曝光台 注意防骗
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stall, the more complete the stall is likely to become,
and the greater the loss of altitude to be expected.
Pilots must recognize the flight conditions that are conducive
to stalls and know how to apply the necessary
corrective action since most gliders do not have an
electrical or mechanical stall warning device. Pilots
should learn to recognize an approaching stall by sight,
sound, and feel. The following cues may be useful in
recognizing the approaching stall.
Vision is useful in detecting a stall condition by noting
the attitude of the glider. This sense can only be relied
on when the stall is the result of an unusual attitude of
the glider. Since the glider can also be stalled from a
normal attitude, vision in this instance would be of little
help in detecting the approaching stall.
Hearing is also helpful in sensing a stall condition. In
the case of a glider, a change in sound due to loss of
airspeed is particularly noticeable. The lessening of the
noise made by the air flowing along the glider structure
as airspeed decreases is also quite noticeable, and
when the stall is almost complete, aerodynamic vibration
and incident noises often increase greatly.
Kinesthesia, or the sensing of changes in direction or
speed of motion, is probably the most important and
the best indicator to the trained and experienced pilot.
If this sensitivity is properly developed, it will warn of
a decrease in speed or the beginning of a settling or
mushing of the glider.
The feeling of control pressures is also very important.
As speed is reduced, the resistance to pressures on the
controls become progressively less. Pressures exerted
on the controls tend to become movements of the control
surfaces. The lag between these movements and the
response of the glider becomes greater, until in a complete
stall all controls can be moved with almost no
resistance, and with little immediate effect on the glider.
Signs of an impending stall include the following.
• High nose attitude.
• Low airspeed indication.
• Low airflow noise.
• Back pressure on sticks.
• Mushy controls, especially ailerons.
• Buffet.
Always make clearing turns before performing stalls.
During the practice of intentional stalls, the real objective
is not to learn how to stall a glider, but to learn
how to recognize an approaching stall and take prompt
corrective action. The recovery actions must be taken
in a coordinated manner.
First, at the indication of a stall, the pitch attitude and
angle of attack must be decreased positively and immediately.
Since the basic cause of a stall is always an
excessive angle of attack, the cause must first be eliminated
by releasing the back-elevator pressure that was
necessary to attain that angle of attack or by moving the
elevator control forward. This lowers the nose and
returns the wing to an effective angle of attack. The
amount of elevator control pressure or movement to use
depends on the design of the glider, the severity of the
stall, and the proximity of the ground. In some gliders,
a moderate movement of the elevator control—perhaps
slightly forward of neutral—is enough, while in others
a forcible push to the full forward position may be
required. An excessive negative load on the wings
caused by excessive forward movement of the elevator
may impede, rather than hasten, the stall recovery. The
object is to reduce the angle of attack, but only enough
to allow the wing to regain lift. [Figure 7-29]
Buffet
Stall
Initiate
Recovery
Increase
Airspeed
Recovery in
Straight Glide
Figure 7-29. Stall recovery.
progresses toward the roots. Thus, the ailerons can be
used to level the wings.
Using the ailerons requires finesse to avoid an aggravated
stall condition. For example, if the right wing
dropped during the stall and excessive aileron control
was applied to the left to raise the wing, the aileron
that was deflected downward (right wing) would produce
a greater angle of attack (and drag). Possibly, a
more complete stall would occur at the tip because the
critical angle of attack would be exceeded. The
increase in drag created by the high angle of attack on
that wing might cause the airplane to yaw in that direction.
This adverse yaw could result in a spin unless
directional control was maintained by rudder, and/or
the aileron control was sufficiently reduced.
Even though excessive aileron pressure may have
been applied, a spin will not occur if directional (yaw)
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Glider Flying Handbook(75)
