Document
REPORT No. 614
P RE SS UR E DISTR I BUTI O N O V E R AN N. A. C. A. 230 1 2
A I RF OI L WI TH AN N. A. C. A. 230 1 2
EXTERNAL-AIRFOIL FLAP
By CARL J. WENZINGER Lang ley Memorial Aeronautical Laboratory 2 291 5 --37------1 I NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS HEADQUARTERS, NAVY BUILDING, WASHINGTON, D. C.
LABORATORIES, LANGLEY FIELD, VA .
Created by act of C ongress approved March 3 , 1915 , for the super v ision and d i rec t ion of the scienti fi c study of the problems of fl i ght (U. S. Code , Title 50 , Sec. 151). Its membe r ship was increased to 15 by act approved March 2, 1929. The members are appointed by the President , and serve as such without compensation.
J O SE P H S. AMES , P h. D., Chai r man, H A RRY F . GUGGE N HE I M , M . A. , Baltimore, Md. P ort Washington, Long Island , N. Y.
DA V ID W . TAYLOR, D . Eng., Vice Chai r man , SYDNEY M . KRAUS , Captain , United States Navy , Washington , D.C. B ureau of Aeronautics, Navy Department.
WILLIS RAY GREGG , SC . h . , Chai rm an, Executive Committee, CHA R L E S A . L I NDBE R GH, LL . D . , Chief, United States Weather Bureau. New York City.
W I LLIAM P. MA C CRA CK EN , J. D . , V i c e Chai r man, Executive A U G U ST IN E W . R OBINS , B rigadier General , United States Committee, Army , Washington , D.C. Chief Materiel Division, Air Corps, Wright Field, CHA RL ES G . A BBO T , Sc. D . , Day t on , Ohio.
Secretary , Smithsonian Inst i tut i on. EDW A RD P. WARNER , M . S., LY M AN J. BRIGGS , Ph.D. , Greenwich, Conn.
Director , National Bureau of Standards. OSCAR WESTO V ER , Major General, United States Army , ARTHUR B . COOK , Rear Admiral, United Sta t es Navy, Chief of Air Corps , War Department.
Chief , Bureau of Aeronautics , Navy Department. ORVILLE WRIGHT, Sc. D._ F RED D . FAGG , JR . , J , D . , Dayton, Ohio .
Director of Air Commerce, Department of Commerce.
GEORGEW . LE W IS , Di r ecto r of Ae r onautical Research JOHN F . VICTORY, Sec r eta r y HENRY J. E . REID, Enginee r- in - Charge, Langley Memorial Ae r onautical Labo r atory, Langley Field, Va .
JOHN J. IDE , Technical Assist a nt in Eu r ope, Paris, France T ECHNICAL COMMI TT E ES AERODYNAMICS AIRCRAFT STRUCTURES POWER PLANTS FOR AIRCRAFT AIRCRAFT ACCIDENTS AIRCRAFT MATERIALS INVENTIONS AND DESI G NS Coordin a tion of Re s ea r ch Needs of Milit ar y and Civil Aviation Preparation of Resea r ch Programs Allocation of P r oblem s P r evention of Duplication Consideration of Inventions LANGLEY MEMORIAL AERONAUTICAL LABORATOR Y OFFICE OF AERONAUTICAL I NTELLIG'ENCE LANGLEY FIELD, VA. W A S HINGTO N , D. C.
Unified conduct , fo r all agencies , o f Collection , classification , comp i lation, scientific research on the fundamental and dissemination of scientific and tech- problems of flight, nical information on aeronautics.
REPORT No. 614
PRE SSUR E D I STR I BUT I ON O VER AN N. A. C. A. 23012 A I RFO I L W ITH AN N . A. C. A.
