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NATIONAL ADVISORY ;:GOMMITTEE
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FOR AERON~:@&~$S
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BOMB DRAG AT HIGH SUBSONIC SPEEDS ; .:,-, ,.. ..-. .. .. .... . . . .._=.... .
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%% F’, “ ;J” ;: * U Vergleich zwi&?&en&4~b’wurf- und Windkanalversuchen hinsichtlich des Widerstandes vonB*omben bei hohen Unterschallgeschwtidigkeiten” .
Nr. 1570 Luftfahrtf orschung, Forschungsbericht ,Deutsche ,, -.
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Washqin@Ok-=y==w=--=--=-= May 1948 .
-v.- * u.”; *<* NATIONAL ADVISORY COMMITTEE FOR AERONAU121CS TECHNICAL Mf!MORAN’UM NO. 1186 ,- . . . . “~i - -.-... .— COMPARISON OF DROP ti’ WIN&TUNNEL EXI?ERIMEN’1% ON BOMB DRAG AT HIGH SUBSONIC SPEElX#3 ‘, Qy B. .G6thert SUMMKRY The drag coefficients of bombs at high velocities (the highest velocity of fall was 97 percent of the speed of sound) are determined by drop tests and compared with measurements taken in the DVL hi~h- speed closed wind tunnel and the open jet at AVA - G&btingen.
1. PURPOSE OF THE DROP EXPERIMENTS 1. Limits of Mensurability in Subsonic ~.indTunnels The upper limit of the a$rsFeed in subsonic wind tunnels at which it is no longer possible to carry over wind-tunnel measurements to free flfght is that velocity at which the supersonic field originating in the flow past the model has spread out to the flow boundary. It is not known how closely this upper limit can be approached, that is, by what amount the airsyeed must remain smaller than the limlting velocitp.
In the closed D’VLwind tunnel.,the variation of preesure on the wall and the v@ocity variation along the test length are measured along with all model measurements taken at high airspeeds so that it can be established each time leyond question when the speed of eounfi, and, therefore, the lmge~t pos~i,ble *“Vergleich zwischen A~wurf- und Windkanalverauchen hinsj.chtlich d.eslJider@xandes von Bomben bei hohen Unterschall~eschwlndi~keiten.” Zentrale fi.ir wissenschaftliches Berichtswesen der”Luftfahrt~orsch~g dee Generalluftzeugmeisters (ZWB) Eerlln-Ad3ershof, Forschungsberlcht Nr. 15’70,April 17, 1942.
lThe DVL would like to take this opportunity to thank the varigus establishments, the Rheinmetal.1-Borseg F5.rmand the Luftwaffe Experimental Station at 32eeneti’nde - Vest especially, for their support in substantially expediting the drop expe~”iments.
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II’ k”* 2 NAC!ATM NO. 1186 airspeed are attatned.. l?orpurposes of evaluation, measurements in the proximity of the upper velocity limit are discarded from time to time. No equiva3,ent sign for the limiting velocity that can be reached in wind tunnels with open test lengths is known.
Since there is no -prospectfor acceptable measurement in wind tunnels in the immediate vicinity of the,speed of sound, it is necessary to extrapolate in this remge from measurements made at lower velocities, However, this requires high reliability of measurement, especially in the critical velocity region,that is,in the vicinity of the limiting airspeed, since, aside from the magni- tude of the individual measurements, the slope of the experimental C1.aiWe iS important, tOO, . ‘“ ,, 2. Correction Factor for the Flow Velocity in Subsonic l?u~els Wind-tunnel experiments hs,veshown that the air drag of the models tested rises considerably if the airspeed is Increased to the neighborhood of the speed of sound.
