NAVWEPS 00-807-80 
BASIC AERODYNAMICS 
PITOT-STATIC SYSTEM 
w / :% . I. q 
PITOT WITH SEPARATE 
STATIC SOURCE 
PRESSURE INDICATED BY GAUGE IS 
DIFFERENCE BETWEEN TOTAL AND 
STATIC PRESSURE, H-p= q 
Figure. 1.5. Airspeed Measurement 
instrument and errors due to position or lo- 
cation of the installation. The instrument 
error must be small by design of the equip- 
ment and is usually negligible in equjpment 
which is properly maintained and cared for. 
The position error of the installation must 
be small in the range of airspeeds involving 
critical performance conditions. Position 
errors are most usually confine,d to the static 
source in that the actual static pressure 
sensed at the static port may be different 
from the free airstream static pressure. 
When the .,aircraft is operated through a 
large range’ of angles of attack, the static 
pressure distribution varies ‘quite greatly 
and it becomes quite difficult to’minimize 
the static source error. In most instances a 
compensating group of static sources may 
be combined to reduce the position error. 
In order to appreciate the magnitude of this 
problem, at flight speed near 100 knots a 
11 
0.05 psi position error is an airspeed error 
of 10 knots. A typical variation of air- 
speed system position error is illustrated in 
figure 1.6. 
(3) The equivalent airspeed (PAS) is the 
result of correcting the (CAS) for compressi- 
bility effects. At high flight speeds the 
stagnation pressure recovered in the pitot 
tube is not representative of the airstream 
dynamic pressure due to a magnification 
by compressibility. Compressibility of the 
airflow produces a stagnation pressure in 
the pitot which is greater than if the flow 
were incompressible. As a result, the air- 
speed indication is given an erroneous mag- 
nihcation. The standard airspeed indicator 
is calibrated to read correct when at standard 
sea level conditions and thus has a com- 
pressibility correction appropriate for these 
conditions. However, when the aircraft is 
operating above standard sea level altitude, 
Revised January 1965