23012 EXTERNAL-AIRFOIL FLAP By C A_LJ . WE_ZING_R SUMMAR Y both the main airfoil and the flap have been investi - Pressure - distribution tests of a n N . A . C . A . 23012 gated; the most promising arrangement thus far airfoil with an N . z t. C . A . 23012 e x te rn al - ai rf oil fla p obtained has the N. A. C . A. 23012 section for both were made in the 7 - by lO - foot wi n d tunnel . The p re s- ma i n airfoil and flap. In addition , a survey of the n a res were measured on the up per and lower surfaces flap hinge-axis location has been made (reference 2 ) at one chord section on both the main airfoil and on the to obtain one that would give low flap-operating mo- flap for several different fl a p deflections and a t several ments and good aerodynamic characteristics.
angles o f attack . A test i _ stallatio n was used in which In order to complete the information required for the airfoil w as mounted horizontally in the wind tunnel structural-design purposes , pressure-distribution tests between vertical end planes so that two - dimensional flow were made to obtain the air-load distribution over the w as approximated , main airfoil and flap. The combination tested has T h e data are presented in the f or m of pressure - dis - the N. A. C. A. 23012 section for both main airfoil and tribution diagrams and as graphs of calculated coe ffi cients flap and u s es the hinge axis previously developed for f or the airfoil - and - flap combination and for t he flap this flap.
alone . T h e pressure - distribution tests showed that, as APPARATUS AND TESTS with other types o f flap, the greater part of the increment MODEL Of total maximum lift due to deflecting the external - The main airfoil was built of laminated mahogany to airfoil fla p downward arises f ro m the increased load the N. A. C. A. 23012 profile and has a span and chord carried by the main airfoil . The maximum normal - force each of 20 inches. The external-airfoil flap was built of coefficient o f the external - air f oilflap w as about the same a s brass , also to the N. A. C. A. 23012 profile , and has a that oJ a split flap . The hinge moments, however, were span of 20 inches and a chord of 4 inches (20 percent of much lower because o f the . axis location used wi th the the main airfoil chord). The flap was supported on the external - airfoilflap . T h e pressure diagrams showed that, main airfoil by metal fittings at each end and by two when the plain airfoil and the fla pp ed airfoil are compared intermediate fittings spaced equally along the span.
at the same total lift, the fla p reduces the adverse p ressu r e The flap hinge axis (see fig. 1) was that previously gradients and the tendency of the main airfoil to stall , developed as described in reference 2, the flap being When the plain and flapped airfoils are compared at the arranged for locking at any desired deflection between same angle of attac k , it i s ap parent that the flap influences -- 10 ° and 60 °.
the air flow around the main airfoil so that the airfoil A main row of Pressure orifices was built into the carries a much greater load wi thout stalling than is upper and lower surfaces of both the main airfoil and p ossible wi thout the flap . the external - airf o il flap at the midspan section. These orifices were located on the model as tabulated in figure INTRODUCTION 1 , the tubes from the orifices being brought through the The external-airfoil flap in combination with a main model and out at one end. The pressures were photo- airfoil appears to be one of the most generally satisfac- graphically recorded by a multiple-tube manometer.
tory high-lift devices investigated up to the present Two auxiliary rows of pressure orifices were also time. Previous investigations of this arrangement (ref- built into the upper and lower surfaces of only the main erences 1 , 2 , and 3) have shown that it is capable of airfoil , one row being located 2 inches and the other row developing high lift coefficients and that it gives lower / 2 inch from the end. These orifices , together with those drag at these high lift coefficients than do plain or at the midspan location , were used incidentally to split flaps, measure the distribution of pressures along the span o f Several different combinations of airfoil section for the model between end planes for a few conditions.
2 REPORT NO. 61_NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
TEST IN S T ALLATI ON record was then taken of the pressures at the orifices The model was mounted in the N. A. C. A. 7- by by means of the photographic manometer.
10-foot open-jet wind tunnel (reference 4) as shown by PRESENTATION OF DATA figure 2. The main airfoil was rigidly attached to two large circular end plates to which the flap was also PRESSURE DIAGRA M S fastened but arranged to rotate so that its setting might Diagrams of the pressures over the upper and lower be changed. The two end plates were supported in surfaces of the main airfoil without flap (fig. 3) are given as ratios of orifice pressure p to dynamic pressure I c w : 20 . 0o "- _ o f the air stream q for the angles of attack investigated.