Tillsdrag blse of the models, according to known measurements in wiu.dtunnel~, has,been larger, in general,’with clo~ed test lengths than in open arrangements. ‘l’his difference is understandable, too, as long as no velocity correction factois’are used as a result of the model obstructing the test length, As a result “of the obstruction of the test length, the air in a closed tunnelnust flow past ‘themodel with a higher velocity than in an.
empty test’lAngth, which produces higher drag and with this, too.,, larger drag coefficients are simulated at velocities that are.too.
low. Conversely, the air in em open jet canbe deflected more .easily thanin the unbounded air space so that the effective flow velocity becomes “smallerand the drag and drag coefficients appear too small.
In the operation of the DVL high-speed wind tunnel a correction factor method was discovered whiah yermits the calculation of the velocity correction factor for closed wind tunnels at high airspeeds, too, in a simple manner with ~he help of the dynemic pressure at the wall measured simultaneously.
Since this semiempi.ricalcorrection factor method can not be taken over for an oyen wind tunnel without further development and, at present, no other metinodhas been worked out yet, a velocity correction factor has bean omitted, up to now,, -, for the open arrangement.
This omission of the velocity correction factor in open Jet experiments, for which only a smaller correction is known to be necessa.qythan for a closed wind tunnel with the same obstruction of the test length, is justified as long as the dimensions of the model which uust be tested near the syeed of sound are chosen small enough.
However, there is no accurate knowledge of what are to be considered sufficiently small.dimensions of the model.
——— ‘Compare B. (lothert: “Windkanalkorr6ktmen bei hohen Unterschall- geschwindigkeiten,” IG&Tagmg~bericht 127, p. 113.
NACATM No. 1186 .
3. Checkhg the Wind-Tu~elResults By Drop:Tests ; .. —.——— ., . ..- .,. - Ta --,..-. ,,4 .. . . . . .. ... . .. . ,.. ” . .
Although valuable evtdence concerning the magnitude of the influence of the”sti’eamboundary andthe li~ting airspeed is acquired by systematic witi-tunnel exoeriments~,’ for ex~ple with large and with %ry ’smallmodels of the same form in the.seinetunnel, there exists the pressing necessity of atleast knowing ‘thevariation ‘, of the aerodynamic forces for several bodies in un3Nnited airspace and thereby ~ossessing, a means of examining the reliability of.the wind-tunnel method ‘ofmeasurement.
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In the present report we will deal with an attempt to determine the drag variation of bonibsat high subsonic speeds by drop tests of original bombs from an airplane. Bom?x3 were selected as test bodies because there were sufficient numbers of them and the supports and release installations were available in guantity$ also.
According to holifavorably these f~.rsttests run off, these tests will be extended to other bodies such as rectangular wings, sweptback wings, and so forth. Among other things, several fallinR bodies axe to be selected with the correct weight and dro_ppedfrom the right altitude to exceed the speed of sound in order to obtain evidence in the same range covered in wind-tunnel experiment~.
II. PERJ?OHMANCE OF DROP EXPERIMENTS The drop tests were carried out by,D~ with the support of the Rheinmetall-Borseg firm. The measurement of the tra.~ectorv was made by the measuring-squad’ of the Luftwaffe research establishment at Peenemiinde.
Several original bomb’sSC-50”and SC-2>0 with and without tail fin struts (fig. 1),weie”released apd observed. The bombs were equipped with flm-es (flare dimensions 190 X 60 millimeters di~e~er) which were installed on the bomb axis behind the corresponding cut out of the %omb tail in ,theS0-50 bombs, somewhat off center in the angle between two fins in ‘th6. SG-250 bombs.
‘B. G?$thert: “HochgeschY~indigl<eits-Untersuchungen an symmetrischen l%?ofilen mit verschi.edenen Dickenvarh31tnissen im DV&Hochgeschwind- igkeits-Windkanal (2.7 m ~) und Tergleich tit llessu.ngen in An.deren Wtndkar&len,” Forschungsbericht Nr. 1506, p. 17.
G. Richter: “Einfluss der Mcdell@%8se in Hochgeschwindigkeitskatialen (Messungen an vier verschieden giossen F1igeln von gleichem Profil imDVL-Hochgeschwindigkeit%-Wind&nal) ,“ LGkTagungsbericht 127, p. 121.
