvery ra.relyle encomtered unless deliberately sought. coectonwtthe problem of d,efiningthe msximum icingcondition in which an ice-preventionsystetislould.be expected toprovide protection,
29、it is necessary to define the mast severe icingcondition likely to be met with a g:-venairplane ra.thei-than the most -.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NACA TN No”.1393 21,. ,.,.-seveiae icing condition which can ever occur. This pres
30、ents a rbler.1. .sinilar to that involved in tho design of airple.ne structures tcresist turbulence. For exmplej it,:i-noocnsdered necessary todesi airplanes to withstand the condi%insocctrfin in tOrndoS,and yt such to.rm are by no n-cans,raye:inth UnitedStates. Thehomiest icing conditions,.though m
31、uelllarger tine.lxxnados, are stillquite .imitedbo%il”inextetit”a.lddura.tionain,ue parts of Cloiadsystems which ordinarily cwneasi.lyhe racoizcd and avoided byexporiencod pilots. Thus, while thwe is reason to lmliove that theconcentira,t.ionof supercooledm,ter near the tos of hmlus cloudsnay occasi
32、onally rec,ch 2.7 gram pm Guiceterin winter in the“PacificlTorthwestand 6 hi-.7 grnns yez-cubcneter in sunner nomthe Gulf Coast, tt is estinated that he hi,gihestvqliielikely to be“encounteredin the course of all-weather transport operations inthe United States is about 2.0 grams per cubic nter, Tho
33、 corre-sponding os.tira,tedvalues of uear.effective drop dianeter and terJper-ature are 20 nicrons and 0 F. Tho nest probable duration of iligh.t(at160nph) in this condition, if it is encountered, is estixa.tedto be qhout 1 ninuto, cnd the nsxirxm duration a little less than2 uinutes, as will be sho
34、wn bc?lOW e rel,tiOnbtween fntnsft:jand naxj.uunextent Of icingconditions.-Becau.seof the facts that heavy icing=onditions wereobserved only in cumlus clouds andths.tcunul.usclouds are G.lWGYSrather linited % horizontal extent a stuQ of the data was made inan effort to define a.relation between the
35、extort antiintensity oficing condj.tions.lTnforturratelyjcla%aon the linear dincnslons ofthe icing conditionswere not obtained. The duration of flight incontinuous icing was therefore used a,sa nea,sureof the extent ofthe conditions although in several cases the airplane was flowm backand forth in a
36、 single cloud torna.tionthus giving.rise to largerdurationtha.nwouldhave been r6quired for a straight flit.Figure 13 shows the relation %etween the duration of erods of flightin continuous icing conditions and the average liquid water contentduring the periods. The plotted points e.rofor individuall
37、y olservedcases and the line represent the estirmted relation tetween averagewa.tevcontont and ?.?exinuiidumtion. It is fully relized that thisesttiate is uncertain due to the linited mount of data upon whichit is based. It should le of sone value, however, in indicating inat least,a roughly qua.n-t
38、ita,tiveway the inVei”serelationship whiclexists between a,specified icing condition and tl%eprobable d-ura.timof flight in that condition. In the application of these results tothe probleu of evaluating the requirements for ice protection in ,1-weathe”transpoi-toperatior-s,it should bc reuez;beredt
39、lna.ton theresearch fiedrcws.- Another important aspectof the problem of the definition of CXiDlnj.cingconditions concernsthe lazzgestdrp diameterslikely to bc cncountored and the probablecorrospondin;valu.csof water content. Exe.mina.tionOf the data.ilifigure 7 indicates that in abcut.1 obscrva.tio
40、nin 50 the mean-effec-ttve diameter is over 35 niczwns. T%is limited amoluntof da%. forclouds with large moan-efsectivodrop diameter indicates that.thewater content is likely ta be low, 0.25 gam per cubic mot= or less.It should be yointed out.I?uwover,that the La.ta,in figure 5 are inProvided by IHS
41、Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-24 NACATNNO. 1393 . . .Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-. .inCOndtiOnSVGUallY OCCW O?Ilyinculus Clouds,but collditis in the i:l) rts un.s;.d.ythick stra
42、,tusor stratcmumulug layers orrasi.oaa.llyreach moderateintetisity5. Icing conditiorisilicumulus iouds are highly variable andin the upper pOitionSof tall cloud-snmy be ver.vsevere.6. During winterj heavy icing conditionsare not likely to heencountered con-tinuouslyfor more tlxm 2 to 3 minutes.Analy
43、sis ofthe a.va.ilahleobservationaldata.sujjplementedbycon”sidera.tionaof the physicalprocesses involved in the formationof icing conditions has led to the following tetativeestimates ofthe most severe icing cond.iticmsltkely tocourse of all-weather cra.nsport0p62?ZLtiO?lSDuration Liquidcloud tme 160
44、 mnh) water. content,Ctmlulus 1 minute 2.0 #n/m3stratus or 20 minutes 0.8 gm/m3stratocumulus or longerSti”atusor 20 minutesstratocumulus op lfmwjer 0,5 gm/msbe encountered in thein the UnitedAveragedrop diameter20 miCrOnS15 microns25 micrsnsAmes Aeronautical Laboratory,Na.tiona.lAdvisory CommitteeMo
45、ffett Field, C%lif.for Aercma.uticssProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-26 NACATl!TNo. 1393 ,.The true tmaperalturesubtracting tiefallowingWhen flying in cleay air:ATof the undistmrbei air was cbtain.bycorrectionfrom the observed tauycr,a
46、t?xa= 1.7”7am-)where AT is the correction in dcaes Fdcrcnhclt, and lly isthe true airspeed in N51OS pm hour, Thc valuo of tke constanta must be determined experimentallyfor any particular instcll,a-tion. For full adiabattc compressionat a stagnationpoint a = 10;for the particular installationdoscibe
47、dheroin a = 0.93 kO.03.This value was determinedby making mccessive runs at varicusspeeds at 3.constant d.titude over the same arm in a.pyroxima.tc?l:.homogeneous air.When the al% contiinsllquid water drops thsre is appar%tl.yno accepted theory for calculating the temperature rise. ymmmctcr suspended cm a cable bc?lowthe blimp was used to make a sounding trough the clcud Iaycr ,just9.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
