is the primary reference in lateral stability as 
well as directional stability considerations. 
The yaw angle, #, is a primary reference for 
wind tunnel tests and time history motion of 
an airplane. From the definitions there is no 
direct relationship between @ and # for an 
airplane in free flight, e.g., an airplane flown 
through a 360° turn has yawed 360” but side- 
slip may have been zero throughout the entire 
turn. Since the airplane has no directional 
sense, static directional stability of the air- 
plane is appreciated by response to sideslip. 
The static directional stability of an airplane 
can be illustrated by a graph of yawing moment 
coe&cient, C., versus sideslip angle, 8, such as 
shown in figure 4.22. When the airplane is 
subject to a positive sideslip angle, static direc- 
tional stability will be evident if a positive 
yawing moment coefficient results. Thus, 
when the relative wind comes from the right 
(+p), a yawing moment to the right (+C.) 
should be created which tends to weathercock 
the airplane and return the nose into the wind. 
Static directional stability will exist when the 
curve of C,, versus fi has a positive slope and the 
degree of stability will be a function of.the 
slope of this curve. If the curve has zero slope, 
there is no tendency to return to equilibrium 
and neutral static directional stability exists. 
When the curve of C. versus /3 has a negative 
slope, the yawing moments developed by side- 
slip tend to diverge rather than restore and 
static directional instability exists. 
The final chart of figure 4.22 illustrates the 
fact that the instantaneous slope of the curve of 
C,, versus @ will describe the static directional 
stability of the airplane. At small angles of 
sideslip a strong positive slope depicts strong 
directional stability. Large angles of sideslip 
produce zero slope and neutral stability. At 
very high sideslip the negative slope of the 
curve indicates directional instability. This 
decay of directional stability with increased 
sideslip is not an unusual condition. However, 
directional instability should not occur at the 
angles of sideslip of ordinary flight conditions. 
NAVWEPS 00-ROT-80 
STABILITY AND CONTROL 
Static directional stability must be in evi- 
dence for all the critical conditions of flight. 
Generally, good directional stability is a ftm- 
damental quality directly affecting the pilots’ 
impression of an airplane. 
CONTRIBUTION OF THE AIRPLANE 
COMPONENTS. The static directional sta- 
bility of the airplane is a result of contribution 
of each of the various airplane components. 
While the contribution of each component is 
somewhat dependent upon and related to other 
components, it is necessary to study each 
component separately. 
The vertical tail is the primary source of 
directional stability for the airplane. As 
shown in figure 4.23, when the airplane is in 
a sideslip the vertical tail will experience a 
change in angle of attack. The change in 
lift-or side force-on the vertical tail creates 
a yawing moment about the center of gravity 
which tends to yaw the airplane into the 
relative wind. The magnitude of the vertical 
tail contribution to static directional stability 
then depends on the change in tail lift and the 
tail moment arm. Obviously, the tail moment 
arm is a powerful factor but essentially dic- 
tated by the major configuration properties of 
the airplane. 
When the location of the vertical tail is set,, 
the contribution of the surface to directional 
stability depends on its ability to produce 
changes in lift-or side force-with changes in 
sideslip. The surface area of the vertical tail 
is a powerful factor with the contribution of 
the vertical tail being a direct function of the 
area. When all other possibilities are ex- 
hausted, the required directional stability may 
be obtained by increases in tail area. How- 
ever, increased surface area has the obvious 
disadvantage of increased drag.