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towards the stall, and remains effective even after the
wing has begun to stall. This enables the pilot to drive
the wing into a deeper stall at a much greater angle
of attack.
At the stall, two distinct things happen. After the stall,
the sweptwing T-tail airplane tends to pitch up rather
than down, and the T-tail is immersed in the wing
wake, which is low energy turbulent air. This greatly
reduces tail effectiveness and the airplane’s ability to
counter the noseup pitch. Also, the disturbed,
relatively slow air behind the wing may sweep across
the tail at such a large angle that the tail itself stalls. If
this occurs, the pilot loses all pitch control and will be
unable to lower the nose. The pitch up just after the
stall is worsened by large reduction in lift and a large
increase in drag, which causes a rapidly increasing
Lift Coefficient
Angle of Attack
Straight
Wing
Sweptwing
Figure 15-13. Stall vs. angle of attack—sweptwing vs. straight
wing.
Figure 15-14. Stall progression—typical straight wing
airplane.
Ch 15.qxd 5/7/04 10:22 AM Page 15-11
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descent path, thus compounding the rate of increase of
the wing’s angle of attack. [Figure 15-15]
The pitch up tendency after the stall is a characteristic
of a swept and/or tapered wings. With these types of
wings, there is a tendency for the wing to develop a
strong spanwise airflow towards the wingtip when the
wing is at high angles of attack. This leads to a
tendency for separation of airflow, and the subsequent
stall, to occur at the wingtips first. [Figure 15-16] The
tip first stall, results in a shift of the center of lift of the
wing in a forward direction relative to the center of
gravity of the airplane, causing the nose to pitch up.
Another disadvantage of a tip first stall is that it can
involve the ailerons and erode roll control.
As previously stated, when flying at a speed in the area
of VMD, an increase in angle of attack causes drag to
increase faster than lift and the airplane begins to sink.
It is essential to understand that this increasing sinking
tendency, at a constant pitch attitude, results in a rapid
increase in angle of attack as the flightpath becomes
deflected downwards. [Figure 15-17] Furthermore,
once the stall has developed and a large amount of lift
has been lost, the airplane will begin to sink rapidly
and this will be accompanied by a corresponding rapid
increase in angle of attack. This is the beginning of
what is termed a deep stall.
As an airplane enters a deep stall, increasing drag
reduces forward speed to well below normal stall
speed. The sink rate may increase to many thousands
of feet per minute. The airplane eventually stabilizes
in a vertical descent. The angle of attack may approach
Pre-Stall
Stall
Figure 15-15. Stall progression sweptwing airplane.
Spanwise Flow of Boundary
Layer Develops at High CL
Initial Flow Separation
at or Near Tip
Area of Tip
Stall Enlarges
Stall Area Progresses
Inboard
Figure 15-16. Sweptwing stall characteristics.
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90° and the indicated airspeed may be reduced to zero.
At a 90° angle of attack, none of the airplane’s control
surfaces are effective. It must be emphasized that this
situation can occur without an excessively nose-high
pitch attitude. On some airplanes, it can occur at an
apparently normal pitch attitude, and it is this quality
that can mislead the pilot because it appears similar to
the beginning of a normal stall recovery.
Deep stalls are virtually unrecoverable. Fortunately,
they are easily avoided as long as published limitations
are observed. On those airplanes susceptible to deep
stalls (not all swept and/or tapered wing airplanes are),
sophisticated stall warning systems such as stick
shakers and stick pushers are standard equipment. A
stick pusher, as its name implies, acts to automatically
reduce the airplane’s angle of attack before the airplane
reaches a fully stalled condition.
Unless the Airplane Flight Manual procedures
stipulate otherwise, a fully stalled condition in a jet
airplane is to be avoided. Pilots undergoing training in
jet airplanes are taught to recover at the first sign of an
impending stall. Normally, this is indicated by aural
stall warning devices and/or activation of the airplane’s
stick shaker. Stick shakers normally activate around
107 percent of the actual stall speed. At such slow
speeds, very high sink rates can develop if the
　
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