NAVWEPS 00-8OT-80 
STABELITY AND CONTROL 
CENTER OF ..-.. ,.-.. _ 
1 VERTICAL AXIS 
2 
Figure 4.4. Airplane Rekre&e Axes 
LONGITUDINAL STABILITY AND 
CONTROL 
STATIC LONGITUDINAL STABILITY 
GENERAL CONSIDERATIONS. An air- 
craft will exhibit positive static Iongitudinal 
stability if it tends to return to the trim angle 
of attack when displaced by a gust or control 
movement. The aircraft which is unstable will 
continue to pitch in the disturbed direction 
until the displacement is resisted by opposing 
control forces. If the aircraft is neutrally 
stable, it tends to remain at any displacement 
to which it is disturbed. It is most necessary 
to provide an airplane with positive staric 
longitudinal stability. The stable airplane is 
safe and easy to fly since the airplane seeks and 
tends to maintain a trimmed condition of 
flight. It also follows that control deflec- 
tions and control “feel” are logical in direction 
and magnitude. Neutral static longitudinal 
stability usually defines the lower limit of 
airplane stability since it ‘is the boundary 
between stability and instability. The air- 
plane with neutral static stability’ may be 
excessively responsive to controls and the 
aircraft has no tendency to return to trim fol- 
lowing a disturbance. The airplane with 
negative sradc longitudinal stability is in- 
herently divergent from any intended trim 
condition. If it is at all possible to fly the 
aircraft, the aircraft. cannot be trimmed and 
illogical control forces and deflections are rc- 
quired to provide equilibrium with a change 
of attitude and airspeed. 
Since static longitudinal stability depends 
upon the relationship of angle of attack and 
pitching moments, it is necessary to study the 
pitching moment contribution of each com- 
ponent of the aircraft. In a manner similar 
to all other aerodynamic forces, the pitching 
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