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National Advisory Committee for Aeronautics, Report - Airfoil Pressure Distribution Investigation in the Variable Density Wind Tunnel

With a view to extending the knowledge of the aero-
dynamics of lifting surfaces, the distribution of pressure
orer one section each of sin airfirils has been measured in
the Variable Density Wind Tunnel of the National Adri~
sory Committee for Aeronautics. The following air-
foils were inrestigated: N .A. 0. A. 85—J, N. A. 0. A. 84—J,
N. A. 0. A 84, N. A. 0. Aid—6, Clark Y, a-ndR.:l.F.30.

Pressure-distribution diagrams, as well as the inte-
grated characteristics of the airfoils, are gi-ee-nfor botha
high and a low dynamic scale or, Reynolds Number Vl/r,
for comparison with flight and other wind-tunnel tests,
respectively. It is concluded that the scale efect is tery
important only at angles of attack near the burble. The
distribution of pressure orer an airfoil having a Joulcow—
slci section is compared with the theoretically derited
distribution. A further study of the distribution of
pressure orer all of the airfoils resulted in the derelopment
of an approximate method of predicting the pressure
distribution along the chord of any normal airfoil for
all attitudes within the working range if the distribution
at one attitude is known.

Two distinct methods have been commonly used in
investigating the aerodynamic characteristics of air-
foils. One consists of measuring the mechanical forces
required to support a wing in an air stream, the other
consists of measuring directly the air pressures acting
on the surface of the wing. Tests of the first type are
spoken of as force tests, and of the second type as
pressure-distribution tests. If all of the air forces were
transmitted to the wing by the action of pressure alone,
the results of a pressure-distribution test would be
more complete than the results of a force test, because
the total forces acting on the wing could be obtained
by integration and, in addition, the results would show
how the forces are distributed. Actually, through the
action of viscosity, shear forces which are not measured
in a pressure-distribution test are transmitted to the sur-
face of the wing. Such forces account for only a small
part of the total airforce acting on swing except at angles
of attack near zero lift, but are sufficiently large at any
angle to make the drag determination as obtained by
integrating the pressures of little practical value.