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National Advisory Committee for Aeronautics, Technical Notes - Calibration and Lag of a Friez Type Cup Anemometer

Tests on a Friez type cup anemometer have been made in the
Variable Density Wind Tunnel of the Langley Memorial Aeronau—
tical Laboratory to calibrate the instrument and to determine
its suitability for velocity measurements of wind gusts. The
instrument was calibrated against a Pitot—static tube placed
directly above the anemometer at air densities corresponding
to sea level, and to an altitude of approximately 6000 feet.
Air—speed acceleration tests were made to determine the lag
in the instrument reading. The calibration results indicate
that there should be an altitude correction. It is concluded
that the cup anemometer is too sluggish for velocity measure—
ments of wind gusts.

One of the most important items under discussion by the
Subcommittee on Meteorological Problems at a meeting early in
1929, was the study of the structure of the atmosPhere and the
measurement of gustiness. At this time consideration Was given
to the question of instrument requirements, and it was the con—
sensus of opinion of the representatives of the Weather Bureau
and the Bureau of Aeronautics-that any instrument used should
be of rugged construction and simple in operation. This is
apparent, of course, in view of the fact that the instrument
may be subjected to all kinds of weather conditions and must
make continuous measurements over long periods of time. Aside
from these requiremsnts any instrument considered must be test—
ed for its ability to measure rapid changes in velocity; that
is, its inertia reactions must be determined. A Friez type
cup anemometer had been loaned to the Bureau of Aeronautics
by Julien P. Friez and Sons, and since the instrument seemed
to meet the requirements of ruggedness and simplicity, it was
proposed for use in the preliminary studies. The National
Advisory Committee for Aeronautics Was requested to make a cal—
ibration and lag study of the instrument.

The investigation was made in the Variable Density Wind
Tunnel by comparing the anemometer air~speed indications with
the air speed as measured by a Pitot—static tube. The purpose
has been twofold, namely, to calibrate the instrument against
a Pitot—static tube, and to determine its lag characteristics.
Calibrations were.made at two air densities (approximately 1,
and 2/3 atmospheres), in order to determine the effect of
change in altitude. The lag characteristics were determined
by a direct comparison of anemometer and Pitot-static tube in—
dications during accelerating air flow.