II1..JIJIl_~1 11- - ­10055 6219

CO MMUN ICATED BY T HE P RI XCIPAL D I RECTOR OF SCIENTIFIC R ESEARCH (A IR) ,)!IXISTRY OF SUPPLY

Summd"..-This report describes measurements of profile drag made on the wing of the King Cobra aircraft . whichhas a low-drag profile of X.A.C.A. design. The profile drag was high with the original surface finish and although itwas improved when the surface was polished the profile drag was still much too high for a low-drag aerofoil.

By reduction of the surface waviness to ± one thousandth of an inch low drag coefficients of the order of 0 ·0028were obtained.

The report describes tile technique used to reduce the waviness and also the effect of flies, dust, water, high .Machnumber an d normal acceleration upon the low drag characteristics of the wing.

Reports and M emoranda No. 2375

A ug ust, 19+5*

By

F. SMITH, M.A. and D. J. H I G T ON

Flight Tests on "King Cobra" FZ.440 to investigateth e Pract ical Requi rements for the Achievement ofL ow Profile D rag Coe ffic ients on a "Low Drag "

Aerofoil

1. I ntroduction.- In the pas t few years a considerable effort has been put into the exami nationof t he t heoretical requirements for the maintenance of laminar flow much further aft on anaerofoil than has hitherto been thought possible. The major requirement has been found tobe t he maintenan ce over the surface of a steadily rising velocity gradient. On t his basis seriesof aerofoils have been designed both in this country and t he U.S.A., maintaining a rising veloci tygradient back as far as 60 per cent. of the aerofoil chord. \Vind-tunn el tests have demons tratedt hat t hese aerofoils are suitable for the maintenance of laminar flow back to 60-70 per cent.

T he t heoretical work, however, assumes t ha t the aerofcil can be manufactured to the requiredprofile. But, in practice, certain inaccuracies such as joints. rivet heads and waviness due toman ufacturing limitat ions and section distort ion under load are bound to occur. Such in­accurac ies cause variations in the velocity gradient which may be sufficiently large to producepermanent transition from laminar to turbulent flow further forward than the opt imum position.

Several t ypes of aircraft incorporat ing wings of " low-drag " design have been produ ced inthe U.S.A., but it has been found t hat standard manufacturing limit s are not sufficient ly closefor the maintenance of laminar flow in flight.

• RA.E . Report No. Aero. 2078 received 13th October. 1945.

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The objec t of t he present test s was to investigate t he criteria for lam inar flow and to exami nethe practicabilit y of meeting the necessary requirements. Measuremen ts of the pr ofile dragof a t est sect ion were made wit h t he wing surface as received , with the roughness reduced to thesmallest practicable limit, by using t he U.S. specification finish and wit h both t he waviness androughn ess greatly reduced by the met hod described in this report. " Lo\v~drag" was achievedand maintained for some thirty fligh ts in the latter condition. The effect of rain, du st , flies andpolish upon the low-drag qu alit ies of t he fina l surface finish was investigat ed . The tests wereext ended to examine t he effect of Mach number up to M = 0 ·7 and of normal acce leration.

The measurements of drag were corre late d wit h t he transi tion point by a series of t est s in whichthe transition was fixed by sur face ridges.

2. Description of Aircraft.- The aircra ft used for t hese t ests was the King Cobra (P. 63) F Z440.Thi s aircraft is a single-engi ned low-wing monoplane wit h t ricycle undercarriage (Figs. 1 and 2);the engine is placed behind t he pilot as in the Airacobra. The wing profile is NACA 662x-116at the root and 662x-216 at the t ip. F urther det ails are given in Table 1. The sect ion chosenfor the experimental work was on the port wing 137 in . from t he aircra ft cent re line (F ig. 4).The choice of section was rather limited by external fitt ings on the wings. In fact , the testsect ion was handicapped by t he wing- fue l tank access panel and by t he aileron (Fig. 2). Theaileron is however , pressure-scaled and , in some tes ts. the ga p was also sealed between the aileronand the shroud (Fig. 6) to confirm t hat t he result s were not affecte d by airflow into or out of theaileron gap.

~l . .Methods of Test.-3.1. Jl easurement of Profi le Drag.- The profile d rag of the test sectionwas measured by a pitot comb 10·4 per cent. chord aft of the wing trailing edge (F ig. 3). A singlecent rally placed stat ic tube was used. The tot al head loss in the wake was obtained by con­necting the pitot tubes of the com b to airspeed indicators mounted in an automatic observe r.The aircraft A.S.1. and altimet er and a desynn recording the aileron position were also includedin the automat ic observer. The usual" top-hat " curves were obtained and the result s wereanalysed by the method described in Ref. 1.

