C.P. No. 251 C.P. No. 251 (17.566)

A.R.C. Technical Report (17.566)

A.R.C. Technical Report

MINISTRY OF SUPPLY

AERONAUTICAL RESEARCH COUNCIL 9 %

CURRENT PAPERS -9

Preliminary Low Speed Wind Tests on Flat Plates and Air

Flow, Vibration and Balance Measurements

R. Fail, T. B. Owen and R. C. W. Eyre

LONDON: HER MAJESTY’S STATIONERY OFFICE

1956

SIX SHILLINGS NET

C.P. No. 251

u.D.c, No.533.G91.~52.3 : 53T.6.048.3

11

II

:‘z

13

‘I3

14

15

Incrw‘clAx! of llf?, k.x& and pitchirg moment. SqLm-e plate on 1L.50 in. dia. bo<y (Scat tall) 5. = 1, c$2 = I. Iv

a 2 5

Increments of lift, 6ra& an? pitchirg raorrant. Cascade brakes on 7 in. 6la. bo?lx v

-2..

LIST OF XLIuSTRA!CIONS

Arrangement of apparatus - Tests on flat plates in 4f%x3ftvci.natunnel

Flat plates -Variation of drag vrith incidence

Rectangular fkt plates (e = 900) - Variation of drsg mth aspect ratio

Flow behind flat plates (8 = 90')

Flov behmd square plate 15.3 in. downstream

Flow behind square plate - spectra of $ 0

Arrangement of square plates on 4.50 m. &a. body

Lift, drag and pitching moment - square plate on 4.50 in. dia. body (Bluff tail) $ = ?z, % = 5

4

Lift, arag and pitching moment - square plate on 4.50 in. dia. body (Boat tail) $ = $, y = i

Lift, drag at-18 pitching mment - square plate on 4.50 ii% dza. body (Faked tail) 5 = 2, y = t

a 3

Lift, drag and patching moment - square plate on 4.50 in.

ka. body (Boat tail) $ = $, y = s

Lift, drag and pitching moment - square plate on 4.50 in

aia. body (Boat tad) t= $, % = i Y

Static pressure d.xstributions on 4.50 in. dia. body

Pitching moment about a range of assumed C.G. pu~ition% e Square plate on 4.50 in. dia. body -= 2, &!=I a3 .e 5

hnalyszs of pitcbxng moment about mid C.G. position (Flg.13) Square plate on 4.50 m. dia. body

Pitching moment about mid C.G. posxti.on (Fig.14). Effect of gap and plate sxze. sqlare plate on4.50 xn. ala. body wxth boat tail

Flow behd square plate (e = 70') - Comparison of spectra of 2

UO

for plate on 7 in. dia. body and isolated plate

2!iiez?

1

2

3

4

5

6

7

8

9

10

II

12

I3

14

15

16

17

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LIST OF ILlXJSTWJ'IOKZ (Contd)

Arragement of brakes on 7 m. cka. body

Brakes on 7 m.da. body - lift anti drag

Dr&es on 7 in, &a. body - patching moment

Flow behind sheet metal cascade (3.54" x 3.54") on 7 i-n. am. body

37'10~ behind brakes on 7 m. dir*. body - spectra of e 0

Pwre

18

19

20

21

22

-L-

.

1 1 Introductzon Introductzon

Tests are bang tide ~.n low sjjed wind tunnels vnth the -n object Tests are being tide ~.n low speed wind tunnels vnth the m&u object of prowdug data for the desxgn of a.zw brakes. of provldxng data for the desxgn of zw brakes. !rhe results are also of !Phe results are also of interest as a contribution to the knowledge of the flow behud bluff interest as a contribution to the knowledge of the flow behud bluff bodies. bodies.

Some prelimrEiry neasw7nent.z have been made of the drag and the nature of the flow behind Isolated flat plates of various shapes over a range of inmdence. Both the mea flow pattern and the long;lttiaual velocxty fluctuatzons hzve been stuZx&.

Lift, drag and pztch3ng moment have been measured on a body fitted xxth a square flat plate br,&e to explore the possibility of getting no charge m pitching moment over a range of br,&e angle. One of these brakes %zs used for an uwesti,nation of vibration and showed a zmzch steafiler flow ?ue to putting the brake on the body. Further wrk on this 1s to be done.

Types of nxr brakes other than simple flat plates will be tested, such as slotted and perforated plates. th& a cwicade3 may have advutages

From Australian tests, it appe0.r~ and some tests have already been made

on such a brake. The results are mcluded m the present note.

2 ~iments vn2 isol.at& flat plates -

2.1 Descriptzon of tests

'he experiments ecu,. be divided roL%hly Into three groups:

(1) Keasurements were made of the drag, and of the mean velocitLe.es behln6 plates uerpendlcular to the wmc?. The shapes tested ware a square, c aisle, a 60b aelta and a rectangle of aspect xtlo 2.15. All of the plates had an area of 25 sq.in.* and the edges were chamfered at 3Go as sholm In Q.1.

(2) Xrther drag measurexcnts were made on plates per$endxular to the r&nd :nth aspect rat%os of 5, IO and 20, mainttiihing an area of 25 so.m.

(3) Tests were made on the sqixare plate over an incidence range frG.1 270 to 90”. ELeasurements were nude of the drag and of the velocity fluctuations m a plane 15.3 in. behind the plate.

fill of these experiments were made VI the 4 ft x 3 f-t tunnel at n qeed of 140 f%/sec. The plates 78x-e mounted on the upstream end of 2 rod dxxlli 3 f?t long an6 0.5 xl. dmneter on the ax~.s of the tunnel. The do7+xstream ed of the rod vas sup-orted by struts attached to the tune1 IKillS; ths rod was also supported just behind the plate by means of three mre s . R sketch of the rq is given in 55g.l.

For drzg measuremE,lts, a a~.11 cappacxty-type balance was constr'u5kd and fatted beWeen the rod anal pl?te as sham m Fig.1. In older to reduce the support Interference vntii the plate at smzll ,angles, a short ler@h of streclnllne rod was interposed between the rod and plate (as shown m Fxg.1) -&en velocity traverses were berg made behind the plate.

