NAVWEPS DD-807-80 
STABILITY AND CONTROL 
of figure 4.7 illustrates the change of wing 
contribution possible between subsonic and 
supersonic flight. The large increase in static 
stability in supersonic flight can incur high 
trim drag or require great control effectiveness 
to prevent reduction in maneuverability. 
FUSELAGE AND NACELLES. In most 
cases, the contribution of the fuselage and 
nacelles is destabilizing. A symmetrical body 
of revolution in the flow field of a perfect fluid 
develops an unstable pitching moment when 
given an angle of attack. In fact, an increase 
in angle of attack produces an increase in the 
unstable pitching moment without the devel- 
opment of lift. Figure 4.8 illustrates the pres- 
sure distribution which creates this unstable 
moment on the body of revolution. In the 
actual case of real subsonic flow essentially 
the same effect is produced. An increase in 
angle of attack causes an increase in the 
unstable pitching moment but a negligible 
increase in lift. 
An additional factor for consideration is the 
influence of the induced flow field of the wing. 
As illustrated in figure 4.8, the upwash ahead 
of the wing increases the destabilizing influence 
from the portions of the fuselage and nacelles 
ahead of the wing. The downwash behind 
the wing reduces the destabilizing influence 
from the portions of the fuselage and nacelles 
aft of the wing. Hence, the location of the 
fuselage and nacelles relative to the wing is 
important in determining the contribution to 
stability. 
The body of revolution in supersonic flow 
can develop lift of a magnitude which cannot 
be neglected. When the body of revolution in 
supersonic flow is given an angle of attack, a 
pressure distribution typical of figure 4.8 is the 
result. Since the center of pressure is well 
forward, the body contributes a destabilizing 
influence. AS is usual with supersonic con- 
figurations, the fuselage and nacelles may be 
quite large in comparison with the wing area 
and the contribution to stability may be large. 
Interaction between the wing and fuselage and 
nacelles deserves consideration in several in- 
stances. Body upwash and variation of local 
Mach number can influence the wing lift while 
lift carryover and downwash can effect the fu- 
selage and nacelles forces and moments. 
HORIZONTAL TAIL. The horizontal tail 
usually provides the greatest stabilizing influ- 
ence of all the components of the airplane. To 
appreciate the contribution of the horizontal 
tail to stability, inspect figure 4.9. If the air- 
plane is given a change in angle of attack, a 
change in tail lift will occur at the aerody- 
namic center of the tail. An increase in lift 
at the horizontal tail produces a negative 
moment about the airplane c.g. and tends to 
return the airplane to the trim condition. 
While the contribution of the horizontal tail 
to stability is large, the -magnitude of the 
contribution is dependent upon the change in 
tail lift and the lever arm of the surface. It is 
obvious that the horizontal tail will produce a 
stabilizing effect only when the surface is aft 
of the c.g. For this reason it would be inap- 
propriate to refer to the forward surface of a 
canard (tail&St) configuration as a horizontal 
“stabilizer.” In a logical sense, the horizontal 
“stabilizer” must be aft of the c.g. and- 
generally speaking-the farther aft, the greater 
the contribution to stability. 
Many factors influence the change in tail 
lift which occurs with a change in airplane 
angle of attack. The area of the horizontal 
tail has the obvious effect that a large surface 
would generate a large change in lift. In a 
similar manner, the change in tail lift would 
depend on the slope of the lift curve for the 
horizontal tail. Thus, aspect ratio, taper, 
sweepback, and Mach number would deter- 
mine the sensitivity of the surface to changes 
in angle of attack. It should be appreciated 
that the flow at the horizontal tail is not of