NAVWEPS 00-8’X-60 
STABILITY AND CONTROL 
attack airplanes. A large patrol or transport 
type airplane would ordinarily show a much 
higher maneuvering stick force gradient be- 
cause of the lower limit load factor. 
When the airplane has high static stability, 
the maneuvering stability will be high and 
a high stick force gradient will result. A 
possibility exists that the forward c.g. limit 
could be set to prevent an excessively high 
maneuvering stick force gradient. As the 
c.g. is moved aft, the stick force gradient de- 
creases with decreasing maneuvering stability 
and the lower limit of stick force gradient 
may be reached. 
The pitch damping of the airplane is obvi- 
ously related to air density. At high altitudes, 
the high true airspeed reduces the change in 
tail angle of attack for a given pitching velocity 
and reduces the pitch damping. Thus, a de- 
crease in maneuvering stick force stability can 
be expected with increased altitude. 
TAILORING CONTROL FORCES. The 
control forces should reflect the stability of 
the airplane but, at the same time, should be 
of a tolerable magnitude. The design of the 
surfaces and control system may employ an 
infinite variety of techniques to provide satis- 
factory control forces. 
Aerodynamic balance must be thought of in 
two different senses. First, the control surface 
must be balanced to reduce hinge moments due 
to changes in angle of attack. This is necessary 
to reduce the floating tendency of the surface 
which reduces the stick-free stability. Next, 
aerodynamic balance can reduce the hinge 
moments due to deflection of the control sur- 
face. Generally, it is difficult to obtain a high 
degree of deflection balance without incurring 
a large overbalance of the surface for changes 
in angle of attack. 
Some of the types of aerodynamic balance 
are illustrated in figure 4.16. Thesimple horn 
type balance employs a concentrated balance 
area located ahead of the hinge line. The 
balance area may extend completely to the 
leading edge (unshielded) or partway to the 
leading edge (shielded). Aerodynamic balance 
can be achieved by the provision of- a hinge 
line aft of the control surface leading edge. 
The resulting overhang of surface area ahead 
of the hinge line will provide a degree of 
balance depending on the amount of overhang. 
Another variation of aerodynamic balance is 
an internal balance surface ahead of the hinge 
line which is contained within ,the surface. 
A flexible seal is usually incorporated to in- 
crease the effectiveness of the balance area. 
Even the bevelling of the trailing edge..of the 
control surface is effective also as a balancing 
technique. The choice of the type of aerody- 
namic balance will depend on many factors 
such as required degree of balance, simplicity, 
drag, etc. 
Many devices can be added to a control 
system to modify or tailor the stick force 
stability to desired levels. If a spring is added 
to the control system as shown in figure 4.16, 
it will tend to center the stick and provide a 
force increment depending on stick displace- 
ment. When the control system has a fixed 
gearing between stick position and surface 
deflection, the centering spring will provide a 
contribution to stick force stability according 
to stick position. The contribution to stick 
force stability will be largest at low flight 
speeds where relatively large control deflec- 
tions are required. The contribution will be 
smallest at high airspeed because of the smaller 
control deflections required. Thus, .the stick 
centering bungee will increase the airspeed 
and maneuvering stick force stability but the 
contribution decreases at high airspeeds. A 
variation of this device would be a spring 
stiffness which would be controlled to vary 
with dynamic pressure, 4. In this case, the 
contribution of the spring to stick force 
stability would, not diminish with. speed. 
A “downspring” added to a control system