1、REPORT NO. 745TESTS OF CONVENTIONAL RADIAL-ENGINE COWLINGSBy RTTSSELLG. ROBINSON and JOEN V. BECKEE.SUMMARYThe drag churacteristica oj eight radial+ngine cowlinghare been determined orer a wide speed range in theNA12A 8#oot high-speed wind tunnel. The pre=ure die-ti-ibution ocer all couling uxw meas
2、ured, to and abore thepeed of thecompressibility burble, as an aid in intwpret-ing the jorce tats. On.e-fifi-scale models oj radial-tmg”ne cowlings on a uing+mcefle combination were usedin the tekg.% speed at which the eectioe nucelle drag abruptlyincreaeed owing to the compressibility burble wai?fo
3、und toLwy jrom S1O miles pm hourfor one of the existing cowlingshupe to l miles per hour jor the beet shape derel.qxdas a remdt of the present investigation. The correspondingspeeds at 30,W0 jeet ahiiude in a. standard atmosphere(a l?) are fi80 and 430 miles per hour, respe -circular, closed-throat
4、turnd. The flow in the testsection has been found by surve s to bc satisfactorilyzsteady and uniform both in spe d and direction. Thetiirspecd is continuously controllable from 76 to. morethah 500 iniles per hour. The turbulence, as deter-mined by sphere tests (reference 5), is approximatIyequivalen
5、t to that of free air.The radiaI-engine cowkgs were mounted on a nacelIewhich was mounted centrtdly on a wing of 2-footchord and NACA 23012 section. The wing com-pletdy spanned the teat scwtion of the hmncl. ThecowIings and the nacelle were one-fifth thu size of thefull-scale cowlings and nacelle re
6、porlxd in reference 3.The wing was metal-covered, unpaintml, and amody-namically smooth; that is, further polishing woukl.FmuBB ?I !i.-K-.- . . J- . .L- . . . . . . . ;:FIGLRI 4.-CUWIIIU profiles.were chosen as the largest that could be used with the2-foot-chord wing and still maintain nomml wing-na
7、ceIle proportions. The ratiocowling diameterwing chord = 0.43for the model is somewhaL larger than for averagepractice but is within the range of prment+iay instal-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-HIGH-SPEED TESTS OF CONVENTIONAL RKOIA
8、L-ENGINE COWLINGS 289lations. The center Iine of the naceIIe lay on tlchord Ike of the wing. The fore-and-aft position the nacelle was such as to locate the propelIer, hathere been one, 40 percent of the wing chord aheaof the Ieading edge.The five cowling-nose shapes (fig. 4) scaled dowfrom the corr
9、esponding full-scale cowlings eraployein the investigation reported in reference 3 me desiinated by the numbers used in that investigatioNose 1 was moditkd progrewhdy by cutting back tkmger radii at the Ieading edge. Noses A, B, and were designed as the tests progressed. They have thsame over-all di
10、mensions as nose 2 but have ditlererintermediate ordinates. Figure 5 presents photographof nose 5 and nose C. A bIank nose with a squalcorner rind the same over-aU dimensions as nose 2 waSISOtested. TabIe I gives the ordinates for all thcowIing noses tested. ,.TABLE I.VALUES OF R IN INCHES FOR EIGHT
11、 MODENOSES OF 10.4MNCH DIAMETER COWLINGSeeflKs.1and 416.)ag.m.4Q.):gL40MZ4QEmseveraltubes were located near tho point of peak negativrpre9sure. The locations are given in table 111.TABLE 111.LOCATION OF PRESSURE ORIFICES ON COWLINGS. SW fig. 11, op.Nom 1 7 :.2 .3 4 6 6 7“(h O% .(;.) (1% (k.) (i:.) (
12、h (1%) (i: J (i:.) (1:.) (/:.1 (f%) (1:.). -. . ,. . . . .L. ,- 0.02. . _. .23 ;$ ?: iti !% ;g ;g fi f; fi :; :; :4. . . _-_ 0 484 .24 : 4.s : L 16 i%s. 13 i%:%J , H M 5.a86.19;:_.70 *4Jh Is.26 .m 1.16 478M 6.19. 4. w 226 hoe 3.00:;: onlynose 5 was tested with skirt 2. :The lifhe drag, and the pitch