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NACA TM No.1186 The bombs were dropped from a height of approximately l-lkilometers and their trajectories recorded with two photothe- odolites set up on the ground.
From time to time after drops a balloon ristng from the,ground was ob~erved to deterrn3,ne wind intensity and direction. With these measurements the true velocity” relative to the air was determined. To continue, during the ascent and descent of the airplane fromwhich the labs were dropped,.the air temperature was measured at various heights with en electric thermometer calibrated prior to the experiment to deteaine the air density and the speed of somd. A median curve was drawn through the ex~erimental temperature points; the experimental points are scattered within 2°0r 3° C of the curve.
The uncertainty, due to this, in the detenuinatiQn of syeed of sound, therefore, is in the order ,, of 1/2 percent.
The choice of the altitude of?release oi’11 kilometers is based on argyments which are explained in detail in the following section III.
111. INTERPRETATION OF EXPERIMENTAL RMXILTS The”evaluation of the phototheodolite measurements gives, as raw data, the position of the bomb at intervals of 1/4 or 1/2 second.
At eve~ instant, the path which the bomb GoVei”edin 1/2 second was calculated by means of’the detximoination of position previously made.
This path for each 1/2 second shows the bomb velocity (measured in meters pe~ 1/2 sefi)which was plotted against the time elapsed and averaged by a suitable curve,’ The ”&zperimentalvalues for acceptable measurements of the velocitT lie within 2 or 3 meters per 1/2 second of the average curve.
By graphical differentiation of the velocity-time curve, the acceleration b~B/at acting on the bomb and from that,the air drag was ascertained from the followin~ equation.
Where VB path velocity of the bomb v’s velocity com~onent in the direction of gravity NACA TM No. 11861 . .
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— .G .-bomb weight F bomb cross-sectional area ‘ tD2. ,.. .
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P~ air density It is seen’from this equation, that the.determirlationof drag is more inaccurate, the,la~ge~ the acce~eratiol~ of tilebomb .?3VB@t d~B relative -to,gravity. For exsmple, if ~ =9.0 meters per second2 and (vSfiB)g = 9.5 meters per seconds2, then the va?.uegoverting the drag is the difference 9.5-9.0 = 0.5 meters pei+second2, Smal~ errors in the determination of the acceleration bVB/&t appear many times larger inthe determination of dra~ in this caae. The range of high accuracy of measurement ~ossiblly,therefore, depends”on the velocities which equal the terminal velocity of the bomb or fall free Of acceleration. To exbencl’this favorable range over the largest possible portion of the drop curve,the bombs were released at the altitude of 11 kilometers previously mentioned, so that the bombs reached their highest velocity at an altitude”of k 01: ~ kilometers and then were decelerated, iwtead of accelerated, on falling through the lower altitudes as a result of the increasing air density.
Corresponding to the difli’erent orders of accu~acy of measurement, the following three ranges of measurement are differentiated in the description of the results and are made reco~nizable on the graphs by individual point designations: ,..
1. Range of small accuracy of measurement.- The acceleration of the bomb is even larger than the a~rarily fixed limiting value of 5.0 meters per second2, that it is at the highest elevation of the drop. Not more than a fe~fpoints were evaluated, trom time to:time in this range, when a good”S-traight variation of the’measurernents pemitted this.
2. Range of increasing Mach number4--- The bomb accele~tion here i.s al~~ady smaller -than S.O meters persecond2 and falls”off to am — = o, ~ossibly.
This range terminates where’the bomb’attairis its at closest approach to the speed of”sotid’in the vicinity of the limiting velocity.
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3. Range of decreasing Mach number.--Zn this rage the bomb —— _—-—___ acceleration is almost always negativ~~ that is, the bombs are retarded % = The ratio of pathvelocity/velocity of sound .— — NACA TM No. 1186 as a result of the drag so that the highest accuracy of measurement is obtained in this range. This range ends on impact with the ground, Good control of the results is obtained, therefore, due to the fact that each drop is made from a high enough altitude so that the range of high Mach numbers is traversed first wtth increasin~ and then with decreasing Mach number, Thereby, two different, mutual~ independent parts of a curve are obtained which must fit together.