__ . 054 G F/ o p hihge oxi b - --_- 03 _ I',_,_ FmUI_E 1 .--Cross section of model showing o ri fice locat i ons used in pressure-distri- bution tests• N . A. C. A. 2 3012 a i rfoil with 0.20 c _ 2 3012 external-airfoil flap•
edge-- I _ k
Ori ce c3 Of wing Of flap (fractions c _) ( f ractions c s ) 1 -- q) . Location from leading 0. 000 0. 0 00 I • 0 2 4 . 025 I • O5 O . O5O 1 • 100 .1 0 0 I •250 .2 5 0 I _3 •450 . 4 50 I I • 625 .625 I •750 .750 I • ssl t Plan •936 circular cut-outs in two large vertical end planes that Fi x e d wooden extended from top to bottom of the air stream and some , /end p/ o n e attack of the model was set by rotating the large cir - _ cular plates and locking them at the desired angle.
Approximately two-dimensional flow is obtained with this type of installation and the section characteristics of the model under test may be determined. _ _ ' _" f ? o totoDle I T E S TS ,) _ I, _,__ / _ end,, o/ e D " ;
_o nhodh + o iii1 2
speed of 80 miles per hour at standard sea-level eondi- ] ti o ns. The average test Reynolds Number , based !
on the sum of the main airfoil and flap chords , was ! / 1,460,000. This test Reynolds Nu m ber , when con- L verted to an effective Reynolds Number (reference 5) _ , Cle v afl bn _-- that takes account of the turbulence in the air stream , is _-2 4 " _ 2 , 040 , 000. (Effective Reynolds Number=test Reynolds " _-- 34 " _ NumberX turbulence factor; turbulence factor for the '- 1020 tunnel is 1.4.) F IG UR E 2 . -- Diagram of m od e l w i th e xt er nal- a irfoil flap i ns t all e d b et we en e n d pla n es i n the 7- by 10-foot wind tun ne l.
The model was tested with the external-airfoil flap set at angles of -- 3 °, 0 ° , 10 ° , 20 ° , 30 ° , and 40 °. The Pressure di a gr a ms for the combin a tion of m a in a irfoil main airfoil was also tested by itself without the flap with extern a l- a irfoil fl a p a re given in figures 4 to 11 for for purposes of comparison. The angles of attack the v a rious fl a p deflections a nd a ngles of a tt a ck tested.
r a nged from -- 16 ° to 16 ° and the lift coefficients in- On the diagr a ms the pressures are plotted norm a l to cluded those from approximately maxi m um n eg a tive the m a in- a irfoil chord a nd to the fl a p chord, the pres- to maximum positive. With the model at a given angle sure v a lues being me a sured from the m a in chord for of attack and with a give n flap setting , a few minutes the m a in-airfoil pressures a nd from th e fl a p chord for were allowed for all test conditions to become steady; a the fl a p pressures.
PRESSURE DISTRIBUTION OVER AN AIRFOIL WITH AN EXTERNAL-AIRFOIL FLAP 3 c _ = 0 . 664 _o =- 6 . 32 ° _o = 8. 8 3 ° e__ = O.930 Upper surtoce 0 3 - u I_ - 4 f A irfoi/ c _=- 0 " 202 - 2 P _ q _& =- 3 " 31 ° " _o =12 . 05 ° c . =0 . 080 i _ / - _ = I. 155 FI GU R E 3,-- P ressure distribution on the N. k . C. A 2301 2 main air(oil without flaps at various angles of altacl_,
gg_og_ go. _
Ai r fo il Flap X , _ 4 _ U p e r - s u r fa c_ o g DIBq_ R IB_ ' _IOI_ OW_IR' AN AII_FOI_ _....
