Document
REPORT No. 446
AIRFOIL SECTION CHARACTERISTICS AS AFFECTED BY PROTUBERANCES By EASTMAN N. JACOBS SUMMARY to study the effects of a small ring protruding from the surface of a sphere. Large negative, or favorable, The drag and interference caused by protuberant interference effects were observed at certain values of from tb surface of an airfoil hum been determinedin the the Reynolds Number because the turbulence produmd ‘N. A. O. A_. varhkkity wind hmt-el d a Reynokh by the protuberance changed the character of the Numbw oj approximately 3,100,000. % e~eci8 oj vari- boundary layer so aa to delay the separation of the flow atimw of thefore-and-afi podiun, heighi, and shupe oj from the surface, thus producing a smaller turbulent the protuberance were meiww.red by determining how t)k wake and a smaller drag. Similar experiments have airfoil 8ecti0ncharacterhticxwere ajected by the addiiion more recently been performed by Ower in England of th variom @otu&ram @5n4i?i~ W th &we with streamline bodies. (Reference 1.) At low values span of the airjoi.t. T/i-5 re4ult8 provide fundamental of the Reynolds Numb6r, when the flow in the bound- dala on which to base the prediction of the e~eri%of actual ary layer of the body is to a considerable extent 8h0rt-8panprotuberanztx. The data may &o be applied laminar, protuberances from the forward portions of to tlu de-signof air brakes and 8poiLer8.
the body cause a transition horn the laminsr to the INTRODU~ON turbulent state of flow in the boundary layer with a resulting increase of drag. This effect is not of great The ideal airplane, aerodynamically, may be con- sidered as one having only the drag due b skin friction practical signifkance, however, because the flow in the and the minimum induced drag associated with its lift. boundaq layer of full-scale bodies is probably, in any Prof. B. M. Jones in England has shown that actual event to a large extent, of the turbulent type. It is airplanes fall far short of such an ideal. Interference advisable, therefore, to make im-estimations involving effects, it seems, must be blamed for a considerable part boundary-interference effects at large vah.ws of the of the energy wasted in producing the turbulence Reynolds Number if they are to be of the greatest msocirkd with the comparatively large drag of actual practical value.
airplanes. Tests have been made in the variabl~density wind The National Advisory Committee for Aeronautics tunnel at large values of the Reynolds Number to determine the effects of protuberance from the surface has planned a series of investigations dealing with the of a streamlined body of revolution. The results have subject of aerodynamic interference. The investiga- tions will, it is hoped, lead to the discovery of the cause not yet been published. The present report deals with of the serious adverse effects and will provide data that another phsse of the investigation; that is, the effects may be applied to the solution of practical problems of on airfoil wction characteristics of protuberances ex- design. An examination of presentiay airplanes, both tending along the entire span from the airfoil surface.
military and commercial, hss led to the belief that a A succeeding report will consider the effects on wing characteristics of protubersmces extending ordy over considerable part of the adveme bt.erference arises portions of the wing span. The tests with which the from small projecting objects, such as fittings, tubes, present report deals were made in the N. A. C. A.
wires, rivet heads, lap joints, butt straps, filler caps, inspection plates, and many other projections from the variabledensity wind tunnel during Mar&, 1932.
main surfaces that may be considered together as pro- The N. A. C. A. 0012 airfoil section was employed tubercumea. A systematic investigation of protuber- throughout the investigation and the dynamic scale of ances differently formed and variously located should the tests was maintained approximately the same indicate the relative msgnitude of such effects and also throughout (Reynolds Number 3,100,000). The effects show the effect of disturbing the flow in the boundary of variations of the position, size, and shape of the pro- tuberance were measured by detenninin layer about otherwise streamline bodies. g how the air- foil section characteristics were aileetad by the addition Some mrly investigations of boundary-interference effects were originated by Prandtl at G6ttingen in 1914 of the various protuberances.