--- -- %xcept for the rcctwgle kich was wtended to be of aspect ratio

2.0 but ms made In error to have an aspect ratlo of 2.15 and an area of 27 sq.ins.

-5-

Traverses behind the plates were made vzith pltot-static tubes and with a hot wire using the apparatus shown inFig.1. An elliptic section tube spanned the tunnel 4 carried a sliding block which could be traversed from outside the tunnel. Vertical movements viere obtaiued by means of cranked holders, ati fore and ai't movements mere made by moving the whole traversing gear bodily, The pitot and static tubes mere kept parallel to the tunnel axis. Ivieasunments of the reverse velocities behind the centre of the plate were made vrith pltot-static tubes attached to the central rod but pouting dobwtrwm.

The hot wue was arranged to be normal to the free stream direction. The associated equipment which included a vibration analyser was basically that described m Refs.1 and 2.

2.2 Velocity measurements

The velocity contours showi XI F1g.4 were drawn from measurements made %ith pitot and static tubes set paKUe1 to the tunr~l axis in planes 3 in, 6 in, 12 in. and Z?J+ in. do:mstrssm of the @at&s and also 18 in. dov?n&resm in the case of the square and rectangular plates. The contours of velocity end velocity fluctuations shown 111 Fig.5 were drawn m measurements made with a hot wire set normal to the tunnel axis in a plane 15.3 in. dow&ream of the plate.

The measurements are therefore in error where the local stream direction is not parallel to the tunnel axu. For the particular pitot and static tubes used in these experiments, a check showed that the measured velocities exceeded the true velocities by z% for IO0 misalign- writ and @ for 20' misalignment. It should be noted that misalignments are small near the axis of the "bubble" (see para 2.3) and near the mid- length of the bubble. The length and maximm~ diameter of the bubble are therefore fairly accurately determined. The hot tire measu~s velocity normal to its length; it is therefore substantially non directtinal in the plane normalto its length tiith a cosine effect in the plane con- taining its length. In the latter case the z-e-s are therefore low by I& for 100 misalignment and 6% for 20° misaligraoent.

The measurem nts are also subject to errors due to the fluotuatina, rdm-e of the CA. fl In order to investigate this effect, comperatlve traverses have been made with hot wires and with the pltot and statu tubes in a plane 18 in. behind the plate. The results which are shown inFig.4g give some *cation of the accuracy of the present measmments. Corrections have been applied to the hot wws measurements for the effect of veloozty fluctuations; the discrepancy between the velocities given by normal and inclined. tires suggests that these corrections are inade- pate. The pitot-static measurements are uncorrected and lie above the hot tire measurements, The maxumm difference between hot tire measure- ments snd pltot-static measurements amounts to about 2Q0 of the local velocity or I@ of the tie stream velocity.

Measurements of the longitudinal veloczty fluctuations were made as \2 follows. Measurements of the mean square of the analyser output, 0

"A u '

mew made over a range of fl-equemy, f. mith the analyser out of $izwit,

measurements were made of the mean sq+sre of the total output, 2 0

. The U

spactlum f'unotion, F(f), IS defined so that P(f)df is the contribution

to u 2

0 v of frequenoies between f snd f+ df, i.e.

-6-

= .I- fF(f) d(lag f)

Since the analyser band?vldth ratlo, sA, is small (= 0.10 x tuned frequ==Y)

u l2 fF(f) = ..A

( ii u s* approximately.

are presented X-I terms of the free stream speed. Contours are drawn of u and spectra are glvczl of By f)

< loof/(r,.

,(=(Fy/ Q) pitdied against

The latter is the frequenoy coGspond.mg to a free stream speed of 100 Pt/sec .

The traversing gear already described was found to be unsatasfactory for holding a hot wire. The slrding block was not a good fit at all poants along the elliptic tube so that a low frequency vibration of the mre may have occurred on some occasions, giving high readings of the velocity fluctuations". A new traversing gear has been mode and found satasfaotory for further experiments.

Outsxk the velocity wake of the plate, lateral oscsllations of the flow have a negligible effect on a normal wire. Obviously lateral velocxty fluctuations may be important; an inclined hot wire is required to measure these components.

The present tests arc therefore lx&ted an scope and crude in character. Nevertheless some important conclusions can be drexn from them. Further experunents are being msde with nxmal and mnclined hot wires mounted on the iTproved traversing gear. An attempt will also be made tc correlate hot wire measurements v&h measurements of the fluctua- ting pressures on an aerod~c surface.

2.3 Eesults

'The drag measurements** on plates at 90' gave values of the drag coefYxcx.ent of 1.15 for the square plate, 1.16 for the 60° delta plate

Wnile measuring the spectra sho7-n in Fig.6, the sliding block was

temporarily fixed rigidly to the tube.

**Tine drags of 25 sq.inch plates an the I+.' x 3’ tunnel have been irmltiplied by a factor (1 - 0.03 %) for blockage, using some tests on plates of dafferent sxes. The correction thus found is larger than xas expected, and further work. is being done. an the 11; ft tunnel.

The correction is negligible

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and 1.13 for the circular plate. (A Rressure plot of the circular plate gave a drag coeffxxent of 1.14).

The variation of drag vrith incidence for the square and delta plates is shown in Figs.2a and 2b. The sq- plate stalls at about 0 = 36O, the drag coefficient falling abruptly fkom 0.80 to 0.60. There is a consider- able hysteresis loop at the stall4 which could not be investigated during the present tests since it was not possible to vary the incidence vath mind on. The drag is reduced as the plate stalls since the reduction in m&xed drag is geater than the increase in profile drag. The delta plate shows a stilar but less marked break in the drag curve.

The variation of drag with aspect ratio for rectan&ar plates at 90' is shown inFig.3. The two-dimensional value for CD is about 1.84 (Ref.5). The present results shorn that between A = 1 and A = IO, the drag coefficient increases only ~from 1.15 to 1.27. At values of A above IO, the drag increases more rapidly; at A = 20 (the highest tested) the drag coefficient was 1.47.

