NAVWEPS 00-SOT-80 
BASIC AERODYNAMICS 
the inherent compensation is inadequate and 
additional correction must be applied. The 
subtractive corrections that must be applied 
to CA$ depend on pressure altitude and CAS 
and are shown on figure 1.6 for the subsonic 
flight range. The equivalent airspeed (EAS) 
is the flight speed in the standard sea level 
air mass which would produce the same free 
stream dynamic pressure as the actual flight 
condition. 
(4) The true airspeed (TAS) results when 
the &4X is corrected for density altitude. 
Since the airspeed indicator is calibrated 
for the dynamic pressures corresponding to 
airspeeds at standard sea level conditions, 
variations in air density must be accounted 
for. To relate EAS and TAX requires con- 
sideration that the EAS coupled with stand- 
.ard sea level density produces the same dy- 
namic pressure as the TAX Soupled with the 
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From this reasoning, it can be shown that: 
(TAS)2p=(EAS)2 po 
d 
- 
or, TAS=EAS 62 P 
TAS= EAS 2 
4 
where TAX= true airspeed 
EAS=equivalent airspeed 
p=actual air density 
PO= standard sea level air density 
n=altitude density ratio, p/pa 
The result shows that the TAX is a function 
of EAS and density altitude. Figure 1.6 shows 
a chart of density altitude as a function of 
pressure altitude and temperature. Each par- 
ticular density altitude fixes the proportion 
between TAX and EAS. The use of a naviga- 
tion computer requires setting appropriate 
values of pressure altitude and temperature on 
the scales which then fixes rhe proportion be- 
tween the scales of TAS and EAS (or TAS and 
CAS when compressibiliry corrections are 
applicable). 
Revlted Jmuoy 1965 
14 
Thus, the airspeed indicator system measures 
dynamic pressure and will relate true flight 
velocity when instrument, position, compress- 
ibility, and density corrections are applied. 
These corrections are quite necessary for ac- 
curate determination of true airspeed and 
accurate navigation. 
Bernoulli’s principle and the concepts of 
static, dynamic, and total pressure are the basis 
of aerodynamic fundamentals. The pressure 
distribution caused by the variation of local 
stack and dynamic pressures on a surface is 
the source of the major aerodynamic forces 
and moment. 
DEVELOPMENT OF AERODYNAMIC 
FORCES 
The typical airflow patterns exemplify the 
relationship of static pressure and velocity 
defined by Bernoulli. Any object placed in an 
airstream will have the a& to impact or stag- 
nate at some point near the leading edge. The 
pressure at this point of stagnation will be an 
absolute static pressure equal to the total pres- 
sure of the airstream. In other words, the 
static pressure at the stagnation point will be 
greater than the atmospheric pressure by the 
amount of the dynamic pressure of the air- 
stream. As the flow divides and proceeds 
around. the object, the increases in local ve- 
locity produce decreases in static pressure. 
This procedure of flow is best illustrated by the 
flow patterns and pressure distributions of 
figure 1.7. 
STREAMLINE PATTERN AND PRES- 
SURE DISTRIBUTION. The flow pattern of 
the cylinder of figure 1.7 is characterized by 
the streamlines which denote the local flow 
direction. Velocity distribution is noted by 
the streamline pattern since the streamlines 
effect a boundary of flow, and the airflow 
between the streamlines is similar to flow in a 
closed tube. When the streamlines contract 
and are close together, high local velocities 
exist; when the streamlines expand and are 
far apart, low local velocities exist. At the