stability. As shown in figure 4.17, a down- 
spring consists of a long preloaded spring at- 
tached to the control system which tends to 
rotate the elevators down. The effect of the 
downspring is to contribute an increment of 
pull force independent of control deflection or 
airspeed. When rhe downspring is added to 
the control system of an airplane and the air- 
plane is retrimmed for the original speed, the 
airspeed stick force gradient is increased and 
there is a stronger feel for airspeed. The down- 
spring would provide an “ersatz” improve- 
ment to an airplane deficient in airspeed stick 
force stability, Since the force increment from 
the downspring is unaffected by stick position 
or normal acceleration, the maneuvering stick 
force stability would be unchanged. 
The bobweight is an effective device for im- 
proving stick force stability. As shown in 
figure 4.17, the bobweight consists of an eccen- 
tric mass attached to the control system 
which-in unaccelerated flight--contributes 
an increment of pull force identical to the 
downspring. In fact, a bobweight added to 
the control system of an airplane produces an 
effect identical to the downspring. The bob- 
weight will increase the airspeed stick force 
gradient and increase the feel for airspeed. 
A bobweighr will have an effect on the 
maneuvering stick force gradient since the bob- 
weight mass is subjected to the same accelera- 
tion as the airplane. Thus, the bobweight will 
provide an increment of stick force in direct 
proportion to the maneuvering acceleration of 
the airplane. Because of the linear contribu- 
tion of the bobweight, the bobweight can be 
applted to Increase the maneuvering stick force 
stability if the basic airplane has too low a 
value or develops a decreasing gradient at high 
lift coefficients. 
The example of the bobweight is useful to 
point out the effect of the control system dis- 
tributed masses. All carrier aircraft must have 
the control system mass balanced to prevent 
undesirable control forces from the longi- 
tudinal accelerations during catapult launching. 
NAVWEPS 00-EOT-80 
STABILITY AND CONTROL 
Various control surface tab devices can be 
utilized to modify control forces. Since the de- 
flection of a tab is so powerful in creating hinge 
moments on a control surface, the possible 
application of tab devices is almost without 
limit, The basic trim tab arrangement is 
shown in figure 4.18 where a variable linkage 
connects the tab and the control surface. Ex- 
tension or contraction of this linkage will de- 
flect the tab relative to the control surface and 
create a certain change in hinge mon~ent coef- 
ficient. The use of the trim tab will allow the 
pilot to reduce the hinge moment to zero and 
trim the control forces to zero for a given flight 
condition. Of course, the trim tab should have 
adequate effectiveness so that control forces 
can be trimmed out throughout the flight speed 
range. 
The lagging tab arrangement shown in figure 
4.18 employs a linkage between the fixed sur- 
face and the tab surface. The geometry is 
such that upward deflection of the control 
surface displaces the tab down relative to the 
control surface. Such relative displacement 
of the tab will aid in deflection of the control 
surface and thus reduce the hinge moments due 
to deflection. An obvious advantage of this 
device is the reduction of deflection hinge 
moments without a change in aerodynamic 
balance. 
The leading tab arrangement shown in figure 
4.18 also employs a linkage between the fixed 
surface and the tab surface. However, the 
geometry of the linkage is such that upward 
deflection of the control surface displaces the 
tab up relative to the control surface. This 
relationship serves to increase the control sur- 
face hinge moments due to deflection of the 
surface. 
The servo tad shown in figure 4.18 utilizes a 
horn which has no direct connection to the 
control surface and is free to pivot about the