PlAVWEPS 00-8OT-80  BASIC AERODYNAMICS  Thus, certain corrections must apply to the  instrumentation as well as the aircraft per-  formance if the operating conditions do not  fit the standard atmosphere. In order to prop-  erly account for the nonstandard atmosphere  certain terms must be defined.  Pressure  .&itudc  is the altitude in the standard atmosphere  corresponditrg to a particular pressure. The  aircraft altimeter is essentially a sensitive  barometer calibrated to indicate altitude in  the staotlard atmosphere. If the altimeter is  set for 29.92 in. Hg the altitude indicated is  the pressure  altitude-the  altitude in the stand-  ard atmosphere corresponding to the sensed  pressure.  Of course, this indicated pressure  altitude may not be the actual height above  sea level due to variations in remperature,  lapse rate; atniospheric pressure, and possible  errors in the sensed  pressure.  This relationship has great application in  aerodynamics and is quite fundamental and  necessary  in certain parts of airplane perform-  ance.  VISCOSITY.  The viscosity of the air is  important in scale and friction effects. The  coefficient of absolute viscosity is the propor-  tion between the shearing stress and velocity  gradient for a fluid flow.  The viscosity of  gases is unusual in that the viscosity is gen-  erally a function of temperature alone and an  increase  in temperature increases  the viscosity.  The coefficient of absolute viscosity is assigned  the shorthand notation I, (mu).  Since many  parts of aerodynamics involve consideration of  viscosity and density, a more usual form of  viscosity measure is the proportion of the co-  efficient of absolute viscosity and density.  This combination is termed the “kinematic  viscosity” and is noted by Y (nu).  cc  coefficient of absolute viscosity  density  v=PlP  The more appropriate term for correlating  aerodynamic performance in the nonstandard  atmosphere is density &it&-the  altitude in  the standard atmosphere corresponding to a  particular value of air density.  The computa-  tion of density altitude must certainly involve  consideration of pressure (pressure altitude)  and temperature. Figure 1.6 illustrates the  manner in which pressure altitude and tem-  perature combine to produce a certain density  altitude.  This chart is quite standard in use  and is usually included in the performance  section of the flight handbook. Many subject  areas  of aerodynamics and aircraft performance  will emphasize density altitude and temperature  as the most important factors requiring con-  sideration.  kinematic viscosity  The kinematic viscosity of air at standard sea  level conditions is 0.0001576 square feet per  second. At an altitude of 40,000 feet the  kinematic viscosity is increased to 0.0005059  square foot per second.  In order to provide a common denominator  for comparison of various aircraft, a standard  atmosphere has been adopted. The standard  atmosphere actually represents the mean or  average properties of the atmosphere. Figure  1.1 illustrates the variation of the most im-  portant properties of the air throughout the  standard atmosphere. Notice that the lapse  rate is constant in the troposphere and the  stratosphere  begins with the isothermal region.  Since all aircraft performance is compared  and,evaluated in the environment of the stand-  ard atmosphere, all of the aircraft instrumenta-  tion is calibrated for the standard atmosphere.  All of the external aerodynamic forces on a  surface are the result of air pressure  or air fric-  tion. Friction effects  are generally confined to  a thin layer of air in the immediate vicinity of  the surface and friction forces are not the pre-  dominating aerodynamic forces. Therefore,  BERNOULLI ’ S PRINCIPLE AND SUBSONIC  AIRFLOW  4 