110 REPORT NATIONAL ADVTSORY 00~ FOB AEBONAUTIOS TESTS The characteristics of the airfoil without protuber- The N. A. C. A variabledensity tunnel and the ances-that is, with all slots iilled-were measured twica during the progress of the investigation as a methods employed for airfoil testing in the tunnel are check on the consistency of the results.
described in detail in reference 2. These tests were For comparison with the results obtained at nega- made in the usual way, measuring the lift, drag, and tive angles of attack, average curves for the negative- pitching moments on a 5 by 30 inch duralumin airfoil angle runs on the plain airfoil have been used. These mounted in the air stream so that the angle of attack differ slightly from the corresponding positive-angle could be varied. The model mounting difEered in one respect from that described in reference 2. Instead curves because of asymmetrical support interference.
of using a sting attached to the lower surface of the When the protuberance was in the leading-edge posi- tion the tests were made at both positive and negative airfoil as part of the airfoil support, a special st@ was angles of attack, but average curves have bedn used employed that was attached near the &ding edge of to present the results. Thus the various curves pre- the airfoil. As the airfoil has symmetrical sections, it senting the results for the plain airfoil do not agree was thus possible to make the airfoil and sting as- exactly. Furthermore, they should not be expected to sembly symmetrical about the plane of the airfoil aggee with other tests of the same airfoil, because the chords.
tare-drag correction applied throughout this investiga- A section of the airfoil employed, the N. A. C. A.
tion did not allow for the lower drag of the special air- 0012 (reference 3), is shown in Figure 1. The pro- foil sting employed.
tuberances were placed in the slots shown, the posi- ,--00125 c pm tubermce in 0.15 c posifim RESULTS AND DISCUSSION The results are presented by means of curves of the lift coefficient CL, pro filedrag coefficient C~O, moment coefficient about a point one-qumter of the chord be- hind the leading edge Cm,, and the angle of attack for Leading edge po=”fion. Sfofions & wdinaks in Z chord iniinite aspect ratio aO.
The results are thus presented as airfoil section characteristics. The most important results, those corresponding to the various heights rmd positions of the protubermce, are presented in Figures 2 to 10. Attention should be here called to the fact, however, that the characteristics thus presented should FIGURE I.-N. A. C.A. 0)12 dlfOil nhowims Protnbemmm not be used with precise strip method calculations as tions being: Directly at the leading edge; 5 per cent though they were true iniinite-aspecl+ratio character- of the chord behind the leading edge; 15 per cent istics, but should be considered as average section (approximately the front spar position); 30 per cent characteristics deduced from the test data by the (maximum ordinate position); and 65 per cent (ap- methods described in reference 2. Differences be- proximately the rear spar position). The protuber- tween these section characteristics and the true ones may probably be neglected as long as all the sections of ances were placed only on “the upper side of the sym- the rectangular wing that was tested were operating metrical airfoil, but the effect of each on the lower surface was determined by testing the airfoil through at effective angles of attack +rithin the range of rLp- the negative angle-of-attack range. proximately normal lift curve slope. Their use is also The protuberance consisted of a strip of sheet du- partly justified by the fact that appro.simately correct rahunin havi.qg the desired height placed in one of the results for a f&span protuberate on a wing of nor- slots indicated in I&me 1 in such a way as to extend mal aspect ratio are obtained from them when the along the entire span of the model. The form that will simple aspect-ratio corrections (reference 2) are Rpplied.
