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National Advisory Committee for Aeronautics, Report - The NACA Variable Density Wind Tunnel

This report describes the redesigned variable—density
wind tunnel of the National Advisory Committee for
Aeronautics; it supersedes a previous report that de—
scribed the original tunnel. The operation of the balance
and the method of testing are explained and the method of
correcting and presenting airfoil data is described. A
summary of the formulas for predicting the characteristics
of finite wings from the airfoil section data as they are
usually presented is also given.
The variable—density wind tunnel was constructed
by the National Advisory Committee for Aeronautics
to provide equipment for testing models wherein the
error resulting from the comparatively low dynamic
scales common to most other model tests could be
eliminated. The necessity of recognizing this source
of error and the practicability of eliminating it by the
use of this type of wind tunnel have been demonstrated
by investigations which have been made in the variable-
density wind tunnel and in flight. A discussion of the
theory of this type of wind tunnel is given in reference
The Reynolds Number (97?) is a measure of the
dynamic scale, and large values of the Reynolds
Number are obtained in this tunnel by increasing the
density (p) while the other factors remain sensibly
constant.
The variable-density wind tunnel was proposed in
1921 (reference 2), and the construction of the tunnel
was completed in March, 1923. A description of the‘
tunnel as it then existed is given in reference 1. The
closed-throat test section was 5 feet in diameter, and
the tunnel was of wooden construction inclosed within
a steel tank. A number of investigations were made
in this tunnel, and the results showed the theory to be
correct.
The tunnel was destroyed by fire in August, 1927,
but the tank in which it was inclosed was not seriously
damaged. The tunnel was rebuilt, using fireproof
construction, and the balance was made more acces-
sible by building the test section of the open-throat
type.