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• November 2, 2016 Create Date
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National Advisory Committee for Aeronautics, Report - Error in Airspeed Measurement Due to the Static Pressure Field Ahead of an Airplane at Transonic Speeds

The magnitude and variation of the static-pressure error for
various distances ahead of sharpmose bodies and' open~nose air
inlets and for a distance of 1 chord ahead of the wing tip of a
muept wing are defined by a combination of experiment and
theory. The mechanism of the error is discussed in some detail
to show the contributing factors that make up the error. The
information presented provides a useful means for choosing a
proper location for measurement of static pressure for most
purposes.

The precision with which airspeed and altitude can be
measured in flight by a pitot—static tube depends upon the
accuracy with which the free-stream total and static pres-
sures are determined. The error introduced by a well—
designed pitot-statie airspeed head is usually negligible both
at subsonic and at supersonic speeds (refs. 1 and 2).
The problem is then resolved into the choice of a location
for the airspeed head at which the total and static pressures
are affected to a minimum extent by the pressure field of the
airplane.

There is no difficulty in locating the total-pressure tube at
subsonic speeds if it is placed well outside of the propeller
slipstream, the boundary layer, and the wake from the
airplane structure. At supersonic speeds, there is a loss in
total pressure when the total-pressure tube is subjected to a
shock wave; however, this loss is negligible at low supersonic
speeds and may be calculated from the normal shock rela—
tions at higher speeds.

The location of static-pressure tubes for minimum static-
pressure error can be realized at subsonic speeds by locating
the tube sufficiently far ahead of the wing tip of the airplane
(usually one chord length for research purposes). Satisfac—
tory measurements may also be obtained in many instances
by fuselage static vents. The choice of a suitable vent
location, however, must usually be made by trial in wind—
tunnel or flight tests.