Authors
Schrenk, OAckeret, JBetz, A
Topics
Plasma and Flow Control in AerodynamicsBiomimetic flight and propulsion mechanismsFluid Dynamics and Turbulent Flows, · • •'-------------".''Ii'I!;:n;r'l"'T1':~ l:EM'JRANDUMSCO!lliIT'I'EE FOR AERONAUTIOScEXPERIJ":E1:TTS '::I'i'H .AS .AIRFOIL FnO'~ TdICH THE BOUl{DARY LA ERFromIS REJO~ED B7 SJC7lCN By J . Ackeret , A. oetz and O. Schrenk u'''or1aufige iii tteilung~ der Aerodynamiscben Versuchsansta1t zu G6ttingen," no. 4 r-~ ________________________ Jove~ber , 1925 ' )R P CATIONS SHOULD BE ADI)RES '[0OR AERONAtJT.Washington ugtlst, 1926NA TIONAL AD"V-I80RY COM!'! ITTEE FOR AERONAUT lCS.TECHN ICAL LriL1LORH:J:-UM NO. 374.EXPERIMENTS WJ:TH An ~I F. :;'IJ FRQM rr'~ CH :rL~ EourJARY LAYERBy J. Acker et, A. Betz and O. Schrenk.Our atterllpts to improve the p rop erties of airfoils by remov ing the b ou. nda ry la,yer by suction, go back to 1922. Tn.isprinc i p le wa s involv d in Profe ssor P r a'ndtl ' s experiments in19 Ok;' , but, s o far as we k..'l10W, no succe s sful app l i cation of thesuct i on ;:'let hod to technical problems ha.s yet been made. Th eobject of t he suction i s chiefly to prevent the detachment oft he b ounda ry layer from the su:rfac e of the airfoil. At l a r g-eangles of attack, such detachment prevents the attainm ent ofthe grea t lif t p romi s ed by the theory , bes ides g reatly increasing the drag, spE)c c. lly o f thiek a i:rf oils. Ofll itting many intermedia te xp~r en s, we will gj.~re a f ew r ecen t results. The experiments we re rendered f inancially poss ible by the Flettnorand Daimler c ompan ies.The i~ oil tested (F .A. 41) was mounted in the small ~indtunn el of t llC I nst i tute, so that one end was secured to a sidewall and t he other end projected into the air stream. This arr angcment , as n E''J!.:'sS::1T.r, in order to p rovid e for the suetion of"V8rsuc:18 ~ ir~r-;!l1 F:: {I',ge l mi,t rE''YJ.z ch~, ta ugungl l froml! r:";"'~'J.f j, ge E l t te:i.lun gen der Ae l'oQyndmi 8cbcll V el'suchsanstal t zu• c G6ttingen, No.4.N .A. C.~i· Technical . .1emorandum No . 374 2the air t hrough the inside of the airfoil and out at one end. It rendered impossible, nowever , the usual mcthod of power deter mination by weighing, for '7hich pressure-distribution measure mente had to bc substituted. The ob j ect of the experiments was to produce a good adher ence of t he boundary l ayer to the airfoil with the minimum su c tion force. We measured the requi s i te quantities Q. and the nega t ive p ressures p produced by the suction . The suction force (rli thout taking account of the pump 'efficiency) was t hen p, 3 L = Qp = z 2 v F, in which P is the air densi ty; v , the wind velocity; F the airf oil a rea (chord x span) ; and z , a nondimensional coef fi cient dcnoting the pov:er . The latter is exactly comparable with the drag coefficients cwi and cwo of a wing. The a irfoil lmd a t h ick Jouko~ski profile (Fig. 3) . The suction v~ s applied only to the top of the wing and onl y through thc three separ~te cha~bers shown in Fig. 3, since it was impracticable to empl oy on the trailing edgc the relativcly great ne~" tive pressur e requ ired near the leading edge. Fi gs. 1-2 show the arrangement in detail. For measuring the p ressure, ther e we re three t ubes at each end of the airfoil, the ones at the left being shown i n the photograph (Fig. 1). For ~eas ring the quantity three measuring nozzles were inserted in the t hTee suction p i pes . Three throttle valves were a l s oN.A . C.A. Techni cal Memo r andum No . 