ACR No. L14.E31 NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS WARTIME REPORT ORIGINALLY ISSUED May 19lt-4 as Advance Confidential Report Ll(E31 PROFHE-DRAG COEFFICIEHTS OF CONVEHTIOML AHD LOW-BRAG • ^ ,t' AIRFOILS AS OBTAHJED IK FLIGHT By John A. Zalovcik Langley Memorial Aeronautical Laboratory Langley Field, Va. ^Pftfl1<'*H*-,^^^„^J:«*3^J£:<»^^*. ,'<'4«CtE9S3 NACA WASHINGTON , I NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of advance research results to an authorized group requiring them for the war effort. They were pre- viously held under a security status but are now unclassified. Some of these reports were not tech- nically edited. All have been reproduced without change in order to expedite general distribution. L - 139 DOCUMENTS DEPARTMENT Digitized by tlie Internet Arcliive in 2011 witli funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation http://www.archive.org/details/profiledragcoeffOOIang NAG A ACR No. L^Ejl NATIOiIAL ADVISOHY COr-IIIITTSE FOR AERONAUTICS ADVANCE CONFIDENTIAL REPORT PROFILE-DRAG COEFFICIENTS OF CONVENTIONAL AND LOW-DRAG AIRFOILS AS OBTAir-IED IN FLIGHT By JoLn A. Zalovcik The resLilts of flight investigations of the profile drag of several carefully finished conventional and lov;- drag airfoils are presented. The results indicated that in all cases lower profile -drag ooefflclents were obtained with the low-drag than with the conventional airfoils over the range of lift coefficient tested and thatj, for coraparable conditions of lift coefficient au'^ • Reynolds numherj the low-drag airfoils may have rsrofile- drag coefficients v/hlch are at laast 27 "percent lower than the profile -drag coefficients of the conventional airfoils. INTRODlPGTION A raijTiher of flight investigations have been conducted by the National Advisory Co^iinittee for Aeronautics during the past several year's to determine the profile drag of various conventional and low-drag airfoils. The p^irpose of this report is to present the principal resiilts of these Investigations in order to provide Information that may be of assistance in judging the relative merits of conventional and low- drag airfoils. AIRFOILS TE: The various airfoils tested were the NACA 27-212, NACA 55-215, NAG A 66,2-2(ll+.7), NACA 6i4.,2- ( 1 ,i|) ( 13 . 5 ) , C0NT^ID31ITIAL KACA AGR No. iJiEjl NAGA 2lj.lli.c5, N-22, and two Rspublic S-3 sections, one 11 -nercent thick and. the other 15 percent thick. These two sections are designated Repnbllc S-3, 11 and Repub- lic 3-5,15 in this paoer. Flight tests of the F.AGA 6"'4.,2--(l.J4.)(15.5) and the FACA 2klli. = S airfoils are ret>orted in references 1 and 2, respectively,. The pro- files of the airfoils tested are shown in figure 1. The ITACA 27-212 and NAGA 55~215 airfoil sections were built into panels around the wings o.f the airplanes on which they were tested. The others were sections of the actual wings of the test airplanes. The arrangement of the test panels and the spanwise positions of the wing sections tested are shown in plan form in figure 2. The airfoil designation NAGA 61|,2-(l.