Aeronautic instruments. Section 2: altitude instruments Page: 3 of 62
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REPORT No. 126.
ALTITUDE INSTRUMENTS.
PART L
ALTIMETERS AND BAROGRAPHS.
By A. H. Murs, H. B. HEzncsON, and W. G. BROMBAOHER.
INTRODUCTION.
This report is Section II of a series of reports on aeronautic instruments (Technical Re-
ports Nos. 125 to 132, inclusive) prepared by the aeronautic instruments section of the Bureau
Of Standards under research authorizations formulated and recommended by the subcommittee
on aerodynamics and approved by the National Advisory Committee for Aeronautics. Much
of the material contained in this report was made available through the cooperation of the
War and Navy Departments.
This part discusses briefly barometric altitude determinations, and describes in detail the
principal types of altimeters and barographs used in aeronautics during the recent war. This
is followed by a discussion of performance requirements for such instruments and an account
of the methods of testing developed by the Bureau of Standards. The paper concludes with
a brief account of the results of recent investigations.
Altimeters and barographs are used for determining the altitude of aircraft above the
ground, the former giving direct indications on a dial, and the latter furnishing a continuous
record on a chart.
For accurate measurements of altitude, reference must also be made to thermometer
readings of atmospheric temperature, since the altitude is not fixed by atmospheric pressure
alone. This matter is discussed in the following section on barometric altitude determination.
Obviously, any instrument which purports to determine altitude solely from observations of
atmospheric condition is liable to some uncertainty unless very complete observations of the
state of the atmosphere are taken throughout the entire period of time and over the entire
region of space corresponding to the flight, including simultaneous observations on the ground.
Thus the determination of the most probable altitude from barometric observations
requires an elaborate system of observations and intricate computations. Fortunately for many
purposes extreme accuracy is not necessary. It is possible to make altitude observations by
direct reading of the altimeter without any computations or supplementary observations, pro-
vided an accuracy of 10 or 15 per cent is deemed sufficient. This is done by having the alti-
meter dial graduated directly in altitude units, based on the assumption of some particular
constant value for atmospheric temperature.
BAROMETRIC ALTITUDE DETERMINATION.
It is a comparatively simple matter to deduce mathematically the relation connecting
pressure, temperature, and altitude for a perfectly stationary atmosphere treated as an ideal
gas. If proper account is taken of the actual variation of temperature vertically throughout
the air column such a relation will be sufficient for most purposes of altitude determination.
This relation may be referred to as the general altitude equation, which may be written in either
of the forms
or
- logey (2)
501
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Mears, A. H.; Henrickson, H. B. & Brombacher, W. G. Aeronautic instruments. Section 2: altitude instruments, report, 1923?; (https://digital.library.unt.edu/ark:/67531/metadc65776/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.