3.2. Flight Test Data.-Almost all t he test s were made in level fligh t at 10,000 ft . altitude.The exceptions were (a) high Mach number tests which were made at 25,000 It. in a sha llow diveand (b) test s to in vestigate t he effect of doubling t he lift /weight ratio for which t he aircra ft wasflown at 10,000 ft. with a steady bank of 60 deg. An aircraft limit ation of 350 m .p .h . A.S.1.gave CL = 0 ·1 as the smallest lift coefficient which could be obtained in level flight. The pilotwas given a desynn indicator to show t he port aileron position and he was instructed to fly withthis aileron neutral durin g the act ua l recording of result s. The aileron link age and tabs wereadjuste d in preliminary flights so that at high speed with t he port aileron neutral t he air craftdid not roll and also no stick force was required to hold the aileron in this posit ion . At slowspeeds in this cond ition putting t he port ai leron neutral gave rise to a very slow roll; this wasnegligible in effect.

3.3. I mprovement of Su rface Finish. - The reduction of roughn ess on t he test sect ion wasobtained by ru bbing the su rface with damp carbo rundum paper, fine grade (400A), held on asoft pad. Th is tec hn ique is similar to that recomm ended by N.A.C.A. for low-drag aerofoils .Careful rubbing removed small excrescences on the surface and also the well known "orangepeel " effect obtained when a sur face is spray painted . The paper te nded to clog easily andit was necessary to renew it frequently to avoid scratching t he surface. If the paper is usedvery wet , as in t he reduct ion of waviness t ech nique, t he tendency to clog is much reduced bu tthe finished surface appears matt. By rubbing wit h damp paper a smooth highly polished surfaceis obtained, in fact, the subsequent use of polish {Sinec Nos. 2 and 3) made little difference inthe appearance of the surface and gave only a small reduct ion of profile drag. The polish didhowever give good protection against wate r and is recommended from that point of view.

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aA . Xl easurement of Surface H"av1lless.- An Ames dial which cou ld be read to one-fifth ofne-thousandth of an inch was used for the investigation of surface waviness. The dial was

nounted cent ra lly on a met al base supporte d on points 2 in , apart (Fig. 7). The deflection ofthe gauge was t hus a measure of curvat ure over a 2-io . base. Thi s t echnique has been criticisedfor two reasons. Fi rstly , the ga uge does not give t ru e local curvature on a wavy surface, andsecondly. if used on regular waves of I-in. pitch t he ins t rumen t would give no reading of curvature.Although it is true that the cu rvat ure shown by the instrument is not the true one, it is at leastrepresenta ti ve of t he curvature, and a reduction in t he inst rument read ing will in genera l corre­spond to a reduction in curvature. The second criticism is not important in t he present testssince the waviness of the Kin g Cobra surface was irregular (see t he traverses of Figs. 8 to 14).For surfaces on which the waves are regul ar, due perhaps to the method of manufact ure, thebase length should be adjuste d to give the max imum variation of reading when being traversedacross the surface.

In addition, any ga uge of this t ype used on a bumpy surface will show apparent wan's ofwavelength eq ual to gauge lengt h corresponding to each hump. Thus the gauge readings t akenon a surface with occasional indiscrimina te bumps will appear to show a succession of waves.Thi s is the case in t he present tests (Fig. 8 to lot). The apparen t waves of z-ln . wavelengt h arenot true waves but represent occasional bumps on the surface.

Using this instrument, stat ic traverses parallel to the flight direction were made on the te stsect ion. It was not possible to measure deflections in flight during these t ests, but an instrumentis being developed for this work. It is apprec ia ted that th e surface waviness in flight is theimportant factor, but , for these tests, it has been assumed t ha t a reduction of wavi ness understatic conditions will represen t a redu ction of waviness in fligh t.

Five spanwise posit ion for these traverses over t he wing were used on both top and bottomsurfaces, namely t he t est sec tion and sect ions -I I- in . and 81·in . in board and outboard of the testsect ion. From the examinat ions of the results of these traverses in standard positions it waspossible to deduce the areas of the surface which needed to he filled .

3.5. Reduction of wQviness.- A fter initial tests on surface polish indicated t ha t low drag couldnot be obtained by simple reducti on of roughness . it was decided to try to reduce t he waviness.An examination of t he surface wavin ess (F ig. 8.) indicated theoretical critical speeds of the wors tbulges to be about 160 to 200 m.p.h . A.S.1. (Ref. 2). Since the tests were being made up to350 m.p .h . A.S.1. theory indicated that our efforts to obtain low drag were being defeated by t hewaviness of the surface, particularly over the first 40 per cent. of t he chord where t he boundarylayer is most sensitive to surface waves.

The test portion of the wing extending- 18 in. eit her side of the test sect ion was first cleaneddO\\TI to the metal surface, and the standard traverses were made (Figs. 9 , 13), showing that thewaviness had been unaffect ed by the paint and filler put on during manufacture. These hadprobably been sprayed on even ly and then rubbed down in the manner described in section 3.3.As we found during the present tests this technique has little or no effect on the waviness andwill result in a smoot h but wavy surface.

The test portion of the wing' was given t wo coats of primer paint and then paint type fi ller wassprayed on. A coat of filler was sprayed on evenly and then local areas shown by the t raversesto be low were filled with further coats, the number depending on the depth of the local hollow.It was found that at least twenty-four hours should elapse between spraying of large areas, andafter the final coat the surface should be left for three days before any work is carried out onit (Figs. 10-12. 14).

