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far aft of the approved limits that control inputs are not
sufficient to stop the nose from pitching up. If this were
the case, the glider could enter a spin from which recovery
would be impossible. Loading a glider with the CG
too far forward also is hazardous. In extreme cases, the
glider may not have enough pitch control to hold the
nose up during an approach to a landing. For these reasons,
it is important to ensure that your glider is within
weight and balance limits prior to each flight. Proper
loading of a glider and the importance of CG will be discussed
further in Chapter 5–Performance Limitations.
FLUTTER
Another factor that can affect the ability to control the
glider is flutter. Flutter occurs when rapid vibrations
are induced through the control surfaces while the
glider is traveling at high speeds. Looseness in the
control surfaces can result in flutter while flying near
maximum speed. Another factor that can reduce the
airspeed at which flutter can occur is a disturbance to
the balance of the control surfaces. If vibrations are
felt in the control surfaces, reduce the airspeed.
LATERAL STABILITY
Another type of stability that describes the glider’s
tendency to return to wings-level flight following a
displacement is lateral stability. When a glider is rolled
into a bank, it has a tendency to sideslip in the direction
of the bank. In order to obtain lateral stability,
dihedral is designed into the wings. Dihedral increases
the stabilizing effects of the wings by increasing the
lift differential between the high and low wing during
a sideslip. A roll to the left would tend to slip the glider
to the left, but since the glider’s wings are designed
with dihedral, an opposite moment helps to level the
wings and stop the slip. [Figure 3-19]
DIRECTIONAL STABILITY
Directional stability is the glider’s tendency to remain
stationary about the vertical or yaw axis. When the
relative wind is parallel to the longitudinal axis, the
glider is in equilibrium. If some force yaws the glider
and produces a slip, a glider with directional stability
develops a positive yawing moment and returns to
equilibrium. In order to accomplish this stability, the
Figure 3-19. Dihedral is designed into the wings to
increase the glider’s lateral stability.
Figure 3-18. The horizontal stabilizer is mounted at a slightly negative angle of attack to offset the glider’s
natural tendency to enter a dive.
3-11
vertical tail and the side surfaces of the rear fuselage
must counterbalance the side surface area ahead of the
center of gravity. The vertical stabilizer is the primary
contributor to directional stability and causes a glider
in flight to act much like a weather vane. The nose of
the glider corresponds to a weather vane’s arrowhead,
while the vertical stabilizers on the glider act like the
tail of the weathervane. When the glider enters a
sideslip, the greater surface area behind the CG helps
the glider realign with the relative wind. [Figure 3-20]
TURNING FLIGHT
Before a glider turns, it must first overcome inertia, or
its tendency to continue in a straight line. You create
the necessary turning force by using the ailerons to
bank the glider so that the direction of total lift is
inclined. This is accomplished by dividing the force of
lift into two components; one component acts vertically
to oppose weight, while the other acts horizontally
to oppose centrifical force. The latter is the
horizontal component of lift.
To maintain your attitude with the horizon during a
turn, you need to increase backpressure on the control
stick. The horizontal component of lift creates a force
directed inward toward the center of rotation, which is
known as centripetal force. This center-seeking force
causes the glider to turn. Since centripetal force works
against the tendency of the aircraft to continue in a
straight line, inertia tends to oppose centripetal force
toward the outside of the turn. This opposing force is
known as centrifugal force. In reality, centrifugal
force is not a true aerodynamic force; it is an apparent
force that results from the effect of inertia during the
turn.
If you attempt to improve turn performance by increasing
angle of bank while maintaining airspeed, you
must pay close attention to glider limitations due to the
effects of increasing the load factor. Load factor is
defined as the ratio of the load supported by the
glider’s wings to the actual weight of the aircraft and
its contents. A glider in stabilized, wings level flight
　
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