1、REPORT No. 860ANALYSIS OF COOLINGON LEVEL-FLIGHTLIMITATIONS AND EFFECT OF ENGINE-COOLING IMPROVEMENTSCRUISING PERFORMANCE OF FOUR-ENGINE HEAVY BOMBERBy FRANK E. MARBLE, hfAHLON A. 311LLEE, and E. BAETON BELLSUMhIARYThe NC.4 haa dewdoped means, including an injectionimpeller and ducted head baj7e8, t
2、o improre the cooling charac-teristics of the $?360-cubic-inch-displacement radial en”neinstulled in a four-engine heaoy bomber. T7ie improvementsafforded proper cooling of the rear-row exhaust-mce seats for awide range of cowl-jfup angles, mixture strengths, and airlane8peed8. The results of$ight t
3、eAe w“th thi8 airplane are u8ed asa baei.sfor a study to det+vmine the manner and the e.rtent to whichthe airplane performance was limited by engine cooling. Bymea.n8 of thi8 analy”8 for both the standard airplane and theairglane un”th en”ne-cooling madijicution8, compam”80n of thespecijic range at
4、particular condition8 and comparison oftie crui8ing-performance limitations were made.The anal8 of Lw./-jlight cruising performance of the air-pfane m“th both the 8tandard- and the ded-en”ne in8talfa-tion8 indicated that the mm-mum crut%ing economy is attainedat the minimum brake 8pec fuel consumpti
5、on when enginetooting under the8e condititm8 is po8et”be. eration at leanzwi.xture8, high altitudes, and large gross weights ms limitedfor the standard airpfane by engt”ne cooling at the paint wherelarger cowl-jap openings increase the power regu.ired for lerel$ight at such a rate that the additiona
6、l cooling air arwilable isinsujicient to cool the engine when developing the additionalu,er. When cooling becomes impossible at the minimum brakespecijc juel conwmptwn, the maximum cruim”ng economy isobtained with a cowl-sap angle of approximately 6 and un”ththe leanest mixture (abore the 8tut”ch.io
7、metric value) giing8atis-fzctary engine cooling.Compam”80n of the calculated performance of the 8tandardand. the modiji.ed airplane indicated that cooling improvementsincreased the mm-mum 8peozj$c range as much aa 38 percentfor operation where wide cowl#ap angles and enriched mixturesare required to
8、 cool the stano%rd airpfune. Correspondingincreases in crwking range were calculated for $ights in whichconditions allouing large increaseg in” cruising economy wereencountered. I%e cooling improvements allow either an in-crease of more than 10,000 feet in operating altitude at a girenairplane uvigh
9、t or a gros8-w and airp-lane weight, lb) where the propulsive efficiency is assumedto be 0.85 and the wing area is 1750 square feet6P 8(w/loo,ooo)/=6.9 X10-5 l+a(. I“o.s.e.7.8.s.4.4 .6 .8 /.0 2.0 4.0 2.35aAEIGuzfr7.-Compnrkon between mmelatfond molbg data for standard aud modfSedeU WWIalarmat 2 (ckm
10、ed)poeftior,levelflightat maxlrnnnrltft-dragratio;rrdnfmmnbrakesottlcfnel coneumptlonforreqnkedpower.The values of the airplane specific range and the valuee ofthe important associated variables are shown in figure 9 forthe reference conditions over the complete range of airplaneweights.RESULTS AND
11、DISCUSS1ONIn the presentation of the relation between the specificrange and the airplane operating conditions as well as in thecomparison of the airplane using the standard- and the modi-fied-engine installations, the specif% range has been expressedas a function of the brake specfic fuel ccmsumpt.i
12、on and oneof the three flight variables: airspeed, altitude, or grossairplane weight. These relations among the variables af-fecting the specfic range of the airplane are represented by”three-dimensional curves.PERFORMANCELIMITATIONSIMPOSEDBY COOLfNGREQUIEEMENTSAND ENGINEOPERATIONThe natyre of the p
13、erformance Iimitdiona imposed by theengine performance and the cooling requirements may hounderstood through graphicnl solution (fig. 10) of tho si-multaneous equations characterizing cruising with propcengine coding. For operation at a given altitude, airplnncgross weight, and cowl-flap angle, the
14、apparent power re-quired is related to the indicated airspeed by equation (4)and the specific range may be found in terms of the indicatwlairspeed and the brake specific fuel consumption. Thisrelation, plotted three-dimensionally in figure 10 (a, isterminated by the minimum attainable brake specific
15、 fuelconsumption, as indicated by the hatchwi area. Inasmuchas the engine power is known, the engine speed, the fuel-nirratio, and the cooling-air pressure drop (figs. 1, 2, and 6,respectively) can be found for a given indicated airspeed Rndbrake specific fuel consumption. This information is suffi-
16、cient for calculating the temperature of the. exhaust-valveseat according to equation (6) or equation (7) and conse-quently any point of the surface representing specific rrmgoat a given cow-l-flap angle (fig. 10 (a) has a definite cylindcr-head temperature. Curves of constant hctid tempwwturccan th
17、en be drawn on the surface, as shown in figure 10 (b).The mtimnm cylinder-temperature criterion prohibited safuengine operation. in a certain area of the specific-range surfacowith the restrictl.on most severe in the vicinity of tho stoichio-metric mixture where the maximum combustion-gas tempera-tu
18、re occurs. The hatched area of figure 10 (b) must t.hcrcforcbe disregarded because of cooling difficulties. A similarsituation exists for each cowl-flap opening; tlwsc othersurfaces and their limiting temperatures lines arc shown in(2) the portion for which linit.-ing head temperature exists for all
19、 cowl-flap angIcs; and (3)the normal cruising economy surface at full-open cowl flaps,continuing untiI limiting heacl temperatures arc reached.Although excessive cooling is available at R1l points withinthis region, the most economical cruising conditions arcrepresented by the upper portion of the s
20、urface and conse-quently only this part need be considered.The operating altitude or the gross weight, as well as theairspeed, could be considered individually indepcmdrnt andsimilar surfaces would be obtained. Surfaces of this typoare shown in figures 11 to 13 for the standard-engine installa-tion.
