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National Advisory Committee for Aeronautics, Technical Notes - An Automatic Speed Control for Wind Tunnels

Since 1901 1t has been good wind tunnel practice to gener—
ate the air flow by some type of suction fan or screw, and to
determine the air speed by means of a Pitot—static tube, or the
tunnel wall pressure, or the rotational speed of the fan. Usual—
ly it is desirable to keep the air speed, or preferably the im—
pact pressure of the stream 9 Vz/Z, constant for some minutes
consecutively, while taking observations. To maintain such con—
stancy the fan can be held at a suitable Speed either by hand
control or by some automatic deyice. The respective advantages
of constant speed and constant head were set forth by the writer
in 1905 (Reference 1), in a paper explaining the inverted—cup “
manometer and various forms of Speed nozzles.

The present article deSOribes an automatic control that has
been used in several forms, since 1921, in the wind tunnels at
the Washington Navy Yard. The structural drawings were perfected
in turn by Messrs. L. H. Crook and R. H. Smith, of the aerodynam~
ics staff, and the apparatus was made in the Construction Depart—
ment. In its original form it was designed by the writer for his
university wind tunnel in 1902. We consider here the form now in
use with the 8—foot wind tunnel at the Navy Yard.

The hand—control system.— Since the completion of this tun—
nel, in 1913, the air current has been generated by a 500 horse—_m
power direct-current motor driving a Sirocco fan placed well af—
ter the working part and discharging into a return circuit. A
Pitot—static tube (Figure l), suspended 18 inches below the tune
nel ceiling, in the working part, has its leads connected to an
inclined tube manometer (Figure 2), on the desk of the aerody—
namic balance in the observation room overhead. To keep fixed
the manometer meniscus that indicates the wind strength, the op—
erator seated at the desk, with thumb and finger on one of the
small bent up reversing—switch handles, shown at the base of the
manometer[ makes the little rheostat motor (Figures 5 and 4), run md
forward or backward, if necessary, to adjust the field resist—
ance in the dynamo that supplies the fan motor. Details of this
reversing switch and its connections need not be given.