To facilitate the reduction of waviness t hree sets of wooden rubbing blocks with curva tureson one surface corresponding to the surface curvatures were used (Fig. 5). St rips of carbo rundumpaper (grade 280C) were held over these curved surfaces, and t he blocks were rubbed chord wisein the manner sandpaper blocks are used. Each block was used over the area in which the meansurface curvature most nearly matched that of the block. The paper and surface were keptthoroughly wet during the rubbing.

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The process of spraying filler and rubbing down wit h the curved blocks was repeated threetimes on the top surface and twice on the bottom surface (Figs. 10-12, 14) at which stage thereduction in waviness was thought to be sufficient to prevent any effect in the t ransition point.. The large fluctuations at the spar on the unpainted surface (Figs. 9, 13) were caused by the

had joint in the surface skin at the spar, the chordwise cross-sect ion being in the form of aninvert ed Vat th e join t. This defect was improved by filling the surface'for about 6 in. on eitherside of th e join t until a smoot h continuous surface was formed. This resulted in a slight modifica­tion to the main sect ion profile which is rep resented in Fig. 12 by a long wave of some 10 to 12 in.wave length covering t he spa r joint in the fina l surface finish. This wave has, however, such alarge wave lengt h that it should preferably be regarded as a small modification to the sect ionprofi le.

:t6. Effect of Flies and other fuseefs .- The flight tests were made during fine weat her inMarch-April . 1945, and as the days became warmer it was found increasingly difficult to avoidthe test s being spoilt by flies and ot her insect s picked up during flights. On landing, these flieswere foun d to be wit h very fevv• exceptions on t he first 10 per cent. of t he section and never aftof :{O per cent. chord. . It was found t hat by flying before about II a. m. (9 a.m. G.~LT. ) thatcontamination of t he surface by flies could genera lly be avoided. \Vith warmer weather thiszero hour became earlier st ill, and event ually it was impossible to fly the aircra ft wit h anyreasonable hope of keeping the surface clean .

The following method was therefore devised to enable th e t ests to be continued. A large sheetof paper was placed over t he lead ing edge of the specially treat ed part of the wing, being about3-ft. wide and extending from 30 per cent. chord on the top surface round the leading edge to~ro per cent. chord on the bot tom sur face. The paper was kept in position by adhesive tape alongthe sides; a string loop was made aro und t he paper along th e lead ing edge, the free end beingled to the cockpit.

Aft er take-off and climb to abou t 5,000 ft . the pilot pulled the st ring t hus tearing the paperalong the lead ing edge. The two ha lves on top and bottom surface were then blown off by theair stream, leaving the surface clean and free from flies.

This technique was used successfully in the latter part of t he present series of tests, and isrecommend ed for tes ts of this type.

a.7. Use of Tapes to fix Transition Point.-To relat e t he drag results obtained in flight to theircorresponding t ransit ion points, drag measurements \v-ere made wit h surface ridges at fixedpercentages of th e chord. These ridges consisted of strips of adhesive tape, rectangular cross­sect ion O · OO~i6-in . thick and i-in. wide, used in one, two or three layers of one st rip and twost rip widt hs . The drag results showed whether tran siti on had been caused by a particularridge, and thus gave the drag corres ponding to fixin g the transit ion at a known position .

4. Results of the Tests.- -4. l. Reduction. of lVaviness.-Traverses t aken wit h the waviness gaugeare given in Figs. &-12 for the top surface ot the test portion of the wing at successive st ages ofthe improvemen t. It will be seen t hat t here is a steady reduction in the maximum fluctuations ­over the surface and a marked improvement at t he spar about 30 in. aft of the leading edge.

Apar t from th e spa r th e general improvement in waviness particularly of the front half of thewing is represented by a redu ction of the fluctuations from ± t wo thousandths of an inch on theunpainted surface to ± one th ousandth of an inch on th e final surface.

The initial and final t raverses for the bottom surface are given in F igs. 13, 14. Here againt he main improvement is at the spar. The reduction in waviness on the bottom surface is ingeneral not so good as that on t he top surface for th e following reasons: firstly, the bottom surfacewas initially bet ter th an the top; secondly, the bottom surface is mu ch more difficult to rub downthan the top surface by the simple t echnique used in this work ; thirdly, th e bottom surface isopera ting in genera l under more favo urable condit ions than the top surface, and a slightly largertolerance was considered to be permissible.

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4.2. Profile Drag.-The profile drag was obtained from the pitct -static comb by th e methoddescribed in Ref. 1. The results in the fonn of section profile-drag coefficient against au-craf tlift coefficient are given in Figs. IS to 21.

\Vith th e origina l surface a steady high drag coefficient of 0· 0075 to 0· 0080 was recorded(Fig. 15) but on removal of the small local surface defects such as grit embedded in the pain t ,a drag coefficient of 0 ·005 to 0 ·006 was obtained in the neighbourhood of CL =-, 0 · 2 (Fig. 15).At higher and lower lift coefficients the drag rose rapidly to 0 ·0075 at Ct- = 0 ·1 and C,. = 0 ·5 .

