NAVWEPS 00-801-80 
BASIC AERODYNAMICS 
rotation will be quite a “curve ball artist” 
the golfer that cannot control the lateral mo- 
tion of the club face striking the golf ball will 
impart an uncontrollable spin and have trouble 
with a “hook” or “slice.” 
While a rotating cylinder can produce a net 
lift from the circulatory flow, the method is 
relatively inefficient and only serves to point 
out the relationship between lift and circula-, 
tion. An airfoil is capable of producing lift 
with relatively high efficiency and the process 
is illustrated in figure 1.8. If a symmetrical 
airfoil is placed at zero angle of attack to the 
airstream, the streamline pattern and pressure 
distribution give evidence of zero lift. HOW- 
ever, if the airfoil is given a positive angle of 
attack, changes occur in the streamline pattern 
and pressure distribution similar to changes 
caused by the addition of circulation to the 
cylinder. The positive angle of attack causes 
increased velocity on the upper surface with 
an increase in upper surface suction while the 
decreased velocity on the lower surface causes 
a decrease in lower surface suction. Also, 
upwash is generated ahead of the airfoil, the 
forward stagnation point moves under the 
leading edge, and a downwash is evident aft 
of the airfoil. The pressure distribution 0” 
the airfoil now provides a net force perpendicu- 
lar to the airstream-lift. 
The generation of lift by an airfoil is depend- 
ent upon the airfoil being able to create circu- 
lation in the airstream and develop the lifting, 
pressure distribution on the surface. In all 
cases, the generated lift will be the net force 
caused by the distribution of pressure over the 
upper and lower surfaces of the airfoil. At 
low angles of attack, suction pressures usually 
will exist on both upper and lower surfaces. 
but the upper surface suction must be greater 
for positive lift. At high angles of attack 
near that for maximum lift, a positive pressure 
will exist on the lower surface but this will 
account for approximately one-third the net 
lift. 
The effect of free stream density and velocity 
is a necessary consideration when studying the 
development of the various aerodynamic forces. 
Suppose that a particular shape of airfoil is 
fixed at a particular angle to the airstream. 
The relative velocity and pressure distribution 
will be determined by the shape of the airfoil 
and the angle to the airstream. The effect of 
varying the airfoil size, air density and air- 
speed is shown in figure 1.9. If the same air- 
foil shape is placed at the same angle to an 
airstream with twice as great a dynamic pres- 
sure the magnitude of the pressure distribution 
will be twice as great but the r&rive shape of 
the pressure distribution will be the same. 
With twice as great a pressure existing over 
the surface, all aerodynamic forces and mo- 
ments will ~double. If a half-size airfoil ib 
placed at the same angle to the original air- 
stream, the magnitude of the pressure distri- 
bution is the same as the origina! airfoi! and 
again the relative shape of the pressure dis- 
tribution is identical. The same pressure act- 
ing on the half-size surface would reduce all 
aerodynamic forces to one-half that of the 
original. This similarity of flow patterns 
means that the stagnation point occurs at the 
same place, the peak suction pressure occurs 
at the same place, and the actual magnitude of 
the aerodynamic forces and moments depends 
upon the airstream dynamic pressure and the 
surface area. This concept is extremely im- 
portant when attempting to separate and ana- 
lyze the most important factors affecting the 
development of aerodynamic forces. 
AIRFOIL TERMINOLOGY. Since the 
shape of an airfoil and the inclination to the 
airstream are so important in determining the 
pressure distribution, it is necessary to properly 
define the airfoil terminology. Figure 1.10 
shows a typical airfoil and illustrates the