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glider from ballooning high above the runway, the
pilot may exert considerable forward pressure on the
control stick. As the glider continues to accelerate,
this forward pressure on the control stick exerts a
rapidly increasing nose-down force on the glider
due to the increasing airflow over the elevator.
When the glider eventually pitches down, the pilot
may exert considerable back pressure on the stick to
arrest the descent. PIOs are likely to result. If
allowed to continue, a hard landing may result, with
potential for glider damage and personal injury.
If the wing flaps are incorrectly set to a negative flap
setting, decreasing wing camber and wing lift, then
takeoff may be delayed so long that the towplane will
lift off and begin to climb out while the glider is still
rolling down the runway, unable to get airborne.
Powerful back pressure on the stick may eventually
assist the glider in leaving the runway, but the relatively
high airspeed at liftoff translates into a very effective
elevator, and ballooning may occur as a result of the
elevator position. The pilot, startled once again by the
magnitude of this pitch excursion, tries to correct by
Figure 8-1. Premature takeoffs and PIOs.
8-3
applying considerable forward pressure on the control
stick. A series of PIOs may result. If the PIOs continue,
a hard landing may occur.
PILOT-INDUCED ROLL OSCILLATIONS
DURING LAUNCH
Pilot-induced roll oscillations occur primarily during
launch, particularly via aerotow. As the towpilot
applies full throttle, the glider moves forward, balanced
laterally on its main wheel. If a wingtip begins
to drop toward the ground before the glider achieves
significant speed, aileron control is marginal and considerable
stick displacement must be applied to elicit a
response from the glider. As the glider accelerates, the
control response improves and the latency of response
from the glider shortens. As acceleration continues, the
pilot must recognize the quickening response of the
glider to avoid over-controlling the glider. [Figure 8-2]
Although roll oscillations can develop during ground
launch operations, they occur less often than during
aerotow operations because excellent aerodynamic
control of the glider is quickly achieved thanks to the
rapid acceleration. Since control improves as acceleration
increases, operations that use a powerful winch
or launch vehicle are less likely to be hampered by
oscillations.
Wing mass also affects roll oscillations. If the wings do
not stay level, the pilot applies considerable aileron
pressure to return the wings to level attitude. Because of
the large mass and considerable aerodynamic damping
that long-winged gliders exhibit, there is a considerable
lag time from the moment pressure is applied until the
moment the wings are level again. Inexperienced pilots
maintain considerable pressure on the ailerons until the
wings are level, then release the pressure. The wings
continue their rolling moment due to their mass, length,
and momentum about the longitudinal axis of the glider.
The pilot senses this momentum too late, and applies
considerable pressure in the opposite direction in
another attempt to level the wings.
After a time, the wings respond and roll back to level,
whereupon the pilot centers the ailerons once again. As
before, the momentum of the wings about the longitudinal
axis is considerable, and the wings continue their
motion in roll. This series of PIOs may continue until
one wingtip contacts the ground, possibly with considerable
force, causing wing damage or a groundloop and
an aborted launch. To reduce the likelihood of this type
of roll oscillation, anticipate the momentum of the
glider wings about the longitudinal axis and reduce
aileron control pressure as the wings approach the level
position.
PILOT-INDUCED YAW OSCILLATIONS
DURING LAUNCH
Pilot-induced yaw oscillations are usually caused by
overcontrolling the rudder. As with roll oscillations, the
problem is the failure of the pilot to recognize that the
glider is accelerating and has considerable momentum.
If the glider is veering away from the towplane, rudder
application in the appropriate direction helps correct
the situation. If the rudder pressure is held too long, the
large yaw momentum of the glider wings and fuselage
results in overshooting the desired yaw position and
veering off in the opposite direction. The alarmed pilot
now applies considerable rudder pressure in the direction
opposite from the original rudder pressure. As the
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Glider Flying Handbook(88)