The characteristic drag-coefficient curve for this section should show a low drag range fromroughly - 0 ·05 to + O·3S Cc rising rapidly outside t his range to (} ·OO6--o ·008 (depending onReyn olds number). The main differences bet ween this and t he curve III Fig. 15 are the rise indrag bet ween 0·1 and 0·2 CL and the large minimum drag obtained in the King Cobra tests.

The tes t surface was next smoothed down to the standards recommended bv the N.A.C.A.for low-drag wings ; t he results are shown in Fig. 16. It will be seen that there is no improvementof the drag minimum or t he rise in drag at low lift coefficients. It thus appeared that smooth nessalone was not the limiting criterion in the achievement of low drag in flight .

Examin ation of t he surface waviness indicated ' t ha t on theoretical groun ds' the local bulgeswere sufficiently large to cause earlier t ransition at speeds greater than 200 m.p.h. AS .£. As aresult th e surface waviness was next reduced by the technique described in section 3.

The profile drag was measured in this condition and t he result s are shown in Fig. 17. Th echaracteristic shape of the drag-lift curve was obtained, and drag coefficients of 0 ·0028 atCL = 0·1 were recorded. Appl ication of polish (Table 3) to t he surface made very lit tle differencein appearance and only a small improvement in drag (F ig. 18). A value of drag coefficient of0 ·00245 was, however , record ed in th is conditio n.

Fig. 19 shows the results of several flights made after an interval of four weeks and fifteenhours flying. No deteriora tion of drag is noticeable.

4.3. Effect of Afach Number on Profile Dra/?, .- A few tests were made at 25,000 ft. to investigateth e effect of Mach number on profile drag. The higher ind icat ed speeds were obtained in shallowdives. Fig. 20 shows t hese result s at 25,000 ft . compared with a typical curve obtained at 10,000ft . The drag curve is in general unchanged except at the lowest lift coefficients (about 0 ,1).Here there is a noticeable rise in profile drag at a Mach number of 0 ·70 as compared wit h that atM = 0 ·52 (10,000 ft .). It is not clear why the drag rises at .11 = 0 ,70, but it may be due toeither an unfavourable modification in t he pressure distribution causing the t ransition poin t tomove forward, or a break away of t he boundary layer on the back part of the wing behind themaximum suct ion poin t giving a rise in form drag, or distortion of t he surface at high Machnumber. The matter is of some importance, since such a drag rise would be undesirable on ahig-h-speed transport designed to fly for long periods a little below the critical Mach number.

4.4. Effect of Normal Acceleration on Profile Drag.-The King Cobra wing is built to fighterspecificat ions and has been designed to break at a lift /weight of about 10. T he distort ion inlevel flight is th us only that corresponding to 1/10 of the break ing stress. Civil tran sport s orbombers are however, designed to break at lift /weigh t ratios of 4 to 5 ; their distortion in levelflight is thus 1/5 of the maxim um. The tests on the King Cobra are th erefore likely to showoptimist ic results. In order to investigate the effect of doubling t he lift /weight rat io the dragwas measured at 10,000 ft. in 6O-deg. banked turns. T he ma ximum lift coefficient that could beobtained was 0 ·2, but t he aero-elast ic disto rtion was, of course, that corresponding to speedsup to 350 m.p.h . A.S.1. The resu lts, although of necessity rather sca ttered. showed that theeffect was negligible, and it is concluded t hat, had the design load of t he King Cobra been doubled ,th e effect on th e present results would have been negligible.

4.5. Effect of Flies on the Profile Drag.-The difficulty of avoiding flies and other insects andthe method adopted have been discussed in section 3.6. Several flights were made however,on which flies were picked up on the wing during and after take-off. It was impossible for thepilot to see whether the t est section was affected and so occasionally results were obtained inthi s condit ion. These results were an alysed and are given in Fig'. 21 for the surface which wouldotherwise ha ve shown low drag characteristics. The levv drag of 0·003 has been raised to valuesranging from 0 ·004 to 0·006 depending upon the position and number of flies picked up. Ingeneral it ca n be said that flies upon th e su rface will rai se the drag over the low-drag range by50 to 100 per cent. If low-d rag sect ions are to be used with any reliability some means must befound to prevent the f lies reaching the surface or to rem ove th em from th e surface a fter take-offand climb. The d evice used in the present tests of protecting the leading edge with paper whichcould he torn off by the pilot was sat isfactory for the smaller section under test but it is not verysuitable for complete wings \vithout a great deal of development . The alternative is to arrangeto clean the surface after con tamination, but from the experience gained on the present testsit is felt that this would be difficult . The flies on hitting the surface disintegrate and are foundin the fonn of small parts of the flies ' bodies st uck t o the surface with a particularly potent glue.Even on the ground it was found to be quite difficult to remove them, and TO do so in flight wouldpresent a la rge problem.

In general it is felt that prevention is better than cure and that the wings should he protectedfrom flies rat her than cleaned a fte r contamination.