13、ing moment of thewing-nacelle-cowling combinations were measured at-intervals of 30 miles per hour at the lower speeds andmore frequently near the critical speeds. The charac-teristics of the wing a30ne were determined in the stimeway, Pressure measurements on the cowlings weremade simultaneously wi
14、th the force measurements.RESULTSCompressibility effects, such as those encountered athigh speeds on the engine cowlings under consideration,are intimately connected with the nondimensional Machnumber M in the same way that scale effects are con-neated with the Reynolds number B. Mach number.a?o.0/8
15、.0/6.0/4.0/2c.010.Ck78.006.004.002u .fu 33 JU .4U .X1M.60 .70FImnB re 6 shows the rdative magnitude of - . .and a are in-tiles per hour. In some cases, for ease in the drag force of the wing aIone and the wing-naceIle -A,hpeed, mph. at sea level (=”F).24.20.16G./2.08.040 Jo .20 .30 .40 50 -60 .70M(m
16、)a-lr.A.+speed, mph, of sea level i59”FJo Jo .20 .30 .40 50 .60 .70MAAspeed, mph, ut stevel o/co.32.28.24.20f%J6.fzI I . -.08 = -. .-. . .-U4(4 I0 .10 a .30 .47 .52 .80 .70M(c) a-lO.m.6.12%.Ch9.040 .fo 20 .30 .# .m .69 .70M-. .-.(b) a- l”. (d) u-2?.FMCEE 7.EfketNe nacetIe drag for nu+ous nCEWS,skh-t
17、 1. The ticks indicate the uiticd .W.visualizing the magnitude of the speeds, a sde of air- combination (both uncorrected for tunneka effects). .=.speed for st audard sea-levr4 conditions (t = 590 F, a = The drag coefficients for this f and nogativc values indi-cate more than free-stream speed.The .
18、prmsuredistribution diagrams for tho moddsA.%peed m R h, of sea kwdid M, 0.41. (b) NC8C4: a, 1: critical M, 0.40. (c) ha?o7; a, 0% mitcal M, 0,44. (d) Nmc B: a, (P; altIml M, CLfLZFJUURE 1.VarMion wltk apccd of premuru avw top of cowlings*.401 I I I 1 I IL11”1bitiwl M(a) ) II I I 1. I 1. 4-= I L I1.
19、60 I I/I I Ia75 a-”-!: i-t-Hi-H1.20 -,.a.f II I II.60 . I I TI /I 0+i. f“l-t+-.40. I I I 0. 0: I I I IH H If -1U.(J. Nose A CrificofM(cv =0”)-/: 1 , + y L + - . the curves offigure 7 therefore indicate cor that is,“.l.fwherea=33.5460 +t rides per hourThe temperature of the standard atmosphere decrea
20、seswith altitude .to -67 F at about 35,000 feet Thedecrease m temperature causes a decrease in the speedof sound a with increasing altitude and results in lowercritical speeds as altitude increases. At 30,000 feet thecritical speeds for the cowlings tested are lowered to therange of 280 to 430 miles
21、 per hour. since the flyingspeed of present-day airplanes generally increases withaltitude, the danger of encountering serious compressi-bility effects is very real unless proper care is taken indesigning the cpwhng nose.As was to be expected, the cowlings with the greatestnegative pressure (for exa
22、mpIe, noses 4 and 5, figs.10 (c) and 10 (d) had the lowest critical speeds. Mao,as would be expected, the pressure mewurementa(fig. 10) showed huger peak negatfve pressurea forrmgles of attack other than zero. The increment dueto angle of attack was appro%mately proportaI tothe mgle change and was g
23、reater for cowhngs on whichthe pressure already had a Iarge negative value. Thecritical speed should be Iower, then, when a cowling ispitched or yawed, especially for nosea like 4 and 5.The results presented in figurm 7) and 7 (c) confirmthis concision. This” behavior illustea the impor-tance of ali