i Airf oil FI_ P o UpP er L o We r l Z EpoRT I_O . 614_ATIOI_AL ADVISOI_Y cOMMITTEE FOR AERONAUTICS ] - 4 Aicfo// Flop D _ Upper _c lFf ac_ 0 X q F lelJl t _ 6. -l_ t esSt tr e distfib t ltton on the N, _k. G . A; 2_0 1 2 airfoil with N . _ - O . .k, 280 1 2 externsl - Mrfoil flap at _u_ious _ngleS of _ tt sck. Flap fietlected 1 0% PRESSURE DISTRIBUTION OVER AN AIRFOIL WITH AN EXTERNAL-AIR F OIL FLAP 7 / / REPORT NO. 614--NATION A L AD V ISORY COMMITTEE FOR AERO NA UTrCS • x c _ 0 . 027 c_ + n= 14 8 8
/
e. , . . n = I . 700 _a - _ ' _-_,_" ' _ o = - _ Z O ° - _'-_ - _ / "\ 3 ('_ _ vp ev, - / I " "0 e _ o 4 0 2 ° Ge_ ,n = 1 . 830 Fm v _ &--Pressu r e ( ] istrlbt_ t ion on t h e N . A . 0 . A . 2801 2 n / rf o il w it h N . A. ( ' . A, 28 012 e x ternal-a h,h )ii flap a t ratio n s angles of att n (;k, l + lnp deflec ' l, e d 3 0 o.
PRESSURE DISTRIBUTION OVER AN AIRFOIL WITH AN EXTERNAL - AIRFOIL FLAP 9 c, v.. :, j = I . 0/4 c_.. .. I =0 . 032 / c . c _ z = / . /82 \ Upp c l _ ul - / cl c _ o x
'\
._: ; ,_ L o we t ' , U et o= -,3. 47 ° c _r_. .f _=O.B 47 _ / e, ,(_+I_= 1 . 565 FiOU]_ : 9.---Prcss n redistribution on tile N. A. C. A, 23012 airfoil with N. A, C. J. 23012 external-airfoilflap at various angles of attack. Flap deflected 40°.
I0 REPORT NO, 614_-NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS - 3 _--- P/el ½ el k' / h i 7 A ii_ foil with flop -2 ----- 30 ° ........ 20 ° / _ // Ii -I / t
__ f l/l/
i I 0 \, '_ / - I FIGURE 10.--Comparison o[ the pressure distribution on an ,N. A. C. A. 23012 air[oil with a 0.20 Cw exterllal-airfoil flap with that on the plain airfoil at the same lift, c_=I,165.
\
\
P lo ln o /r fo il Airfo/l wil h flop 6, /0 ° ........ 20 0
,,\
',,\ . f
\_-.-\ //
FIGURE ll.--ComD_rison of the prosslire dis t rib_lti o n o n an N . A. C . A . 2 ,3 012 a ir foil with a 0.2 0 c, _ external-airf o il flap with that on the plain airfoil at the same angle o f attack, ( x 0 _8.5 ° , PRESSURE DISTRIBUTION OVER AN AIRFOIL WITH AN EXTERNAL-AIRFOIL FLAP 11 C O EFFICIE N TS The c enter-of-pressure positions and the pitching- The pressure diagrams were mechanically integrated moment coefficien t s were derived from the normal to obtain data from which section coefficients could be forces , the chord forces being neglected except for the effect of the flap , in which case the flap deflection was computed. The section coefficients are defined as follows: taken int o account.
The calculated results from the present tests were all nw corrected to infinite aspect ratio characteristics in c nw=-- , normal-force coefficient of main airfoil alone, accordance with methods given by Glauert (reference qC , , j 6) th a t have been found satisfactory from other tests n ( w +f) , normal-force coefficient of main airfoil of a simil a r arrangement in the 7- by 10-foot wind tunnel c' _('_+1)-- qC ( w +I) with flap. (reference 7). Another check on the theoretic a l cor- = m _ pitching-moment coefficient of main airfoil _ c Crow qCw2 ' alone about quarter-chord point. _._ ' _ _ m (_+_ , pitching-moment coefficient of main _ _ Q c' q_+I) qc < w +_)2 airfoil with flap about quarter- _ u< chord point of combination. _ _ _ . / _ , normM-force coefficient of flap.
c ), f = ) @ v hinge-moment coefficient of flap about hinge k cf axis.
/ . 2
centerof pressure of
k % 0/ main airfoil alone in . 8 percentage of chord _ d from leading edge.