4.6. Effect of Dust and lJ'ater.-Before flight tests the test sec tion was dusted off but a certainamount of dust was picked up during taxying and take-off. It was felt , however, that if thesurface were wet a considerably larger amount of dust would be picked up during taxying. Testswere made wit h th e surface thoroughly wet before taxying and it was found the drag was un ­affected both by dust and water . Examination of the surface afte r landing showed it to heclea n except for a few -mall white spo ts presumably due to residue left when the water dried.On one occasion the aircraft was flown through rain before the test measurements were made.Low drag coefficients were recorded again, indicating that water has no effect on the sur face.

when the surface had been polished with .. Sinec .. polish the water did not penetrate thepaint . as with the unpolished surface, but rolled oft leaving only a few local drops of water anda clean wing . Polish of t his type with a wax base is thus recommended mainly as a protectionfrom water , an d if it is used , the wings can be washed down with water before take-off in muchth e sa me way as o motor car is washed down. It is felt that this represents a cheap and simpleway o f maintaining aircraft with low-d rag wings. It is recommended that the water is filte redbefore use to prevent residue being deposited on t he wing surface.

4.7. Profile Drag 1/;·;tll F ixed T ransif ion.-The low drag obtained with the hes t surface finishcorrespo nds according to t heoretical drag-transition relations (Fig. 25) to transition at about0 ·75 chord. This was felt to be appreciably better than could have been anticipated for thisparticular section, and so it was decided to make test s in which the transition was fixed by su rfaceridges, and thus to establish a transition point-drag relationship for the actual test section.Ridges of varying heights and widths were put on the bottom and top surfaces of the test portionof the wing (sect ion 3.7). Tests were made with these ridges at 10 per cent., 30 per cent . and50 per cen t. of the wing chord : th e results are shown in Fig. 23. These were analysed in the formof drag rise due t o fixing transition (Fig. 24) and plotted against the cri terion" height of ridgedivided by the root of the width of ridge. The resu lts show that in all cases (except the lowestwidth rid ge a t 50 per cen t. chord) the ridges gave a drag rise almost independent of the height.It was concluded that the ridge had fixed transition, and the centre line of the ridge was takenas the transit ion po int . Th e sectio n profile drag correspo nding to fixed transition is given inFig . 2.=i together with t he theoretical relation . The lowest drag ob ta ined in flight now appearsto correspond to abou t 6..:; per cent. chord transition point. This position is more reasonable and ,in fact, corresponds to the best that could he expected with maximum suction at 60 per cent .of the chord.

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\\'e now have, however, an appreciable discrepancy between the theoretical' and measureddrag-transition relation. It was suggested that this might be due to a flow of air into the ailerongap. Since the ailerons are pr essure-sealed th ere can be no flow from the bottom to top surface.There is th e possibility of a spanwise flow in the aileron gap towards th e tip or root, but sincethe thickness/chord ratio is const an t along the span this was unlikely. To examine this possibilitya few tests were made in th e fixed transiti on condit ion with rubber seals actually in the ailerongap on top and bottom surfaces over th e test portion of th e wing. The results (Fig. 25) showedthe effect to be small. The discrepancy between fl ight tests and the theoretical drag-transitionrelation is thus unsolved. .

A summary of the drag results in the various condi tions is given in Fig. 22 together with themeasured and theoretical drag-transition relations. Using the flight transition values it ,,;11he seen that the results of this series of tests redu cing waviness and roughness has been to movethe transition point from the leading edge to HO....65 per cent. chord over the Cr. range 0 ·1 to 0 '2'2­The low drag has not been kept up to the theoretical value of Cr.. -= 0 ·3;:; but this is probablydue to the appreciable waviness and surface inaccuracies st ill left which would te nd to reducethe critical lift coefficient.

.... .8. .\1aintenance of the Surface. - It is of general interest to know how satisfactorily the specialfinish used in these t ests is and how difficult it was to maintain during the tests. No trouble wasexperienced with cracking of th e surface filler and there appeared to be no tend ency to dry outor !ose its flexibility. At the time of writing this report the surface is six months old and is stillsatisfactory.

Only one slight difficulty was experienced and t ha t was at th e skin joint on the main spar.The gap between the two skins was fi ve to ten th ousandths of an inch and was filled with filler.During flight , owing to small varia tions in the gap under load, there was a tendency for thefiller to squeeze out of th e gap. and it was necessary to rub down th e slight ridge so formedevery few flights. This difficulty could have been avoided if the skin had been chamfered onthe under side before assembly, thus forming a dove-t ail joint for the filler.

Apart from rubbing down t his ridge occasiona lly , noth ing was done to th e surface throughoutthe tests.

5. Conclusions.:-.The main conclu sion of this report is that low-drag coefficient s can be achievedand maintained in flight on a low-drag wmg under load up to a Reynolds number of 18 »; 106

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To obtain low-drag coefficient s of the order of O·()()2R it was necessary to redu ce t he surfacewaviness over the front portion of the surface to ± 1 th ousandth of an inch : a larger tolerancecan be allowed further back on th e wing. Roughness has to be red uced to reasonable standa rdssuch as t he N.A.C.A. recommendat ions for low-drag wings.

