angle of the surface must be sufficiently great 
to prevent stall and subsequent loss of effec- 
tiveness at ordinary sideslip angles. The high 
Mach numbers of supersonic flight produces a 
decrease in lift curve slope with the consequent 
reduction in tail contribution to stability. In 
order to have sufficient directional stability at 
high Mach numbers, the typical supersonic 
configuration will exhibit relatively large 
vertical tail surfaces. 
The flow field in which the vertical tail 
operates is affected by the othei components 
of the airplane as well as powe; effects. The 
dynamic pressure at the vertical tail could 
depend on the slipstream of a propeller or the 
boundary layer of the fuselage. Also, the 
local flow direction at the vertical tail is in- 
fluenced by the wing wake, fuselage crossflow, 
induced flow of the horizontal tail, or the 
direction of slipstream from a propeller. Each 
of these factors must be considered as possibly 
affecting the contribution of the vertical tail 
to directional stability. 
The contribution of the wing tb %tatic direc- 
tional stability is tisually small: The swept 
wing provides a stable contribution’depending 
on the amount of sweepback but the contribu- 
tion is relatively weak when compared with 
other components. :. The contribution of the fuselage and nacelles 
is of primary importance since these compo- 
nents furnish rhe greatest destabilizing in- 
fluence. The contribution of the fuselage and 
nacelles is similar to the longitudinal case 
with the exception that there is no large in- 
fluence of the induced flow field of the wing. 
The subsonic center of pressure of the fuselage 
will be located at or forward of the quarter- 
length point and, since the airplane c.g. is 
usually considerably aft of this point, the 
fuselage contribution will be destabilizing. 
However, at large angles of sideslip the large 
destabilizing contribution of the fuselage di- 
minishes which is some relief to the problem 
of maintaining directional stability at large 
displacements. The supersonic pressure,. dis- 
tribution on the body provides a relatively 
NAVWEPS OO-ROLRO 
STARIUTY AND CONTROL 
greater aerodynamic force and, generally, a 
continued destabilizing influence. 
Figure 4.23 illustrates a typical buildup of 
the directional stability of an airplane by 
separating the contribution of the fuselage 
and tail. As shown by the graph of C. versus 
6, the contribution of the fuselage is de- 
stabilizing but the instability decreases at 
large sideslip angles. Tbe contribution of the 
vertical tail alone is highly stabilizing up to 
the point where the surface begins to stall. 
The contribution of the vertical tail must be 
large enough so that the complete airplane 
(wing-fuselage-tail combination) exhibits the 
required degree of stability. 
The dorsal fin has a powerful effect on pre- 
serving the directional stability at large angles 
of sideslip wliich would produce stall of the 
vertical tail. The addition of a dorsal fin to 
the airplane will allay the decay of directional 
stability at high sideslip in two ways. The 
least obvious but most important effect is a 
large increase in the fuselage stability at large 
sideslip angles. In addition, the effective 
aspect rario of the vertical tail is reduced 
which increases the stall angle for the surface. 
By this twofold effect, the addition of the 
dorsal fin is a v useful’ device. 
Poluer effects on static directional stability 
are similar to the power effects on static 
longitudinal stability. The direct effects are 
confined to the normal force at the propeller 
plane or the jet inlet and, of course, are de- 
stabilizing when the propeller or inlet is 
located ahead of the c.g. The indirect effects 
of power induced velocities and flow dirkccion 
changes at the vertical tail are quite significant 
for the propeller driven airplane and can pro- 
duce large directional trim changes. As in