374 3 provided for regula t i ng t he quantity and pressu r e in the sepa rate chambers . I n Fi g. 1, t he suction siev e is partially cov ered (with pasted t h i n b l a ck paper) , si nc e i t was found that, with certain c overings , onl y a small suct ion fo r ce was needed to prevent the detachment. The four whit e t hr eads in the air stream (Fig. 1) show t he def lect ion of t he a i r current by the airfoil. The above- ment ioned pre ssure-d i s t r i bution measurements could not be made in the usual manne r , by means of measurin.g holes from the inside of the a irfo i l , but by means of a pres sure gauge (Sonde ) placed on the surface (error ±5%). Thus the nonnal pressures along the air fo il surface were obtained and the resulting fo r ce (coeffici ent cr ) could be calculated, which comprises most of the l ift and i nduc ed drag, but not the tangential forces p rodu ced b y t he SK i n frict ion, wh i ch consti tute the principal pa rt of t he prof ile d r ag . We giv e t wo re sults obtained wi th suitabl e cov erings: c r == 2 . 9 5 3. 6 z == 0 . 03 0 . 12 a == 17 . 5 0 28 (computed fo r i nfi n i t c asp ec t ra tio) The ::mgle of attack a wa s mea sured, as usua l , with r eference to the chord . Greater c r va l ues ( 5- 6 ) could be ob t ained by increasing the suction f or ce to a mul t i ple . The profile drag was a l so determi ned fo r mode r a t e values• • N.A.C . A. Techn ica l ~0mo andum No . 374 4:of cr by the impul s e mcthod (IfZcits chrift filr Fl ugtechnik und Motor1uftschiffahrt lf 1925, p . 42) . The reduc tion in the profile drag by suction is shovm to a certain extent by Fig. 4. The total p ressure g along a straight 1 ine perpendicular to the chord, 7 cm (2.76 in . ) behind the trailing edge of the airfoil (i.e., the total energy at every point across the stream) was measured and exp ressed in its ratio to the undisturbed total energy go of the flow. The ratio g/go has values sma ller thaw 1 in the region which i s filled with the flow coming from the b-oundal'Y layers of the airfoil. It can be demonstrated t hat the arcas between the straight lines g/ go = 1 and the curves are proportional to the profile drags, up to an err or whose upper limit can be estimated. In our cases, the neces sary corrcctions a l wa ys rema in under 10.% . In Fi g . 4, four such measurements a rc pl otted, of which curve 1 was obtained with the strongest, curv e 3 with the weakest, and curve 4 without suction, and indeed with ex. = 4 0 and c r = 1 . 2 . For c r = 0.9 in one case we obtained: Profile drag Vii thou t suction,If " with " - Requiai te suction force cwo lTranslation by Dwight M. l~ in , NQtioDal Advis ory Committee for Ae r onQutics .cwo = 0~029 , crI~ = 0.014, z = 0 . 007 + Z = 0.021:~ .A •• A. echnical .t.iern crandum '·io. 374Fig. IInfo k e con eN eq . pr<!' 5'sure meo _- u r e m en t ____ ..1 ___ _ E-rEo 2 L('\ - - - - - - - - - -"" 1+--- 500 _m _", _ _F.A. 4-1.f ig.2-~~~----------~-------M e fer.sThroffle valvesWallFigs. 1 & 2To e)(i]ous1 fonN.A.C.A . Technical Memorandum Ho . 374: Figs.3 & 4:- - ...... "Fig.3 Pro f ile showing division into compartments.1.0,..--.. - - I I . , I \'~ \\ 111' / V ,2,' / \ \~\ \ [II 1I' /3 / ir . I \) I /4 \\ I j7 V ,-I1\!1- 0.6 0.4 0.2 -I ! I , .-. L - . o , r 0 -5 ~8 <-S -G -1 1 2 3 4 5 6Fig.4:Lower or pressure side of a..irfoil .cm Upper or suction side of a irfoil .Reduct ion of profile drag by suct i on.- - ----I -._-7
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