Velocity distributions behind the low aspect ratio plates at 90' are shown in Fig.4. It is possible to define a closed "bubble boundary" by saying that, within this boundary, the total axial flow across any section perpendicular to the axis of the plate 1s zero. These boundaries are shown chain dotted in Figs.&+d. The "wake boundary", defined by saying that outside this boundary there is no loss of total head, lies outside the bubble boundary. Along the v&e boundary the static pressure coefficient is constant and equal to -0.42 from the edge of the plate to about the midlength of the bubble. The corresponding velocity is 1.2 U,. The mean velocity is negative near the centre of the bubble cand positive near the outside; the flow is very unsteady. The pressure coefficient on the downstream face of the circular plate nas found to be constant and equal to -0.42. The static pressure coefficient falls to -0.5 three inches behind the plate and to -0.6 six inches behind (Fig&f). At twelve inches pressure recovery has begun and the coefficxnt is -0.35. By eighteen inches, pressure rcoovery is complete, At any one distance behind the plate the static pressure appears to be constant v,zithin the bubble @Lg.&e) although it varies as stated with distance from the plate, and there is a static pressure gradient between the bubble boundary and the make boundary.

Velocity distributions in a single plane behind the square plate over a range of incidence were obtained from hot tire readings and are given on the right hand side of Fig.5. These measurements were made 15.3 in. dovmstmem of the plate, i.e. Just behind the bubble at 0 = 90'). At 9 = 90° the v&e is nearly ciroular in section rsith minirmun velocities of &o&c 0.3 U, in the centre. At 6 = 60' the viz&e is similar in character although distorted and displaced dov?nwards due to the downwash behind the plate. Detailed traverses were not made at 0 = 50' but analysis of the velocity fluctuations at a single point (see below) suggests a flow similar to that at .3 = 900 and 60~. At e = 40°, Just above the stall, the Take is f'undar~entally changed. There are two separate regions of low velocity. At 6 = 3G" , just below the stall, the v&e is again of this type.

Contours of velocity fluctuation are shovm on the left hand side ofFig.5. There is a change from a ring of high velocity fluctuations at 900, to a crescent shaped region at 60° (and probably 50') and finally to two regions at 40' and 36’.

Analyses of the velocity fluctmtions over the incidence mange at a single poxnt* (xndxated in Fig.5) are shorn inFig.6. This paint VGS chosen to have large velocity fluctuat.tlons over the incCience range and the hot tire pias fuced rlgldly to avo~3 vibration. 'The spectra are of two types. At the hIghher angles (~3 = 900, 60' and 50°) each spectnzn includes a hsgh peak value** of f@'(f) at a particular frequency (which varies with incidence) superimposed on a continuaus spectnm. This is due to the shedding ol turbulent eddies in a regulex manner. At 0 = 40' and 36' the spectra are cont~~ous pnth no peaks, and with large random fluctuations minly at relatively tigh frequency. It appears that loge random fluctuations at low frequency are associated with a regular shedding.

3 Ewerti&ents u;lth a plate on a fuselaze

3.1 Lift, dzx.~ end pltcb moments due to plate

It has been found that a single air brake mounted behind the wing under a fuselage ten, in general, only be in trim at one angle; below this angle there is a nose downmoment, and above it, a nose up moment. %xperiments ware made to find what variables affect this behaviour.

A cyl~X1nca1 body 4.5 XI. diameter ivlth faired nose was tested in the 4 rt x 3 ft tunnel:

(4 cut off ix%th a bluff end of full alameter,

the f&?)diameter with a Paired tail cut off with a bluff end of 0.59 times

, (c) with a fklly faired tail.

These are called "bluff", 'boat tail" and "f'aired tail" (Fig.7). The pressure distributions on the bodies without brakes (Fig.13)

agree until. near the modified rear ends.

The brakes used were square flat plates sisular to those tested done, and could therefore only be used at fazly large angles when on the body. For most of the tests the length of the side of the brake, 8, was Z/3 of the body diameter. The hinge line was tangential to the body, and the gap, measured fkom the hzxge was normally 0.24. Tpvro fkrther tests xw-e made on the body with boat tail:

(a) with a larger gap (0.4.4)

(b) vnth a smaller bralie (6 = + body dtitieter) and the normal gap.

It was found that adding the pointed tail to the boat tail does not alter the fore ~318 aft posrtion where the major change m prtching moment of a brahe occurs (Pigs.9 and IO), so it is considered more convenient to keep the ssme reference point for these two bodies. Brake positions are

qxcept that the spectrum for 0 = 90' is an early measurement made 11.8 XI. downstream and 5.0 ID, from the centre line.

**The shapes (inoluding the heights) of the peaks shown in Fig.6 are determined by the characteristics of the analyser. Strictly the results should be presented in the form of the continuous spectrum and the mean square value of the velocity fluctuation at the single frequency. For the present purpose it is sufficient to recognise the existence of peaks.

-Y-

specified by giving the distance of the brake hinge from the end of the bluff or boat toil bodies and, in the case of the body with faired tail, the distance of the brake hinge from the position on the fair& tail corresponding to the end of the boat tail (Fig,-/).

Lift, drag and pitching moment increments due to the larger plate with gap = h/5 are given in Tables I-IV and Figs.8-10 in the form of oo- efflomnts based on the brake dimensions, the moments being about the brake hinge. These renllts show that if the brake is more than about 34 forward of the end of the body (or the datum described above in the case of the body with faired tail) the lift and drag increments are unchanged by further forward movement. The moments at large angles are also unchsnged but at small angles the moments change in a nose down direction with forward movement. As the brake is moved back, beyond the position 34 forward of the end of the body, the lift increases, the drag increases to a smaller extent and the moments at large angles decrease rapidly. The details of these changes depend on the shape of the rear body.

In Figs.l4-16 the moments are shown recalculated about various assumed positions of the centre of gravity, the mid position on the body with boat tail corresponding roughly to the layout of a Hunter. The second scale (of AC,) which has been added to Figs.14 and 16 expresses the moment changes in terms of Hunter dunensions, taking the body diameter as the connecting variable, i.e. the model is assumed to be l/II.67 scale, Fig.14 shows the small effects of moving the centre of gravity and varying the shape of the rear body. An enelysas of the pitching moment about the centre of gravity is given inFig. .sboxing its dependence on lxft. In all oases the brakes give zero trim change at an angle near 70'.

