CHANGE IN LIFT 
DUE TO UPGUST 
NAVWEPS D&801-80 
BASIC AERODYNAMICS 
CHANGE IN LIFT 
DUE TO UPGUST 
C:G. 1 
O.C. 
t (UNSTABLE) 
C:G. 
t LIFT 
1 WEIGHT 
Figure 1.23. Application to Stability 
AC. as a limit but as stall occurs, the drop in 
suction near the leading’ edge cause the c.p. to 
move aft. 
Of course, if the airfoil has negative camber, 
or a strongly reflexed trailing edge, the moment 
about the aerodynamic center will be positive. 
In this case, the location of the aerodynamic 
center will be unchanged and will remain at 
the quarter-chord position. 
The aerodynamic center is the point on the 
chord where the coefficients of moment are 
constant-the point where all changes in lift 
take place. The aerodynamic center is an cx- 
tremely important aerodynamic reference point 
and the most direct application is to the longi- 
tudinal stability of an airplane. To simplify 
the problem assume that the airplane is a 
tailless or flying wing type. In order for this 
type airplane to have longitudinal stability, 
the center of gravity must be ahead of the 
aerodynamic center. This very necessary fea- 
ture can be visualized from the illustrations of 
figure 1.23. 
If the two symmetrical airfoils are subject 
to an upgust, an increase in lift will take place 
at the 4.c. If the c.g. is ahead of the ax., the 
change in lift creates a nose down moment 
about the c.g. which tends to return the air- 
foil to the. equilibrium angle of attack. This 
stable, “weathercocking” tendency to return 
to equilibrium is a very necessary feature in 
any airplane. If the c.g. is aft of the a.~., the 
change in lift due to the upgust takes place at 
the AC. and creates a nose up moment about 
the c.g. This nose up moment tends to displace 
the airplane farther from the equilibrium and 
is unstable-the airplane is similar to a ball 
balanced on a peak. Hence, to have a stable 
airplane, the c.g. must be located ahead of the 
airplane rl.c. 
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