\Vater can be used to wash down wings before take-off without any detrimental effects. Rainduring flight has no subsequent effect on the low-drag qualities of an aerofoil.

Polishing the surface with wax polishes reduces th e drag only slightly but facilitates washingdown the surface and is recommended for this purpose. Polish would only need to be applied. atinfrequent intervals, e.g. during inspection.

Over the period of the tests th e filler used to produce a satisfacto ry surface gave no troubleand would probably be suitable for production aircra ft . Accurate manufacture of the metalsurface to ± 1 thousandth of an inch would give equally satisfactory results and would obviouslybe the better way of obtaining an accurate and more lasting surface.

Any metal skin joints requiring subsequent fiUing should be chamfered during manufactureto give a dove-tail joint for the filler.

The surface must be kept free from surface irregularities such a.';; inspection doors, accesspanels, etc., as far as possib le and protection against damage from boots and shoes duringservicing and from stones t hrown up by wheels must be provided.

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Tests at a Mach number of 0 ·70 indicated a rise in drag as compared with the drag at Al = 0 ·52 .This may be due to a variety of causes, but it is of some importance for aircraft designed tocruise at high Mach numbers.

The tests made with increased lift /weight ratio indicated that the increased surface distortionin the condition did not affect the drag coefficient.

The most important result is the effect flies and other insects have 'upon the drag. The dragrise when the surface is contaminated is 50 to 100 per cent. of the basic low drag coefficient.It is clear that the surface must be protected from flies either by cleaning the surface aftercontamination or by covering the sur face near the leading edge with some form of protectivecover which can be jettisoned after climbing above th e maximum height at which flies arenormally encountered (about 5,000 ft . in thi s country).

Tests with surface ridges chosen to fix transit ion indicated that th e transition with the testfinish was at 60-65 per cent. chord. The drag-transition relation, however, differs appreciablyfrom the theoretical relation for low drag aerofoils. ,

To summarise, t he results of these tests show that low-drag coefficients can be achieved andmaintained on an aircraft of low-drag design without exceptional difficulty, and it is felt thatproduction of the wing struc ture to the requisite limit s is quite pract icable.

REFERENCES

3 Winterbottom and Squire

N u.

I Thompson

2 Fage

Author Title, etc.

A Simple Met hod of Computing CD from wake Traverses at High Speeds.RAE. Report No. Aero. 2005. A.R. C. 8462. December, 1944.(Unpublished.I

Th e Smallest Size of a Spanwise Surfa ce Corrugat ion which affects Bou ndaryLayer Transit ion on an Aerofoil. R. & :M. 2120. J anuary , 1943.

Note on F ur ther Wing P rofile D rag Calculatio ns. R.A .E . Report No.B.A.l634. A.R.C. 4871. October. 1940. (Unpublished}

TABLE I

A ircraIt Data

Aircraft wing areaAircraft weight (all-up)Aircraft C.G. positionAerofoil section (root) ..Aerofoil section (tip)Distance of section from aircraft centre-lineTest sect ion chord lengthMaximum thickness/chord ratioPositi on of maximum thicknessDistance of comb behind t railing edge

8

248 sq . ft.8,000 lb.27 ·5 per cent. A.M.e.N.A.C.A. 662x- 116N.A.C.A. 662x-216137 in.74 ·37 in .15 ·91 per cent.45·3 per cent. w ord7 ·70 in .

TABLE 2

Ordinates of test section

Per cent . chord y /c upper y lc lotrer y lc total

- -21 2·29 2·26 4 ·5,i5 3 ·23 3·05 6·2871 4 ·05 3 ·6 1 7·66

10 4 '71 4 · 17 8 ·81'15 5 ·87 4·96 10·8320 6 ·74 5·64 12 ·3830 8·02 6 ·54 14·5640 8 ·74 7·01 15·7550 8 ·80 7·04 15·8480 R·24 6·69 14·9370 6 ·69 5· 34 12· 0380 4·30 3·51 7 ·8 100 }·85 1·51 3 ·3695 0 '78 0 ·54 1·32

100

Max . thickness 15 ·91 per cen t . at 45 ·3 per cent chord .

TABLE 3

Ref erence to materials used when improving roughness and waviness

MaJerial

PrimerFillerCelluloseCarborundum paperPolish

Reference

Type " UP " Ref. No. 33B j208.. Delco " Ref. No. KAF 4362D.T.D . 83A Ref. x e. 33B/468.. Hydro-D urexsil " Glades 280C and .,\t)tl .\

" Since " Nos. 2 and 3

9

~~\J

Z W0'";:: 1--'""'Illa' t-

••(Jr-:

,;-e-~

i;;e.,

• 0u Uu "- ~-0u '0J• -" ""• E

0

tf""<:

"E"""U...;

~

~ ~#i>' -

FIG. 2. King Cobra FZ 4-10.