The effect of a change in gap from J/5 to Ze/5 is shown by comparing Figs.9 and II snd in Pig.16. The dragisa lxttle less with the bigger ap and the moments arc o.n trim at a slightly bigger brake angle (about 73 ). %

The effect of a smaller brake is shown by comparing Figs.9 and $2. The change in pitching moment slope as the brake approaches the end of the body agrees if the distance from the end of the body is measured in terms of the brahe dimensions, and the moment coefficients of the brake about its hinge are more positive. In Fig.16 the moments are compared in terms of an assumed wing area and C.G. position and the brake angle to trim is about 66'.

The result is that a sin&e brake under a fuselage is in balance at about 70° and gives variations in pitching moment as it is opened agreeing with t'nose found in practice. It is not at all apparent how to vary this except by putting the brake Just ahead of the centre of gravity (which means under the wing, v&en these results are not applicable) or by putting it near the end of the body where it can be in trim at a larger angle but has an increased variation as it is opened and closed.

3.2 Vibration

A 5 in. square plate was tested at 70' on a 7 in. diameter body in the 4 ft Y 3 ft tunnel (Fig.18). The resulting spectrum is compared with that measured behind an isolated plate at 70° inFig.17. (The latter spectrum was produced by interpolation from Fig.6). Although both spectra include peeks due to a regular shedding of turbulent eddies, the smplitude of the fluctuations is reduced by the presence of the body and the shedding frequency 1s creased. This suggests that the angle below which shedding does not OOCUT may be higher when the plate is mounted on a body and further experiments will be made to investigate this.

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4 Experiments uvlth cascade brakes

4.1 Introduction

The use of a cascade as an air brake has been suggested by the Aeronautical Research Laboratory, Kelbourne3 as giving higher drag, and probably less vibration, than a. flat plate. It was thought worth while to follow up this suggestion and to investigate the velcczty fluctuations 3.n tlha w&e of a cascade.

The first cascade brake was made from an oxistlng wind tunnel COITET cascade ~.n w%uch the vanes were of an aercfoll section designed to leave passages of con&ant area betmeen them. It was tested (zith a fla'b plate for comparison) under the body of a Javelin model Just forward of the trailing edge of the izing. Msxirilum amtg coefficients of I.67 for the flat plate (at 90") and 1.62 for the cascade (at 600) were measured. It was clear that the high drag of the flat plate mas due to Interference with the wng, and It seened likely that the interference due to the cascade brake was nuch sm,aIler. The cascade brake rmght therefore have a greater advantage in drag on a body, clear of the Wang field. The tests described below were rnzde to check this conclusion.

4.2 Description of tests

The model used in section 3 was of too small a scale for making cascade orakcs convelucntly, partlsularly as two brakes of half the flat plate area were requued. A body of msxiwm dmmeter 7 m. was therefore made (Fig.18). It represents a fighter fiselage with a shortened nose. A fin T&S fitted but no sting or tailplane. If the model is regarded as of In.5 scale, the 5 u. square flat plate represents roughly the Iiunter under ftzsclage brake and it was fitted m a corresponding poslticn relative to the C.G. and fin.

The brakes a2e also sholvn InFlg.18. The 5 In. square plate VBS used as a basu for comparison and the following cascade brakes were tested.

(a) The brake already mentioned, made from a wind tunnel cornBY cascade. This will be referred to as the "aerofoll cascade", The dzwn- sicns (5.54 in. x 5.75 In.) were chosen to give a reason&k. number (7) of vanes and span.

The other cascade bra&s, described below, iere all of the sheet metal construction shown in Fig.18 an? had vanes of smaller chord than the aercfoil cascade.

(b) A brake of apprcxmtely the same duwnsions as the aercfoil cascade, to afford. a direct ccmpamson between the two types of construction.

(c) A bra&e appmxktely .5 in. square for duect comps.r~.son with the flat plate.

(d) A par of brakes (on omosite sides of the body)w.th a total area approximately the same as the 5 In. square br&e.

%easurements of the Irft, drzg and pitchug moment increments due to these brakes over a range of angles were made In the No.1 11% f't tunnel at 120 ft/sec with the body at zem inoldeme.

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The body with the pair of sheet metal cascade brakes was then mounted in the 4 f?t x 3 ft tunnel and measurements of the velocity and velocity fluctuations in a plane 15 in. downstream of the brake hinges were made with a normal hot &.re. This plane corresponds roughly to the fore and aft position of the IIcnter tail plane.

4.3 Results

Values of the lift, drag and pitching moment increments due to the brakes are given in lb and lb. ft at 100 ft/sec in Table V. It is con- sidered that the various brakes are best compared by forming coefficients of lift and drag based on the gross area of each brake, Z$, i.e. the area of out-out required to stow the brake. in Fig.19.

The results are given in this form The pitching moment increments given in Fig.20 are expressed

in terns of Iiunter dimensions, assuming the model to be l/7.5 scale.

Fig.lVb shows that the cascade brakes produce rmxim~ drag at about 50' oompa.1~3 with 90' for the flat plate. The maximum values of AQP are given in the following table tinhlch also gives the values of SP on &ich the coefficients are based.

Type of Brake

Flat plate Aerofoil Cascade Sheet Netal Cascade Sheet Uetal Cascade Sheetldetal Cascade

P,TldiTh 0-n)

5 5.75 5.75 5 3.54

I i -T

No. of vanes

; 20 15 II

Nominal "Ty$

5 6.54

~ ;:I$; ~ 3.54

s, (sq.fi) 0.174 0.304 0.280 0.180 0.188

I 2

1.28 1.76 I.42 f.41

,I.47

h the absenoe of a '~&ng the flat plate compares less favourebly with the aerofoilcascade since the interference drag of the flat plate is much reduced. The sheet metal cascades are consCierauly less effective than the aerofoilcascade but still have some advantage over the flat plate.

Lift increments due to the brakes are plotted inFig.iga. It will be seen that around g= 50' the cascades produce about as much lift ae drag, while the flat plate produces maxiram lift at about t3 = 30' and zero lift at 8 = 75'. This is in good agreement with the results obtained on the 4.5 in. diameter body (Fc.g.10) the value of X/4 for the tests on the 7 in. daemeter body being 3.87,

Patching moment increments are shown in Fig.20. The flat plate gives a msximuru trim change at 0 = 30' and zero trim change at 0 = 73'. These results are also in good agreement with the measurements made on the 4.5 in. body and given inFig.14. The oascade brakes give very large trim changes so that it would always be necessary to use them in pars.

