wC.f3

FOR AERONAUTICS .

TECHNICAL NOTE 2530

WIND-TUNNEL INVESTIGATION OF ~ SHIELDED TOTAL-PRESSURJZ

TUBES AT EIGH ANGLES OF ATTACK

By

#

SUBSONIC SPEEDS

R. Russell, tViUiam Gracey, William I&ko,and Paul G. Fournier -

Langley Aeronautical LaboratoryIkngley Field, Va.

Washington

November 1951

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Iwmm.AL ADVISORYiiiiiniiifil%i

C@MITTEE FOR AERONAUTICS DDL5474

TIHXU?ICALNOTE2530

WnD-mnulla RIVIWPIGATIW OF SIX SHIELDED

TUBES AT HIGH ANGIJH3OF

suBsmIc sPIaKcs

TOTAL-PRESSURE

ATrAcK

By Walter R. Russell, Willi8m Gracey,

and Paul G. Fournier

Willism Letko,

The effect of inclination of the air stream on the measured pres.sures of six shielded total-pressure tubes (all designed for end-mountingon a horizontal boom) has been determined for an angle-of-attack rangeof approximateely -300 to 65o. The tests were conducted at a Mach numberof 0.26 in the Ia.ngleystability tunnel and at Mach numbers of O.~, 0.70,0.90, and 0.95 in the Langley high-speed 7. by 10-foot tunnel.

The results of the tests showed that curved venturi entries are lesssensitim to inclination of the air stream than the conicsl entry of thestandard IH.eldesign. Of the three curved entries tested, the tube havingthe bluntest entry proved the least sensitim to inclination. Thecritical angle (that is, the sngle.at which the total-pressure errorequals 1 percent of the impact pressure) for this tube was *63. 2° at aMach number of 0.26.

The tests also showed that the critical angle of a shielded tubecould be extended to higher positive angles of attack by means of aslant profile. The critical sngle of a tube having a conical entry witha 10° slant profile, for exsmple, was found to average 9° greater than thatof a similar tube having a square profile. Although the critical sngleof this tube at sngles of yaw was less thsn that at angles of attack,the critical angle in yaw was equal to that of the tube with the squareprofile.

Tests of the effect of vzrqdng the position of the total-pressureprobe in the shield showed that, in general, no advantage was to begained by changing from the position used in the standard Kiel design.Limited tests of the effect of varying the internal diameter of the shield

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2

showed that anan increase insmall increase

NACA TN 2530

increase in the di-ter of the throat correspondingtothe internal area of m percent resulted in a relatively(2.5°) in the critical angle.

The effect of Mach number on the sensitivity of the tubes to incli-nation was shown to be appreciable. An increase in Mach nwiber frm 0.26to O.~, for example, resulted in a decrease in the Critical.angle of mostof the tubes of about 4°.

The National Advisory Committee for Aeronautics is conducting aseries of wind-tunnel investigzztionsto deterndne the effect of inclina-tion of the air strem.uon the measured pressures of a xnniberof total-pressure tubes through a wide ‘raqp of sngle of attack in both the sub-sonic and supersonic speed ranges. These investigations are being con-ducted for the purpose of providing fixed or rigid total-pressure tubesfor use on preseti-&zy high-perfomnance airplanes ha_v3ngthe capabilityof maneuvering to high sngles of tiack at supersonic speeds. In aprevious paper (reference 1) the results of tests of 39 total-presmtubes through an angle-of-attackrange of +!+Y at subsonic speeds werepresented. A subsequent paper (reference 2) presented the calibrationsof 20 of these tubes at supersonic speeds.

The results of these tests indicated that the tube which remainedinsensitive to inclination of the air stream over the greatest range ofangle of attack was a shielded total-pressure tube (ICLeltype) desi~edfor end-mounting on a horizontal born. This tube remained insensiti=(to within 1 percent of the impact pressure) over an angle-of-attackrange of *41.50 at subsonic speeds snd *370 at supersonic speeds. Othertests (reference 3) on small-scale shielded total-p= ssure tubes designedfor engine qd dud survey work indicated that a significant increase .inthe range of insensitivity could be achfeved by the -use of a curved ratherthan a conical venturi entry of the Kiel design (described in reference h).