In the manner described, for each drop only that portion of the drag curve is obtained which is well placed, that is, located in the vicinity of the limiting Mach number. If the drag curve for a larger Mach number range should be determined, the limiting Mach number would have to he shifted according~~. This could be accom- plished ly dropping more models o,fdifferent weights but the same external form. Corresponding experiments on bombs, which are partly unloaded, partly more or less heavily loaded with weighty materials are in preparation.
The accuracy of evaluation can be increased further, if, instead of the graphical method employed here, that is graphic differen- tiation of the average curve drawn through the experimental values, an ‘averageis determined by mathematical averaging calculations and then differentiated. However, it is not to be expected that a considerable improvement will be obtained in the range of high Mach numbers. The advantage of these refined methods of evaluation is seen principally in the range which is termed “The range of small accuracy of’measurement” in the foregoing.
IV. RESULT OFDROl? TESTS AND COMPARISONt iTEWlND- TUNNEL WU13U4ENTS The drag coefficients Cw obtained by the drop tests are shown as functions of the Mach numker in figures 2 and 3. The drap tests made are shown as follows: 2SC-50bombs. . . . . . . . . , . , . . ~0 . , . . . . Infigure2 1 SC-250 bomb without tail fin struts foicomparison. . . . . . . . . . . . . l Oo1nfi~res2and 3 1 SC-250 bomb with tail fin struts . , , . , . . . . . . In fi~re 3 The SC+O bomb used in carrying out the experiment,has no tail fin struts. The original SC-250 bomb had tail fin struts as ,..
;’ NACA.TM,NO. 1186 standard equipment ~n order to stiffen the tail surfaces..: The tall — .-, fin s@uts have a .diameter.of..l6 milli@$e,~@ for.,abomb.diameter of 368 millimeters.
‘, The closest approach to the velocity of sound was made by the SG250 bomb without tail fin wtruts wi.tha velocity 97 percent of the . . .
speed.of sound, ‘All oft,he dnag.curves o%tained from the drop tests “showa very steep increaee of drag on,approaching the speed of sound. This agrees very well with the experimental curves from the’ closed DVL high-speed. wind tunnel which are,drawn in for comparison.
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Admittedly, the wind-tunnel and drop-test curves are displaced. “bya definite ~ount of dzzagfrom one another; however, the increase of drag on approaching the speed .of, sound .showsvery good agreement; the increase ,ofdrag, incidentally, was observed especially “clearly in this experiment.
The measurements from the DKL high-speed wind tunnel, cited for comparison,, h@.vebeen carried out for a model of the &q-2>0 bomb which had a diameter of’123 m!.llimeters. TWO fuse+openings and.a suspension lug for horizontal mounting of the bomb were added.to the model. The variation of drag for the SG>O born%!naGnot been measured in the wind tunnel as yet.
Tlfie. meaaurernents are now being prepared for.5 However, as a result of ‘the~reat similarity between the SC-50 and SC-250 bonlls(compare fig. 1), It is to be expected that the drag curves for the.two bombs would.differ from one another by only a small amount.
In f-igure.4the variation of drag of the tombs investigated in the closed DVL h~gh--speed wind ‘tunnel has been compared with that of the open jet, AVA - Giittinge1106 The experimental curves have been extrapolated somewhat beyond the measured range to larger Mch numbers in.conformity with the slope at the end of the curve.
The experimental curves for the same tombu could not always be used for purposes of comparison of bomb drag in these illustrations. However, sinc~ the bomb shapes are extraordinarily alike (compare fig. 1.),for example, the SC-250 and S0.-500 bombs without tail fin struts have ., .
