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greater than one G. Like a compressed coil spring seeking
release, the wing spars reflex downward, lofting
the fuselage higher. When the fuselage reaches the top
of this motion, the wing spars are now storing elastic
energy in the downward direction, and the fuselage is
sprung downward in response to the release of elastic
energy in the wing spars. The pilot now experiences
reduced G, accompanied perhaps by a head bang
against the top of the canopy. During these excursions,
the weight of the pilot’s hand and arm on the control
stick may cause the control stick to move a significant
distance forward or aft. With positive G the increased
apparent weight of the pilot’s arm tends to move the
control stick aft, further increasing the angle of attack
of the main wing and increasing the positive G factor
as a result, in a type of vicious circle. During negative
G the reduced apparent weight of the pilot’s arm tends
to result in forward stick motion, reducing the angle of
attack and reducing the G factor still further; once
again, in a sort of vicious circle. In short, the effect of
rapid alternations in load factor is to increase the intensity
of load factor variations. One protection against
this is to slow down when cruising through turbulent
air. Another protection is to brace both arms and use
both hands on the control stick when cruising through
turbulent air at high speed. It is worth noting that some
glider designs incorporate a parallelogram control stick
linkage to reduce the tendency toward PIOs during
high-speed cruise.
PILOT-INDUCED PITCH
OSCILLATIONS DURING LANDING
Instances of PIO may occur during the landing
approach in turbulent air for the same reasons previously
stated. Landing the glider involves interacting
with ground effect during the flare and keeping precise
control of the glider even as airspeed decays and
control authority declines. A pilot can cause a PIO by
over-controlling the elevator during the flare, causing
the glider to balloon well above the landing surface
even as airspeed is decreasing. If the pilot reacts
by pushing the stick well forward, the glider will soon
be diving for the ground with a fairly rapid rate of
descent. If the pilot pulls the control stick back to arrest
this descent while still in possession of considerable
airspeed, the glider balloons again and the PIO cycle
continues. If airspeed is low when the pilot pulls back
on the stick to avoid a hard landing, there is not likely
to be sufficient lift available to arrest the descent. A
hard or a nose-first landing may result.
To reduce ballooning during the flare, stabilize the
glider at an altitude of 3 or 4 feet, and then begin the
flare anew. Do not try to force the nose of the glider
down on to the runway. If airspeed during the ballooning
is slow and the ballooning takes the glider higher
than a normal flare altitude, it may be necessary to
reduce the extension of the spoilers/dive breaks in order
to moderate the descent rate of the glider. Care must be
taken to avoid abrupt changes. Partial retraction of the
spoilers/dive breaks allows the wing to provide a bit
more lift despite decaying airspeed.
Another source of PIOs during the approach to landing
is a too abrupt adjustment of the spoilers/dive breaks
setting. The spoilers/dive breaks on most modern gliders
provide a very large amount of drag when fully
deployed, and they reduce the lift of the wing considerably.
Over-controlling the spoilers/dive breaks during
the approach to land can easily lead to oscillations in
pitch attitude and airspeed. The easiest way to guard
against these oscillations is to make smooth adjustments
in the spoilers/dive breaks setting whenever spoilers/
dive breaks adjustment is necessary. This becomes
particularly important during the landing flare just prior
to touchdown. If the spoilers/dive breaks are extended
further with anything less than a very smooth and sure
hand, the resultant increase in sink rate may cause the
glider to contact the runway suddenly. This can lead to a
rebound into the air, setting the stage for a series of
PIOs. As before, the cure is to stabilize the glider then
resume the flare. If the spoilers/dive breaks are retracted
abruptly during the flare, the glider will likely balloon
into the air. Pilot reaction may result in over-controlling
and PIOs may result. During the flare, it is best to leave
the spoilers/dive breaks extension alone unless the
glider balloons excessively. If you must adjust the spoilers/
dive breaks, do so with smooth, gentle motion.
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Glider Flying Handbook(90)
