NAVWEPS OO-ROT-80 
STABILITY AND CONTROL 
with control surface deflection. While it is 
desirable to have a strong centering of the 
stick near the neutral position, the amount of 
force required to create an initial displacement 
must be reasonable. If the control system 
“break-out” forces are too high, precise control 
of’the airplane at high speeds is diflicult. As 
the solid friction of the control system con- 
tributes to the break-out forces, proper mainte- 
nance of the control system is essential. Any 
increase in control system friction can create 
unusual and undesirable control forces. 
The trim of the powered control system is 
essentially any device to produce zero control 
force for a given control surface deflection. 
One system may trim off bungee force at a 
given stick position while another system may 
trim by returning the stick to neutral position. 
Flight at high supersonic Mach numbers 
might require a great variety of devices in the 
longitudinal control system. The deteriora- 
tion of pitch damping with Mach-number may 
require that dynamic stability be obtained 
synthetically by pitch dampers in the control 
system. The response of the airplane to 
longitudinal control may be adversely affected 
by flight at high dynamic pressures. In such 
conditions of flight stick forces must be ade- 
quate to prevent an induced oscillation. Stick 
forces must relate the transients of flight as 
well as the steady state conditions. Such a 
contribution to control system forces may be 
provided by a pitching acceleration bobweight 
and a control system viscous damper. 
DIRECTIONAL STABILITY AND CONTiOL 
DIRECTIONAL STABILITY 
The directional stability of an airplane is 
essentially the “weathercock” stability and 
involves moments about the vertical axis and 
their relationship with yaw or sideslip angle. 
An airplane which has static directional sta- 
bility would tend to return to an equilibrium 
when subjected to some disturbance from equi- 
librium. Evidence of static directional sta- 
bility would be the development of yawing 
moments which tend to restore the airplane 
to equilibrium. 
DEFINITIONS. The axis system of an air- 
plane will define a positive yawing moment, 
N, as a moment about the vertical axis which 
tends to rotate the nose to the right. As in 
other aerodynamic considerations, it is con- 
venient to consider yawing moments in the 
coefficient form so that static stability can be 
evaluated independent of weight, altitude, 
speed, etc. The yawing moment, N, is de- 
fined in the coefficient form by the following 
equation: 
or 
N = C,qSb 
C,=N 0 where 
N=yawing moment, ft.-lbs; 
positive to the right 
q= dynamic pressure, psf 
S=wing area, sq. ft. 
b=wing span, ft. 
C,=yawing moment coefficient, positive 
to the right 
The yawing moment coefficient, C,, is based on 
the wing dimensions $ and 6 as the wing is the 
characteristic surface of the airplane. 
The yaw angle of an airplane relates the dis- 
placement of the airplane centerline from some 
reference azimuth. and is assigned the short- 
,hand notation I& (psi). A positive yaw angle 
occurs when the nose of the airplane is dis- 
placed to the right of the azimuth direction. 
The definition of sideslip angle involves a sig- 
nificant difference. Sides&p angle relates the 
displacement of the airplane centerline from 
the relative wind rather than some reference 
azimuth., Sideslip angle is’provided the short- 
hand notation p (beta) and is positive when 
ihe rela&e wind is displaced to the right of 
the,airplane centerline. Figure 4.22 illustrates 
the definitions of sideslip and yaw angles. 
The sideslip angle, 8, is essentially the di- 
rectional angle of attack of the airplane and 
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