NAVWEe3 OO-BOT-80  BASIC AERODYNAMICS  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  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).  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.  Ambient air temperature  =Standard sea level air temperature  @=TITtl  ,+273  288  6 = PIP0  a = PIP0  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  Altitude pressure  ratio  Ambient static pressure  =Standard sea level static pressure  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,  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  2 