be referred to as the faired protuberance was produced, as indicated in Figure 1, by forming over the protuber- Protuberance position.-The results for the largest ance a plaster*f-Paris fairing the cross section of which protuberance (0.0125c) in the various positions on the approximated a small half airfoil section on the surface airfoil surface are. shown in Figure 11. Considering of the main airfoil. The slots in the airfoil when not in 6rst the effects of the protuberance on the lift at low use were fled with duralumin strips carefully filed to angles of attack, it will be seen that the effect of the protuberamx is to decrease the lift slightly for nll the surface apd polished to present a continuous smooth surface. The protuberance was used in only one slot upper-surface positions and to increase it slightly for at a time, start@ with the highest protuberance RU lower?yrfac~ positions. As regards the lift at 0.0125c, and then reducing the height consecutively to higher angles of attack and the maximum value of the 0.0050c, 0.0020c, and in some cases to O.OO1OCand lift, the protuberances on the lower surface have little 0.0004c, by filing off the top of the projecting strip. effect, whereas the adverse effect of those on the upper AIRFOIL SEOTION CHAIL4CTEHSTIOS AS AIWEOTJ?AD BY PEtOTUBFiR&NOES FIf3UEE 2-9ectlon rimracteristiu for various protubmanra helgbk Pmtnbmum on ImrUng edge @ition indicated by arrow) 1.8 .36’ .09 1.6 .32 .08 1.4 .28 .07 be 1.2f24 .06%.
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halglk Protnterarm on lower #mfacej CL30C Mind leadlng M@ @siUon lndkated by arrow) FIGURE S.-mmon -Ckri6tfcs for Vaiiom PrOtabmma AIRFOIL SEOTION CI13AR40!LWEQ3TIC43 AS AEFEOTED BY PIiOTUBEMNOES FIOIJF!E @.-&ctlon olmmoterbtica for VMOIM pmtnkarrm hdgbk Pmtukance on lower mrfaq O.O& beldnd kadlng CS+3EE @ftirm lndidod by arrow) 1.8 .36 .09 /.6 .32 .08 /.4 .28 .07 & Lz&’4 “04 t? ,8 .&l d , gtog.a .os~ ~ % ~:_ b Fh3ww 7.-&d0n chraddd m for varfous protnkomnm heights Protubamnco on UPW mrface 116.Zx behfnd kadlng edge (pcaitiorr Lndfcatal by arrow) EEFOILT NATIONAL ADVISORY COMMDTEE FOB AERONAUTICS 1.8 1.6 L4 FIGURE S.-.%tlon cimracterktks for verieos protnkanm hdghta. Protukm.rm on nPIMT snr’faq 0.31 behind leading edge @Yklon Indkatal by arrow) .UJ o .63.12 k .02 .4 .08 1 @ 1 I 1 1 I T I f I .01 .2 .04
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.Liff we fficienf, CL Angle of shack &h%% as&f roi;o, do ,degrees Fmwm k-ktlon cbnracia+stb for -a-iom @mbemnm helghta. Pmtnkarmo on upper sorfaq @J& behind lredlng wige @dtlen Indhmtcd by arrow) AIRFOIL SECTION CHAR4CTERISTICIS AS AFFEOTED BY PBOTUEEBANOES ~amE Ia-$ectlon obmacterktica for varions protnberanm heights Protulxmnca on UPW .mrfac% (M!& behind Ieadlng edge Qx8ition indicated by arrow) 1.
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AI Fmmm 11.-$ect[on cbaracimktks far vorfom protukance LK8Mona Height Of PIotnbmanc& 0.012& @&iona Indicated by arrows) , , BEPOBT NATIONAL ADVISOBY CIOMMJTIEE BOB AERONAUTICS surface becomes increasingly serious as the protuber- ante is shown by the curves in Figures 2 to 10. Them ance approaches a point near the leading edge. figures give completa teat data for the various protuber- Considering now the effect of the protuberance on ance positions and heights. The effect on the drag of the drag, it will be seen fxom the plots of the proflle~ varying the height, however, is shown more advan- drag coeilicient in Figure 11 that the effect is drastic tageously in Figure 12, where the proil.le drag coeffi- cients corresponding to CL= O and CL= 0.5 are plotted for any position of the protuberant and attitude of against protuberance height. Straight lines repre- the airfoil except for the nose position at low angles of .W .004 .006 .006 .010 .012 0 .002 .004 .C06 .W8 .010 .012 .014’ Akight of pro tubemce x c FIQUBE U—Variation Of (IIW with P@IIb0r8nw hel@t attack and the lower-surface positions behind the nose senting a calcdated variation in drag with protuber- at the higher angles of attack. The protuberances in ance height are also included for comparison.
the most critical positions, on the upper surface near The calculated lines were obtained by computing the leading edge, produce very large increases of the the additional profile drag due to the protuberance profile drag even at comparatively low angles of from the formula attack.
