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National Advisory Committee for Aeronautics, Technical Notes - Tests of Six Symmetrical Airfoils in the Variable Density Wind Tunnel

This paper is the first of a series covering an in—
vestigation of a family of airfoils all formed from a
basic profile. It gives in preliminary fcrm the results
obtained from tests in the N.A.C.A. Variable density wind
tunnel of six symmetrical airfoils, differing only in max-
imum thickness. The maximum.thickness—tu-chord ratios are
0.06, 0.09, 0.12, 0.15. 0.18, and 0.21. The results are
analyzed with a view to indicating the variation of the
aerodynamic characteristics with profile thickness.

The forms of the airfoil sections tha_t are in common
use todav arle the result of a more or less systematic in-
vestigation made at Gottingen of a large number of air-
foils. Previously airfoils such as the_R. A. F. 15 and
U.S.A.'2?, developed from airfoil profiles investigated
in England, were widely used. Because most airfoils have
been developed from low—scale tests, the forms developed
may not be the optimum for full—scale values of the
Reynolds Number. A number of airfoils have been investi-
gated in the variable density wind tunnel at values of
the Reynolds Humber approaching those of flight .(refer-
once 1), but with the exception of the M series and a
series of propeller sections, the airfoils have not been
related in such a way that the results could be satis—
factorily correlated.

The object of an investigation now being carried out
by the National Advisory Committee for Aeronautics is to
obtain the characteristics, at large values of the Reynolds
Number, of a wide variety of related airfoils. The bene-
fits of a systematic investigation of airfoil profiles at
large values of the Reynolds Number are so self~evident
that it is hardly necessary to point them out. Not only
do the results of such investigations greatly facilitate
the choice of the most satisfactory airfoil for a given
application but, because the results may be correlated to
indicate the trends of the aerodynamic characteristics
with changes of shape, they may point the way to the de-
sign of new shapes having better characteristics.

Airfoil profiles may be‘considered as made up of
certain profile thickness forms disposed about certain
mean camber lines. The major shape variables then be—
come two:‘ the thickness form and the mean camber line
form. The thickness is of particular impdrtance from a
structural standpoint. 0n the other hand, the form of the
mean camber line determines'almost independently some of
the most important aerodynamic properties of the airfoil
section, a. g., the pitching moment characteristics and the
angle of zero lift.