weight if the airplane is flown at the angle 
of attack for (L/D),. Of course, the gross 
weight would affect the glide airspeed neces- 
sary for this particular angle of attack but the 
glide ratio would be unaffected. 
AIRFOIL DRAG CHARACTERISTICS. 
The total drag of an airplane is composed of 
the drags of the individual components and 
the forces caused by interference between these 
components. The drag of an airplane con- 
figuration must include the various drags due 
to lift, form, friction, interference, leakage, 
etc. To appreciate the factors which affect 
the drag of an airplane configuration, it is 
most logical to consider the factors which 
affect the drag of airfoil sections. In order to 
allow an objective consideration of the effects 
of thickness, camber, etc., the properties of 
two-dimensional sections must be studied. 
Airfoil section properties are derived from the 
basic profile in two-dimensional. flow and are 
provided the lower case shorthand notation 
to distinguish them from wing or airplane 
properties, e.g., wing or airplane drag coe5- 
cient is C, while airfoil section drag coefficient 
is c,. 
The drag characteristics of three illustrative 
airfoil sections are shown in figure 1.14. The 
section drag coe&cient, c,, is plotted versus 
the section lift coefficient, cr. The drag on 
the airfoil section is composed of pressure drag 
and skin friction. When the airfoil is at low 
lift coe&cients, the drag due to skin friction 
predominates. The drag curve for a conven- 
tional airfoil tends to be quite shallow in this 
region since there is very little variation of 
skin friction with angle of attack. When the 
airfoil is at high lift coefficients, form or 
pressure drag predominates and the drag co- 
efficient varies rapidly with lift coefficient. 
The NACA 0006 is a thin symmetrical profile 
which has a maximum thickness of 6 percent 
located at 30 percent of the chord. This 
section shows a typical variation of cd and cr. 
The NACA 4412 section is a 12 percent thick 
airfoil with 4 percent maximum camber at 
NAVWEPS OO-EOT-RO 
BASIC AERODYNAMICS 
40 percent chord. When this section is com- 
pared with the NACA 0006 section the effect 
of camber can be appreciated. At low lift 
coefficients the thtn, symmetrical section has 
much lower drag. However, at lift coeffi- 
cients above 03 the thicker, cambered section 
has the lower drag. Thus, proper camber and 
thickness can improve the lift-drag ratio of 
the section. 
The NACA 63,412 is a cambered 12 percent 
thick airfoil of the ‘“laminar flow” type. 
This airfoil is shaped to produce a design lift 
coe5cient of 0.4. Notice that the drag curve 
of this airfoil has distinct aberrations with 
very low drag coefficients near the lift coeffi- 
cient of 0.4. This airfoil profile has its camber 
and thickness distributed to produce very low 
uniform velocity on the forward surface (mini- 
mum pressure point well aft) at this lift coeffi- 
cient. The resulting pressure and velocity 
distribution enhance extensive laminar flow 
in the boundary layer and greatly reduce the 
skin friction drag. The benefit of the laminar 
flow is appreciated by comparing the minimum 
drag of this airfoil with an airfoil which has 
one-half the maximum thickness-the NACA 
ooo6. 
The choice of an airfoil section will depend 
on the consideration oftmany different factors. 
While the cI, of the section is an important 
quality, a more appropriate factor for con- 
sideration is the maximum lift coefficient of 
the section when various high lift devices are 
applied. Trailing edge flaps and leading edge 
high lift devices are applied to increase the 
cr,, for low speed performance. Thus, an 
appropriate factor for comparison is the ratio 
of section drag coe5cient to section maximum 
lift coefficient with flaps-cd/crm,. When this 
quantity is corrected for compressibility, a 
preliminary selection of an airfoil section is 
possible. The airfoil having the lowest value