CONTRIBUTION OF THE COMPONENT 
SURFACES. The net pitching moment about 
the lateral axis is due to the contribution of 
each of the component surfaces acting in their 
appropriate flow fields. By study of the con- 
tribution of each component the effect of each 
component on the static stability may be ap- 
preciated. It is necessary to recall that the 
pitching moment coefficient is defined as: 
M 
‘“=qS(MAC) 
Thus, any pitching moment coefficient-re- 
gardless of source-has the common denomi- 
nator of dynamic pressure, q, wing area, S, and 
wing mean aerodynamic chord, MAC. This 
common denominator is applied to the pitch- 
ing moments contributed by the fuselage and 
nacelles, horizontal tail, and power effects 
as well as pitching moments contributed by 
the wing. 
WING. The contribution of the wing to 
stability depends primarily on the location 
of the aerodynamic center with respect to the 
airplane center of gravity. Generally, the 
aerodynamic center-or a.c.-is defined as the 
point on the wing mean aerodynamic chord 
where the wing pitching moment coefficient 
does not vary with lift coefficient. All changes 
in lift coefficient effectively take place at the 
wing aerodynamic center. Thus, if the wing 
experiences some change in lift coefficient, the 
pitching moment created will be a direct 
function of the relative location of the a.c. and 
c.g. 
Since stability is evidenced by the develop- 
ment of restoring moments, the c.g. must be 
forward of the a.c. for the wing to contribute 
to positive static longitudinal stability. As 
shown in figure 4.6, a change in lift aft of the 
c,g. produces a stable restoring moment de- 
pendent npon the lever arm between the a.c. 
and c.g. In this case, the wing contribution 
would be stable and the curve of CM versus CL 
for the wing alone would have a negative slope. 
If the c.g. were located at the a.c., C, would 
NAVWEPS OO-BOT-BO 
STABILITY AND CONTROL 
not vary with C, since all changes in lift would 
take place at the c.g. In this case, the wing 
contribution to stability would be neutral. 
When the c.g. is located behind the a.c. the 
wing contribution i,s unstable and the curve 
of C, versus CL for the wing alone would have 
a positive slope. 
Since the wing is the predominating aero- 
dynamic surface of an airplane, any change in 
the wing contribution may produce a sig- 
nificant change in the airplane stability. This 
fact would be most apparent in the case of the 
flying wing or tailless airplane where the wing 
contribution determines the airplane stability. 
In order for the wing to achieve stability, the 
c.g. must be ahead of the a.c. Also, the wing 
must have a positive pitching moment about 
the aerodynamic center to achieve trim at 
positive lift coefficients. The first chart of 
figure 4.7 illustrates that the wing which is 
stable will trim at a negative lift coefficient if 
the C,,, is negative. If the stable wing has a 
positive C,,, it will then trim at a useful posi- 
tive CL. The only means available to achieve 
trim at a positive CL with a wing which has a 
negative C,,, is an unstable c.g. position aft of 
the ax. As a result, the tailless aircraft 
cannot utilize high lift devices which incur 
any significant changes in C,,,. 
WhiIe the trim lift coefficient may be altered 
by a change in c.g. position, the resulting 
change in stability is undesirable and is unsat- 
isfactory as a primary means of control. The 
variation of trim CL by deflection of control 
surfaces is usually more effective and is less 
inviting of disaster. The early attempts at 
manned flight led to this conclusion. 
When the aircraft is operating in subsonic 
flight, the a.c. of the wing remains fixed at the 
25 percent chord station. When the aircraft 
is flown in supersonic flight, the ax. of the 
wing will approach the 50 percent chord sta- 
tion. Such a large variation in the location 
of the a.c. can produce large changes in the