warning, and preserves lateral control effec- 
tiveness at high angles of attack. Also, the 
airplane must maintain positive static longi- 
tudinal stability at high lift coe&ients and 
should demonstrate satisfactory stall recovery 
characteristics. 
In order to visualize the principal effects of 
an airplane entering a spin, suppose the air- 
plane is subjected to the rolling and yawing 
velocities shown in figure 4.32. The yawing 
velocity to the right tends to produce higher 
local velocities on the left wing than on the 
right wing. The rolling velocity tends to 
increase the angle of attack for the downgoing 
right wing (a,) and. decrease the angle of 
attack for the upgoing left wing (al). At 
airplane angles of attack below the stall this 
relationship produces roll due to yaw, damping 
in roll, etc., and some related motion of the 
airplane in unstalled flight. However, at 
angles of attack above the stall, important 
changes take place in the aerodynamic char- 
acteristics. 
Figure 4.32 illustrates the aerodynamic 
characteristics typical of a conventional air- 
plane configuration, i.e., moderate or high 
aspect ratio and little-if any-sweepback. 
Ifs this airplane is provided a rolling displace- 
ment when at some angle of attack above 
the stall, the upgoing wing experiences a 
decrease in angle of attack with a correspond- 
ing increase in C, and decrease in C,,. In other 
words, the upgoing wing becomes less stalled. 
Similarly, the downgoing wing experiences 
an increase in angle of attack with a corre- 
sponding decrease in CL and increase in CD. Es- 
sentially, the downgoing wing becomes more 
stalled. Thus, the rolling motion is aided 
rather than resisted and a yawing moment is 
produced in the direction of roll. At angles 
of attack below stall the rolling motion is 
resisted by damping in roll and adverse yaw 
is usually present. At angles of attack above 
the stall, the damping in roll is negative and 
a rolling motion produces a rolling moment 
in the direction of the roll. This negative 
NAVWEPS OO-BOY-BO 
STABIUTY AND CoMml 
damping in roll is generally referred to as 
“autorotation.” 
When the conventional airplane is stalk4 
and some rolling-yawing displacement takes 
place, the resulting autotiotation rolling mo- 
ments and yawing moments start the airplane 
into a self-sustaining rolling-yawing motion. 
The autorotation rolling and yawing tenden- 
cies of the airplane at high angles of attack 
are the principal prospin moments of the 
conventional airplane configuration and these 
tendencies accelerate the airplane into the 
spin until some limiting condition exists. 
The stabilized spin is not necessaray a simple 
steady vertical spiral but may involve some 
coupled unsteady oscillatory motion. 
An important characteristic of the mote 
conventional airplane configuration is that the 
spin shows a predominating contribution of 
the autorotation tendency. Generally, the 
conventional configuration has a spin motion 
which is primarily rolling with moderate yaw. 
High directional stability is favorable since it 
will limit or minimize the yaw displacement 
of the spinning airplane. 
The fundamental requirement of the spin is 
that the airplane be placed at an excessive 
angle of attack to produce the autorotation 
rolling and yawing tendencies. Generally 
speaking, the conventional airplane must be 
stalled .before a spin can take place. This 
relationship establishes a fundamental p&r- 
ciple of recovery-the airplane must be un- 
stalled by decreasing the wing angle of attack. 
The most dfective procedure for the conven- 
tional configuration is to use opposite rudder 
to stop the sideslip, then lower the angle of 
attack with the elevators. With sufficient 
rudder power this procedure will produce a 
positive recovery with a minimum loss of 
altitude. Care should be taken during pullout 
from the ensuing dive to prevent excessive 
angle of attack and entry into another spin.