NAVWEPS OO-EOT-80 
BASIC AERODYNAMICS 
FORWARD STAGNATION AFT STAGNATION 
POINT POINT 
AIRSTREAM AHEAD STAGNATION PRESSURE 
HAS AMBIENT STATIC IS AIRSTREAM TOTAL 
PRESSURE AND DYNAMIC PRESSURE 
PRESSURE P+q 
Ftgure 1.4. Flow Pattern on a Symmetrical Object 
surface anflow continues to the aft stagnation 
point where the local velocity is again zero. 
The important point of this example of aero- 
dynamic flow is existence of the stagnation 
point. The change in airflow static pressure 
which takes place at the stagnation point IS 
equal to the free stream dynamic pressure, q. 
The measurement of free stream dynamic 
pressure is fundamental to the indication of 
airspeed. In fact, airspeed indicators are sim- 
ply pressure gauges which measure dynamic 
pressure related to various airspeeds. Typical 
airspeed measuring systems are illustrated in 
figure 1.5. The pitot head has no internal 
flow velocity and the pressure in the pitot tube 
is equal to the total pressure of the airstream. 
The purpose of the static-ports is to sense the 
true static pressure of the free airstream. The 
total pressure and static pressure lines are 
attached to a differential pressure gauge and 
the net pressure indicated is the dynamic 
pressure, q. The pressure gauge is then cali- 
brated to indicate flight speed in the standard 
sea level air mass. For example, a dynamic 
pressure of 305 psf would be realized at a sea 
level flight ,speed of 300 knots. 
Actually there can be many conditions of 
flight where the airspeed indicator does not 
truly reflect the actual velocity through the 
air mass. The corrections that must be applied 
are many and lisred in sequence below: 
(1) The indicated airspeed (IAS) is the 
actual instrument indication for some given 
flight condition. Factors such as an altitude 
other than standard sea level, errors of the 
instrument and errors due to the installation, 
compressibility, etc. may create great vari- 
ance between this instrument indication and 
the actual flight speed. 
(2) The calibrated airspeed (CM) is the 
result of correcting IAS for errors of the 
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