NAVWEPS 00-BOT-80 
STABILITY AND CONTROL 
at which equilibrium will occur. As the ele- 
vator is fixed in various positions, equilibrium 
(or trim) will occur at various lift coefficients 
and the trim CL can be correlated with elevator 
deflection as in the second graph of figure 4.13. 
When the c,g. position of the airplane is 
fixed, each elevator position corresponds to a 
particular trim lift coefficient. AS the c.g. is 
moved aft the slope of this line decreases and 
the decrease in stability is evident by a given 
control displacement causing a greater change 
in trim lift coefficient. This is evidence that 
decreasing stability causes increased controlla- 
bility and, of course, increasing stability de- 
creases controllability. If the c.g. is moved 
aft until the line of trim CL versus elevator de- 
flection has zero slope, neutral static stability 
is obtained and the “stick-fixed” neutral point 
is determined. 
Since each value of lift coefhcient corresponds 
to a particular value of dynamic pressure re- 
quired to support an airplane in level flight, 
uim airspeed can be correlated with elevator 
deflection as in the third graph of figure 4.13. 
If the c.g. location is ahead of the stick-fixed 
neutral point and control position is directly 
related to surface deflection, the airplane will 
give evidence of stick podion mbility. In 
other words, the airplane will require the 
stick to be moved aft to increase the angle 
of attack and trim at a lower airspeed and to 
be moved forward to decrease the angle of 
attack and trim at a higher airspeed. To be 
sure, it is desirable to have an airplane demon- 
strate this feature. If the airplane were to 
have stick position instability, the airplane 
would require the stick to be moved aft to trim 
at a higher airspeed or to be moved forward to 
trim at a lower airspeed. 
There may be slight differences in the static 
longitudinal stability if the elevators are 
allowed to float free. If the elevators are 
allowed to float free as in “hands-off” flight, 
the elevators may have a tendency to “float” 
or streamline when the horizontal tail is given 
a change in angle of attack. If the hot&ma1 
tail is subject to an increase in angle of attack 
and the elevators tend to float up, the change 
in lift on the tail is less than if the elevators 
remain fixed and the tail contribution to 
stability is reduced. Thus, the “stick-free” 
stability of an airplane is usually less than the 
stick-fixed stability. A typical reduction of 
stability by free elevators is shown in figure 
4.14(A) where the airplane. stick-free demon- 
strates a reduction of the slope of CM versus Cs. 
While aerodynamic balance may be provided 
tu reduce control forces, proper balance of the 
surfaces will reduce floating and prevent great 
differences between stick-fixed and stick-free 
stability. The greatest floating tendency oc- 
curs when the surface is at a high angle of 
attack hence the greatest difference between 
stick-fixed and stick-free stability occurs when 
the airplane is at high angle of attack. 
If the controls are fully powered and actu- 
ated by an irreversible mechanism, the sur- 
faces are not free to float and there is no differ- 
ace between the stick-fixed and stick-free 
static stability. 
The control forces in a conventional air- 
plane are made up of two components. First, 
the basic stick-free stability of the airplane 
contributes an incremem of force which is 
independent of airspeed.. Next, there. is an 
increment of force dependent on the trim tab 
setting which varies with-the dynamic pres- 
sure or the square of ‘equivalent airspeed. 
Figure 4.14(B) indicates the variation of 
stick force with airspeed and illustrates the 
effect of tab setting on stick force. In order 
te trim the airplane at point (1) a certain 
amount of up elevator is required and zero 
stick force is obtained~ with’the nse of the tab. 
To trim the airplane for higher speeds corre-