10

••• •. . . .• •

II

FIG. 3. Photographs of Test Section and Comb.

101. <:01. .ot soi. 701. 601. 90t 100X

L/. RADIUS 153% CHORD

~f-- --r-------

FOR ORDINATES SEE TABLE 2 MAXIMUM T HICK NESS ' IS'91Z' CHORD

POSITION or MAXIMU M THICKN ESS ' 4 S-3%

FIG. 4. Profile of Test Section.

BItASS ST!\Il'$ Of~A!\Y ING GAUGl FITTl OIilRE TO GIVE OJHUI(NTCU!\VATl,/lI l l TO 8tOC~

F IG. 5. Sketch of Curved Block used forRubbing Down Wing Surfac~.

~-""'n~----__,~, '~~B&l\ SUI. 5TU'~ TO S ~ RO~O~ .. RUll IN(; SMOTHl.Y ~VCI/. AlI.tRON

-F IG. 6. Diagrammatic Sketch Showing :Methodof

Sealing Aileron Gap.

---

oT'.

FIG. 7. Surface Waviness Gauge

12

00 1 \ I I I I

lJ\ ~' ,.1 -\J ---

OF T£'JT ~eT10N

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a-n. _ _ AAU

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F iG.8. Surface Waviness as Received at RA.E. (Both surfaces). FIG. 9. Top Surface Waviness-After Removal of Paint .

60

~

50

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FIG. 11. Top Surface Waviness-One Coat Primer.Two Coats Filler, with Local Patches of Filler .

- I I I II ! Io ..... ......

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FIG. 10. Top SUI face Waviness-One Coat of Primer,One Coat of Filler, with Local Pa tches 0 1 Filler.

,.1 I

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• 0«>,.

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F Ir.. 13. Bott om Surface w aviness -c-An er Removal of Paint.

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C) F1 --'-:t- .•J, V·S _ _~J 'J r<OT ..

\~ to JO .~ if ~(

'" G I" l 1'\ ~ _ - i - - ~ ' TC5T S E. 1

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eF IG. 12. Top Surface Wavillf'$s-One Coat Primer, Three Coats

ot Filler , with Local Patches of Filler.(Waviness of final surface with which low drag was ol.tained.j

e

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,

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.,

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Section' Profile Dra HCoeflicieut -c-Rougbncss Reduced to• ~.A .C .A . Specifica t ion Finish.

FIG. 16.

. uJ ' '''CI '- l t c lIVI D 1 T A.....l / --" .00.•

~V/-'

~ ~-;~?" ""~, UCt.VIO. WITH

S LIG HT I5l,l AMCIDHIoCT8 JlIMCl'fIO

IEACH TYP~ Of SYl"I150LRIHAS TO ... SIP...R...TI f l.'G KT

00' - 1REYf'IOLOS '1U"U A•• • .. " " •• .. . .

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I ACH TYP I Of lIYlIiI&OL I1llHA 6- TO ... UPAA...TI n iG HT

.,. .. " W ll.UNOlO, " UI"I8U'._,0' o-e .,. , n ·:\ ,.' ••

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FIG. IS. Sect ion Profile Drag Coeffi cien t , with Original Surface Finish.

o

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~ I~CTUAL DEFU CTION . _ _

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,DISTANCEROUNDS URS:CI' F"AOM

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\TnT SErCTlON .

)40 ' ." 0

,,.h rJs

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V I)

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FIG. 14. Bott om Surface Waviness-One Coat Primer, Two CoatsFi ller, with Local Patches of F iller .

(Waviness of final surface with which low drag was obta ined.)

cAuerRUCINCi11.1000

(I NCHES)

­en

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AIIlCRAFT L IF T COH F ICI l NToa..

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oee

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/ EACH T YPE or SYMBOLREfERS TO A S EPAIlAT E rLlG-lofT

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FIG. 17. Section Profile Drag Coefficient- Waviness Red uced by theTechnique Describ ed in Thi s Repo rt , Roughness to N.A.C.A .

Specification Fi nish .

11- 1 02 0'3 0 '4 D,S 0-& 0-1AIRCRA f T Li fT COE f f ICI ENT

Fig. 19. Section Profile Drag Coefficient- Waviness Reduced by theTechnique Described in This Report , Roughness to N.A.C .A. Spec ifi­cation Finish, Surface Polished with Since Nos. 2 and 3. Repeat ofTests Shown in Fig. 18 after an Interval of Four Weeks and Fifteen

Hours Flying.

,,,LlC! TVP l o r lSYMBOL Il!FERS rcA S t PA II.AH n IGH T I, ..... - -._ _ TYPI CAL CURVE f ROM flG. 19

. -- .-- ::z.'oe - -

.If".~:\ .~

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, IIH NOLiS NUMeli ; 10

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()I ce ()J ()" D'S 0' 6 0"AIIICRAFT L IF T COE frrCI £NT

F IG. 20. Section Profile Drag Coefficient-Surface in ConditionDescri bed in Fig. 19. Test s at 25,000 ft . to Investiga te Effect

of Mat h Nu mbe r.

c

FIG. 18. Section P rofile Drag Coefficient- Wa viness Red uced by theTechnique Described in This R eport. Roughn ess to N.A.C.A. Speci­

fication Finish, Surface Polished with Since Nos. 2 and 3.