[i.) ( 15 .5 ) , which is the test section of the NACA-NAA (North American Avia- tion, Inc.) compromise lovz-drag wing, was based on the m.aximujn thickness and on the pressure-distribution characteristics computed from the measured ordinates of the test section'. The designation NAGA 66,2-2( 1I1.7) was sirailarly determined. The NAGA 2[:ll|.5, Republic S-5,11, Republic S-5,15, and F-22 sections may be clsssified as conventional airfoils and the NAGA 64.,2- (I.I4.) (15 ,5 ) , TTACA 27-212, NAGA 55-215, and NAGA 66,2-2(14.7) sections, as lew- drag airfoils. All the airfoils tested were carefully smoothed and faired to eliminate perceptible protuberances due to rivets, skin .ioints, and access doors. S\:rface waviness, hov;ever, was present to various degrees on the different airfoils. Surface waviness was measured by use of a curvature gage of the type shown in fig- \ire 5 or the uroer surfaces of the NAGA 55~215 snd Republic S-5,15 airfoils and on the unper and lower surfaces of the NAGA 6if ,2- ( 1 .li) ( 15 .5 ) , NAGA 66, 2-2( l[i..7 ) , and Republic S-5,11 airfoils. No v;avlnes3 measurements were obtai.ned for the other airfoils. The curvature-gage measurements on the NAGA 55"215, NAGA 61;, 2- (1.1+) (15.5), NAGA 66,2-2(lk.7 ) , Republic S-5,11, and Republic S-5,15 airfoils were made with the legs of the gage spaced 1.2, 5.8, I|..0, 1^.0, and 5.0 percent of the section chord, respectively. In order to present these tr^easurements on a comnarable basis, the measurem.ents on the NAGA 55-215, NAGA 6[i.,2- ( 1 .1; ) ( 15 -5 ) , and Republic S-5,15 airfcils were reduced to values d GONFIDHNTIAL NACA ACR No. ri|S31 GONPIDEITTIAL 3 that a gage would give if the legs virere spaced I4..O per- cent of the section chord c. This reduction was made to the first order of approximation on the assumption that the readings of a curvature gage were proportional to the square of the leg spacing. The reduced measure- ments together v/ith the measurements on the MCA 66,2-2(li)..7) and Republic S-3,11 sections are presented in figure k as plots of d/c against s/c, where s is the distance along the surface from the leading edge. The dashed lines in figure l\. indicate the approximate curvature-gage readings that would be obtained if the surfaces were free of waviness. It should be pointed out that wing distortion ift ^flight may introduce v/aviness considerably different from"' that m.easured. This effect is probably adverse and may be expected to vary considerably with v/ing construction. The destabilizing effect on the laminar boundary/ layes due to v-zaviness of a given magnitude increases as the chordwise velocity gradient becomes less favorable (or more adverse). The chordwise velocity distribution for the various airfoils at a section lift coeffi- cient c^ of 0.20 have therefore been included in figure 14.. The velocity distributions v/ere calculated for the undistorted airfoil profiles by the method of reference 3. The velocity distrlbiitions are given as a plot of the ratio U/Uq against s/c, where U is the local velocity outside the boimdary layer and Uq is the free-stream velocity. PROFILE DRAG The profile-drag coefficients were evaluated from v/ake surveys of the various airfoils by the method of reference I4. and compressibility corrections were applied as in reference 5« I^"^ figure 5 'tlie section profile- drag coefficients c^j and the corresponding Reynolds numbers R are plotted against section lift coeffi- cient ci , The Mach numbers of the tests were less than 0,55. prom figure 5 it may be seen that all the low-drag airfoils gave lov/er profile-drag coefficients than the CONFIDENTIAL 1^ CONFIDENTIAL NAG A ACR No. l1+S31 conventional airfoils over the range of lift coefficient tested. The lowest profile -drag coeff Icieiit, a value of O.OOiiO, was ineasured on the NAG A 27-212 section at a