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glider to yaw the nose to the left or right. Yaw is movement
that takes place around the vertical axis, which
can be represented by an imaginary straight line drawn
vertically through the CG. When you move the ailerons
left or right to bank, you are moving the glider around
the longitudinal axis. This axis would appear if a line
were drawn through the center of the fuselage from
nose to tail. When you pull the stick back or push it
forward, raising or lowering the nose, you are controlling
the pitch of the glider or its movement around the
lateral axis. The lateral axis could be seen if a line were
drawn from one side of the fuselage to the other
through the center of gravity. [Figure 3-15]
STABILITY
A glider is in equilibrium when all of its forces are in
balance. Stability is defined as the glider’s ability to
maintain a uniform flight condition and return to that
condition after being disturbed. Often during flight,
gliders encounter equilibrium-changing pitch disturbances.
These can occur in the form of vertical gusts, a
sudden shift in CG, or deflection of the controls by the
pilot. For example, a stable glider would display a tendency
to return to equilibrium after encountering a
force that causes the nose to pitch up.
Static and dynamic are two types of stability a glider
displays in flight. Static stability is the initial tendency
to return to a state of equilibrium when disturbed
from that state. Three types of static stability
are positive, negative, and neutral. When a glider
demonstrates positive static stability it tends to return
to equilibrium. A glider demonstrating negative static
stability displays a tendency to increase its displacement.
Gliders that demonstrate neutral static stability
Figure 3-15. The elevator controls pitch movement about the lateral axis, the ailerons control roll movement
about the longitudinal axis, and the rudder controls yaw movement about the vertical axis.
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have neither the tendency to return to equilibrium nor
the tendency to continue displacement. [Figure 3-16]
Dynamic stability describes a glider’s motion and time
required for a response to static stability. In other
words, dynamic stability describes the manner in which
a glider oscillates when responding to static stability. A
glider that displays positive dynamic and static stability
will reduce its oscillations with time. A glider demonstrating
negative dynamic stability is the opposite situation
where its oscillations increase in amplitude with
time following a displacement. A glider displaying neutral
dynamic stability experiences oscillations, which
remain at the same amplitude without increasing or
decreasing over time. Figure 3-17 illustrates the various
types of dynamic stability.
Both static and dynamic stability are particularly
important for pitch control about the lateral axis.
Measurement of stability about this axis is known as
longitudinal stability. Gliders are designed to be
slightly nose-heavy in order to improve their longitudinal
stability. This causes the glider to tend to nose
down during normal flight. The horizontal stabilizer
on the tail is mounted at a slightly negative angle of
attack to offset this tendency. When a dynamically
Figure 3-16. The three types of static stability are positive, negative, and neutral.
Figure 3-17. The three types of dynamic stability also are referred to as neutral, positive, and negative.
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stable glider oscillates, the amplitude of the oscillations
should reduce through each cycle and eventually settle
down to a speed at which the downward force on the
tail exactly offsets the tendency to dive. [Figure 3-18]
Adjusting the trim assists you in maintaining a desired
pitch attitude. A glider with positive static and dynamic
longitudinal stability tends to return to the trimmed
pitch attitude when the force that displaced it
is removed. If a glider displays negative stability,
oscillations will increase over time. If uncorrected,
negative stability can induce loads exceeding the
design limitations of the glider.
Another factor that is critical to the longitudinal stability
of a glider is its loading in relation to the center of gravity.
The center of gravity of the glider is the point where
the total force of gravity is considered to act. When the
glider is improperly loaded so it exceeds the aft CG limit
it loses longitudinal stability. As airspeed decreases, the
nose of a glider rises. To recover, control inputs must be
applied to force the nose down to return to a level flight
attitude. It is possible that the glider could be loaded so
　
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