曝光台 注意防骗
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Fig 7.8 Schematic -variation ofside force variation in crassflow.
+
O
┏━━━━━━┓
┃ /~ ┃
┃~--~ ~ ┃
┃ - 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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