PlAVWEPS 00-8OT-80 
BASIC AERODYNAMICS 
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). 
kinematic viscosity 
cc coefficient of absolute viscosity 
density 
v=PlP 
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. 
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. 
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. 
BERNOULLI’S PRINCIPLE AND SUBSONIC 
AIRFLOW 
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, 
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