Standard Atmosphere in Aeronautics: Definition, Adoption, and Formulas, Slides of Earth, Atmospheric, and Planetary Sciences

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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 atmos-

phere 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 investiga- tions 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 Stand- ards, National Advisory Committee for Aero- nautics, 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)

MOSPHERi-

Phyiidit

(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.

Z 55 = 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 (^) tem- peratures plus 273. (^) p is in the same unit of (^) pres-

Tho 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 Na- tional Advisory Committee for Aeronautics Techni- cal 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 .

3614

. 3090 2642 2250 1931 . 02256 . 01929 . 01649 . 01410

30000 35000 40000 45000 50000

. i ;^ s; 3705 2oos. 01872 . 01474 . 01161