( c . p .)(_+ f )= 0.25 c, ,(_+ f ) ×100 , center of pressure _
(
Cn ( w T f ) / of main airfoil _ _ with flap in _ -. 4 percentage of chord of com- bination from -' 8 - /6 - /2 - 8 - 4 0 4 _ / 2 leading edge. C _o , nee .
FmUR_ 1 2 .--Section characteristics of the plain N. A. C. A. 23012 airfoil.
( c . p .)s= ( O. 2 5-- c _h--!_×lO0 , center o f pressure o f flap rectionis showu in figure 12 , where the corrected results \ _n f/ in percentage o f flap of the pressure-distribution tests are compared with chord from leading force-test results (reference 8) for a 10- by 60-inch edge.
N. A. C. A. 23012 plain wing corrected to infinite aspect ratio by the usual methods.
where the forces and moments per unit span are n . , normal force of main airfoil. For the case of the pressure-distribution tests n (_+_ , normal force o f main airfoil with flap. a 0= a -]-A_ row, pitching moment of main airfoil about quarter- where chord point.
m ( w +. r ), pitching moment of main airf o il with flap A a (deg.) =-- ( 0.25_ c _)X 57.3 \ l _ ' / about quarter-chord point Of combination.
nr , normal force of flap. c is the total chord.
h r , hinge moment of flap about hinge axis. h, the height of the jet.
and (The quantity c _ is substituted for C _ in the present q , dynamic pressure, correction and the substitution results in only a slight c _ , main-airfoil chmd. error because of the small difference in value between cz, flap chord, the two quantities.) Curves of the various calculated c (_+i) chord of combination ( Cw -_-c_). coefficients are given in figures 12 to 18.
12 REPORT NO. 614--NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS .S ° NOL LK C_o " " L. _ EJ I ---o )......- > -- _o---_ .-o 0 _-_ , i x >-- _ -_ i i " / I ,_ _
/
,,
" °'h I
, ................. 7- /
i • _ i --_ . ,
I "_ _ _-- '_-t--_
--. z ! ............. J i . s ___ -- i
! _ / '
I
2 i / / I
_ . 4 i / /
P_ 8 // 7
..... t. t/ i /
x i /
: j - " --- 00 / (i) _ _L o.._ >t _> - 4 0 . 4 s -,' z -8 - 4 0 4 e i. - iz - e -# o 4. a d o , deg . [ (a) Airfoil with flap. (a) Airfoi l with flap. (a) Airfoil with flap .
- 40 ? __ __ _: .2 o ....
_. i # o II> / " _ -" _ ......
_'._ , r /4 - -
_ i_ • _ -- _,_ ",_ ..-.__ _._,, .___ _ k,___ _, _>__ _ > _._ o o__ _ ._ __t_. _ < _ ,
_ -._
. 4 / " ° fo-- - _ . 2 J , / o _ i / , ". 0 . ' .7 . / __
ok_) -. 2 _ " <> " /
-. 4 , I\_ _ (_') , > I o>) (!)
t
- .8 -.4 0 . 4 . 8 /2 Z - .8 -.4 0 . 4 . 8 L2 l F : 8 ':4 0 . 4 ,8 L2 N o/f, o/- force c oeff/c/en l o f c on T bi b o/i ; Dn c, _(_v) (b) Flap alone. (b) Flap alon e . (b) Flap alone.
FmVEE 13.--Section characteristics Of the N. A. C.A. FIG U RE 14.--Se c tion characteristics of the N , A. C. A . Fm V EE IS.--Section characteristics of the N. A. C. A.
23012 airfoil with a 0 , 20 c _ N. A. C. A. 2 3 012 external- 2 3012 airfoil with a 0.20 c _ N. A. C. A. 23012 external- 23012 airfoil with a 0.20 c _ N. A. C. A. 23012 external- airfoil flap set at --3 °. airfoil flap set at 0° . airfoil flap set at 10% PRESSURE DISTRIBUTION OVER AN AIRFOIL WITH AN EXTERNAL-AIRFOIL FLAP ]3 = 40 ° LO
k_
. 2 = 4 0 . 4 . 8 L2 L6 .4 . 8 0 . 4 . 8 L2 L6 c oeff/ c ien/ +s) (b) Flap alone. (b) Flap alone. (b) Flap alone.
