NAVWEPS OO-SOT-SO 
BASIC AERODYNAMICS 
An additional requirement of stability is 
that the airplane must stabilize and be trimmed 
for flight at positive lift. When the c.g. is 
located ahead of d.c., the weight acting at the 
c.g. is supported by the lift developed by the 
section. Negative camber is required to pro- 
duce the positive moment about the aerody- 
namic center which brings about equilibrium 
ot balance at positive lift. 
Supersonic flow produces important changes 
in the aerodynamic characteristics of sections. 
The aerodynamic center of airfoils in subsonic 
flow is located at the 25 percent chord point. 
As the airfoil is subject to supersonic flow, the 
aerodynamic center changes to the 50 percent 
chord point. Thus, the airplane in transonic 
flight can experience large changes in longitu- 
dinal stability because of the large changes in 
the position of the aerodynamic center. 
FRICTION EFFXTS 
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counter resistance to flow over a surface. The 
viscous nature of airflow reduces the local 
velocities on a surface and accounts for the 
drag of skin friction. The retardation of air 
particles due to viscosity is greatest immedi- 
ately adjacent to the surface. At the very sur- 
face of an object, the air particles are slowed to 
a relative velocity of near zero. Above this 
area other particles experience successively 
smaller retardation until finally, at some dis- 
tance above surface, the local velocity reaches 
the full value of the airstream above the sur- 
face. This layer of air over the surface which 
shows local retardation of airflow from vis- 
cosity is termed the “boundary layer.” The 
characteristics of this boundary layer are illus- 
trated in figure 1.24 with the flow of air over 
a smooth flat plate. 
The beginning flow on a smooth surface gives 
evidence of a very thin boundary layer with 
the flow occurring in smooth laminations, 
The boundary layer flow near the leading edge 
is similar to layers or laminations of air slid- 
ing smoothly over one another and the obvi- 
ous term for this type of flow is the “laminar” 
52 
boundary layer. This smooth laminar flow 
exists without the air particles moving from 
a given elevation. 
As the flow continues back from the leading 
edge, friction forces in the boundary layer 
continue to dissipate energy of the airstream 
and the laminar boundary layer increases in 
thickness with distance from the leading edge. 
After some distance back from the leading 
edge, the laminar boundary layer begins an 
oscillatory disturbance which is unstable. A 
waviness occurs in the laminar boundary layer 
which ultimately grows larger and more 
severe and destroys the smooth laminar flow. 
Thus, a transition takes place in which the 
laminar boundary layer decays into a “turbu- 
lent” boundary layer. The same sort of 
transition can be noticed inthe smoke from a 
cigarette in still air. At, first, the smoke 
ribbon is smooth and laminar, then develops 
a definite waviness, and decays into a random 
turbulent smoke pattern. 
As soon as the transition to. the turbulent 
boundary layer takes place, the boundary 
layer thickens and grows at a more rapid rate. 
(The small scale, turbulent flow within the 
boundary layer should not be confused with 
the large scale turbulence associated with 
airflow separation.) The flow in the turbu- 
lent boundary layer allows the air particles to 
travel from one layer to another producing an 
energy exchange. However, some small lami- 
nar flow continues to exist in the very lower 
levels of the turbulent boundary layer and is 
referred to as the “laminar sub-layer.” The 
turbulence which exists in the turbulent bound- 
ary layer allows determination of the point of 
transition by several means. Since the turbu- 
lent boundary layer transfers heat more easily 
than the laminar layer, frost, water, and oil 
films will be removed more rapidly from the