As a means of determining the effectiveness of the curved entry on atile of the size suitable for airspeed measurements and, at the same time,determining the effect of va@ng other pertinent dhensions of the.E1.eldesign, additionalwind-+amnel tests have been conducted on a number ofdifferent configurations of the shielded total-pressure tube.

This paper presents the results of the tests of these tubes at severalMach numbers ranging fxm 0.26 to 0.95.

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NACA TN 2530 3

SYMBOLS

outside diameter of shield

inside diameter of shield

distance from leading edge of probe to leading edge of shield

total pressure of free stresm

total pressure measured by tube

total-pressure error (H~ - H)

free-stresm impact pressure

free-stream Mach number

angle of attack of total-pressure tube, degrees

angle of yaw of total-p= ssure tube, degrees

angle of attack at which ~ = 0.01%

angle of yaw A which # = 0.01

APPARATUSAND TEsTS

Six shielded total-pressure tubes designed for end-mounting on ahorizontal boom were tested during this investigation. Design detailsand dimensions of tube l-a, the design of which was based on the Eelshielded total-pressure tube (reference 4), are given in figure 1.Pertinent details and dimensions of tube l-a, as well as of the othertubes, are given in table I and in the calibration charts at the end ofthe paper. In addition, enlarged sections of the various shield entriesm presented in figure 2 for cmparison purposes.

The calibration curves for tube l-a were used as a basis for com-paring the effects of changing various design features. The design featureswhich were investigated include the shape of the shield entry, the posi-tion of the total.-pressureprobe relative to the leading edge of the shield,the inside Usmeter of the shield, and the shape of the profile of theshield. The vent area of all the tubes tested was 1.5 times the frontalarea and the outside U.smeter was 1 inch.

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4 I?MA TN 2530

The tests were conducted at M = 0.26 in the 6- by 6-foot testsection of the Langley stability tunnel and at M = 0.50, 0.70, 0.90,and 0.95 in the Langley high-speed 7- by 10-foot tunnel.

The tube support used in the stability-tunnelinvestigationwas aU-tube swivel mechanism (fig. 3) mounted on the side wall of the tunnel.This support was designed with the axis of rotation of the swivel amnin line with the leading edge of the total-pressure tube so that thetotal-pressure entry would remain at the same point in the air stream forall angles of attack. Possible errors caused by deflection of the su~ortmechanism were investi@ed and were found to be negligible.

The tubes were tested at a Mach nmiber of 0.26 through an angularrange of -300 to 65o in so increments. For each setting of the tube thetotal-pressure error was determined by measurdng the prss~e differencebetween the test tube and a fixed pitot-static tube mounted on the oppo-site side of the tunnel. The accuracy of the measurements of AH/~was esttiated to be within M. 002.

In the Langley high-speed 7- by 10-foot tunnel the total-pressuretubes were mounted on a sting which was supported by a vertical strut in

-1

the diffuser section of the tunnel (fig. 4). The angle of attack wasvaried by a cmbined rotation and translation of the sting on the strut k?as shown in figure 4. The motion of the sting was such that the leadingedge of the total-p=ssure tube remained at approxhately the same pointin the tunnel cross section for all angles of attack. The angular r-of the sting was a~roximately +13°. In order to shift this range tohigher angles of attack, couplings were @serted in the sting (fig. 4).Since the total-pres~ error of the tubes was shown by the stability-tunnel tests to be negligible to within a few degrees of the criticalangle, the tests in the 7- by 10-foot tunnel were restricted to a smallangular range around the critical angles as detemined by the stability-tunnel tests. The sngle of -ack was varied in 5° increments through-out the test range. In some cases readings at intemnetiate angles werealso taken.

The Mach numbers at which the various tubes were tested in the 7-by 10-foot tunnel are given in table I. At each Mach number and angularsetting, the total-pressure error was detemined as the difference betweenthe pressure measured by the test tube and the pressure in’the sett13ngchsniber. The accuracy of-the measurements of AH ~/ varied with Machnumber. At M = O.~, the accuracy was estimated to be within +0.0045andat M= 0.90, to within Ml.002. Errors resulting frcrmdeflectionof the sting were estimated to be less thsn 1/4° for a = 60° atM= O.go.

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NACA TN 2530 5

RESULTS AND DISCUSSION

.

