NAVWEPS 00-SOT-80 
BASIC AERODYNAMICS 
If the surface of the plate is smooth and the 
original airstream has no turbulence, the plate 
at low Reynolds Numbers will exist with pure 
laminar flow. When the RN is increased to 
approximately 530,000, transition occurs on 
the plate and the flow is partly turbulent. 
Once transition takes place, the drag coefficient 
of the plate increases from the laminar curve 
to the turbulent curve. As the RN approaches 
very high values (20 to 50 million) the drag 
curve of the plate approaches and nearly equals 
the values for the turbulent curve. At such 
high RN the boundary layer is predominantly 
turbulent with very little laminar flow-the 
transition point is very close to the leading 
edge. While the smooth, flat plate is not ex- 
actly representative of the typical airfoil, basic 
fluid friction phenomena are illustrated. At 
RN less than a half million the boundary layer 
will be entirely laminar unless there is extreme 
surface roughness or turbulence induced in the 
airstream. Reynolds Numbers between one 
and five million produce boundary layer flow 
which is partly laminar and partly turbulent. 
At RN above ten million the boundary layer 
characteristics are predominantly turbulent. 
In order to obtain low drag sections, the 
transition from laminar to turbulent must be 
delayed so that a greater portion of the sur- 
face will be influenced by the laminar bound- 
ary layer. The conventional, low speed air- 
foil shapes are characterized, by minimum 
pressure points very close to the leading edge. 
Since high local velocities enhance early 
transition, very little surface is covered by 
the laminar boundary layer, A comparison 
of two 9 percent thick symmetrical airfoils is 
presented in figure 1.25. One section is the 
“conventional” NACA C!UO~ section which 
has a minimum pressure point at approxi- 
mately 10 percent chord at zero lift. The other 
section is the NACA 66039 which has a 
minimum pressure point at approximately 60 
percent chord at zero lift. The lower local 
velocities at the leading edge and the favor- 
able pressure gradient of the NACA 66-009 
delay the transition to some point farther aft 
on the chord. The subsequent reduction in 
friction drag at the low angles of attack ac- 
counts for the “drag bucket” shown on the 
graphs of cd and cI for these sections. Of 
course, the advantages of the laminar flow 
airfoil are apparent only for the smooth airfoil 
since surface roughness or waviness may pre- 
clude extensive development of a laminar 
boundary layer. 
AIRFLOW SEPARATION. The character 
of the boundary layer on an aerodynamic 
surface is greatly influenced by the pressure 
gradient. In order to study this effect, the 
pressure distribution of a cylinder in a perfect 
fluid is repeated in figure 1.26. The airflows 
depict a local velocity of !zero at the forward 
stagnation point and a maximum local velocity 
at the extreme surface. The airflow moves 
from the high positive pressure to the minimum 
pressure point-a favorable pressure gradient 
(high to low). As the air moves from the 
extreme surface aft, the local velocity decreases 
to zero at the aft stagnation point. The static 
pressure increases from the minimum (or max- 
imum suction) to the high positive pressure 
at the aft stagnation point-an adverse pres- 
sure gradient (low to high). 
The action of the pressure gradient is such 
that the favorable pressure gradient assists 
the boundary layer while the adverse pressure 
gradient impedes the flow of the boundary 
layer. The effect of an adverse pressure gradi- 
ent is illustrated by the segment X-Y of figure 
1.26. A corollary of the skin friction drag is 
the continual reduction of boundary layer 
energy as flow continues aft on a surface. * The 
velocity profiles of the boundary layer are 
shown on segment X-Y of figure 1.26. In the 
area of adverse pressure gradient the bound- 
ary layer flow is impeded and tends to show a