NAVWEPS OCMOT-60 
HIGH SPEED AERODYNAMICS 
It is important to note that compressibility 
effects are not limited to flight speeds at and 
above the speed of sound. Since any aircraft 
will have some aerodynamic shape and will 
be developing lift there will be local flow 
velocities on the surfaces which arc greater 
than the flight speed. Thus, an aircraft can 
experience compressibility effects at flight 
speeds well below the speed of sound. Since 
there is the possibility of having both subsonic 
and supersonic flows existing on the aircraft 
it is convenient to define certain regimes of 
flight. These regimes are defined approxi- 
mately as follows: 
Subsonic-Mach numbers below 0.75 
Transonic-Mach numbers from 0.75 to 
1.20 
Supersonic-Mach numbers from 1.20 to 
5.00 
Hypersonic-Mach numbers above 5.00 
While the flight Mach numbers used to define 
these regimes of flight are quite approximate, 
it is important to appreciate the types of flow 
existing in each area. In the subsonic regime 
it is most likely that pure subsonic airflow 
exists on all parts of the aircraft. In the 
transonic regime it is very probable that flow 
on the aircraft components may be partly sub- 
sonic and partly supersonic. The supersonic 
and hypersonic’ flight regimes will provide 
definite supersonic flow velocities on all parts 
of the aircraft. Of course, in supersonic flight 
there will be some portions of the boundary 
layer which are subsonic but the predominating 
flow is still supersonic. 
The principal differences between subsonic 
and supersonic flow are due to the cmprrs- 
Jibi& of the supersonic flow. Thus, any 
change of velocity or pressure of a supersonic 
flow will produce a related change of density 
which must be considered and accounted for. 
Figure 3.2 provides a comparison of incom- 
pressible and compressible flow through a 
closed tube. Of course, the condition of con- 
tinuity must exist in the flow through the 
closed tube; the mass flow at any station along 
the tube is constant. This qualification must 
exist in both compressible and incompressible 
cases. 
The example of subsonic incompressible flow 
is simplified by the fact that the density of 
flow is constant throughout the tube. Thus, 
as the flow approaches a constriction and the 
streamlines converge, velocity increases and 
static pressure decreases. In other words, a 
convergence of the tube requires an increasing 
velocity to accommodate the continuity of 
flow. Also, as the subsonic incompressible 
flow enters a diverging section of the tube, 
velocity decreases and static pressure increases 
but density remains unchanged. The behavior 
of subsonic incompressible flow is that a con- 
vergence causes expansion (decreasing pressure) 
while a divergence causes compression (in- 
creasing pressure). 
The example of supersonic compressible flow 
is complicated by the fact that the variations 
of flow density are related to the changes 
in velocity and static pressure. The behavior 
of supersonic compressible flow is that a con- 
vergence causes compression while a divergence 
causes expansion. Thus, as the supersonic 
compressible flow approaches a constriction 
and the streamlines converge, velocity dc- 
creases and static pressure increases. Con- 
tinuity of mass flow is maintained by the 
increase in flow density which accompanies the 
decrease in velocity. As the supersonic com- 
pressible flow enters a diverging section of the 
tube, velocity increases, static pressure de- 
creases, and density decreases to accommodate 
the condition of continuity. 
The previous comparison points out three 1 
significant differences between supersonic corn- 1 
pressible and subsonic incompressible flow. 
(a) Compressible flow includes the addi- 
tional variable of flow density. 
(b) Convergence of flow causes expansion 
of incompressible flow but compression of 
compressible flow. 
(c) Divergence of flow causes compression 
of incompressible flow but expansion of