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Fig 7.8    Schematic -variation ofside force variation in crassflow.
+
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┏━━━━━━┓
┃   /~       ┃
┃~--~ ~      ┃
┃       - l~ ┃
┃~~\         ┃
┣━━━━━━┫
┃            ┃
┗━━━━━━┛
C~ [ C)
Antispin effect                     Basic                           Prospin effect
Fig. 7.9    Effect of fuselage cross-sectional shape on spin behavior.
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646            PERFORMANCE, STAB;LITY, DYNAMICS, AND CONTROL
7.4 AirplaneSpin
      staivspin problems have been encountered since the very beginning of aviation.
The very low altitudes at which early aircraft were flown precluded the progression
of stall to a fully developed spin prior to the ground impact. As a result,it was not
possible to clearly ascertain the tme causes of these early crashes.
      The stall-spin is one of the major causes oflight airplane accidents even today.
Such an accidentis characterned by an inadvertent stall and a spin entrr3r at an alti-
tude that is too low to effect a successful recovery.l0,11 There a~'three approaches
to preventing an inadvertent stall departure:12 l) pilot training, 2) stall warning,
and 3) increased spin resistance. Although FAA places significant importance on
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.
　
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