24、ing the coding with the air direction whenthe, airplane is in the high-speti attitude, mpeciallyif the cowliug is blunt or is near its critical speed.The rapid increase m drag with nosea 4 and 5 at air-speeds below 200 miles per hour for 2 angle of attackg. 7 (d) is uot to br attibuted to the compre
25、ssiblHityburble. The pressure diagrams (figs. 10 (c) and 10 (d)show radicrd chnges in pressure distribution andshow small peak negative pressures at 2 angIe ofattack, indicat.mg a flow breakdown; but, from thefact that the maximum local speeds wwe lee9 than halfsonic speed before the change in flow-
26、 occurred, thebreakdown is attributed to ordinary stalling over thetop of the cowling and not to a compribility burble.This effect is disctl.ssed in detail later.The curves of figure 11 show the way in which thestatic pressure over typical cowlings varies as the speedis increased above the critical
27、speed but fail to show .-. uniform tendencies for all cowlings above the criticaJ ill.The blunter cowIings show a decided reduction in the . .magnitude of the negative pressure coefficients but the . “- ”reduction occurs at a vahe of M appreciably higherthan the critical value. The cowlings of bette
28、r shapeshow a less dedided change in pressure coefficient abovethe crithxd speed and, in some cases, even-an increasein negative presanr e (.-.-. -.- .POFmcm 17.-Voriaffon .wIth peak LUYSSWOcoefEdent ofmitfcal spwd.The total-pressure measurements of figure 12 alsoshow marked effects for the blunter
29、cowlings and smaller “or negligible effec for the cowlings of improved shape.For the blunt cowlings, the lOSS in total pressure islarge ahd occurs almost immediately .after the criticalspeed is reached; for the improved cowinga, the lossoccurs latrr (fig. 12 (c) or is of a negligible magnitud(fig. 1
30、2 (d). Thus the totaI-pressure-tube measure- -.ments frd to indicate the occurrence of a shock on the .improved cowlings, although the corresponding forcemeasurements show increases in effective nacelle drag -with these cowlings. Shocks of a diflerent nature fromthose recorded for the blunter cowlin
31、gs, however, may _possibly have occurred. If the shocks extended aconsiderable c (2) at high lift (noses 4 and 5)” or (3) beyondmaximum lift, that is, stalled (nose 1), as shown by thepressure diagrams of figure 10. This comparison indi-cates the reason why mme cowlings have a greateruscfuI angle ra
32、nge without stalling than others.In the present test aehup, as in the case of actualnacelles near the center of a wing or even of the enginecowling of a single-engine airplane, the relative anglebetween the oncoming air and the nose of the cowlingis increased by the induced upflow in front of the wi
33、ng.The effective angle of attack of a cowIing always beinggreater than the geometric angle, a cowling may stallat a comparatively small an in spite of the fact thatit is a body of revolution with three-dimensional flow.The likelihood is greater when large negative pressuresme present at zero angle.F
34、guree 10 (c) find (d) show the large negotive pres-suree for nosee 4 and 5 at a=O” and the increase ofnegative pressure with angle. The stall is seen tohave occurred before 2 was reached and apparentlya negative pressure of about P= 3.2 was the mostthat could be maintained before the stall cwcurred.
35、(See fig. 14.) Figurm 10 (f), (g), and (h) show” thesmall negutive pressures for noses A, B, and C at a= O”;and iigures 10 and 14 indicate that the rate of increaseof negative pre9sure with rmgle was proportionatelysmaller. than for noses 4 and 5. If a pressure coef-ficieniW of ubout 3.2 is still i.
