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NASA, Technical Memorandum - Development of a Computer Program to Obtain Ordinates for NACA 6- and 6A-Series Airfoils

The NACA 6-series airfoil sections were developed in the early 1940's, and discus—
sions of the method of the derivations and the resulting ordinates have been published in
references 1 and 2. As aircraft speeds increased, more attention was focused on the
thinner airfoils of this series. However, difficulties were encountered in the structural
design and fabrication of these thinner sections because of the very thin trailing edges.

As a result, the NACA (SA-series airfoil sections were developed, and details of these
have been published in reference 3. Essentially, the modification consisted of a near-
constant slope from about the 80—percent chord station to the trailing edge and an increase
in the trailing—edge thickness from zero to a finite value.

Recently, parametric theoretical studies have been made to investigate the use of
these airfoil sections for both rotorcraft and conventional aircraft. The results of one
investigation are presented in reference 4 and Show advantages of an NACA 6-series
cambered airfoil for use as a tail rotor on helicopters. It was tedious and expensive to
make these studies because no method was available for calculating the desired ordinates
rapidly and accurately. Because the 6-series airfoils do not have an analytic expression
for the ordinates, use must be made of the ordinates published in references 1 to 3.

Also, the ordinates are not linear with variations in thickness-chord ratio so that airfoils
obtained by linearly increasing or decreasing the ordinates of an originally derived shape
will be approximate, as mentioned in reference 1. The published ordinates have been
cross-plotted as a function of thickness and published in reference 5, but the values must
be read off the graphs and only 26 longitudinal locations from nose to tail were available.

An attempt, using a derivative of the NACA 4-digit series, to provide a computer
program for ordinates of the NACA 6-series airfoils was made in reference 6. However,
as stated in the reference, the resulting accuracy of only 3.5 X 10'3 chord is not sufficient
for many applications.