NAVWEPS 00-8OT-80 
STABILITY AND CONTROL 
high angles of attack is required to effect re- 
covery during a spin. Since the effectiveness 
of the vertical tail is reduced at large angles of 
attack, the directional control power neces- 
sary for spin recovery may produce a critical 
requirement of rudder power. 
SLIPSTREAM ROTATION. A critical di- 
rectional control requirement may exist when 
the propeller powered airplane is at high 
power and low airspeed. As shown in figure 
4.26, the single rotation propeller induces 
a slipstream swirl which causes a change in 
flow direction at the vertical tail. The rudder 
must furnish sufficient control power to balance 
this condition and achieve zero sideslip. 
CROSSWIND TAKEOFF AND LANDING. 
Since the airplane must make a true path down 
the runway, a crosswind during takeoff or 
landing will require that the airplane be.con- 
trolled in a sideslip. The rudder must have 
sufficient control power to create the required 
sideslip for the expected crosswinds. 
ASYMMETRICAL POWER. The design 
of a multiengine airplane must account for the 
possibility of an engine failure at low airspeed. 
The unbalance of thrust from a condition of 
unsymmetrical power produces a yawing mo- 
ment dependent upon the thrust unbalance 
and the lever arm of the force. The deflection 
of the rudder will create a side force on the tail 
and contribute a yawing moment to balance 
the yawing moment due to the unbalance of 
thrust. Since the yawing moment coefficient 
from the unbalance of thrust will be greatest 
at low speed, the critical requirement will be 
at a low speed with the one critical engine 
out and the remaining engines at maximum 
power. Figure 4.26 compares the yawing 
moment coeflicient for maximum rudder deflec- 
tion with the yawing moment coefficient for 
the unbalance of thrust. The intersection of 
the two lines,determines the minimum speed 
for directional control, i.e., the lowest speed 
at which the rudder control moment can equal 
the moment of unbalanced thrust, It is usually 
specified that the minimum directional control 
speed be no greater than 1.2 times the stall 
294 
Revised January 1965 
speed of the airplane in the lightest practical 
takeoff configuration. This will provide ade- 
quate directional control for the remaining 
conditions of flight. 
Once defined, the minimum directional con- 
trol speed is not a function of weight, altitude, 
etc., but is simply the equivalent airspeed (or 
dynamic pressure). to produce a required yaw- 
ing moment with the maximum rudder deflec- 
tion. If the airplane is operated in the critical 
unbalance of power below the minimum con 
trol speed, the airplane will yaw uncontrolla- 
bly into the inoperative engine. In order to 
regain directional control below the minimum 
speed certain alternatives exist: reduce power 
on the operating engines or sacrifice altitude 
for airspeed. Neither alternative is satisfac- 
tory if the airplane is in a marginal condition 
of powered flight so due respect must be given 
to the minimum control speed. 
Due to the side force on the vertical tail, a 
slight bank is necessary to prevent turning 
flight at zero sideslip. The inoperative engine 
will be raised and the inclined wing lift will 
provide a component of force to balance the 1 
side force on the tail. 
In each of the critical conditions of required 
directional control, high directional stability 
is desirable as it will reduce the displacement 
of the aircraft from any disturbing influence. 
Of course, directional control must he sufficient 
to attain zero sideslip. The critical control 
requirement for the multiengine airplane is 
the condition of asymmetrical power since 
spinning is not common to this type of airplane. 
The single engine propeller airplane may have 
either the spin recovery or the slipstream rota- 
tion as a critical design condition. The single 
engine jet airplane may have a variety of