The results of the tests of the total-pressure tubes are presentedin figures 5 to 13. A sumaqg of the results is given in table I. Thesymbol AH shown in these figures is defined by the relation AH = Hi - Hwhere H! is the pressure registefid by the tube at a given sngle ofattack and H is the free-stresm total pressure. The total-pressureerrors are presented as a fraction of the impact pressure ~ and areplotted as a function of the sngle of attack a (or angle-of yaw V )of the tubes. The basis for cmparison of the tubes is the range of angleof attack over which the tube reads the total pressure correctly -towithin1 percent of the impact pressure. This range is called the range of insen-sitivity and the angle at which the total-p~ssure error reaches 1 percentof the impact pressure is called the cfitical angle (%= or %-). Forthe five symmetrical tubes, the cfitical angles at negative angles ofattack were assumed to be the same as those at positive angles.

The calibrations for tube l-a (~el design) sre given in figure 5for Mach numbers of 0.26, 0.m, 0.’70,end 0.90. This tube, which issimilar to tube A-13 of references 1 and 2 except for a larger outsidediameter D, has a range of insensitive@ at M = 0.26 of al .3°. Thisrange is almost identical to the range of tube A-13 of reference 1atM= 0.26. As the Mach nuuiberis increased the range of insensitiviiqdecreases until at M = 0.90 the range is &38.60.

The effect of vaqing the axial position of the total-pressure probein the entrance cone of the Hel design was detemnined for probe positionsa/D of O.k~, O.~0, and O.~ at a Mach number of 0.26.(figs. 5 and 6).The use of O.W as the value for the rearward position of the probe inthis tube is somewhst arbitrary, because it is based on the positionchosen for similar tests (~scussed subsequently) on a tube with a slantprofile. The value 0.456 was used for the forward position because itplaced the probe at the smne distance ahead of the standard position(O.WO) as O.~ had to the mar. Caparison of figures 5 and 6 showsthat the ranges of insensitivity for the three probe positions, 0.456,0.500, snd O.~, are Al. 5°, #+1.3°, and ~. 1°, reS~Cti~ly. Asindicated by these results, moving the probe to the rear of the standardposition decreases the critical angle by 1.2°, whereas the effect ofmoving the probe dead of the standard position is negligible. No advanbage is to be gained, therefo~, in varying the position of the probe frcmthat used in the standard Kiel design.

The effect of vaqdng the inside.diameter of the shield of the standardKlel design mqy be seen by compa&tng the calibrations of tubes l-a end 2-aor tubes l-b snd 2-b (figs. 5, 6, and 7). The value 0.591 for the inter-nal diameter of tubes 2-a and 2-b represents an increase in the area ofthe vent passage through the throat of 50 percent above that of the standard

6 NACA TN 2530

Kiel design. (Note that this increase in internal diameter is acco-mpaniedby small decrease in the angle of the entrance cone.) For probeposition 0.%)0 (tubes l-a and 2-a) this increase in size of the internalpassage results in an increase in the range of insensitivity of fromtkl.s” -to M4..oo. For probe position 0.544, the range is increased from +b.l” ~to +42.5°. The effect of increasing the area of the internal passage by50 percent, therefore, is to increase the critical sngleby about 2.5°.

The effect of probe position on the range of insensitivity of tube 2is shown in figure 7. As indicatedby this figure, the range of insen-sitivity of probe position 0.500 is +44.0°, while that for position 0.544iS *42.5°. The effect of moving the probe frmn 0.500 to 0.544, therefore,is to decrease the critical.angle by 1.5°, which agrees well with the 1.2°decrease noted for tubes l-a snd l-b.

The effect of varying the leading-edge profile of the shield msybeseen by caqxming the calibratio~ m fi~s 51 6) 8(a)~ ~d 9= Forprobe position 0.500, the effect of changing the leading-edge profilefram square to a 10° slant is to increase the critical angle of attackfrom41.3° to 51° at M = 0.26 sndfrom 38.6°to 46.5° at M= 0.90 (figs. 5,and 8(a)), an average increase of about 9°. When the position of the probeis set at 0.544, the 10° slant profile has the effect of increasing thecritical angle from kl.l” to 47.7° at M = 0.26 (figs. 6 and 9). The Mvslue 0.54-4was chosen for the resrwsrd position of the probe becausethis position placed the leading edge of the probe midwsy between theresr of the cone and the leading edge of the slanted shield (at thepoint of intersection with the center line of the tube) and thus corre-sponded to the probe position (0.500) of the standard Kiel design (tube l-a).The results of these tests indicate that the effect of a 10o slantat M= 0.26 is to increase the critical angle 9.70 for ~obe positionO.WO and only 7.6o for probe position O.5kk. If probe position 0.544can properly be considered as corresponding to the standard position (0.~0)of the Klel design, the results of these tests indicate that moving theprobe forward of the “standard”position aP the tube with the slant pro-file has a beneficial effect on the performance of this tube.

