曝光台 注意防骗
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values of the angular velocity l02. Let 92 : SZc when CfZ - O. We observe that
for 0 < S2 < S2c, CfZ > O, which implies that the induced yawing moment wiU_
assist the imparted disturbance, and, for t2 > 92c, Cfl < 0, the induced yawing
moment will oppose the rotation. Hence, the angular velocitjr <2c when Cn - 0 is
the steady-state or equilibrium autorotational speed. If the variation of Ccz curve
is of type II, then the body does not autorotate. It is resistant to autorotat:ion.
The side force variations that can give type I'and t}rpe n variations of the
yawing moment are shown in Fig. 7.8. The type A variation of side force has a
positive value of side force forlow values ofthe crossflow angle, and the side force
becomes'negative for higher values of the crossflow angle. This type of side force
variation generates autorotative (yawing) moments on sections close to the center
of gravity and damping (yawing) moments for sections away from the center of
gravity. Equilibrium or steady autorotation speed is reached wvhen the autorotafive
moments balance the damping moments so that the net yawing moment on the
J
body is zero. For type B, the side force coe:fficient is always negative, leading
to damping (yawing) moments at all cross sections. This corresponds to type II
variation of Cn vs 02 as shown in Fig. 7.7.
The cross-sectional shapes having rounded bottom srrrfaces (Fig. 7.9) generally
have type B side force coePfficient variation, and those having flat bottom surfaces
have ty;e A side force coefficient variation. Rounding the top surface while the
bottom surface is kept fiat enhances the autorotational tendency.
The Reynolds number is known to have a sigruficant infiuence on the side force
characteristics of noncircular cylinders.9 For certain cross-sectional shapes, the
side force variation changes from type A to type B as the Reynolds number is
increased. Thus, it is quite possible that a spin-tunnel model may indicate that
the configuration is prospin, whereas full-scale airl,lane may exhibit an antispin
behavior.
+
O
INERTIA COUPLING AND SPIN
┏━━━━━━━━━┓
┃/---< ++ ┃
┣━━━━━━━━━┫
┃ ~\l@r~~ 1t - ┃
┗━━━━━━━━━┛
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.
~
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
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