AC~O= CD (V’/V)z h/c Protuberance height.-’l%e effect on the fioil baaed on charactwistics of varying the height of the protubey- CDis the drag coefficient of the protuberance ,.- AIRFOIL SEOTION OHAEACTEEUSTICS M AFFECTED BY PROTUBER4NOES ll”i its frontal area. Weiselsberger (reference 4) giws the In the third region the curves tend to become drag coefficient for flat pldes of very large aspect ratio paraUel to the calculated lima. The actual drag as approximately 2. The value 2 was therefore used influences, however, are much smaUer than the calculated ones.
for the calculations. The term (V’/V)i represents the square of the ratio of the local velocity at the airfoil Some of the curves show a fourth region where the surface at the position of the protuberance to the free- protuberance produces a marked interference with the stream velocity. Values of this ratio claculated by flow over the airfoil. This region is not shoivn by any the method of reference 5 are given in Table I for the of the curves corresponding to CL= O, and only by positions on the surface corresponding to those of the those corresponding to CL= 0.5 for the protuberance , protuberance. The ratio h/c is the ratio of the pro- positions on the upper surface forward of the 0.65c tuberance frontal area to the airfoil area. h other position. Very rapid increases of drag with pro- words, AC~. is the drag the plate would be expected to tuberate height are indicated in this region for have expressed as a coefficient based on airfoil area protuberances higher than 0.005c. The conclusion is neglecting the interference of the plate on the flow over that protuberances extending horn the upper surface the airfoil and the effects of the reduced velocity in the forward of the maximum-thickness position, having a boundary layer of the airfoil on the drag of the plate. height greater than 0.005c, should be particularly The lines plotted in Figure 12, obtained by adding avoided. These protuberances may, however, have AC~Oto the profile drag of the wing without protuber- a useful application as spoilers or air brakes.
ance, are of value for comparison with the actual For the &timation of the drag due to protuberances experimental curves in connection with practical applications, a simpler method of calculating the drag due to protuberance TABLE I.—RESULTS OF CALCULATIONS OF VELOG based on the data given in the following table wiU ITY AT SURFACE OF N. A. C. A. 0012 AIRFOIL probably be more satisfactory than the previous St8tfon, px rant c discussion. In the table are presented the important
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results at a lift coefficient of 0.2 corresponding to vat afrfou : fOr cL-o..—___ L%3 L41 LM L 14 high-speed @ht. The results are given M coefficients v undJ&r~o#e8rn 2 Onf:P~=o~ca- of drag due to the protuberance, the coefficients being ,24 229 LW ,.fl, 8tlWlll vat Ofrfofl ‘on lower mrfaca based on the protuberance frontal area and the free- Mdktlllkt .9tl’Wm fm cL=&8____ ..$3 .Ea L(M L(33
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stream dynamic pressure, so that the drag due to a protuberance may be obtained simply as the product A comparison of the lines with the experimental of the protuberance frontal area, dynamic pressure, curves indicates that four regions may be co&idered and the coefficient from the following table: as the protuberrmce height is increased.
