Since the only control forces are those of the 
tab, this device makes possible the deflection 
of large surfaces with relatively small control 
forces. 
A variation of the basic servo tab layout is 
the sprirzg tab arrangement of figure 4.18. 
When the control horn is connected to the 
control surface by springs, the function of the 
tab is to provide a given portion of the required 
control forces. The spring tab arrangement 
can then function as a boost to reduce control 
forces. The servo tab and spring tab are 
usually applied to large or high speed subsonic 
airplanes to provide tolerable stick forces. 
The spring Zoadcd tab of figure 4.18 cotisists 
of a free tab preloaded with a spring which 
furnishes a constant moment about the tab 
hinge line. When the airplane is at zero air- 
speed, the tab is rotated up to the limit of 
deflection. As airspeed is increased, the aero- 
dynamic hinge moment on the tab will finally 
equal the spring torque and the tab will begin 
to streamline. The effect of this arrangement 
is to provide a constant hinge moment to the 
control system and contribute a constant push 
force requirement at speeds above the preload 
speed. Thus, the spring loaded tab can im- 
prove the stick force gradient in a manner 
similar to the downspring. Generally, the 
spring loaded tab may be more desirable 
because of greater effectiveness and the lack of 
undesirable control forces during ground 
operation. 
The various tab devices have almost un- 
limited possibilities for tailoring control forces. 
However, these devices must receive proper 
care and maintenance in order to function 
properly. In addition, much care must be 
taken to ensure that no slop or play exists in 
the joints and fittings, otherwise destructive 
flutter may occur. 
LONGITUDINAL CONTROL 
To be satisfactory, an airplane must have 
adequate controllability as well as adequate 
NAVWEPS OtWOT-80 
STABILITY AND CONTROL 
stability. Ati airplane with high static longi- 
tudinal stability will exhibit great resistance 
to displacement from equilibrium. Hence, 
the most critical conditions of controllability 
will occur when the airplane has high sta- 
bility, i.e., the lower limits of controllability 
will set the upper limits of stability. 
There are three principal conditions of 
fli~ght which provide the critical requirements 
of longitudinal control power. Any one 
or combination of these conditions can de- 
termine the longitudinal control power and 
set a limit to forward c.g. position. 
MANEUVERING CONTROL REQUIRE- 
MENT. The airplane should have sufficient 
longitudinal control power to attain the maxi- 
mum usable lift coefficient or limit load factor 
during maneuvers. As shown in figure 4.19, 
forward movement of the c.g. increases the 
longiturjinal stability of an airplane and 
requires larger control deflections to produce 
changes in trim lift coefficient. For the 
example shown, the maximum effective de- 
flection of the elevator is not capable of trim- 
ing the airplane ‘at C,,,, for c.g. positions 
ahead of 18 percent MAC. 
This particular control requirement can be 
most critical for an airplane in supersonic 
flight. Supersonic flight is usually accom- . panied by large increases in static longltu- 
dinal stability and a reduction in the effective- 
ness of control surfaces. In order to cope with 
these trends, powerful all-movable surfaces 
must be used to attain limit load factor or 
maximum usable C, in supersonic flight. This 
requirement is so important that once satis- 
fied, the supersonic configuration usually has 
sufficient longitudinal control power for all 
other conditions of flight. 
TAKEOFF CONTROL REQUIREMENT. 
At takeoff, the airplane must have sufficient 
control power to assume the takeoff attitude 
prior to reaching takeoff speed. Generally, 
for airplanes with tricycle landing gears, it 
is desirable to have at least sufficient control 
power to attain the takeoff attitude at 80 
275