NAVWEe3 OO-BOT-80 
BASIC AERODYNAMICS 
percent water vapor, argon, carbon dioxide, 
etc. For the majority of all aerodynamic con- 
siderations air is considered as a uniform 
mixture of these gases. The usual quantities 
used to define the properties of an air mass are 
as follows: 
STATIC PRESSURE. The absolute static 
pressure of the air is a property of primary 
importance. The static pressure of the air 
at any altitude results from the mass of air 
supported above that level. At standard sea 
level conditions the static pressure of the air 
is 2,116 psf (or 14.7 psi, 29.92 in. Hg, etc.) 
and at 40,000 feet altitude this static pressure 
decreases to approximately 19 percent of the 
sea level value. The shorthand notation for 
the ambient static pressure is “p” and the 
standard sea level static pressure is given the 
subscript “a” for zero altitude, pa. A more 
usual reference in aerodynamics and perform- 
ance is the proportion of the ambient sta~tic 
pressure and the standard sea level static 
pressure. This static pressure ratio is assigned 
the shorthand notation of 8 (delta). 
Altitude pressure ratio 
Ambient static pressure 
=Standard sea level static pressure 
6 = PIP0 
Many items of gas turbine engine perform- 
ance are directly related to some parameter 
involving the altitude pressure ratio. 
TEMPERATURE. The absolute tempera- 
cure of the air is another important property. 
The ordinary temperature measurement by the 
Centigrade scale has a/datum at the freezing 
point of water but absolute zero temperature 
is obtained at a temperature of -273“ Centi- 
grade. Thus, the standard sea level tcmpera- 
ture of 15” C. is an absolute temperature of 
288”. This scale of absolute temperature using 
the Centigrade increments is the Kelvin scale, 
e.g., o K. The shorthand notation for the 
ambient air temperature is “T” and the stand- 
ard sea level air temperature of 288’ K. is 
signified by Ta. The more usual reference is, 
the proportion of the ambient air temperature 
and the standard sea level air temperature. 
This temperature ratio is assigned the short- 
hand notation of 0 (theta). 
Temperature ratio 
Ambient air temperature 
=Standard sea level air temperature 
@=TITtl 
,+273 
288 
Many items of compressibility effects and jet 
engine performance involve consideration of 
the temperature ratio. 
DENSITY. The density of the air is a prop- 
erty of greatest importance in the study of 
aerodynamics. The density of air is simply 
the mass of air per~cubic foot of volume and 
is a direct measure of the quantity of matter 
in each cubic foot of air. Air at standard sea 
lcvcl conditions weighs 0.0765 pounds per cubic 
foot and has a density of 0.002378 slugs per 
cubic foot. At an altitude of 40,000 feet the 
air density is approximately 25 percent of the 
sea level value. 
The shorthand notation used for air density 
is p (rho) and the standard sea level air density 
is then pO. In many parts of aerodynamics it 
is very convenient to consider the proportion 
of the ambient air density and standard sea 
level air density. This density ratio is assigned 
the shorthand notation of c (sigma). 
density ratio= ambient air density 
standard sea level air density 
a = PIP0 
A general gas law defines the relationship of 
pressure temperature, and density when there 
is no change of state or heat transfer. Simply 
stated this would be “density varies directly 
with pressure, inversely with temperature.” 
Using the properties previously defined, 
density ratio= Pressure rat’o. 
temperature rat10 
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