21、 The extension of operation toward high speeds, ahi-tudes, or gross weights will be eventwdly limited by cmginopower, whereas the limitation at rich mixtures (largo brakoProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-(aEFFE OF ENf31NE-COOIJNG IMPROV
22、EMENTS ON CRUISING J?=FORN OF HEA= BOMB 421.A OKIermion o+whibited b v-r“excessivehead temp er;tue2( i Cowl-flap on?fe required8.$4fo obtain limiting headL imi f e d head fern ra *ureaFtemperature .7for kt?rioue cowl- lap an.s ,.4, pg: / .-% -,4A-,. b-. - - Surface on Micb limifing. -.-.:.-:. ,. . h
23、ead enpera ture- -:.=: ;+.-: is attained .2 . .,Fbps are full aen -I+w /1 ,., 1/ r u ”/ etiine is overcabfed(e, “w%(a) Specbbranse mrfocefff constantcowl-flape. (b) SpeebWrmUemrfnm for mnstant cowl-flap e showing curves d constant heed(o) speelae-nulgESurfacesfor vdm Colvl-ffapangles. temperature.(d
24、) Completespeclflc-ransesnrfwa.FIEBE10.Developmentof smfaces showtusspeclf!omngewith properenginecoolfnsasfnnctfonofJlfghtandensfnemrfnbk.specific fuel consumption) is very indefinite. Operation atvery low speeds is aerodamicall unstable. Becausethese Limitations are indefinite and of littIe importa
25、nce herein,the figures are terminated arbitrarily at low speeds and richmixtures.For a given brake specific fuel consumption, the airspeed(fig. 11), the altitude (fig. 12), and the gross weight (fig. 13)are limited by the availabe cooling facilities. Coolinglimitations of airplane performance are mo
26、st severe nearthe stoichiometric mixture; that is, where the maximumvalue of the combustion-gas tempemture h cotered.Satisfactory engine cooling can usually be attained at en-riched mitures but can or cannot be attained at rnkturesleaner than the stoichiometric, depending on the severityof the cooli
27、ng requirements and on the mixture at whichengine operation becomes unsatisfactory.V7hen a cooling knit exists, it can be physically observedby noting the response of specfic range to the progressiveIeaning of a rich mixture at a giv airplane speed. ?n_the fuel-air ratio (or brake specific fuel ccmm
28、mption) is de-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-FQg$Qo 10 2f7 30 40Fuei weigh f, W, fhousan d lb(8) Standard+ngfneinablletfon.(b) Mcdltled+ngtnefnetrdletion.FIGUEE lS.-Effect of improvedeoolfngpfrformanwon esthnat.ederufaingrangeateever
29、aleltltndeq thexetfmlestfmateabaaedonnraxhrmmcalanlritedsLMcfdcrengaforboth hr.Walletlonairpleneweightlesefuelweight,W,WOpounds.Calculations of level-flight cruising range for a basic air-plane weight of 90,000 pounds (airplane gross weight lessfucI weight) made for various fuel weights and alt.itud
30、es arepresented in figure 18. The results of the calculations indi-cate that improvement as great as 17 percent in the cruisingrang e of the airplane may be achieved by the use of theNACA injection impeller and the ducted head baffles andthat the greatest improvement in range results from thepossibi
31、lity of using lean instead of rich mixtures.Extension of operating conditions,-In general, the air-speed at which the specific range was optimum was unaf-fected by the cooling improvements, The values of airspeedfor which cooling of the hottest rear-row exlutusbvalve seaLis possible have, however, b
32、een grca,tly extded. (See fig.14.) The operating altitudes and the airphum weight thtmay be used without exceeding the arbitrarily chosen limii-ing temperature for the rear-row fxhaust-valve scats of560 F have been markedly increased (figs. 15 and 16). lhisimprovement is shown more clearIy in figure
33、 17 where theapproximate hrniting altitude of operation for various vttlucsof airplane weight may be obsened for both the stan discussion,pp. 25-30.7. iVyatt, DeMarquis D., and Coma& E. AllIiam: An Investigationof CoW1-Flapand CowI-Outlet Designs for the B-29 PoirePlant ,Installation. NACA MR No. E5
34、K30a, Army Air Forces, 1946.8. Katzoff, S., and Finn, Robert S.: Determination of Jet-BoundaryCorrections to CowIing-F1ap-Outlet Preesm by an ElectricalAnalogy Method NACA ARR NO.4B23, 1944.9.Pinkel, Benjamin: Heat-Transfer Procee.sesin &-Cooled EngineCylinders. NACA Rep. No. 612, 1938.10. Pinkel, B
35、enjamin, and Rubert, Kennedy F.: Correlation of WrightAeronautical Corporation Cooling Data on the W335&14 Inter-mediate Engine and Comparbon m“th Data from the Langley16-Foot High-Speed TunneL NACA ACR NO. IZ5A18, 1945.8429515%29Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