FIGURE16.--Section characteristics of the N. A . C. A. : FIG U EE 17.--Section characteristics of the N. k. C.A. FI O URE18.--Section characteristics of the N. A. O. A.
23012 airfoil with a 0.20 c _N. A. C. A. 23012external- 2 3012 airfoil with. a 0.20 c _N. A. C. A. 23012externaI- 23012 airfoil with a 0.20 c . N. A. C. k . 2 3012external airfoil flap set at 2 0°. airfoil flap set at 30°. airfoil flap set at 40% 14 REPORT NO. 614--NATIONAL ADVISORY COMMITTEE FOR AEI%ONAUT1C_ PRECISION Comparison of pressure diagrams for the plain airfoil No air-flow alinement tests were made in the wind and for the airfoil-flap combination at the same angle tunnel with the test arrangement used in the investiga- o J attac k (fig. 11) shows that the flap increased the nega- tive pressure over the entire upper surface of the main tion , so the absolute setting of the angle of attack may be slightly in error; the relative angles are , however, airfoil and increased the positive pressure on the lower accurate to ±0.1 °. The flap deflections were set to the surface near the trailing edge. The pressure gradients specified angles to within ±0.1 °. The orifice pressures remained about the same except at the trailing edge of based on check tests in which both the angle of attack the main airfoil , where they were reduced. The pres- and the flap settings were independently changed sures on the upper and the lower surfaces of the flap showed that they agreed to within ± 2 percent , with the both increased with flap deflection. The important exception of upper-surface pressures near the leading effect of the flap in this case was its ability to influence edges , which , at high angles of attack , checked to within the air flow around the main airfoil so that the airfoil ± 5 percent. The dynamic pressure recorded on each carried a much greater load without stalling than was diagram was accurate to within ±0.25 percent for all possible without the flap.
tests. One other interesting item is suggested by the progres- The distribution of pressures along the span of the sire increase in flow velocity over the main-Mrfoil upper model indicated that two-dimensional flow was ob- surface relative to free-stream velocity as the flap is rained with the installation used. The pressures , for a deflected. This characteristic suggested that the use given location along the chord of the airfoil , were the of this type of flap would increase the rolling effective- same from midspan to within at least _ inch of the ends ness of ailerons located on the trailing edge of the main (the row of orifices nearest the end of the model), airfoil. An investigation of such an arrangement (reference 3) recently completed in the N. A. C. A. 7- RESULTS AND DISCUSSION by 10-foot wind tunnel indicated that such an improve- SECTIO N PR E SSURE DI S TRIBUTION merit could be realized.
The pre s sure-distribution diagrams (figs. 3 to 9) are SECTION L O AD S A N DM O M EN TS useful to show the chordwise distribution of the air loads on the main airfoil and on the flap and may be The section coefficients are plotted in figures 12 to 18.
regarded as satisfactory for application to rib and flap It will be noted that the flap loads build up rapidly at design. The diagrams also illustrate certain special relatively low lifts of the combination and that they also features of the action of external-airfoil flaps, increase rapidly with flap deflection (figs. 13 to 18).
Comparison of pressure diagrams for the plain airfoil The maximmn flap loads appear , in general , to reach and for the airfoil-flap combination at tlm same li f t somewhat higher values than are obtained with an air- (fig. 10) shows the effect of the flap. Increasing the foil of the same size tested alone at the appropriate flap angle and decreasing the angle of attack to main- Reynolds Number. (Test Reynolds Number for flap tain constant lift had the following effects: (1) The alone based on flap chord and free-stream velocity= magnitudes of the peak pressures at the leading edge of 244,000.) The greater part of the increment of total the main airfoil were progressively reduced , and (2) the C _ma _ due to deflecting the flap downward , however , magnitudes of both positive and negative pressures at arises from the increased load carried by the main air- the trailing edge of the main airfoil and at the leading foil.