The angular range covered in these tests was not large enough todetezmine the critical negative angle of attack for the tubes with a slantprofile. The results of the tests of anunvented, cylindrical tube(tube A-6) reported in reference 1, however, showed that the total range al?insensitivityremains essentially constant when the profile is ch&mgedfrom square to a 10° slant. If the same result is assumed to hold forshielded tubes, the range of insensitivity for tube 3-a would be fran -31.6°to 51° at M= 0.26 and from -30.70 to 46.7 at M= 0.9. For tube 3-bthe range wouldbe -32.Y to 47.70at M = 0.26.

.

The characteristics of the tube with the slant profile at angles ofyaw (a = Oo) are shown in figure 8(b) for tube 3-a. At M = 0.90 the rangeof insensitivity of this tube in yaw is +38°. This range is approxbnately

NACA TN 2530 7

.

the same as that of tube l-a, which has the same design features exceptfor a square profile. A 10° slat profile, therefore, has the effectof extending the range of insensitivity to higher positive angles ofattack (~ . 0°) without 10SS in perfommce at angles of yaw (a= OO).

CakLbrations of three tubes with curved entries are presented infigures 10 to 13. For each of these tubes a/D and d/D equal 0.500.The slope of the entry curve of tubes k-a, k-b, and k-c is 60° at theleading edge and 0° at a distance O.~OD behind the leading edge, whilefor tube 5 the slope is 30° at the leading edge and Oo at a distance Dbehind the leading edge. The entry for tube 6 is an adaptation of an ‘NACA nose inlet. Tube 6 has a rounded leadh.g edge while tu%es k-a, k-b,k-c, and 5 have sharp leading edges.

The results of the tests of tube k-a at M = 0.26,0.X, 0.70,0.w,and 0.95 are presented in figure 10. For each of these Mach nuniberstherange of insensitivity is +63.20,*63.3°, +60. 6°, +58.4°, and @3.00,respectively. Comparison of these values with those obtained with tube l-ashows that the range of insensitivity of this tube is about 20° higherthan that of the Kiel design.

The effect of probe position on the peflormance of a tube tith acurved entry (tubes k-a, k-b, and k-c) was detemnined for probe positions0.456, 0.mO, and O.X at a Mach number of 0.26 (figs. 10 and 11). Asindicated by these calibrations,the rauge of insensitivity for the threeprobe positions was *63.8°, *63.2°, and *63.8°, respectively. It iS apparentfrom these results that the effect of,varying the position of the probeon a tube of this design is negligible for the range of positions tested.

Calibrations of tube 5 are given in figure 12 for Mach numbers 0.26and 0.90. At M = 0.26 the range of insensitivity is &jl.4° whereasat M = 0.90 the range is i?+5.50. The “criticalangles, therefore, areabout 10° higher than that of tube l-a at M = 0.26 and about 70 higherat M = 0.90.

The calibrations of tube 6 at M = 0.26 and 0.90 are presented infigure 13. For each of these Mach numbers the range of insensitivityis *53.Y and m. 6°6 respectively. The corresponding critical anglesare appro-tely 12 greater than those of tube l-a.

A comparison of the results of the tests of the three tubes withcurved entries shows that the range of insensitivity increases as thebluntness of the entry increases. The range of the tube with the bluntestentry (tube k-a), for exemple, is the highest (~3. 2°) while that of thetube with the least bluntness (tube 5) is the lowest (*Z. 4°). The testsalso show that in every case the range of insensitivity of tubes withcurved entries is considerably greater than that of the c“onicalent~of the ~el design (tube l-a).