lift coefficient of 0.28 end. a Reynolds nuraher of 7,14. X 10^. The NACA 27-212 airfoil, hov.'ever, is not considered a particularly desirahle airfoil becaitse, as indicated "by wind-tunnel tests, low drag is obtained only over a relativel'^'" small range of lift coefficient ^Si^ and the pressure gradient at the trailing edge is o unnecessarily severe. At Reynolds n-uinhers in the range from 15 X 10*-^ to 20 x 10°. new coinTrionly encountered oy fighter-type aircraft, rrofile-drsg coefficients of 0,00^5 and 0.0052 y;ere Pleasured on the NAGA 6^. ,2-2( 1I4..7 ) and NACA 61;., 2- ( 1 .[]-')( 15 . 5 ) airfoils, respectively. At ReT^olds nvd-ihers fro^. 22^ x 10^ to 51 x lOo a profile - drag coefficient of 0.0014.9 ^-'^^ obtained on the NACA 55-215 airfoil. The lowest profile-drag coofficient obtained on the conventional wing sections vi^as O.OO62 and was measured on the Republic 3-5,11. The lowest profile- drag coefficients obtained en the other conventional s-^ctions were O.OO67 for the Republic S-3,15 and 0.0066 for the NAGA 2kll4..5. All these values were obtained at low lift coefficients in the range of Reynolds number fro;n IS x 10^ to 20 x 10^. On the N-22 section only one value of profile -drag coefficient, 0.0070, was obtained, which v;as at the relatively high lift coeffi- cient of 0.50 ^^d the low Re;;,Tiolds n-ijaaber of I1..4 x 10^. The results for the Nj^GA 66,2-2(li=..7 ) and Repub- lic S-3jll sections were obtained for the aost nearly comparable test conditions - that is, lift coefficient, Reynolds niu^iber, and wing-surface preparation - and are therefore best suited for the coraparison of the profile- drag characteristics of low-drag and conventional air- foils. At a lift coefficient o£ 0.20 and a Reynolds number of 16 x 10^ the profile-drag coefficients for the NACA 66 , 2-2 ( l[;.7 ) and Republic S-5,11 sections were O.OO4.5 and 0.0062, respectively. The profile-drag coef- ficient of the NAGA 66,2-2(lk.7) section is thus O.OOI7, or 27 percent, lower than the profile-drag coefficient of the Republic 3-5,11 section. Unpublished tests in the NACA two-di^iensional low- t'^rbulence pressure tunnel of a section apDroximatlng CONFIDENTIAL NACA ACR Fo. lIlSJI CONFIDENTIAL 5 the HACA 66, 2-2( I4 .7 ) indicated a -prof lie -drag coeffi- cient of O^OO^k at a^lift coefficient of 0.20 and a Reynolds nurrber of I6 x 106. Similar tests (unpublished) of NACA 250-3erie3 airfoils indicated a Drofile-drag coefficient of O.OO65 for an NACA 2^011 section at a lift coef ."Cicient of 0.20 and a Reynolds number of 9 ^' 10"^. The Republic S-J sections have pressure- distribution characteristics that are very nearly those of the NA.CA 250-series sections and may therefore be expected to have the same drag characteristics. Inas- much as Che surfaces of the NACA 66, 2"2( lij..7 ) airfoil tested in f].lght were carefully finished to give a very low degree of waviness (figs, h^ig) and (h)), probably comparable writh that cf the tunnel model, the con- siderably greater drag m.easured in flight as compared with the value obtained in the tunnel is believed to be due to an increase in surface waviness associated with v;lng distortion under air leads. The better agreement between the flig>"t and tunnel results for the conven- tional sections may indicate that the position of transition is so far forward on these sections that it is not materially affected by an increase in surface waviness resulting from loads imposed on the wing