COEFFICIENTS OF DRAG DUE TO PROTUBERANCE The fkst is that region extending from h= O to BASED ON PROTUBERANCE FRONTAL AREA approximately h= 0.00Ic, where the rate of increase (CL=O.2) of drag with protuberance height is low as compared Hefght in with that indicated by the lima representing the ml- tenm of A chord O.m awl am )Sm Lola bohfnd culated values. The relatively slow increase of drag Imm with protuberance height in this region is probably ~ due to the fact that the protuberance is in the low- L1 L8 24 5 Upp ~------------------------ L9 “ 1 + $ 23 20 29 15 Tlpw ~-_-----–-----. --..-.--...:..
veloci~ part of the wing boundary layer. Even in L2 22 30nprfu ~—----------. -------. -!....-.. Lb .–..– .9 L4 Gripper ~- . . . . . ..------------l-.. –..
this region, however, the drag should not be consid- :: 5 IOww ~------------------------ 11 <~ ii L3 i; 15lower Smfam.-.--., . . . . . . . . . ---- ------- ered as negligible, M shown by the fact that the drag .7 L1 L1 L5 xl 10w6T~.--. --.---. --- . . . . ..--. -l....-..
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increase due to the 0.00Ic protuberance expressed as a drag coefficient based on the free-stream dynamic pressure rmd the protubermce frontal area is in no As a rule, the drag due to most of the Protuberances case less than 0.7 at CL= O. ; investigated could be roughly estimated as equal to or The forward positions particularly show a second greater than the product of the protuberance frontal region extending from approximately 0.00Ic to 0.002c area and the free-stream dynamic pressure. A lower where the drag increases rapidly with protuberance drag results from protuberances on the leading edge or height. In this region the protuberance is probably near the leading edge on the lower surface, and from producing serious disturb@~ effects on the airfoil other smaU protuberances, but the rule may be found boundary layer. From a practiwd standpoint, it is useful. The higher drags may be seen from the table therefore concluded that a special eilort should be to correspond to protuberances having a height of made to eliminate from a wing surface protuberances 0.002c or more, particularly when they are on the for- that exceed a height of 0.00Ic. On a wing of 70-inch ward portion of the upper surfacb.
chord this height corresponds to 0.07 inch, or little As a practical application, consider a j&neh thick more than one-sixteenth inch.
butt strap at a position on the upper surface 0.05c \ COMMITTEE FOB AERONAUTICS REPORT NATIONAL ADVISORY behind the leading edge extending along the span of a Figure 14 to a scale corresponding approximately to wing having a 70-inch chord and a 35-foot span, the full scale for medium-size airplanes The general con- frontal area of the protuberance is then 0.091 square clusion that may be drawn from this phase of the in- vestigation is that the airfoil leading edge must be feet. If the velocity is 200 miles per hour, the dy- smooth and fair if high maximum lift coe5ciente are namic pressure for standard air is 102.32 pounds per square foot. Applying the above rule, or taking the to be obtained.
coefficient 1 from the preceding table, the drag is esti- Faking,-The effects of fairing the 0.005c protuber- mated as 102 times 0.091, or 9.3 pounds. The corre- ance are shown in Figures 15 to 23. Each figure pre- sponding power consumption at the speed considered sents the airfoil section characteristics corresponding to one protuberance position for the plain airfoil, the would be approximately 5 horsepower.
The effects on maximum lift of the protuberances of airfoil with the normal 0.005c protuberance, and the various heights are also shown in Figures 2 to 10. The airfoil with the faired protuberance.
effect can be seen more easily, however, horn the curves The results showing the effects on drag of faking the protuberances are shown by the profle-drag curves at of Figure 13 representing the variation of mtium the right of each figure. It is concluded from these lift with protuberance height for the vE&ous positions results that the adverse drag effects of the protuberance 1.61 I I i I I I I I I I I I I I may be greatly reduced but not entirely eliminated by emplofig a simple f airing over the protuberance as shown in Figure 1.
As regards the adverse eflects of the protuberance on the maximum lift, it may be concluded that they can be practically eliminated by a simple fairing of the type employed except where the protuberance is near the leading edge. With the protuberance in the leod- ing-edge position, it is obvious that a suitably formed fairing would eliminate the adveme effects. In this position, therefore, the fairing was applied to only one These results, which me side of the protuberance.
presented in Figure 15, indicate that the faking haa little effect when it is employed on only one side of the protuberance. For the first position behind the lead- ing edge on the upper surface the simple f airing em- ‘I—tti—t ..
ployed apparently was not adequate, as the full value of maximum lift coefficient (fig. 23) was not regained after the fairing had been applied.