edge of the flap were progressively increased. It is interesting to note that the maximum normal- The flap , in addition, obstructed the flow of air below force coefficient of the external-airfoil flap tested has the airfoil and caused the pressures to build up on the about the same value as that attained by split flaps in a lower surface. The air flowing through the slot over previous investigation (reference 9). Owing to the use the upper surface of the flap produced a higher average of the hinge axis chosen, however, the hinge moments of velocity and increased the negative pressure on the flap the external-airfoil flap are much smaller than those of upper surface. Thus , the influence of the flap was to corresponding sizes of split flap.
reduce the adverse pressure gradients and the tendency CONCLUSIONS of the main airfoil to stall.
The external-airfoil flap itself had a pressure distribu- 1. Pressure-distribution tests show that, as with tion similar to that of a plain airfoil , so that the flap other types of flap , the greater part of the increment of would have a small wake as long as it remained un- total maximum lift due to deflecting the external-airfoil stalled. The wake of the combination would therefore flap downward arises from the increased load carried be small, particularly near the stall; this small wake by the main airfoil.
permitted the development of high lift together with 2 . The maximum normal-force coefficient of the low profile drag. In this respect slotted flaps , in gen- external-airfoil flap investigated had about the same eral , appear better than plain or split flaps, value as that attained by split flaps. The hinge mo- PRESSURE DISTRIBUTION OVER AN AIRFOIL WITH AN EXTERNAL-AIRFOIL FLAP 15 ments , however , were much lower because of the axis 2. Platt, Robert C., and Abbott, Ira H.: Aerodynamic Charac- location used with the external-airfoil flap. teristics of N. A. C. A. 2301 2 and 23021 Airfoils with 20- Percent-Chord External-Airfoil Flaps of N. A. C. A. 2301 2 3. The pressure diagrams showed that , when the Section. T.R. No. 573 , N. A. C. A. , 1936.
plain airfoil and the airfoil with the external-airfoil 3. Platt, Robert C., and Shorta], Joseph A.: Wind-Tunnel In- flap were compared at the same total lift, the flap re- vestigation of Wings with Ordinary Ailerons and Full-Span duced the adverse pre s sure gradient s and the tendency External-Airfoil Flaps. T.R. No. 603, N. A. C. A. , 1937.
of the main airfoil to stall. When these plain and 4. Harris, Thomas A.: The 7 by 10 Foot Wind Tunnel of the National Advisory Committee for Aeronautics. T. R.
flapped airfoils were compared at the same angle o f No. 412, N. A . C. A., 1931 .
at t ack, it was apparent that the flap influenced the air 5. Flatt, Robert C.: Turbulence Factors of N. A. C. A. Wind flOW around the main airfoil so that the airfoil carried a Tunnels as Determined by Sphere Tests . T.R. No. 558, much greater load without stalling than was possible N.A.C.A., 1936 .
without the flap. 6. Glauert, H.: Wind Tunnel Interference on Wings, Bodies, and Airscrews. R. & M. No. 1566 , British A. R. C. , 1933.
7. Wenzinger , Carl J.: Wind-Tunnel Investigation of the Aerodynamic Balancing of Upper-Surface Ailerons and Split Flaps. T.R. No. 549, N. A. C. A. , 1935.
8. Wenzinger, Carl J.: Wind-Tunnel Investigation of Ordinary LANGLEY MEMORIAL AERONAUTICAL LABORA T ORY, and Split Flaps on Airfoils of Different Profile. T.R. No.
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS, 554, N. A. C. A., 1936.
9. Wenzinger, Carl J.: Wind-Tunnel Measurements of Air LANGLEY FIE L D , VA. , July 29, 1937.
Loads on Split Flaps. T.N. No. 498, N. A. C. A., 1934.
REFERENCES 1. Platt , Robert C.: Aerodynamic Characteristics of Wings with Cambered External-Airfoil Flaps, Including Lateral Con- trol with a Full-Span Flap. T.R. No. 541, N. A. C. A. , 1935.