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8 NACA TN 2530

The variation of tube sensitive@ with Mach nuiiberis given infigure 14 for those tubes for which a/D .and d/D equal O.~0. Asshown by this figure, the cfitical angles for all the tubes decreasewith ticreasimgMach number. The magdtude of the decrease, however,is not the same for each of the tubes, because it varies frm 30 fortube l-a to 6° for tube 5 (for a Mach number range of 0.26to O.90).Furthermom, the variation of critical angle over the Mach number rangeis not the-same for the two tubes (l-a and k-a) for which data wereobtained at more thsn two Mach nunibers. The critical angle for tube l-a,for exsmple, varies linearly with Mach number, while that for tube &ais constant between M = 0.26 and O.n and decreases as M increasesto 0.95. Despite these differences, the results are still significant inshowing that the effect of increasingMach number is to cause a decreasein the critical angle and that the magnitude of the decrease may beappreciable (an am–rage of over 4° fo; five tubes tested). -

CONCLUDING RIMARE!

A wind-tunnel investigation of six shielded total-pressure tubeshas been conducted over an angle-of-attackrange of approximately -30°to 65° at Mach m.mibersof 0.26,0.50, 0.70, 0.90, and 0.95.

The results of the tests showed that the shape of the venturi entryhas pronounced effect on the range of angle of attack over which thetubes remain insensitiveto inclination of the alr stream. In thisregard, curved entries were shown to be superior to the conical entry ofthe standard ELel design. Of the three curved entries tested, the tubehaving the bluntest entry proved the least sensitive to inclination.The critical angle (that is, the angle at which the total-pressure errorequals 1 percent of the impact pressure) for this tube was 63.2°at aMach number of 0.26.

The tests also showed that the critical angle of a shielded tubecould be extended to higher positive angles of attack by means of a slantprofile. The critical angle of a tube having a conical entg with a 10°slant profile, for example, was found to average 9° gmeater than thatof a similar tube having a square profile. Although the critical angleof this tube at angles of yaw was less than that at angles of attack, thecritical angle in yaw was egyal to that of the tube with the square profile.

Tests of the effect of varying the position of the probe in theshield showed that, in general, no advantage was to be gained by changingfrom the position used in the standard Kiel design. Idmited tests ofthe effect of varjdng the internal diameter of the shield showed that anincrease in the internal diameter corresponding to an increase in theinternal area of 50 percent resulted in a relatively small increase (2.5°)in the critical angle.

2

.

.

NACATN 2530 9

The effect of cumpressibil.i~ on the sensitivity of the tubes toinclination was shown to be appreciable. An increase in Mach xnnuberfrm0.26 to 0.90, for example, resulted in a decrease in the critical anglesof most of the tubes of about 4°.

Langley Aeronautical LaboratoryNational Adtisory Committee for Aeronautics

~ky J?ield,Va., lwgust I-6,1951

INFERENCES

1. Gracey, Willism, Letko, Williamj and Russell, Walter R.: Wind-TunnelInvestigation of a Number of Total-Pressure Tubes at High Anglesof Attack. Subsonic Speeds. NACA TN 2331, 1951.

2. Gracey, Willism, Coletti, Donald E., and Russellj Walter R.: Wind-Tunnel Investigation of a Nmnber of Total-Pressure Tubes at HighAngles of Attack. Supersonic Speeds. NACA ~ 2261, 1951.

3. Moffat, Marston: Report on Ki.el Proles. Rep. No. PWA-576, Pratt& Whitney Aircraftj Oct. 17, 1945.

k. Kiel, G.: Total-Head Meter with Small Setiitivity to Yaw. NACATM 7’75,1935.

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TA33LE I. - ANGULAR RANGE OVER WEICH SEtELDED TO!CA&PRWSURE TUEFS

HMAJIJ IIV!?J31?SI!l?IW?TO ~CXLNATION !152WITHDJ 1 PERCENT ~

bbe FllgureBhield $ d

RmI@ of imensltivlty, &g, at -

ent~ shape 5M = 0.26 M.o. w M- O.70 M= 0.90 M= O.95

l-a 5 conical 0.500 O. Xx) *41.3 *horh *39.q @.6 —l-b 6 conical .54J+ *500 *40.1 —-— --—----- -----l-c 6 Cotical .4$ .500 *41.5 .--= :1 --w---- -----

