NAVWEPS 00-801-80 
STABILITY AND CONlRO’L 
speed, If the airplane were given a large nose 
down tab setting the pull force would in- 
crease with airspeed. This fact points out the 
possibility of “feel” as not being a true indi- 
cation of airplane static stability. 
If the c.g. of the airplane were varied while 
maintaining trim at a constant airspeed, the 
effect of c.g. position on stick force stability 
could be appreciated. As illustrated in figure 
4,14(C), moving the c,g. aft decreases the 
slope of the line of stick force through the 
trim speed. Thus, decreasing stick force 
stability is evident in that smaller stick forces 
are necessary to displace the airplane from 
the trim speed. When the stick force gradient 
(or slope) becomes zero, the c.g. is at the 
stick-free neutral point and neutral stability 
exists. If the c.g. is aft of the stick-free 
neutral point, stick force instability will 
exist, e.g. the airplane will require a push 
force at a lower speed or a pull force at a higher 
speed. It should be noted that the stick force 
gradient is low at low airspeeds and when 
the airplane is at low speeds, high power, 
and a c.g. position near the aft limit, the 
“feel” for airspeed will be weak. 
Control system friction can create very un- 
desirable effects on control forces. Figure 
4.14(D) illustrates that the control force 
versus airspeed is a band rather than a line. 
A wide friction force band can completely 
mask the stick force stability when the stick 
force stability is low. Modern flight control 
systems require precise maintenance to mini- 
mize the friction force band and preserve 
proper feel to the airplane. 
MANEUVERING STABILITY. When an 
airplane is subject to a normal acceleration, 
the flight path is curved and the airplane is 
subject to a pitching velocity. Because of 
the pitching velocity in maneuvering flight, 
the longitudinal stability of the airplane is 
slightly greater than in steady flight condi- 
tions. When an airplane is subject to a pitch- 
1 ing velocity at a given lift coefficient, the air- 
plane develops a pitching moment resisting 
the pitch motion which adds to the restoring 
moment from the basic static stability. The 
principal source of this additional pitching 
moment is illustrated in figure 4.15. 
During a pull-up the airplane is subject to 
an angular rotation about the lateral axis and 
the horizontal tail will experience a component 
of wind due to the pitching velocity. The 
vector addition of this component velocity to 
the flight velocity provides a change in angle 
of attack for the tail and the change in lift on 
the tail creates a pitching moment resisting 
the pitching motion. Since the pitching mo- 
ment opposes the pitching motion but is due 
to the pitching motion, the effect is a damping 
in pitch. Of course, the other components of 
the airplane may develop resisting moments 
and contribute to pitch damping but the 
horizontal tail is usually the largest contri- 
bution. The added pitching moment from 
pitch damping will effect a higher stability 
in maneuvers than is apparent in steady flight. 
From this consideration, the neutral point for 
maneuvering flight will be aft of the neutral 
point for unaccelerated flight and in most cases 
will not be a critical item. If the airplane 
demonstrates static stability in unaccelerated 
flight, it will most surely demonstrate stability 
in maneuvering flight. 
The most direct appreciation of the ma- 
neuvering stability of an airplane is obtained 
from a plot of stick force versus load factor 
such as shown in figure 4.15. The airplane 
with positive maneuvering stability should 
demonstrate a steady increase in stick force 
with increase in load factor or “G”. The 
maneuvering stick force gradient-or stick 
force per G-must be positive but should be 
of the proper magnitude. The stick force 
gradient must not be excessively high or the 
airplane will be difficult and tiring to maneuver.