The results of the measurements with a normal hot wire in a plane 15 in. downstream of the brake hinges are sho%n zn Figs.21 and 22. Fig.21 shows that the wahe of the cascade brake (at 50°) is split anto two regions of low velocity; Fig.22 shows that the longitudinal velocity fluctuations are entirely random. One anslysis of the longitudinal velocity fluctuations behind the square flat plate at 70' was made in a position (shown m Fig.22) where the fluctuations are a mauimum. The resulting spectrum is shown in Fig.22. This clearly shows the regular shedding of turbulent eddies with associated random low frequency fluctuations larger than those behind the cascade.

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The longitudinal fluctuations ~31 not directly affect the aeroplane except via fluctuating forces on the brske itself; the lateral fluctua- tions have not yet been measured, though work. on this is now in hand. On the assumptions that the lateral fluctuats.ons vary directly with the longitudinal fluctuations and that low frequency vibrations are the most important, the cascade brakes look very promwing.

If t;yo flat plates, of the same total area, had been used on either aide of the body, the spectrum would have been moved to the right by log 42 and the fluctuation maxima would have been beyond the trslling edge of each brake, extending over the widtln of the brake, in much the sat&e ~a as for the cascade brakes. of fF(f $

At lOOf/u = IO the relative values would have been 0.002 for the flat p&es against 0.0008 for

the cnscaces.

If the fluctuations affect the tailplane, the single flat plate may have an advantage ~tl having its maximum fluctuations below the aimraft and further away from the tail.

5 Discussion

The present results suggest that considerable improvements in l.oW frequency vibration due to flat plate szw brakes muld result from using the brakes at sm&ller angles than is usual. A single under fuselage brake has been considered because it interferes least vmth a tailplane if this is above the fksclage. Such a brake used at small angles suffers from twz disadvantages: in order to maintain the same drag a greater area 1s required, and it is impossible to avoid large tram changes. A possible solution is to use a pair of brakes (on opposite sides of the body, for exsmplle) but this will generally bring the w&es of the brakes nearer to the tailplane and increase the blockage effect on trim and elevator hinge moment, and also increase vibration. From stability considerations, tail planss behind swept vcings should be low and nn obvious solution would be exteni&.ng the jet pipe and carrying a sym- metrical arrangement of brakes behind the tail. Vhen symmetrical arrangements of brakes are conside~d, cascades offer the advantages of high drag coeffLcz.ents with relatively small low frequency velocity fluctuations.

6 Conclusions

The nature of the wake behind an isolated sqoare flat plate changes m character between 0 = W" end 50'. At larger angles there is a regular shedding of turbulent eddies which gives large velocity fluctua- tions m the w&e at a single frequency. Large random low frequency fluctuations are nssoclated with this shedding. At saaller angles there is no regular shedding and the largest random fluctuations oocur at relatively high frequency.

Results obtained wth a spare plate on a body suggest that under these conditions the angle at vtis.oh the flow changes is greater than that for the isolated plate.

A single brake under a body can give ecro trim change only at an angle of about 70'. At angles jmrch different from 70' It is necessary to use a pair of brake-.

A cascade brake has a higher maximum drag coefficient than a flat plate and there is no regular shedding. The lift on a cascade brake produces large ohCanges of trim unless a pair of brakes is used.

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List of S.ymbols

e

c

d

M

X

x

z

%

c

A

uo

U

u

ALI 00

ADI 00

QGOO

*%B

*%3

f

= Incidence of plate (or brake) relative to free stream or body axis (degrees)

= Length of suk of square breke

= M . 2 s.xzmm diemeter of body

= Width of cascade brake

= Distance of brake hinge plane from end of body or in the case of the body inth faxed tal, the distance of the brake hinge plane f?eom the datum sho%n 1~ Fig.7

= Distance of C.G. fixxn end of body or, in the case of the body with faired tall, the distance of the C.G. from the datum shown in Flg.7

= Distance f'rom end of body, or, m the case of the body tith faired tall, the distance from the datum shown in Fig.7

zz Distance of br&e hinge below body axis

= Area of plate or brake (see para 4.3) sq.ft

= Cross sectional area of tunnel v~ork.i.ng section (sq.ft)

= Aspect ratio of plate

= Tunnel speed (uncorrected for blockage) ft/sez

= Local mean 1ongltudlns.l velocity (ft/sec)

= r.m.s. value of longxtudinal velocxty fluctuation (ft/sec)

= Lift increment due to plate or breke (lb at 100 ft/sec K.S.)

= Drag increment due to plate or brake (lb at 100 ft/sec M.S.)

= Pitching moment increment due to plate or brake (lb f't at 100 ft/sec) U.S.

AL = Lift increment coefYicient = - @B

= Drag increment coeffuxent =

= Pitching moment increment coeffxclent

= Frewency (cycles/sea)

” 14 -

List of Symbols (Contd)

IOOf .I_ = Frequency correspo~ to tunnel speed of 100 ft/sec

UO

F(f) = Spectrum function such that

es I = PwF(fj ar I- I

-m(f) dilog f) Y

0 -co

- 15 -

No. Author

1 R.H. James and J.H. Mitchell

2 H. Scnuh and X. G. Wmt er

3 i?.B. Atkins and R.m. Muqhy

4 Von 0. Flacnsbart

5 A. Fage and F.C. Johansen

6

7 E.C. Maskell

RE?ERXWCES

Title, etc.

Turbulence Measuring Apparatus R.A.E. TeckNote No. Inst.949 March, 1946

R.A.E. 4 ft x 3 ft Experimental Love Turbulence Wind Tunnel Part II Measurements of longrtudinal mtensity of turbulence R.A.E. Report No. Aero.2285 August, 1948 ARC 11829

Loiv Speed wind tunnel tests on a new type of au brake suitable for Vampire aircrsft Aeronautical Research Laboratories, Australia. Aero Note 106 August, 1952

Messungen an Xneoenen und Gewalben Platten Ergenblsse der Aer-cdynamischen Versuchsanstalt eu Gottingen IV Lelferung 1932 p.96

On the flow of air behind en inclined flat plate of infinite span R& Iv1 No.1104 February 1927

Modern Developments in Fluid Dynamics Vol.1 para.