2-a 7 Conical .m .591 *44,0 --- -.—-. —-. --—.. -----2-b .%4 .591 &2.5 ----- ----------- ----3-a 8~a) cz~ ?00 .W .W *-31.6, % 1:: -— *-30.7, 46.P “.-s-

lant profilej-a 8(~) Conical, 10° .500 .503 ---”---–-– ----- ----- v = *38.o -----

slant profile3-b 9 conical.,100 .544 .W +-32.5, 47.7 ----- ----- ------------ -----

slant profile4-a 10 Curvad .m .W *63.2 63.3 60,6 k9,4 @.o4-b 11 .%4 .W 63,84-c 11

----- ---- -—- —----- --..—

Clu-vea .4s .500 K3.8 ---- ---- .——-— - —--

Curved .W .m m.4 ----- ----- &5. 52 E

-----

alrvecl ●XQ .W 83.9 ----- -— B.6 ------

— —*EBtimated. =5=

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Figure 1.- Section view of 8hielded total-pressure tube l-a.

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-—----t-

-–J—–—–———–—-––--—.——— —— —____ ---

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Figure 2.- Comparison the shield entries of the sti total-pressuretubes tested.

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I

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1

t .

1

TOtxil-presanre tube\ I

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I

L -d6“10”Ill

10”

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Tunnel Wdl

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Ill I IIII

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Ik‘1Control em

To Iasncmleter

Figure 3.- Diagram of swivel mechanism us~d h Langley stability tunnel forohangtig angle of attack of total-pressure tubes.

I-J.P-

Figure 4.- Diagram of meahaniem ueed in Langley high-speed

tunnel to support total-pressure tubaa.

r atitlg / , H.”1+1 d- \l AZ-’ . !T-’1 1 I

If I //2/ w’

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=qSy

-30 -’a -lo 0 10 m

Angle or dhink, a%

Figure ~,- Variation of total-preaeure error

0.50, 0.70, and 0.90.

30 40 x 60

with angle of attaok. M = 0,26,

Tube l-a,G

o G

-.2

-.4

-.6

-.8

-30 -’m -lo 0 10 m 30 ho 90 &

w Qfatti, w

Mgure 6.- Variation of total-pressurew-ror with angle of attack. M = 0.26.Tubes l-b and 1-o.

.1

-1

0

-. 2

-, 6

-. 8

+1”+

l—~

30 -m -lo 0 10 m 30 40 w 6!)

A@O of attack, m

Figure 7.- Variation of total-pressure error with angle of attaok. M = 0,26.Tubes 2-a and 2-b.

+ t

m

o

-.2

-.4

-.6

-.8

–30 -m –lo o 10 .?0 30 40 % m

An@ of attack, (leg

(a) Angle of attack (V E OO).

Figure 6.- Variation of total-pressure error with tiohation to the air

etream. M = 0.26 and 0.90. Tube 3-a.

El

I

-. 6

-.8

E

-30

.

, ,

h

--l

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mah [nmbr, N

.5Q

y

-m -lo 0 10 K1 30 &

of yaw (a = OO).

8.- Concluded.

,

1

+ 1“+

-. 2

0

q# -.4

-.6

8-. I

-30 -al -10 0 lo m 30 & P&l

An@ of atbak, W

Figure 9.- Variation of total-pressure error with angle of attack ($ = OO).M = 0.2b. Tube 3-b.

,

1 , , ,

o

-.Q

I

-. 4

—--- ---—

-.6

-.0

-30 -m -lo 0 lo m 30 & m & 70

w of atw, deg

RLgurt? 10.- Variation of tots.1-p~sa~e error with angle of attack.M = 0.26, O.~0, 0.70, 0.90, and 0.95. Tube b-a.

Angle of 8*, dog

Figure 1.1.- Variation of total-premure error with angle of attack.

H = 0.26. Tubes b-b and 4-c .

, .

o G n ,-, A wn

w v Y*

.* .

w

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Figure 12s-

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Variation of total-pressure

H = 0.26 and 0.90.

30 40 Y &

error with angle of attack.

Tuba ~.

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I

I

al$

1

I

-. 2

-.4

-.6

-.8

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Figure 13.- Variation of total-preseure error with angle of attack.

H= 0.26 and 0.90. !bibe 6.

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m

70

60

50

40

30

20

10

0

u

Tubeo l-a

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0 .2 .4 .6 .8 1.0

Figure 14.-

Jkch number, M ~. .

Variation of critical angle with Mach number for tubes with a/nand d@ equal.to O.@O.

NACA - Mngley Field, VJ.

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