A radar method of calibrating airspeed installations on airplanes in maneuvers at high altitudes and at transonic and supersonic speeds Page: 2 of 7
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REPORT 985-NATIONAL ADVISORY COMMITTEE FOR AERONAiUTICS
Airplane equipment.-For the calibration tests the air-
plane should be equipped with the following instruments: a
pitot-static installation, a recording normal accelerometer,
an airspeed recorder, a recording altimeter, a chronometric
timer, and a radio to communicate the timing signals to the
ground equipment. The airspeed recorder is used to record
the impact pressure, or the difference between total and
static pressures, measured by the pitot-static installation.
The recording altimeter is used to record the static pressure
measured by the static-pressure source. A measure of free-
stream temperature by means of a calibrated thermometer
is also very desirable but not absolutely necessary. The air-
speed recorder and the recording altimeter should be the
only instruments connected to the static-pressure source and
should be located as near to it as possible in order to minimize
the pressure lag of the recording system. The magnitude of
the pressure lag may be determined by methods described
in reference 1. If the lag is appreciable, corrections must be
made to the measured static pressure.
Ground equipment,-The required ground equipment
consists of a radar unit, a phototheodolite, and a chronograph.
The radar unit is directed on a target through the use of the
phototheodolite. The radar unit is equipped within a motion-
picture camera to photograph the radar scope, a target
camera, and an elevation-scale camera., The scope photo-
graphs give the slant range and the target photographs, the
correction to _the elevation scales. The_three cameras and
the airplane equipment are synchronized by means of a
chronograph that records timing impulses from the ground
cameras and the airplane timer.
CALIBRATION PROCEDURE AND ACCURACY
The calibration procedure -consists essentially in surveying
the atmospheric pressure over the desired range of altitude
and then flying the airplane to be calibrated through this
region and recording the static pressure measured by the
static-pressure source,. Th _.static-pressure error of the
pitot-static installation is the difference between atmospheric
pressure and the static pressure measured by the static-
The survey of atmospheric pressure may be made with the
airplane being calibrated or with another airplane similarly
instrumented. In either case the airplane is tracked with
the radar-phototheodolite unit while measurements _of static
pressure are being made with the static-pressure installation
at airplane speeds for which a calibration is available or may
be obtained by other techniques such as described in refer-
ence 1. The atmospheric pressure so determined is in error
by the static-pressure error of the static-pressure source and
must be corrected for it. Because of the variation of atmaos-
pheric pressure with time, the survey should be repeated
frequently (for instance, between calibrating maneuvers if
necessary) when the calibration tests are expected to take a
long time. Variations in sea-level atmospheric pressure
with time of as much as 0.3..inch of water per hour are
possible under some conditions.
When the airplane is also equipped with a thermometer
for determining free-stream temperature, measurements of
temperature are made simultaneously with the measurements
of static pressure during the surveys. From the variation
of free-stream temperature with altitude, a fairing of the
static-pressure data with altitude may be obtained since
P=v Ae R
where _, is arbitrarily selected for the best fairing of the
In the calibration maneuvers the airplane should be flown
through the region surveyed (slant range, elevation, and
azimuth) or as near to it as possible. For maneuvers in
which the airplane cannot be restricted to the region surveyed
as a result of the high speed of the airplane, corrections may
be necessary to the data obtained in the survey and in the
maneuver for the following: refraction of light, curvature
of the earth, error in alinement of the plhototheodolito refer-
ence plane with the horizon, and the variation of atmospheric
pressure at a given altitude with distance along the earth's
surface. The error in altitude due to refraction of light, as
determined from equations derived in the appendix and the
notation in figure 1, is presented . in figure 2 and tihe error
due to the earth's curvature is given in figure 3. A check of
the sea-level atmospheric pressure as measured at U. S.
weather stations I0 to 30 miles apart indicated that on days
with surface winds of 20 to 25 miles per hour the horizontal
pressure gradients may be as much as 0.05 inch of water per
mile along the earth's surface. At an altitude of 10,000 feet,
weather maps indicated horizontal pressure gradients as
much as 0.02 inch of water per mile.
An SCR-584 radar unit with phototheodolite and cameras
(scope, target, and elevation scales) has an estimated accur-
acy of about -45 feet in slant. range andl -0.2 nil in eleva-
tion for a single observation. For a series of observations
where the data may be faired, the accuracy, of course, is
improved. The error in altitude due to tie error in slant
range and elevation angle is shown in figure 4. Consistent
errors in the tracking equipment should have no effect on
the accuracy of the calibration, provided that these errors
occur throughout the calibration procedure.
An NACA recording altimeter has a random error of about
X percent of full-scale value for a single observation. This
error corresponds to errors of -I and =0.2 inch of water
for altimeters built to measure pressures including sea-level
pressure and to measure pressures at and above an altitude
of 40,000 feet, respectively. The combined maximum prob-
able error in a single observation due to errors in measuring
static pressure and in determining altitude (+45 ft or 0.2
in. of water at 40,000 ft) is -1 1 inch of water for the altim-
eter with a range including sea-level pressure and 0.3 inch
of water for the altimeter with a range for altitudes above
about 40,000 feet. Since in the calibration procedure a series
of observations is made over a range of altitude, the fairing
of the data reduces the magnitude of the error.
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Zalovcik, John A. A radar method of calibrating airspeed installations on airplanes in maneuvers at high altitudes and at transonic and supersonic speeds, report, August 31, 1949; (digital.library.unt.edu/ark:/67531/metadc60324/m1/2/: accessed October 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.