Out-of-Control Flight
Aerodynamic Principles
A familiar analogy is a falling leaf. The leaf spins to the ground in a balanced autorotation: aerodynamic
forces are acting on the leaf in concert to produce a regular spin motion.
Figure 11 illustrates how these differential aerodynamic forces cause autorotation. In a spin, the aircraft is
rolling and yawing at some AOA above stall. In an upright spin, the aircraft rolls and yaws in the same
direction; in an inverted spin, the aircraft rolls and yaws the opposite direction. The up-going wing will
experience a decrease in angle of attack with an increase in lift and decrease in drag relative to the down-
going wing. Therefore, the up-going wing will become less stalled. The resulting autorotation rolling
moments and yawing moments start the aircraft into a steady-state spin. For this reason, roll damping
(resistance to roll) is negative at angles of attack above stall; the rolling motion produces a rolling moment
in the direction of the roll. This negative damping is referred to as “autorotation.” Figure 11 shows a
graphic depiction of roll and yaw autorotation.
Direction
Higher C L
Up-Going Wing
Lower C L
Difference in Lift
Causes Roll
Autorotation
Down-Going Wing
ROLL AUTOROTATION
Yaw Direction
Down-Going Wing
Up-Going Wing
Lower C
D
Higher CD
Difference in Drag
CausesYaw
Autorotation
YAW AUTOROTATION
Figure 11: AUTOROTATION IN T-45 AIRCRAFT
(9-98) Change 1
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