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pilot training, theo~emonstration of competency in spin recovery does not form a
part of the private or commercial pilot licensing in the United States. Stall waming
systems have helped improve the safety oflight general aviation airplanes, but the
pilot is still required to take some form of conective action to prevent the airplane
from becoming uncontrollable.lmproving theinherent spin resistance offers great
potential to make the configuration spinproof and improve the safety of general
aviation airplanes.
The straight-wing, light propeller-driven airplanes usually have good longitu-
dinal and directional stabilit)r at stall. The critical aerodynamic characteristic of
such airplanes at stallis the autorotative tendency. Also, such airplanes experience
an asymmetric stall. In other words, both wings do not stall at the same time, or
one wing stalls earlier than the other. One ofthe possible causes for an asymmetric
stallis the propeller sidewash effect. Following an asymmetnic stall, the nose drops
and the airplane rolls in the direction of the fallen wing and continues to roll owing
to the autorotative tendency of the stalled wings. In this process, yawing motion
develops due to the aerodynamic roll-yaw coupling. (Note that we are not talk-
ing of the inertial roll-yaw coupling here.) Such a motion of the stalled airplane
involving combined pitch, roll, and yaw is often called poststall gyration. As the
yaw rate builds up, the nose rises, the flight path steepens, and the aircraft starts
losing height The airplane is now in a spin.ln9pin, the airplane descends vertically
downward in a helical path as schematically shown in Fig. 7.10. If a steady-state
spin develops, the descent velocity, pitch, roll, and yaw rates attain constant values.
The radius of the helix or the spin radius is usually of the order of one half
of the wing span. However, as the spin becomes flatterJ the spin.radius decreases
further and, in the limiting case, the spin axis may pass through the airplane center
of gravity.
Whether an airplane develops a steady-state spin depends on the balance between
the inertia couples and the a9rodynamic moments. If such a balance cannot be
achieved, the spinning motion remains oscillatorjr. Sometimes, this type of spin is
also called incipient spin.
In contrast-to a straight-wing light airplane, modem aircraft with highly swept
wings and long, slender fuselages experience loss of longitudinal and directional
stabilities as well as directional corUrol at stall. As a result, the motion following a
stallis predominantly in yaw invoMng directionalinstability and divergence that
may lead to a spin entry.
The spin is not of any tactical value for a military aircraft. Also, it serves no
useful purpose in civil aviation. The spin is different from a spiral dive. What
distinguishes a spin from the spiral dive is the low air speed and operation of the
INERTIA COUPLING AND SPIN
t
\
Y-- SpinAxis
1
i
Fig.7.10 Schematic diagram of a spinning airplane.
647
wings well beyond the stalling angle. In a spiral dive, the airplane is not stalled
and the air speed is continuously increasing. Further, the spin axis is much closer
to the airplane center of gravity than the axis of the spiral dive. The airplane has
a much higher yaw rate in a spin compared to that in a spiral dive. Typical yaw
rates for straight-wing light general aviation airplanes are about 120-150 deg/s in
a developed spin.
The spin may be in either direction,i.e., to the right or to the left. However, both
the right and left spins may not be identical because of the effect of rotating engine
masses or the propellers. The spin may be due to an inadvertent stall or could
be intentionally initiated by stalling the airplane and applying prospin controls.
Initially, the pitch, roll, and yaw rates oscillate, but these oscillations decay if the
airplane establishes a steady spin,in which case all the parameters tend to assume
constant values.
648 PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
The spin may be erect or inverted.ln erect spin, the roll (as seen visually by the
pilot) and yaw (as indicated by the turn indicator needle) are in the same direction.
In an inverted spin, the two are in opposite directions.
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