NAVWEPS 00-BOT-BO 
STABILITY AND CONTROL 
type of flight control system is decided by the 
size and flight speed range of the airplane. 
The conventional control system consists of 
direct mechanical linkages from the controls 
to the control surfaces. For the subsonic 
airplane, the principal means of producing 
proper control forces utilize aerodynamic bal- 
ance and various tab, spring, and bobweight 
devices. Balance and tab devices are capable 
of reducing control forces and will allow the 
use of the conventional control system on large 
airplanes to relatively high subsonic speeds. 
When the airplane with a conventional 
control system is operated at transonic speeds, 
the great changes in the character of flow 
can produce great aberrations in control sur- 
face hinge moments and the contribution of 
tab devices. Shock wave formation and 
separation of flow at transonic speeds will 
limit the use of the conventional control 
system to subsonic speeds. 
The power-boosted control system employs a 
‘mechanical actuator in parallel with the 
mechanical linkages of a conventional control 
system. The principle of operation is to pro- 
vide a fixed, percentage of the required control 
forces thus reducing control forces at high 
speeds. The power-boosted control system 
requires a hydraulic actuator with a control 
valve which supplies boost force in fixed 
proportion to control force. Thus, the pilot 
is given an advantage by the boost ratio to 
assist in deflecting the control surface, e.g., 
with a boost ratio of 14, the actuator provides 
14 lbs. of force for each 1 lb. of stick force. 
The power-boosted control system has the 
obvious advantage of reducing control forces 
at high speeds. However, at transonic speeds, 
the changes in control forces due to shock 
waves and separation still take place but to a 
lesser degree. The “feedback” of hinge 
moments is reduced but the aberrations in 
stick forces may still exist. 
The power-opsrdted, irreversible control system 
consists of mechanical actuators controlled 
by the pilot. The control surface is deflected 
by the actuator and none of the hinge moments 
are fed back through the controls. In such 
a control system, the control position decides 
the deflection of the control surfaces regardless 
of the airloads and hinge moments. Since the 
power-operated control system has zero feed- 
back, control feel must be synthesized other- 
wise an infinite boost would exist. 
The advantages of the power-operated COR- 
trol system are most apparent in transonic and 
supersonic flight. In transonic flight, none of 
the erratic hinge moments are fed back to the 
pilot. Thus, no unusual or erratic control 
forces,will be encountered in transonic flight. 
Supersonic flight generally requires the use of 
an all-movable horizontal surface to achieve 
the necessary control effectiveness. Such con- 
trol surfaces must then be actuated and posi- 
tively positioned by an irreversible device. 
The most important item of an artificial feel 
system is the stick-centering spring or bungee. 
The bungee develops a stick force in proportion 
to stick displacement and thus provides feel 
for airspeed and maneuvers. A bobweight 
may be included in the feel system to develop 
a steady positive maneuvering stick force 
gradient which is independent of airspeed for 
ordinary maneuvers. 
The gearing between the stick position and 
control surface deflection is not necessarily a 
linear relationship. The majority of powered 
control systems will employ a nonlinear gear- 
ing such that relatively greater stick deflection 
per surface deflection will occur at the neutral 
stick position. This sort of gearing is to 
advantage for airplanes which operate at flight 
conditions of high dynamic pressure. Since 
the airplane at high 4 is very sensitive to small 
deflections of the control surface, the nonlinear 
gearing provides higher stick force stability 
with less sensitive control movements than