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National Advisory Committee for Aeronautics, Report - Calculated and Measured Pressure Distributions Over the Midspan Section of the NACA 4412 Airfoil

Pressures were simultaneously measured in the variable-
density tunnel at 64 orifices distributed over the midspan
section of a 5— by 30-inch rectangular model of the N. A.
C. A. 4.413 airj'oil at 1?’ angles of attach: ranging from
—20° to 30° at a Reynolds Number of approximately
3,000,000. Accurate data were thus obtained for study-
ing the deviations of the results of potential-flow theory
from measured results. The results of the analysis and
a discussion of the experimental technique are presented.

It is shown that theoretical calculations made either at
the eflectioe angle of attack or at a gioen actual lift do not
accurately describe the observed pressure distribution over
an airfoil section. There is therefore developed a modified
theoretical calculation that agrees reasonably well with
the measured results of the tests ofthe N. A. C. A. 4412
section and that consists of making the calculations and
evaluating the circulation by means of the experimentally
obtained lift at the efiective angle of attack; i. e., the angle
that the chord of the model makes with the direction of the
flow in the region of the section under consideration. In
the course of the computations the shape parameter 5 is
modified, thus leading to a modified or an efiectioe profile
shape that difi’ers slightly from the specified shape.

Pressure-distribution measurements over an airfoil
section provide, directly, the knowledge of the air-force
distribution along the chord that is required for some
purposes. In addition, such data, when compared with
the results of potential-flow (nonviscous fluid) theory,
provide a means of studying the effects of viscous forces
on the flow about the airfoil section.

The results of experimental pressure measurements
for a few miscellaneous airfoils may be found in various
publications. The general application of this method
of obtaining design data, however, is limited because of
the expense of making such measurements.