0 .002 .004 -w CONCLUSIONS Heighf of pro fuberme x c ~GuEE 13.-Varfation of =RIIn l&ttiA@kanc3 hekht. ROtII- The following conclusions of immediate practical value may be drawn from the results in regard to the on the upper surface of the airfoil. It will be remem- effects of full-span protuberances.
bered that the protuberance on the lower surface pro- 1. For most of the unfaired protuberances investi- duced only a slight change in the maximum lift coeffi- gated except those very near the lending edge, the cient. Figure 13 indicatw that the 10s9 of maximum drag resulting from the addition of the protuberance lift due to the protuberance is nearly proportional to could be roughly estimated as equal to or greater than, the protuberance height except for the positions near the product of the free-stream dynamic pressure and the leading edge on the upper surface. For these posi- the protuberance flontal area.
tions the small protuberances produce disproportion- 2. The greater drag increnaes may rcsndt from pro- ately large effects. In the nose position the protuber- tuberances the height of which exceeds 0.00Ic, par- ance having a height of only 0.0004c reduced the maxi- ticularly when the protuberances are from points mum lift by approximately 15 per cent. This pro- along either surface forward of the maximum-thickness tuberance was so small that it might rather be position.
classed as a surface roughness. Because considerable 3. Very large increases of drag may result from the diflicu& was experienced in ‘forming it, the shape of interference of a protuberance having a height ex- the protuberance was not maintained exactly as de- meding 0.005c if it is on the forward portion of the sired. Sections of the airfoil nose, including the pro- upper surface of the profile.
tuberance, were measured after the protuberance had 4. A simple fairing over the protuberance greatly been reduced in height to 0.0004c. The results of reduces but does not entirely eliminate the adverse these measurements for four sections are shown in effect.
AIRFOHJ SECTION CHARACTEEISTIC!S AS AFFEOTED BY PEOTUBEEANOES i19 ~GUEE 14.-Nme pmffl~ m@smW3 at four repi%emtatfve statfona along ~, ehowfng O.W protubruanm at kadfng edge
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Lifi,coe&ienf, CL Angle of off act% for in fihife aspecf rcdio, ti~,degrees ~GUEE 23.-Effwt of fafrfng O.C@& PIwtubaranca cm UPp@r smfam O.O& behind lea% M@ @ftfk fndkated by arrow) ---- - ----—-——- A-. . . . , - REPOBT NATIONAL ADVISORY CIOMMITTEE FOB AEBONAUTIOS REFERENCES 5. The efkt of a protuberance on the maximum lift is unimportant when the protuberance is” on the 1. Ower, E.: Interference. Roy. Aero. Sot. Jour., July, 1932, pp. 531-77.
lower surface, but becomee very important, even for a 2. Jacoba, Eastman N., and Abbott, Ira H.: The N. A. C. A.
protuberance so small that it would ordinarily be Variable-Denaity Wind Tunnel. T. IL No. 416, N. A.
classed ns a surface roughness, ss the position ap- C. A., 1932.
proaches the-lending edge along the upper surface.
3. Jacobs, Eastman N.: Ted-s of Sk Symmetrical Airfoils in the Variable-Density Wind Tunnel. T. N. No. 3S6, N. A. C. A., 1931.
4 Wieselsberger, C., and Betz, A.: Ergebniwe der Aero- dynarniachenVersuclwmstalt zu GMingen. Oldenbourg kGLEY MEMORIAL kMLONAUTICAL LABORATORY, (Mtlnchen), 1923. II Lieferung, pp. 33-34.
NATIONAL &wIsoEY COMMITTEE FOR AERONAUTICS, 5. Theodomen, T.: Theory of Wing Sections of Arbitrmy T. R No. 411, N. A. C. A., 1931.
LANGLEY FIELD, VA., Jdy Ii, 19%?!. Shape.