36、ho limit, these cowlingswill have a wide useful rango of angIe of ttdc whoutstalling. Figm.” 13 corroborates time conclusions inindicating a rise in whereas the drag ofnose B does not rise correspondingly, even at 6, whichwas the limit of the t=ts. Noses A and C undoubtedlyhave characteristics si.mi
37、hw to .B. Theso offccts areimportant not only for controlling the drag of an air-plane for cowing uttitudes other than zero but aIsofor air scoops or any other construction depending onsmooth flow over the top of the cowling.COMPARATIVE DRAG EESULTSThe results presented in figure 7 indicate no largo
38、variation of the effecive mwelk drag with speed unt.iIthe critical speed was reached. The favorable SCR.ICeffccts were balanced at the higher spmds by the un-favorable compressibility effects. The rcsulti show,however, appreciable erencm in effective naccllodrag for the various nose forms, With noac
39、 5, theeffective riaceJle drag was approximately 30 pcuwmtgreater than with nose C. In general, the noses of lowcurvature, low peak negative pressure, and low localspeeds had lower drags and higher critical spmds thanthosG of high curvature and correspondingly high Iocalspeeds. The lower dcin-fricti
40、on drag for the models oflow local spwIds may account in part for the lower dragsof noses A, B, and C. A comparison of thr prmsurc-distribution curves for noses 2, A, B, uml C (fig, 10)shows the extent to which the peak ncgativo pressureswere lowered and the pressure, or the vdocit,y, distri-bution
41、was made more uniform by succmsive chtingegin nosrcurvatur; .The saving in the intqnal drag due to cooling-flowlosses that may bc effcctud by passing rxactly thucorrect quantity of cooling air through a cowling atcvmy speed instead of using opmings Dnd exits of tlxudsize for the entire speed rfmgo h
42、ave hem previouslydiscusmd (reference 3). The results proscntcd byfigure M. show the drag rwluction clue to chunghg theexit opening .frgn O. 25 inch to O. 11 inch. The fticLthat the cowling with the smaller exit opening (skirt 2)shows a lower critical speed at a= 0 suggests that upart of the air whi
43、ch fornwrly passed inside the cowling(with the larger exit opening) now pasws outside the:owling to incrase the 10CWIspeed on tho rowlingmse.rhe increased speed outside the cowliigj or the cqualy.mportant factor of inmcasing angle of rrltitivc windkt the cowling nose with reduced flow though the:owl
44、ing, also appears as a dctrimcntul effect in reducing;he useful angle-of-attack range of a ;owling. lfitbnose 5, the cowling stalled at 1. (Cf. figs. 10 (d) an(l10 (i).) Both the lower critical speed and the smallwusefuI angle-of-attack ,range emphasize the relativeimportance of using the bust possi
45、ble nose shape whenthg internal flow is most restrnctcd, se is the case inProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-HIGH-SPEED TESTS 03 C!ONVENTIONAL RADIAL-ENGINE COWLINGS 299a71; .-high-speed flight with the optimum amount of coolingair.Both
46、the effective nacelle hg for nose 1 (fig. 8) andthe pressure distribution (fig. 10 (a) indicate that thisnose was stalled at all angles of attae inchding OU.An attempt was made to improve the flow over the noseby succvely cutting back the nose to form profileswith circdar arcs of larger radii inscri
47、bed in the Iead-FIGURE 16.-CompsrlSS C4effestive narelle drsg OCnme 5 with skkt 1 and SW 2.ing edge on the assumption” that a radius would bereached at which the flow would be unstalled. Thedrag for each rrmdiiication, howev, was found to belarger than for the preceding condition. The changein cbag
48、with increa of angle of attack for nose lf,as ahown in figure 13, indicates that khe decrease ineffective angle of attack on the-bottom of the cowhgcaused a consiclmabIe improvement in the flow at thatpoint which was not at first counteracted by increasedseverity of the stalI on top of the cowling.
49、The modi-fications to nose 1 were ineffectie, probabIy becausethe sIope of the chord line of the nose decre=. aS thenose radius was increased. In critical cases, it appearsto be much more important to dine the slop-e of thenose with the relative wind than to increase the noseradius.CONCLUSIONS1. The 31ach numbers at which the dleotive
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