in flight. CONCLUDING REMARKS The results of profile -drag tests of various smoothed airfoils indicated that in all cases lower profile-drag coefficients were obtained on low-drag airfoils than on conventional airfoils over the range cf lift coefficient tested. The results also indicated that, for comparable conditions cf lift coefficient and Reynolds num.ber, the low-drag airfoils m^ay have profile- drag coefficients which are at least 27 Dercent lov/er than the profile-drag coefficients for the conventional airfoils » Langley Memorial Aeronautical Laboratory National Advisory Coiamittee for Aeronautics Langley Field, Va. CONPID'-'^NTIAL CONFIDENTIAL NACA AGR No. Lli.S51 REFERENCES 1. ZaloTCik^ John A.: A Profile-Drag Inx'estigation in Flight on an Experlnental Fighter-Type Air- plane ~ The North American XP-5I (Air Corps Serial No. i^l-jS). NAGA AGR, Nov. 19i+2 . 2. Bicknell, Joseph; De ter nination of the Profile Drag of an Airplane 'A'ing in Plight at High RejTP-clds N^Jinhers. NAGA Rep, No. 667, 1939. 3. Theodorsen, T., and ;3-arrick, I. E, ; General Poten- tial Theory of Arhitrar;^ Wing Sections. NACA Rep. No. Ii.52, 1933. k. The Camhridge University Aeronautics Laboratory: The Measurement of Profile Drag hy the Pi tot- Traverse Meth.od. R. ?: M. No. l638, British A.R.C., 1956. 5. Silverstein, A., and Katzoff, S.; A Simnlified Method for Determining Wing profile Drag in Flight. Jour. Aero. Scl., vol. 7, no. 7* May I9I1O, pD. 295-3C'l. ONFIDENTIAL NACA ACR No. L4E31 CONFIDENTIAL Fig. 1 NACA Z414.5 REPUBLIC 5-3,// REPUBLIC S-3jl3 NACA 64,2- (1.4X1 3.5) NACA 66^2-2(14.7) CONFIDENTIAL NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS. Figure 1.- Profiles of various airfoil sections tested in flight. NACA ACR No. L4E31 CONFIDENTIAL Fig. 2 4- Panel hav\n q T KIACA 35-215 airfoi l E V) :Test section N-Z2 ■\' t Panel havin g V~ NACA ll-l\l airfoil r N-2.2 wing Test section ^R epublic S-S^ll Test section MKCA 66,Z-Z(\4.7) NACA 67(ll5)-2l3,a=0.7 KJACA 66(215) -1(16.5), a-l.O sT est section approy . NACA 64,2-(\AH\3.5) NACA-NAA comprom\se low-drag w\ng V^_Republ\c S-3 wing Test_sect\on Republic 3-3^13 CONFIDENTIAL 50 100 150 I I I I 3calej in. \_ Republic S-3 wing Testsection V NACA Z4-I4.5 NACA 2409 -NACA 2415 NATIONAL ADVISORY COMMITTEE F0« AERONAUTICS. Figure 2.- Plan forms of various wings of vrhich tests were made. NACA ACR No. L4E31 Fig. 3 ►J < I— I Eh td O >— 1 •z, o o ►J' < I— I Eh O I— I 2; o u fclD 0) Li 3 > u o to u NACA ACR No. L4E31 CONFIDENTIAL Fig. 4a,b,c ^ .oooz .0010 .0008 .0006 .0OO4 .OOOZ .10 zo \ -Al r. K \ ih\ \ \ \ / -s V" .^^ /: -U/Uc 1 1 \ / — W :20 40 .50 J^O .70 s/c 1.4- 1.0^ .8 § .0010 .0008 .0006 .0004- .oooz I CONFIDENTIAL .50 .&0 ,70 40 S/C 1.4 >8 NATIONAL ADVISORY COMMITTEE FOU AERONAUTICS. (aJ Republic S-3,11 airfoil, lower surface. (b) Republic S-3,11 airfoil, upper surface. (c) Republic S-3,13 airfoil, upper surface. Figure 4.- Surface waviness and velocity distribution (at c^ = 0.20) over various airfoils. (Dashed lines indicate approx. gage readings for surfaces free of waves . ) NACA ACR No. L4E31 CONFIDENTIAL Fig. 4d,e,f,g,h .0002. \-JH' ^c (d 1 r-TT/TT. ^^\ S,<^ — / / ^ '^ -^ ^ J- ,\0 ,Z0 .30 .40 3/C .50 .60 .70 /.4 .8 .0004^ ^ .0002 .OOOA-r .0002. \ ■'■■ \ / d/c (g ^ V "^^ >Z- UA ;. ^ -^ r^ ^\^ ^^ -^=~ |- ,/0 20 .30 .40 .-50 ,60 ,70 5/c /,4 '■% 1.0^ .8 NATIONAL ADVISORY 'CBMHITTEE FOB AERONAUTICS. ^ (d) (e) (f ) ig) (n) NACA NACA NACA NACA NACA O WLJKJ't Ix a. [K 0OO2.