Pressure distributions illustratirq flow reattachment behind a forward mounted flap C.P. 211 March 19%

- 16 -

WBLEI --

Increments of lift, drag and pitching moment L-2 gg,:! Square plate on 4.50 in. dia. body (Bldf tad) ;; - --,

4 5

4.167

3.583 0.767

0.767

0.767

2.083 0.767

0.767

0.767

0.323

I t-

i 1

0.767

0.767

0.767

90

0 50

::

1% 130 -

90

30

:: 90 - 30 50 70 TO

-

,'z 70 90 90

.so 50

::

30

:: TO

-

,

0.639 , 0.611 0.370 0.864

-0.045 l.lGl -0.458 1.303

0.5% 0.610 0.360 0.872

-c.O37 1.183 -0.446 1.342

-0.507 I.353

0.555 0.625 0.292 0.895 0.298 0.892

-0.'116 A.213 -0.509 1.338 -0.788 1.213 -0.856 0.969

-0.522 1.339

0.584 0.642 0.224 0.931

-0.145 1.187 -0.503 1.308

0.510 0.679

0.271 I I.300

I:::;: j :::;;

ii:;2 ' 0818 j 1:327

"o:gi ; ::g3

0.960 0.800 1.187 1.401 1.062 1.714 0.750 I.798

*QB

-0.436 0.043 0.970 1.325

-0.297 0.073 0.909 I.302

1.289

-0.254 0.227 0.277 I .061 1.264 1.080 0.491

1.311

-0.207 0.389 I.057 I.149

-0.164 0.444 0.903 I .008

-0.107 0.326

",*'6;: 01716

0.251 0.651 0.902 0.793

0.545 0.734 0.965 1.165

- 17 -

Increments of Lrrft, drag and pitching umment

Square plate on 4.50 in. dia. body (Boat tsll) $ = ;, T = - 'and: 5 !

a.707 0.767

0.689 0.455 0.447

-0.016 -0.445

0.611 0.844 0.851 1.141 1.285

-0.757 -0.317 -0.299

0.870 1.391

0.673 i -1.231 0.868 -0.664

I

*%B

0.872 0.519

0.074 -0.373

1 1.105; 0.258

L 0.930 --

8.123 0.767 -0.44’ -0.674 -0.859

q .274 I.153 0.929

1.286 1 .ooo 0.615

6.363 0.767

0.667 0.433

-0.010 -0.425

O.@Jt 0.878 1.165 1 .%y8

-0.470 0.894 0.030 0.500 0.905 0.046 I.373 -0.381

0.633 0.416

0.632 0.9’9

-0.278 u4.0

4.040 0.733

50 30

;i 70

;“o 110 I30 - 30

;: 70 70 90

ITo” 130 - 90 90

110 110 130 130 - 30

::

0.031 -0.370 -0.364 -0.657 -0.848

4.236 1.356 1.350 I.196 0.982

I.109 1.432 1.383 1.128 0.565

0.895 0.892

%~ 0:064

-0.316

0.683 i- -1.008 0.872 -0.4-E I.113 0.546 I.256 0.959

0.722 -0.793 0.720 -0.77’ 0.891 -0.062 ?.I66 0.868 I.165 0.857 1,306 1 .oov

0.716 0.667 -0.198

0.451 0.954 2.357 0.663 0.226

-0.138 -0.121 -0.519 -0.76f

I.231 1.328 1.333 1 .I75 0.929

0.793 o.aa4 0.906 0.809 0.206

0.9'17 0.764 -0.634 0.491 0.916 0.037 0.486 0.923 0.013 0.272 1.201 0.451 0.266 1.208 a42

-0.098 2.320 0.694

I.007 0.567 i, 1,

0.371 0.387

-0.a.o -0.033 -0.490 -0.487

1.510 1.510 1.290

0.309 0.289 0.382

/ T

I.290 0.820 1 "0:::;

0.217 @.834 (

0.867 0.547

0.873 1.018 0.812

/ ’ 0.449

I.056 1 .OOl 0.807

0.8uC -0.451 1,220 -0.420 I.536

I -0.245 -0.133

2 5 1

0.432 ’ 1.597 1 90

- 18 -

Imrements of lift, drag, and pitching moment.

Square plate on 4.50 in. dm. body (Faired tail) e = 2, @ = !- a3 4 5

“4 z/ e j e” *Ck / As>, / *%I~

30 0.661 0.618 -0.706 8.707 0.767 / ;o” 0.419 0.860 -0.154

-0.045 I.131 I.032 / 90 -0.426 1.263 l.342

-- *

30 0.652 0.600 -0.530 6.363 0.767 ‘7; 0.409 0.869 0.061

-0.036 I.149 I.059 90 -0.425 q.267 I.334

30 0.676 0.622 -0.367 4.040 0.733 50 0.437 0.891 I 0.=4

:: -0.350 0.039 i 1.200 I.328 I.113 1.419 ,

:I 0.720 ‘ 0.630 0.488 0.931 i -K2:

2.357 0.663

;:

;.1’%8” I 1.205 1 0:788

-0:223 ::;;; / 0.802 1 .oov / I

0.867 0.547 :: 70

v” 8

Inorements of lift., drag and pitching moment

Square plate 0114.50 In. dia. body (Boat tail) i = i, T = $

xh z/e e” “$3 *?E A%3

:: oo::z ::z -0.757 -0.165 II .6Og 1.022 70 -0.087 1 I.183 1.565

. 70 -0.079 I.169 1.516 90 -0.436 1.295 I.976

30 0.497 0.565 -0.3qs

73.484 1.022 :o" 0.370 0.882 0.103

-0.043 I.196 I.479 70 -0.044 1.195 1 .a 90 -WI.% I.301 2.057

30 0.462 0.563 0.063 5.387 0.978 :: 0.326 0.925 0.747

-0.008 I.267 1,766 90 -0.407 1.372 2.120

30 0.531 0.602 0.161

3.142 0.884 1 50 0.370 0.9&6 0.839 ;: 0.365 0.958 0.84.1

0.099 1.271 1 A.86 90 -0.282 I.361 1 l 724

0.850 0.766 0.289

'1.155 0.729 :"o 1 .O36 1.338 0.605

1 70 go 0.500 0.872 1.608

! 0.767

1.612 0.927

- 20 -

Increments of Iaft, drag and pitching moment. Cascade brakes on 7 in. illa. body

6' 1 AL,, 1 AD,00

j in. Square Plate

1.17 0.48 -2.35 1.72 1.00 -3.07 I.84 1.35 -3.w l.GO 1.44 -3.07 I.40 1.42 -2.59 1.33 1.54 -2.34 0.97 I.90 -1.79 0.17 2.43 -0.09

