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National Advisory Committee for Aeronautics, Report - A Radar Method of Calibrating Airspeed Installations on Airplanes in Maneuvers at High Altitudes and at Transonic and Supersonic Speeds

A method of calibrating the static-pressure source of a pitot
static airspeed installation on an airplane in level flight, dices,
and other mane-urers at high altitude and at transo-nic and
supersonic speeds is described. {lie method principally
involves the use of radar—phototheodolite tracking equipment.
The various sources of error in the method are discussed and
sample calibrations are included.

The speed of aircraft is still most readily determined by
means of a pitct—static installation. The pitot-static installa-
tion may consist of a single tube containing both total-
p1 essure and static-pressure elements or it. may consist of a
pitot tube and a separate source of static pressure in the form,
perhaps, of a fuselage static—pressure vent. A pitot tube can
usually be easily located to measure the correct total pressure
at subsonic speed of the aircraft. At supersonic speeds the
pitot tube should be located ahead of shock waves emanating
from any part of the airplane if the usual normal-shock rela-
tions are to be used to evaluate the airspeed. At low super-
sonic speeds approaching a Mach number of 1.0, shock waves
emanating from the airplane and passing in front of the
pitot tube may be tolerable since the loss in total pressure
due to shock at such speed is negligible. The principal diffi—
culty in the determination of airspeed from measurements
with a pitot-static installation lies in the measurement of the
correct static pressure.

For installations intended primarily
for transonic and supersonic speeds the static—pressure source
should be located, if possible, in a region where compressi-
bility shocks are absent or, if present, where they are suffi-
ciently weak to avoid large discontinuities in the variation
of static pressure, and hence indicated Mach number, with
airspeed. Greater freedom in the selection of a suitable loca—
tion of the static-pressure source exists at subsonic speeds;
however, in general the location chosen for transonic and
supersonic speeds is also satisfactory for subsonic speeds.
Since the static—pressure source does not. always provide an
exact. measure of free-stream static pressure, the error in
static pressure may be determined by calibration or by com-
putation for some special cases. For installations in which
intense compressibility shocks pass over the static-pressure
source, a single-value calibration may be impossible to obtain.