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Standard Atmosphere
The earth's atmosphere is a dynamically changing system
The pressure and temperature of the atmosphere depend on
altitude, location onthe globe (longitude and latitude), time of
day, season, and even solar sunspotactivity. To take all these
variations into account when considering the design andperformance
of flight vehicles is impractical.
A standard atmosphere is defined in order to relate flight
tests, wind tunnelresults, and general airplane design and
performance to a common reference . Itgives mean values of
pressure, temperature, density, and other properties asfunctions of
altitude; these values are obtained from experimental balloon
andsounding-rocket measurements combined with a mathematical model
of the
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atmosphere.
The tables in Apps. A (SI units) and B (English units) at the
end of the book.
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Geometric altitudegeometric height above sea level.
Absolute altitude ha. If r is theradius of the earth, thenha = h
G + r.
From Newton's law of gravitation,
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Hydrostatic Equation
Now assume that (where h is the geopotential altitude)
divide
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Geopotential Altitude
There is little difference between h and h G for low altitudes.
For such a case, h G r, r/ (r + hG) ~ 1; hence h ~ h G .
Earth radius r = 6.356766 x 10 6 m (at a latitude of 45), for hG
= 7 km
Principles of Flight (MEC 2710)
h = 6.9923 km 0.001 error
Only at altitudes above 65 km (213,000 ft) does the difference
exceed 1 percent.
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DEFINITION OF THE STANDARD ATMOSPHERE
The keystone of the standardatmosphere is a defined variation of
Twith altitude, based on experimentalevidence.
cons s s o a ser es o s ra g nes,some vertical (called the
constant-temperature, or isothermal regions )and others inclined
(called the gradientregions ).
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PRESSURE, TEMPERATURE, AND DENSITY ALTITUDES
start at sea level (h = 0)
PRESSURE, TEMPERATURE, AND DENSITY ALTITUDESSuppose you are
flying at actual pressure 6.16 x 10 4 Pa, and actual T= 265.4 K
If actual (measured) pressure = 6.16 x 10 4 Pa App. A hG = 4 km
pressure altitude of 4 km.
for actual measured tem erature = 265.4 K A . A h = 3.5 km
Principles of Flight (MEC 2710)
temperature altitude of 3.5 km.
Thus you are simultaneously flying at a pressure altitude of 4
km and a temperaturealtitude of 3.5 km while your actual geometric
altitude is yet a different value. Thedefinition of density
altitude is made in the same way.
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Chap 2 : 1 5* 11 14 Chap 3 : 1 2 4* 9 12
Principles of Flight (MEC 2710) 10
