and high power, the dynamic pressure in the 
shaded area can be much greater than the free 
stream and this causes considerably greater 
lift than at zero thrust. At high power con- 
ditions the induced flow also causes an effect 
similar to boundary layer control and increases 
the maximum lift angle of attack. The typical 
four-engine propeller driven airplane may have 
60 to 80 percent of the wing area affected by 
the induced flow and power effects on stall 
speeds may be considerable. Also, the lift of 
the airplane at a given angle of attack and air- 
speed will be greatly affected. Suppose the 
airplane shown is in the process of landing 
flare from a power-on approach. If there is 
a sharp, sudden reduction of power, the air- 
plane may drop suddenly because of the reduced 
lift. 
The typical jet aircraft does not experience 
the induced flow velocities encountered in 
propeller driven airplanes, thus the only sig- 
nificant factor is the vertical component of 
thrust. Since this vertical component con- 
tributes to supporting the airplane, less aero- 
dynamic lift is required to hold the airplane 
in flight. If the thrust is small and the thrust 
inclination is slight at maximum lift angle, 
only negligible changes in stall speed will re- 
sult. On the other hand, if the thrust is very 
great and is given a large inclination at maxi- 
mum lift angle, the effect on stall speed can 
be very large. One important relationship 
remains-since there is very little induced flow 
from the jet, the angle of attack at stall is 
essentially the same power-on or power-off. 
DEVELOPMENT OF AERODYNAMIC 
PITCHING MOMENTS 
The distribution of pressure over a surface 
is the ,source of the aerodynamic moments as 
well as the aerodynamic forces. A typical 
example of this fact is the pressure distribution 
acting on the cambered airfoil of figure 1.21. 
The upper surface has pressures distributed 
which produce the upper surface lift; the lower 
surface has pressures distributed which pro- 
duce the lower surface lift. Of course, the 
NAVWEPS 00-801~0 
BASIC AERODYNAMICS 
net lift produced by the airfoil is difference 
between the lifts on the upper and lower sur- 
faces. The point along the chord where the 
distributed lift is effectively concentrated is 
termed the “center of pressure, c.p.“ The 
center of pressure is essentially the “center of 
gravity” of the distributed lift pressure and 
the location of the c.p. is a function of camber 
and section lift coe&cient 
Another aerodynamic reference point is the 
“aerodynamic center, d.e.” The aerodynamic 
center is defmed as the point along the chord 
where all changes in lift effectively take place. 
To visualize the existence of such a point, 
notice the change in pressure distribution with 
angle of attack for the symmetrical airfoil 
of figure 1.21. When at zero lift, the upper 
and lower surface lifts are equal and located 
at the same point. With an increase in angle 
of attack, the upper surface lift increases while 
the lower surface lift decreases. The change 
,of lift has taken place with no change in the 
center of pressure-a characteristic of sym- 
metrical airfoils. 
Next, consider the cambered airfoil of 
figure 1.21 at zero lift. To produce zero lift, 
the upper and lower surface lifts must be equal. 
One difference noted from the symmetrical air- 
foil is that the upper and lower surface lifts are 
not opposite one another. While no net lift 
exists on the airfoil, the couple produced by 
the upper and lower surface lifts creates a nose 
down moment. As the angle of attack is in- 
creased, the upper surface lift increases while 
the lower surface lift decreases. While a 
change in lift has taken place, no change in 
moment takes place about the point where 
the lift change occurs. Since the moment 
about the aerodynamic center is the product 
of a force (lift at the c.P.) and a lever arm 
(distance from c.9. to a.~.), an increase in lift