NAVWRPS DD-807-80 
STABILITY AND CONTROL 
destabilizing and the greatest contribution 
will occur at high power and low dynamic 
pressure as during a waveoff. 
As in the case of longitudinal static stability, 
freeing the controls will reduce the effective- 
ness of the tail and alter the stability. While 
the rudder must be balanced to reduce control 
pedal forces, the rudder will tend to float or 
streamline and reduce the contribution of the 
vertical tail to static directional stability. The 
floating tendency is greatest at large angles of 
sideslip where large angles of attack for the 
vertical tail tend to decrease aerodynamic bal- 
ante. Figure 4.24 illustrates the difference be- 
tween rudder-fixed and rudder-free static di- 
rectional stability. 
CRITICAL CONDITIONS. The most criti- 
cal conditions of,staric directional stability are 
usually the combination of several separate 
effects. The combination which produces the 
most critical condition is much dependent upon 
the type and mission of the airplane. In addi- 
tion, there exists a coupling of lateral and di- 
rectional effects such that the required degree 
of static directional stability may be deter- 
mined by some of these coupled conditions. 
Center of gravity position has a relatively 
negligible effect on static directional stability. 
The usual range of c.g. position on any air- 
plane is set by the Jinits of long&d&a/ stability 
and control. Within this limiting range of 
c.g. position, no significant changes take place 
in the contribution of the vertical tail, fuselage, 
nacelles, etc. Hence, the static directional 
stability is essentially unaffected by the varia- 
tion of c.g. position within the longitudinal 
limits. 
When the airplane is at a high angle of a$tack 
a decrease in static directional stability can be 
anticipated. As shown by the second chart of 
figure 4.24, a high angle of attack reduces the 
stable slope of the curve of C,, versus 8, The 
decrease in static directional stability is due in 
great part to the reduction in the contribution 
of the vertica1 tail. At high angles of attack, 
the effectiveness of the vertical tail is reduced 
because of increase in the fuselage boundary 
layer at the vertical tail location. The decay of 
dir&ctional stability with angle of attack is 
most significant for the low aspect ratjo air- 
plane with sweepback since this configuration 
requires such high angles of attack to achieve 
high lifr coefficients. Such decay in directional 
stability can have a profound effect on the re- 
sponse of the airplane to adverse yaw and spin 
characteristics. 
High Mach ntrmbers of supersonic flight reduce 
the contribution of the vertical tail to direc- 
tional stability because of the reduction of lift 
cnrve slope with Mach number. The third 
chart of figure 4.24 illustrates the typical decay 
of directional stability with Mach number. To 
produce the required directional stability at 
high Mach numbers, a viziy large vertical tail 
area may be necessary. Ventral fins may be 
added as an additional contribution to direc- 
tional stability but landing clearance require- 
ments may limir their size or require the fins to 
be retractable. 
Hence, the most critical demands of static 
directional stability will occur from some 
combination of the following effects: 
(1) high angle of sideslip 
(2) high power at low airspeed 
(3) high angle of attack 
(4) high Mach number 
The propeller powered airplane may have such 
considerable power effects that the critical 
conditions may occur at low speed while the 
effect of high Mach numbers may produce the 
critical conditions for the typical supersonic 
airplane. In addition, the coupling of lateral 
and directional effects may require prescribed 
degrees of directional stability. 
DIRECTIONAL CONTROL 
In addition to directional stability, the air- 
plane must have adequate directional control 
to coordinate turns, balance power effects,