attack would produce an approximate 0.5 
change in lift coefficient. Evidently, lift,~curve 
slope is not a factor important in the selection 
of an airfoil. 
An important lift property affected by the 
airfoil shape is the section maximum lift co- 
efficient, ci-. The effect of airfoil shape on 
ci- can be appreciated by comparison of the 
lift curves for the five airfoils of figure 1.12. 
The NACA airfoils 63X06,63-009, and 63i-012 
ate symmetrical sections of a basic thickness 
distribution but maximum thicknesses of 6, 
9, and 12 percent respectively. The effect of 
thickness on ~1% is obvious from an inspec- 
tion of these curves : 
NACA 63-005 .~. :. Cl.82 9.0° 
NACA 6Mo9. 1.10 10.5~ 
NACA 63‘-01?,. 1.40 13.80 
The 12-percent section has a cr- approxi- 
mately 70 percent greater than the 6-percent 
thick section. In addition, the thicker airfoils 
have greater benefit from the use of various 
high lift devices. 
The effect of camber is illustrated by the lift 
curves of the NACA 4412 and 631-412 sections. 
The NACA 4412 section is a 12 percent thick 
airfoil which has 4 percent maximum camber 
located at 40 percent of the chord. The 
NACA 63i-412 airfoil has the same thickness 
and thickness distribution as the 631-012 but 
camber added to give a “design”’ lift coefficient 
(c, for minimum section drag) of 0.4. The 
lift curves for these two airfoils show that 
camber has a beneficial e&t on cl-. 
ScCdO” %.I a0 for “&* 
NACA 6h-312 (symmctricd) :. 1.40 13.e 
NACA 631-412 Whmd). 1.73 IS. z” 
An additional effect of camber is the change 
in zero lift angle. While the symmetrical 
NAVWEPS OO-BOT-BO 
BASIC AE,RODYMAMlCS 
sections have zero lift at zero angle of attack, 
the sections with positive camber have nega- 
tive angles for zero lift. 
The importance of maximum lift coefficient 
is obvious. If the maximum lift coefficient is 
high, the stall speed will be low. However, 
the high thickness and camber necessary for 
high section maximum lift coefficients may 
produce low critical Mach numbers and large 
twisting moments at high speed. In other 
words, a high maximum lift coefficient is just 
one of the many features desired of an airfoil 
section. 
DRAG CHARACTERISTICS. Drag is the 
net aerodynamic force parallel to the relative 
wind and its source is the pressure distribution 
and skin friction on the surface. Large, thick 
bluff bodies in an airstream show a predomi- 
nance of form drag due to the unbalanced pres- 
sure distribution. However, streamlined 
bodies with smooth contours show a ptedomi- 
nance of drag due to skin friction. In a 
fashion similar to other aerodynamic forces, 
drag forces may be considered in the form of a 
coefficient which is independent of dynamic 
pressure and surface area. The basic drag 
equation is as follows: 
D=GqS 
where 
D=drag, lbs. 
C,= drag coefficient 
q= dynamic pressure, psf 
UP =z (V in knots, TAS) 
S= wing surface area, sq. ft. 
The force of drag is shown as the product of 
dynamic pressure, surface area, and drag co- 
efficient, C,. The drag coefficient in this 
equation is similar to any other aerodynamic 
force coefficient-it is the ratio of drag pres- 
sure to dynamic pressure. If the drag co- 
efficient of a conventional airplane were plotted 
versus angle of attack, the result would be 
typical of the graph shown in figure 1.13. At 
low angles of attack the drag coefficient is 
low and small changes in angle of attack create 
only slight changes in drag coefficient. At 
29