capable of producing sufhcient yawing moment 
for the critical conditions of flight. 
The effect of rudder deflection is to produce 
a yawing moment coefficient according to 
control deflection and produce equilibrium at 
some angle of sideslip. For small deflections 
of the rudder, there is no change in stability 
but a change in equilibrium. Figure 4.25 
shows the effect of rudder deflection on yawing 
moment coefficient curves with the change in 
equilibrium sideslip angle. 
If the airplane exhibits static directional 
stability with rudder lixed, each angle of side- 
slip requires a particular deflection of the 
rudder to achieve equilibrium. Rudder-free 
directional stability will exist when the float 
angle of the rudder is less than the rudder 
deflection required for equilibrium. However 
at high angles of sideslip, the floating tend- 
ency of the rudder increases. This is illus- 
trated by the second chart of figure 4.25 where 
the line of rudder float angle shows a sharp 
increase at large values of sideslip. If the 
floating angle of the rudder catches up with 
the required rudder angle, the, rudder pedal 
force will decrease to zero and rudder lock will 
occur. Sideslip angles beyond this point pro- 
duce a floating angle greater than the required 
rudder deflection and the rudder tends to float 
to the limit of deflection. 
Rudder lock is accompanied by a reversal of 
pedal force and rudder-free instability will 
exist. The dorsal fin is a useful addition in 
this case since it will improve the directional 
stability at high angles of sideslip. The re- 
sulting increase in stability requires larger 
deflections of the rudder to achieve equilibrium 
at high sideslip and the tendency for rudder 
lock is reduced. 
Rudder-free directional stability is appre- 
ciated by the pilot as the rudder pedal force to 
maintain a given sideslip. If the rudder pedal 
force gradient is too low near zero sideslip, it 
will be difficult to maintain zero sideslip dur- 
ing various maneuvers. The airplane should 
NAVWEPS 00-SOT-80 
STABIUTY AND CONTROL 
have a stable rudder pedal feel through the 
available range of sideslip. 
DIRECTIONAL CONTROL REQUIRE- 
MENTS. The control power of the rudder 
must be adequate to contend with the many 
unsymmetrical conditions of flight. Gener- 
ally, there are five conditions of flight which 
provide the most criticalrequirements of di- 
‘rectional control power. The type and mission 
of the airplane will decide which of these 
conditions is most important. 
ADVERSE YAW. When an airplane is 
rolled into a turn yawing moments are pro- 
duced which require rudder deflection to main- 
tain zero sideslip, i.e., coordinate the turn. 
The usual source of adverse yawing moment is 
illustrated in figure 4.26. When the airplane 
shown is subject to a roll to the left, the down- 
going port wing will experience a new relative 
wind and an increase in angle of attack. The 
inclination of the lift vector produces a com- 
ponent force forward on the downgoing wing. 
The upgoing starboard wing has its lift in- 
clined with a component force aft. The re- 
sulting yawing moment due to rolling motion 
is in a direction opposite to the roll and is 
hence “adverse yaw.” The yaw due to roll is 
primarily a function of the wing lift coefficient 
and is greatest at high C,. 
In addition to the yaw due to rolling motion 
there will be a yawing moment contribution 
due to control surface deflection. Conventional 
ailerons usually contribute an adverse yaw 
while spoilers may contribute a favorable or 
“proverse” yaw. The high wing airplane 
with a large vertical tail may encounter an 
influence from inboard ailerons. Such a con- 
figuration may induce flow directions at the 
vertical tail to cause proverse yaw. 
Since adverse yaw will be greatest at high 
C, and full deflection of the ailerons, coordi- 
nating steep turns at low speed may produce 
a critical requirement for rudder control power.