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National Advisory Committee for Aeronautics, Report - Aircraft Speed Instruments

This report presents a concise survey of the measure-
ment of air speed and ground speed on board aircraft.
Special attention is paid to the Pitot-static air—speed
meter which is the standard in the United States for air-
planes. Air-speed meters of the rotating vane type are
also discussed in considerable detail on account of their
value as flight test instruments and as service instruments
for airships. Methods of ground¥speed measurement
are treated briefly, with references to the more important
instruments. A bibliography on air—speed measure-
ment concludes the report.
In 1922 the National Advisory Committee for Aero-
nautics issued Report No. 127, Aircraft Speed Instru-
ments, by F. L. Hunt and H. O. Stearns.’ The rapid
progress of aviation, both military and civil, since
that time has brought about so many changes in the
technique of measuring the speed of aircraft, particu-
larly the speed with reference to the air, that the report
is now largely of historical value only. The evident
need for another presentation has resulted in this new
report which was prepared at the aeronautic instru-
ments section of the Bureau of Standards under a
research authorization and with the financial support
of the National Advisory Committee for Aeronautics.
The primary purpose of the report is to present in
concise form a complete summary of the art of measur-
ing the speed of an aircraft, both with reference to the
air and with reference to the ground. To this end,
data now widely scattered in various publications,
many of which are not easily obtainable, are brought
together, and available new material is included in so
far as possible. Although attention is devoted
chiefly to American instruments and methods, fre-
quent references to foreign practice have been required
in order to cover the subject.
The speed of the aircraft with reference to the air
is designated simply the air speed; the speed relative
to the ground, the ground speed. The ground speed
can be obtained from the air speed by applying a cor-
rection for the wind velocity, if the wind velocity is
known.