TEO_XOAL NOTES

NATIONAL ADVIBORY 0C_J,ITT_.E FOR AERONAUTIOS

No. _70

_,.T_CH&_aTZ_USTZCSOF _ N.A.a.A.gT, 0LA_ Y,N.A.O.A.-M6 AZRFOILS WITH PARTIOULAR REFERENOE

TO THE ANGLE OF ATTACK

By George J. HigglnzLe.ngley Memorial Aeronautiual L_boza%ol'y

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NATIONAL ADVXSORY COmmITTEE FOR AERDNAUTIO8.

TEC_2_ICAL NOTE NO. 270.

THE CP.%RACTERISTIOS OF TD_ N.A.C.A. %7, CLARK Y, AND

N.A.0.A.-M6 AYRFOILS W_'IH PARTICULAR REFEKENOE

TO T._E _NQL2: OF ATTACK.

By George J. Higgins.

Sttmmazy

This note gives the aerodynamic charaoterls_ios of _he

r.A.C.A. 87, Clark Y, and N.,&.C.A.-_6 airfoil see%ions as de-

3_mined in _hs variable density wlnd _nnel at Langlsy Fiel_,

inla. Particular attention is called to tho relation of

characteristics to the angle of attack in thei_ use in

_esi _n.

In _he adaptation of a certain airfoil seotlon to an alto-

design, it is necessary to make a careful study of thee

oll ohazaote_istlos in o_dsz that the best performance mayk

!_e attained on the oomplete_ alrplane. Speed range and pay

Ioa_ are the Important factors _hlch are del3endent _ire_tly on

a careful selection and use of the wln_ section. The angle of

wlng setting or incidence on an airplane should be known in

te_s of the absolute angle of attack.

S, ET_

.A.a.A. Technical N_te No. 270

,, The an_le between any reference line on an airfoil sec_-

_lon. at any tlmo and that llne when _he direction of motion is

oh that the llft iB zero, is calle_ the abBolute angle of

tack (soc Figure l). It is this an_le that is used in thoo-

tical formulas for de_ermining airfoil characterlstios. One

inds from experimental tests _hat the llft of an airfoil is

y proportional to the absolute angle of attack, being

_oximately the same for all sections. The minim_ drag

oefficient occurs at about zero lift or at about zero absolute

of attack. The value of the minimum drag is, however,

_endent on the choice of the section. Likewise, the maximum

is determined by the burblin_ characteristics of _he In-

airfoil.

_2he ordinary or geometric anEle of attack,measured from

line is, with "_oet airfoils, dlffezent f_c_. the abso-.

!9.angle of attack. For checking rigging and angl'B of inol-

it is most convenient to use _he geometric angle as it

.:eaSily be measured. The designer, however, should be care-

that he does not confuse the two, particularly if employing

formulas.

To illustrate the above points, suppose that the following

airfoil sections are chosen as suitable for use in _esign:

N..A.O.A. 97 - high camber,

/Olark Y - medium camber,

_.A.O.A.-M6 - low camber.

_T

.A.C.A. Technical Note 11o. 2?0

;These sections are all medium thick _nd the structural details

_f the wings wo_Id be very similar.e_

A medium sized alrplmne (say, a braced monoplane) carrying

gross load of about 5000 pounds with suitable wing ares an_

ipowered with a 400 KP. engine will have, say, a landing speed

between 50 and 60 _.P.H. and a speed r_nge of about 2,5.

The high speed condition will ,eke i_ neceszary, therefore,

that the airplane fly with the wings set to give a llft corre-

eponding to a coefficient OL of about 0._2. Should the

Y section be adopted, the geometric angle of _ttack for

tel flight at high speed would be -2.2 ° (see Figure _),.

the M6 o_ 9? section been chosen, the respective angles

;I'+ "

rould be +_.3 ° and -5.? °. However, on _he basis of _beoiute

e of attack, the correct angle would be +3.1 ° for all +

sections (Figure _). The three airfoil sections a_e Shown

Ti res 2 3 intheabove athlete fc, hi@ ,peed "ihe appearance of the sections is very misleading. It would

eOarcely be suspected that the _? or Ola_k Y would give the

lift &s the M6 in the attitx_de shown.

Referring to Figures 4 and 5, it may be seen that the drag

Coefficient curves as well a_ the lift curves based on _bsolute

angle of a_tack are similar. Pol=r curves and curves of pro-

file Ir_g coefficient ODp plo_'t_i against OL are given in

J¥1gures 8 _nd 7, respectively. The use o_ either of these lat-

i.fez curves for design eliminate_ possible errors due to the +

iN.A.Q.A. Technical Note No. 270 4

_wrong use of the angle of attack. FixAte 7 includes curves of

_ ced drag coefficient ODi, for convenlenoe in obtaining

the correct 0D for a wing of any aspect ratio. Cp curves

fare given (Figure 8) for the Clark Y and M6 to complete the

Lata. 81milar information for the $7 is not available; how-

'eve_, for a section with a high camber like the 97, the 0p

is more than for a section like the Clark Y.

i?ir

N,A.0.A. Teohnloal Note No.270

irection of fll the llft iS zero.

N.A.0.A. 97

rig. I

)Irec_lon of flight.

01_rk Y.

p.,.

_hor_

ion of flight.

@eometric_i Angle of A_t5ok

Abeo!u_e Angle of Attaok

N.A.O ,A.-}L6

97 0-Y M 6

ag -5. ?o -a. a° +8.3 °

c_ ÷S.I ° +S.I ° +3.1°

Figure I.Airfoil Attitude At High Speed

enme lift on eaoh Airfoil, CL=O.S_

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,A.C,A. TeohnlcLl Note No.B?O

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N.A.0.A. Technical N_te Ne.270

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