%Phe report.on the”v~d-tunnel measurements for all.bombs will be published as ’,soon as the measurements on’the model of-the “S&50 bomb have been completed, 6A. Roth: “Untersuchungen von Bomben im komvressiblen ,,..
lJnterschallgebiet”~ AVA-Bericht 41./8/8, September 1941. ~ On the basis of more recent calibrations of the wind tunnel at Gottingen, the experimental results pre~ented in the AVA report had been corrected before they were cited for the comparison in figure 4.
This conversion is In the direction to reduce the differences between the DTL and the AVA measurements.
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NACA TMNo. 1186 The curve~, therefore, can be compared practically the same shape: with one another and be used satisfactorily for the comparison in mind.
The reproduction of’ the experimental curves obtained in the drop tests ha~ not been made in figure 4 because the drop+test measurements agree well with the measurements of the closed DVL high- speed wind tunnel~ (Compare fig~, 2 and ~.)
The conrparisonof the curves shows that the measurements in the open jet do not exhibit the sharp drag increase like those of the closed DVL wind tunnel and, therefore, are also unlike the drop tests. The cause of the deviation may be looked for in the fact that no velocity correction factors were ayplied in the open-jet measure- ments to take care of the effects of the obstruction of the test length by the model, or that the 13eynold~number in the open-jet measurements were extraordinarily low as a result of the limited wind- tuririel dimensions (the bomb model dismeter was 25 millimeters in the AVAmeasuremen-ks! ).
v. SUMIW.RY 1. “Drop tests were made by dropping original bombs from a high altitude and by taking measurements along the drop curve.
The largest velocity of fall in these experiments cuuoumtedto 97 percent of the speed of sound.
2. The variation of the drag coefficients for bombs obtained from the drop tests agreed closely with the measuiwuents in the closed high-speed wind tunnel of DVL. In particular, according to drop and wind--tunnel measurements there is an extraordinarily steep &rag increase when the velocity of fall a~yroaches the velocity of sound.
3. A comparison of drop measurements with drag measurements of,the same boubs in the open jet of AVA .-G6ttingen shows that the increase of drag is undervalued on approaching the speed of sound in the open-jet measurements.
Translated by Dave $’eingold National Advisol~ Comnittee for Aeronautics — NACA TM No. 1186 ..- --- ..— .
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\ Figure 1.- Comparison of the shapes SC.-5O , SC-250 and SC -500.
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NACA TM No. 1186 Comparison of the drag coefficientsobtained from Figure 2.- - Wind tunnel and release experiments for SC-bombs without tail fin struts for various Mach numbers.
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Drag coefficient
Cw =
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F=:D2 Bomb frontal area M = Trajectory speed Mach number Sonic speed —- N.ACA TM No. 1186 — -- ...
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— — — Figure 3.- Comparison of the drag coefficients obtained from wind tunnel and release experiments for the bomb SC-250 with and without tail fin struts for various Mach numbers. ,
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Drag coefficient ‘w = P/2V2F ~ D2 Bomb frontal area F= M = Traj eCtOry speed Mach number Sonic speed IL—-.
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NACA TM No. 1186 L$ C)y / SC-bombs without tall ~im struts i : f’ j j~~- [C1OE8Q ~lnd t.... I SC-/j’5O without tail fin struts ; , ,., /’ A -_ /4yA - OP@Jn j.t JC-50 \ ,/ ‘ / ‘ Extrapolated o“rves .-------- N = velocity of trf4jeat0ry/.0nic velocity: I I I I I I , 1 1 1 I - ?
44 &f L& 47 5 48 6 9 ,’ bomb SC-250 without and with tail fin struts
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i — ., ~~ - closed wind tunnel (p’ $t-500 withOut tail fin struts ~ q4 Without tail fin struts M = Ye locity of trajectory sonic velocity
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45 (M (V Q$ 49 ( Figure 4.- Comparison of bomb drag coefficients from measurements in the closed DVL wind tunnel and the open jet, AVA -Gottingen.
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