-0.58 2.64 I.19 -1.04 2.38 I.84

;heet Metal Cascade (rn 1 5.75”)

30.3 2.20 40.3 3.59 45.3 4.04 50.3 4.22 60.3 4.14 70.3 3.50 90.3 2.39

3.66 4.46

Aif1 00

-1.77 -6.19 -6.82 -7.42 -7.39 -6.50 -4.97

Sheet Metal Cascde

2 Sheet Metal Caaodes (W = 3.54”)

- 21 -

kt.2078.CPZSI.X3 - Przntea ZT Great BT-*t.tnzn

MA TU u-v \Y I I\ii SEC-MN)

TRAVERSINq 5LlDiNij BLOCK

PLATE AT 8 = 40’ MOUNTED PLATE MOUNTED ON STREAMLINE STRUT. ON DRAG BALANCE

2L-- f&*

-4 --f----

SCRAP SECTION THROUGH PLATE

FIG.1 ARRANGEMENT OF APPARATUS.TESTS ON FLAT PLATES IN 4FT. X 3 FT: WIND TUNNEL.

(a> SQUARE PLATE (SB = 25 SQ IN.)

I %

I I I 1 20 40 0” 60 80 100

@I DELTA PLATE (S, - 25 SQ.IN~

FIG. 2.(0&b FLAT PLATES - VARIATION OF d D AG WITH INCIDENCE.

I I I I I I

0 05 0-1 0.5 I A 5 10 50

FIG. 3. RECTANGULAR FLAT PLATES (e =90’) VARIATION OF DRAG WITH ASPECT RATIO.

-.- VELOCITY CON

10

(a> SQUARE PLATE (A = I.0 S, =25 SO.INJ

I I -- BUBBLE BOJNDARY if-1 - VELOCITY CONTOURS *ho

_---- REVERSED FLOW VEL CONTOURS IO- I I I

PI RECTANGULAR PLATE (A = 2.15 SB= 27 SQ. IN.)

FIG. 4 (a&b) FLOW BEHIND FLAT PLATES (0 -903

10 IN

FROM $ PLAT

IO I I 20 IN. I

/‘I ~1’ n AS-r nc

--- --- REVERSED FLOW VEL CONTOURS

IO

IN. FROM

PLATE

60”DELTA PLATE (iB= 25 SQ. INJ

- BUBBLE BOUNDARY - VELOCITY CONTOURS “/uo ‘O-

------ REVERSED FLOW VEL CONTOURS

w ‘O

I I I

CIRCULAR PLATE (Se= 25 I SQIN;)

FIG. 4.(ckd) FLOW BEHIND FLAT PLATES (e =90’)

-0.6

0 e TOTAL HEAD, STATIC PRESSURE AND VELOCITY DfSTRlBUTlON 6 IN. BEHIND SQUARE PLATE. (%=25 Q.lN.)

0 0;5lANCB’ 0EHl;D PL,4tE (IN)

20 24

-0.2 s + PUoZ

-0.4

-0.6

SQUARE PLATE 0 SQUARE PLATE Q 60” DELTA PLATE

60”DELTA PLATE RECTANqUL4R PL

-0.8

(0 STATIC PRESSURE BEHIND PLATES NEAR PLATE AXIS.

FIG.4(e o f) FLOW BEHIND FLAT PLATES (0=90)

PITOT STAT C MEASUREMENTS

WIRE

Oo I I I I I I I z 4 6 8 10 12 14 1 6

. \ DISTANCE FROM t OF PLATE (IN)

(9) VELOCITY DISTRIBUTION 18-O IN. BEHIND SQUARE PLATE. 10’

z- %!i kw 5’ Es &$A z 0

FIG. 4.(gkh) FLOW BEHIND FLAT PLATES (e = 907

. Nn

FIG 5(aab) FLOW BEHIND SQUARE PLATE 15.3 INCHES DOWNSTREAM

0 026+--+++#

a.024 I I I I I

0 022

0~020 \

0.018 .

0.006

0,004

FIG 6. FLOW BEHIND SQUARE PLATE. SPECTRA OF %J. MEASURED IN PLANE 15.3 IN DOWNSTREAM OF PLATE AT POINTS INDICATED ON FIG 5.

(SPECTRUM FOR 8= 9dMEASURED II S IN DOWNSTREAM AT POINT 51~ FROM ,$ )

0 OE -

WIND

,

FAIRED TAIL

SQUARE PLATE

FIG. 7. (a-c). ARRANGEMENT OF SQUARE PLATES ON 4.50 IN. DIA. BODY.

r A t BODY d

.

BRAKE HlN& X--L

(a) LIFT.

0 8 6 4 x/e 0

(9 DRAG.

-l *o

(c) PITCHING MOMENT.

FIG.8 (a -c) LIFT, DRAG AND PITCHING MOMENT SQUARE PLATE ON 45olN. DIA BODY (BLUFF TAIL) ‘/d = ‘/& @$f= %..

1 .d

1

BRAKE HINGE

(0) LIFT.

(c) PITCHING MOMENT.

20

n CD 0 I.0

0

:o

*o

W-Q

'I 0

FIG.9 (a-c) LIFT, DRAG AND PITCHING MOMENT SQUARE PLATE ON 450lN DtA BODY (BOAT TAIL) x//d = %, v = %

BRAKE HINGE ‘~------l

(a) LIFT.

