If the potential energy is represented by the 
static pressure, p, the sum of the potential and 
kinetic energy is the total pressure of the air- 
stream. 
H=p+% P V’ 
where H=total pressure, psf (sometimes re- 
ferred to as “head ’ pressure) 
p=static pressure, psf. 
p=density, siugs per cu. ft. 
V= velocity, ft./set. 
This equation is the Bernoulli equation for 
‘incompressible flow. It is important to ap- 
preciate that the term >$pV2 has the units of 
pressure, psf. This term is one of the most 
important in all aerodynamics and appears so 
frequently t&it is given the name “dynamic 
pressure” and the shorthand notation “4”. 
q= dynamic pressure, psf 
= jgpv2 
With this definition it could be said that the 
sum of static and dynamic pressure in the flow 
tube remains constant. 
Figure 1.3 illustrates the variation of static, 
dynamic, and total pressure of air flowing 
through a closed tube. Note that the total 
pressure is con,stant throughout the length 
and any change in dynamic pressure produces 
the same magnitude change in static pressure. 
The dynamic pressure of a free airstream is 
the one ‘common denominator of all aero- 
dynamic forces and moments. Dynamic pres- 
sure represents the kinetic energy of the free 
airstream and is a factor relating the capability 
for producing changes in static pressure on a 
surface. As defined, the dynamic, pressure 
varies directly as the density and the square of 
the velocity. Typical values of dynamic pres- 
sure, 4, are shown in table l-1 for various true 
airspeeds in the standard atmosphere. Notice 
that the dynamic pressure at some fixed veloc- 
ity varies directly with the density ratio at any 
altitude. Also, appreciate the fact that at an 
altitude of 40,oM) feet (where the density ratio, 
b, is 0.2462) it is necessary to have a true air 
velocity twice that at sea level in order to 
product the same dynamic pressure. 
NAVWEPS 00-801-80 
BASIC AERODYNAMICS 
TABLE l-l. Effect of Speed and Altitvde on Dwzmnic Prerrure 
True air 
speed 
(fr./scc.) 
m= 
169 
338 
507 
616 
845 
I, 013 
- 
,I I 
c 
_- 
AIRSPEED MEASUREMENT. If a sym- 
metrically shaped object were placed in a 
moving airstream, the flow pattern typical of 
figure 1.4 would result. The airstream at the 
very nose of the object would stagnate and the 
relative flow velocity at this point would be 
zero. The airflow ahead of the object pos- 
sesses some certain dynamic pressure and 
ambient static pressure. At the very nose of 
the object the local velocity will drop to zero 
and the airstream dynamic pressure will be 
converted into an increase in static pressure at 
the stagnation point. In other words, there 
will exist a static pressure at the stagnation 
point which is equal to the airstream total 
pressure-ambient static pressure plus dynamic 
pressure. 
Around the surface of the object the airflow 
will divide and the local velocity will increase 
from zero at the stagnation point to some 
maximum on the sides of the object. If fric- 
tion and viscosity effects are neglected, the 
9