•""o"D,. 0 '0',st•

,"

,

~.&

~-I [At'" TYPE Of $Yl1 60l-

" R( H RS TO fit. SEPARAT E fL IGHTV >I

•

III 16 " " • , REYNOLD S NUMBER ~

c

""~: 0-00<:',

~C"•

§";: Q-COlI~-e~0 0 0041

0-01

00

-...,

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..Jo.

--" ~F\. / """.~,

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~ z-; P'~<, • tAtH TYPE: 01=' SVMaOL. ~£FE:RS• ~

TO A. SE:PARATE FLICHT

•

~ .." " 10 B REYNOU)S NUMBE:R X. 10

'0

o

o 0 ·1 Q·t 0-3 0-4 0 ·5 ace. 0 ·7 'A.1R.c:.ItAFT LIFT c:.oEFJ:IC:IENT

F IG. 21. Section Profilc Drag Coefficient-EffC'Ct of flies and Other Insects Picked Up on theWing During Flig-ht . The Surface Otherwise Had Low Drag Characteristics as Sho\\T1 in

Figs. 17, 18 and 19.

o

UCEOTHI S REPORT

(H 0 ·2 0 <30 0 '4 o-s 0 ·6 0 ·7,AUU·q.AFT LIFT COEFFICIENT

Comparison of the Section Profile Drag Coeffidents Obtained with various Surface Finishes.

TRANSITIO~ AT-- ----I--------------- . .

" ------lfS RECEIVED AT RAJ.--

o1.e ...- ---/ - ------- ----B ----

20'%C~'-;;'::- ' .»:V "-O%c'~. _ _• :.-=-:.~:. - - - ----P' - -zrile . ROOOHNE.SS REDUCE.

60% --' , V

- --_. _ -~4 e

40% ~-- .-. -!------;/' ---- - -------60% -- -- - -- - 7 1

~-WAVINESS AND ROUGHNt$S REOBY THE TECHNIQUE D£'SCRIBED IN

Gore I I I- LOWEST OJOBTAINED OURINQ TESTS

Z

------ THEORY_._.-

rROM FLIGrT TESTS

o

0 '00

FIG. 22.

0-010

"z:! o-oo

"~~~ve~O'004o

'"~~

o«~ oocZoe'"~

18

,,,,

.. ..,,/

TAPES fI XED AT soX CHORDTOP AND BOTTOM SURFACES

TAPE WIDTH TAPE HEIGHT SYHBOL

;y.... 0-0036 · X

3/..... 0 -0072," +'\14" 0 ·0108 AI~' 0 -0036 Vl)l" 0 ·0072. El

O-~~ l

000oi--~uQ•~ 00061

8

~ou~o...1 1-~

•~..~~oo,1 I I I§~

TAPES FIUD AT 30 1. CHORDTOP AND BOTTON S URFACES

•4 ;.--

/1--.-;4L,.-- - _ y ' ''''''

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___ _ ·r -- ,,,•• •

,,, /---'-----

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I 8 __~ I~_--::

_.--_.----- - - - CURVE FROM fiG. 17

_ ._.- AILERON GAP SEALtO

TAPEi fiXED AT 107. CHORDON TOP AND BOTTOM SURFACE

0' I I I0·1 D'~ 0 ·3 ~I ~t W 0 ,' O-l 0 ·3

AIRCRAFT LifT COUFICIENT AlRCRAFT LIfT COEFFICIENT AIRCRAF"T LIfT COEFFICIENTFIG. 2:1. Section Profile Drag Coefficient, with Tapes Oil Top and Bottom Surface to Fix Transition I'olnt.

0' I I I

~OIOI I

~OO'I I _~ I

~o-CJ04'

u~

~og: 0'00l1

~ 1- I 1~~oo.I---u

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seu~

-'"

- -i: AT J

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• ~•---7 I

'UL("~ QAP Ul\IIi[AL(O •u. ...t. £ O Q

I>OOS 0.010 oOtS141'( HUGHT+{rAP£ WID1 tt") " -(iJ«:H(~ '"

Fro. 24. Increase in Drag Coefficient-Tape Height.

ATION fORDIU. Rif.J

0

LI~ COEffICIENT · 0' 15REYNOLDS NO • 15"'10·

'"".--,

<, -<,<,

"-.K THEORETICAL RELLOWDRAG AEROf-,

r-, -.• "-.., -,, "

LOWDRAG 08TAlNED IN Tl.ST& .LOWEST DRAG RECORDED

AILERON GAP UNSEALED X. . SEALED 0o

0 '00

c-a 0·4 0·6 0-8DISTANCt Of TRANSITION PT. AfT OF" L.t. . ";"CHORO

F IG. 25. Drag Coefficient-Transition Position.

20(lIo:lIlI22l WI. ISIlI08 K5 S/$O Hw.