I I

‘Tb) ’ DRAG.

I I I 6 'V< 4 2 0

(C) PITCHING MOMENT.

FIG. IO (a-c) LIFT, DRAG AND PITCHING MOMENT SQUARE PLATE ON 4’50lN DIA. BODY (FAiRED TAIL) ‘/d= %, y = &

W 6 4 2 x/f

:0

3 CD0

I.0

3

e=90” --- -------I*0

70”

+’ /-q, AC

% et-

0 C PITCHING MOMENT.

FIG II (a-c) LIFT, DRAG AND PITCHING MOMENT SQUARE PLATE ON 450lN BODY (BOAT TAIL)

-f/d= 73, y-‘/5

(a) LIFT.

t

‘ib, ’ DRAG.

I I I I I

(C) PITCHING MOMENT.

FIG.l2.(a-c) LIFT, DRAG AND PITCHING MOMENT SQUARE PLATE ON 4’5oIN. DIA. BODY (BOAT TAIL) e/DC.=4 GAP= Ys.

FAIRED TAIL

x BLUFF TAIL Q BOAT TAIL A FAIRED TAIL

FIG. 14.(a - c). PITCHING MOMENT ABOUT A RANGE OF ASSUMED C. G. POSITION.

SQUARE PLATE ON 4.50 IN. DIA BODY. “/d - 73, ‘*p = &.

LIFT MOMENTS DRAG MOMEI\ITS MOMEN% ABOUT HINGE

(a) BLUFF TAIL, e/d= %, y=%

(C) FARED TAIL,

5

FIG 15 (a-c) ANALYSIS OF PITCHING MOMENT ABOUT MID C G POSITION OF SQUARE PLATE ON 4 501~ DIA BODY (SEE FIG 14)

UE a0

?I f

0 a”

r

I

FIG. 16 (a-c) PITCHING MOMENT ABOUT MID C.G. POSITION (FIG. 14.) EFFECT OF GAP AND PLATE SIZE. SQUARE PLATE ON 4.5 INS.

DIA. BODY WITH BOAT TAIL.

5 IN SQUXRE PIATE ON 7 IN DIA BODY (3 = POINT 15 IN WWNSTREAM OF HINqE(FIq

PLATE@ -70’) POINT I5 3 IN 30WNSTRE4M

OF PLATE (FIG 6)

WERE ME?SUREt) 3R INTERPOLATED

SKETCHES SHOWING F’OINTS AT WHICH

FIG. 17. FLOW BEHIND SQUARE PLATE (8 ~70”) COMPARISON OF SPECTRA OF -/U,FOR PLATE ON 7 IN. DIA. BODY 8

ISOLATED PLATE

PLANE OF BRAKE HIN’2,fS ( BODY DIA = 6.47’)

IN MODEL SC”’ -

L-7 0369

c 17a4Zfl

I t7 00’ C% DIA ’

;-.”

-

ELLIPTIC NOSE

I

PLANE OF HOT bog d-w-l

WIRE TRAVERSE5 (,,.&)“;;“,,“”

~$@cHAMFER

5 IN. SQUARE PLATE

WIND

Imy- c I5 VANE5

SHEET M&TAL CASCADE SHEET METAL CASCADE

/. , - -,.\ (w = 5.00”)

Ofl~1~5 OF SHEET METAL SHEET METAL

CASCADES FULL MODEL SIZE CASCADE (W=3 54”)

FIG. IS. ARRANGEMENT OF BRAKES ON 7 IN. DIA. BODY.

(CASCADE BRAKES SHOWN AT 8 = 45)

0.5

C

r I Z SWEET METAL SWIADES

(354’%354”) 1 I I

AEROFOIL i 452ADE

(+ 56 x 5 ,,

5 IN ?, (4 FT x 3 F-r TUhNEL )

5 IN SQUARE FLAX

I ,I I I 1

0 20 30 60 0” 80 100 II

FIG. I9(a~b) BRAKES ON 7 IN DIA BODY.

-I/( I I 5 IN SQUARE PLATE 5 IN SQUARE PLATE

-0 02

-004

AC,

-0 oc

-O.OE

-0 IC

-0-12

& METAL CASCADE METAL CASCADE

PITCHING MOMENT.

AC, BASED ON HUNTER WINq AREA (346 59 FT)AND MEAN CHaRD (IO.33 FT) ASSUMING MODEL TO BE -jjs SCALE

FIG. 20. BRAKES ON 7 IN DIA. BODY.

VELOCITY -CONTOURS OF ; 0

POINTS AT WHICH SPECTRA OF N_ WERE MEASURE0 INDICATED T%LS @

VELOCITY FLUCTUATIONS -CONTOURS OF z 0

FIG. 21. FLOW BEHIND SHEET METAL CASCADE (354”X 3%i)ON 7 IN. DIA BODY.

MEASUREMENTS 15 IN DOWNSTREAM OF BRAKE HINGE 8:50’

SHEET METAL CASCADE (3,54” x 3.54”) e ’ 50”

5 IN SQUARE PLATE 8-70”

SKCTCHES SHI)WINq POIN-rS AT WHICH SPECTRA WERE MEASURED 15 IN BEi-lIND HINqE PLANE (FIG 18)

0~028

0 0’26

ofx4

O~OZZ

0 020

0 018 f F(f)

0016

0.014

0,012

o,o IO

0 008

0.006

0,004

0~002

0

-ON BRAKE G-1 3 54 IN

t

---- 2.5 IN BELoT I

BRAKE &, I

, -

A

FIG. 22. FLOW BEHIND BRAKES ON 7 IN.

DIA. BODY SPECTRA OF $

Wc.lO78.K3

C.P. No. 251 (17,566)

A.R.C. Technical Report

HER MAJ~STY’S STATIONERY OWICC

To be purchased from York House, Kings-way, London w c 2

4~3 Oxford Street, London w.r P 0 Box 569, London S.E.I

IDA Castle Street. Edmburgh z q St. Mary Street, Cardiff

39 Kmg Street, Manchester z Tower Lane, Bristol I

z Edmund Street, Burningham 3 ’ 80 Ckchester Street. @elf&

or through any ba&seUer

S.O. Code No. 23-9009-51

C.P. No. 251