to zero. When an aircraft is in equilibrium, 
there are no accelerations and the aircraft 
continues in a steady condition of flight. If 
the equilibrium is disturbed by a gust or deflec- 
tion of the controls, the aircraft will experi- 
ence acceleration due to unbalance of moment 
or force. 
The static stability of a system is defined by 
the initial tendency to return to equilibrium 
conditions following some disturbance from 
equilibrium. If an object is disturbed from 
equilibrium and has the tendency to return 
to equilibrium, positive .rtatic Jtability exists. 
If the object has a tendency to continue in the 
direction of disturbance, negative static stability 
or static instability exists. An intermediate 
condition could occur where an object dis- 
placed from equilibrium remains in equilibrium 
in the displaced position. If the object subject 
to a disturbance has neither the tendency to 
return nor the tendency to continue in the dis- 
placement direction, ncutrnl Jtatic stability ex- 
ists. These three categories of static stability 
are illustrated in figure 4.1. The ball in a 
trough illustrates the condition of positive 
static stability. If the ball is displaced from 
equilibrium at the bottom of the trough, the 
initial tendency of the ball is to return to the 
equilibrium condition. The ball may roll 
back and forth through the point of equilib- 
rium but displacement to either side creates 
the initial tendency to return. The ball on a 
hill illustrates the condition of static insta- 
bility. Displacement from equilibrium at the 
hilltop brings about the tendency for greater 
displacement. The ball on a flat, level surface 
illustrates the condition of neutral static sta- 
bility. The ball encounters a new equilibrium 
at any point of displacement and has neither 
stable nor unstable tendencies. 
The term “static” is applied to this form of 
stability since the resulting motion is not 
considered. Only the tendency to return to 1. eqmlibrtum conditions is considered in static 
stability. The static longitudinal stability of 
an aircraft is appreciated by displacing the 
NAVWEPS 00-802-80 
STABILITY ,AND CONTROL 
aircraft from some trimmed angle of attack. 
If the aerodynamic pitching moments created 
by this displacement tend to return the air- 
craft to the equilibrium angle of attack the 
aircraft has positive static longitudinal 
stability. 
DYNAMIC STABILITY 
While static stability is concerned with the 
tendency of a displaced body to return to 
equilibrium, dynamic stability is defined by 
the resulting motion with time. If an object is 
disturbed from equilibrium, the time history 
of the resulting motion indicates the dynamic 
stability of the system. In general, the system 
will demonstrate positive dynamic stability 
if the amplitude of motion decreases with 
time. The various condirions of possible 
dynamic behavior are illustrated by the time 
history diagrams of figure 4.2. 
The nonoscillatory modes shown in figure 
4.2 depict the time histories possible without 
cyclic motion. If the system is given an initial 
disturbance and the motion simply subsides 
without oscillation, the mode is termed “sub- 
sidence” or “deadbeat return.” Such a motion 
indicates positive static stability by the tend- 
ency to return to equilibrium and positive dy- 
namic stability since the amplitude decreases 
with time. Chart B illustrates the mode of 
“divergence” by a noncyclic increase of ampli- 
tude with time. The initial tendency to con- 
tinue in the displacement direction is evidence 
of static instability and the increasing ampli- 
tude is proof of dynamic instability. Chart C 
illustrates the mode of pure neutral stability. 
If the original disturbance creates a displace- 
ment which remains constant thereafter, the 
lack of tendency for motion and the constant 
amplitude indicate neutral static and neutral 
dynamic stability. 
The oscillatory modes of figure 4.2 depict the