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STANDARD

In aeronautics a standard atmosphere is an assumed condition of the atmosphere which is defined by an altitude- temperature-pressure relation. The chief difference anions the various atmospheres which have been proposed lies in the selection of the altitude-temperature relation. A standard atmosphere is an aeronautic necessity in evaluating the performance of airplanes and for the calibration of instruments. For example, the performance of an

airplane, such as its rate of climb or ceiling, in order

to be comparable with that of other machines is stated in terms of altitudes which correspond to the same standard conditions of ail* temperature,

pressure, and density. Further, altimeters are pressure-measuring instruments graduated in terms

of altitude according to the altitude-pressure relation

of a standard atmosphere. Air-speed indicators are calibrated to indicate true air speed at a standard

air density. The results of wind-tunnel investigations are also reduced to a standard density of air. A standard atmosphere has been officially adopted

for use in the United States for the above and other

aeronautic purposes by the following Government

organizations: Army Air Corps, Bureau of Standards, National Advisory Committee for Aeronautics, Navy Bureau of Aeronautics, and the Weather Bureau. This atmosphere is now in general use throughout the United States in aero- nautics except in the evaluation of the altitude of

llights made to break international records, in which

case the Federation Aei'onautique Internationale standard atmosphere is the basis of comparison.

The altitude-temperature assumption of the United

States standard is a slight modification of that propped \jj Toudsal.-l and closely u^rtxiiuaiw

the average observed values of air temperature at

all altitudes at latitude 40° in the United States. The formulas defining the United States standard atmosphere are given below.

(a) For all standard altitudes

P Ta P=P°F,T

T,, = 288° absolute = 15° C.

2>„ = 760 mm of Hg = 29.921 in. of Hg. K= 19,413.3 for Z in meters. = G3,691.S for Z in feet.

p = 1.2255 for p in kg/nv'. = 0.07651 for pin lbs./ ft.3

(1)

(2)

MOSPHERiPhyiidit

(b) Up to the isothermal layer (below

(4) \~aZ

for Z in meters. = 0.0019812 for Z in feet,

(c) At the lower limit of the isothermal layer (10,769 m)

T=218° absoIute= -55° C. ZS5 =35,332 feet = 10,769 meters.

7'„5S = 251.378 absolute. In the isothermal layer (above 10,769 m)

r=21S° absoluto= -55° C.

(a!)

_Z« z-z^

Tma 218

In the above formulas Z = Standard altitude. Z55 = Altitude at tho lower limit of i isothermal layer. T=Absolute temperature of the air altitude Z. To = Standard sea-level temperature

degrees absolute.

!T„ = Mean temperature of the air colut

7SS = iMoan temperature it lute for Zss . p = Pressure of the air at altitude Z.

2>o = Standard sea-level pressure.

P = Density of the air at altitude Z.

p = Staiidard density at sea level. Absolute temperatures are ecpual to centigrade temperatures plus 273. p is in the same unit of presTho chart on the reverse of this card gives the temperature, pressure, and density for standard

altitudes up to 50,000 feet. A brief table of the values of these quantities is given below. See National Advisory Committee for Aeronautics Technical Reports Nos. 147, 218, and 246 of the committee

for further data and complete tables.

Standard Atmosphere Table

Meters Feet mm Hg in Hg kg/m> Lbs./ft.'

3281

9842

16404

1120 0068

. 9094

26247 29528 32808

. 5896

. 5252

4664 .4127

3614

. 3090

2642 2250

1931

. 02256

. 01929

. 01649

. 01410

30000

35000

40000

45000

50000

. i

; s; 3705

2oos . 01872

. 01474

. 01161