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National Advisory Committee for Aeronautics, Report - Airfoil Section Data Obtained in the NACA Variable Density Tunnel as Affected by Support Interference and Other Corrections

The results of an investigation of the efi'ect of support
interference on airfoil drag data obtained in the variable-
de-nsity tunnel are presented. As a result of the support
interference, previously published airfoil data from. the
variable—density tunnel hare shown too large drag coefi‘i-
cients and too large a rate of increase of drag coefiicient
with. airfoil thickness. The practical eject of the correc—
tions on the choice of the optimum section is briefly
considered and corrected data for a selected list of airfoils
are presented as a convenience to the designer. Methods
of correcting published data for other airfoils are presented.

Air-foil data obtained in the variable-density tunnel
(reference 1) have been published (references 2 to 6) in
forms that were considered at the time of publication
to be most useful to the airplane designer. In the
earlier publications (references 1 and 2), no corrections
other than those for tunnel—wall effects and to infinite
aspect ratio were applied to the data, and emphasis was
placed on the pressing problem of. obtaining good com-
parative data for judging the relative merits of airfoils
rather than on obtaining absolute accuracy.

It was recognized that certain consistent errors were
present in the data, but it was thought that the effect
of these errors on the comparative value of the data
was not of primary importance. Support-strut inter-
ference, for example, was considered to be a possible
source of systematic error, but it was thought that this
interference would not affect the order of merit of the
airfoils tested except possibly in the case of very sensi-
tive airfoils, which might also be similarly affected by
the wing-strut intersections of biplanes common at the
time. The turbulence of the air stream was thought
not seriously to impair the comparative value of the
data and, perhaps, even to be desirable, because the
extensive turbulent boundary layers occurring on the
models in the tunnel as a result of the turbulence would
also be found in practice at high values of the Reynolds
Number on conventional airfoils with the usual mod-
erately rough surfaces.