and maintain it in the displaced position, a 
force must be supplied in rhe direction of 
displacement co balance the inherent tendency 
to return to equilibrium. This same stable 
tendency in an aircraft resists displacement 
from trim by pilot effort on the controls or 
atmospheric disturbances. 
The effect of increased stability on con- 
trollabilicy is illustrated by rhe ball in a 
steeper trough. A greater force is required to 
“control” the ball to the same lateral dis- 
placement when the stability is increased. 
In this manner, a large degree of stability tends 
to make the aircraft less controllable. It is 
necessary to achieve the proper balance be- 
tween stability and tontrollability during rhe 
design of an aircraft because the ~ppcr limits 
of stability arc set by the lower 1imitJ of controlla- 
bility. 
The effect of reduced stability on .controlla- 
bility is illustrated by the ball on a flat surface. 
When neutral static stability exists, the ball 
may be displaced from equilibrium and there 
is no stable tendency to return. A new point 
of equilibrium is obtained and no force is 
required to maintain the displacement. As 
the static stability approaches zero, controlla- 
bility increases to infinity and the only resist- 
ance to displacement is a resistance to the 
motion of displacement-damping. For this 
reason, the lower Limits of stability may be Set 
by the upper limits of controllability. If the 
stability of the aircraft is too low, control 
deflections may create exaggerated displace- 
ments of the aircraft. 
The effect of static instability. on controlla- 
bility is illustrated by the ball on a hill. If 
the ball is displaced from equilibrium at the 
top of the hill, the initial tendency is for the 
ball td continue in the displaced direction. 
In order to “control”~the ball to some lateral 
displacement, a force must be applied oppo& 
to the direction of displacement. This effect 
would be appreciated during flight of an un- 
stable aircraft by an unstable “feel” of the air- 
craft. If the controls were deflected co in- 
NAVWEPS DD-8OT-80 
STABILITY AND CONTROL 
&ease the angle of attack, the aircraft would 
be trimmed at the higher angle of attack by 
a push force to keep the aircraft from con- 
tinuing in the displacement direction. Such 
control force reversal would evidence the aii- 
plane instability; the pilot would be supply- 
ing the stability by his attempt to maintain 
the equilibrium. An unstable aircraft can be 
flown if the instability is slight with a low 
rate of divergence. Quick reactions coupled 
with effective controls can allow the pilot to 
cope with some degree of static instability. 
Since such flight would require constant at- 
tention by the pilot, slight instability can be 
tolerated only in airships, helicopters, and 
certain minor motions of the airplane. How- 
ever, the airplane in high speed flight will 
react rapidly to any disturbances and any in- 
stability would create unsafe conditions. Thus, 
it is necessary to provide some positive static 
stability to the major aircraft degrees of 
freedom. 
AIRPLANE REFERENCE AXES 
In order to visualize the forces and moments 
on the aircraft; it is necessary to establish a 
set of mutually perpendicular reference axes 
originating at the center of gravity. Figure 
4.4 illustrates a conventional right hand axis 
system. The longitudinal or X axis is located 
in a plane of symmetry and is given a positive 
direction pointing into the wind. A moment 
about this axis is a rolling moment, L, and the 
positive direction for a positive rolling moment 
utilizes the right hand rule. The vertical or 2 
axis also is in a plane of symmetry and is estab- 
lished positive downward. A moment about 
the vertical axis is a yawing moment, N, and a 
positive yawing moment would yaw the air- 
craft co the right (right hand rule). The 
lateral or Y axis is perpendicular to the plane 
of symmetry and is given a positive direction