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glider continues to accelerate, the power of the rudder
increases and the lag time decreases. In extreme cases,
the glider may veer off the runway and collide with
Figure 8-2. Pilot-induced roll oscillations during take- runway border markers, airport lights, parked glider, or
off roll.
8-4
other obstacles. The cure for this type of yaw oscillation
is to anticipate the momentum of the glider wings
and fuselage about the vertical axis and reduce rudder
pedal pressure when the nose of the glider begins to
yaw in the desired direction in response to rudder
inputs. [Figure 8-3]
When a wingtip contacts the ground during takeoff
roll, an uncommanded yaw results. The drag of the
wingtip on the ground induces a yaw in the direction
of the grounded wingtip. The yaw usually is mild if
the wingtip is on smooth pavement but much more
vigorous if the wingtip is dragging through tall grass.
If appropriate aileron pressure fails to raise the
wingtip off the ground quickly, the only solution is to
release the towline and abort the takeoff attempt
before losing all control of the glider.
The greater the mass of the wings and the longer the
wingspan, the more momentum the glider will exhibit
whenever roll or yaw oscillations arise. Some very
high performance gliders feature remarkably long and
heavy wings, meaning once in motion, they tend to
remain in motion for a considerable time. This is true
not only of forward momentum, but yaw and roll
momentum as well. The mass of the wings, coupled
with the very long moment arm of large-span wings,
results in substantial lag times in response to aileron
and rudder inputs during the early portion of the takeoff
roll and during the latter portion of the landing rollout.
Even highly proficient glider pilots find takeoffs
and landings in these gliders to be challenging. Many
of these gliders are designed for racing or cross-country
flights and have provisions for adding water ballast
to the wings. Adding ballast increases mass, which
results in an increase in lag time.
If there is an opportunity to fly such a glider, study the
GFM/POH thoroughly prior to flight. It is also a good
idea to seek out instruction from an experienced
pilot/flight instructor in what to expect during takeoff
roll and landing rollout in gliders with long/heavy
wings.
GUST-INDUCED OSCILLATIONS
Gusty headwinds can induce pitch oscillations because
the effectiveness of the elevator varies due to changes
in the speed of the airflow over the elevator.
Crosswinds also can induce yaw and roll oscillations.
A crosswind from the right, for instance, tends to
weathervane the glider into the wind, causing an
uncommanded yaw to the right. Right crosswind also
tends to lift the upwind wing of the glider. When crosswinds
are gusty, these effects vary rapidly as the speed
of the crosswind varies.
Local terrain can have a considerable effect on the
wind. Wind blowing over and around obstacles can be
gusty and chaotic. Nearby obstacles, such as hangars,
groves or lines of trees, hills, and ridges can have a
pronounced effect on low altitude winds, particularly
on the downwind side of the obstruction. In general,
the effect of an upwind obstacle is to induce additional
turbulence and gustiness in the wind. These conditions
are usually found from the surface to an altitude of
three hundred feet or more. If flight in these conditions
cannot be avoided, then the general rule during takeoff
is to achieve a faster than normal speed prior to liftoff.
The additional speed increases the responsiveness of
Figure 8-3. Pilot-induced yaw oscillations during the controls and simplifies the problem of correcting
8-5
for turbulence and gusts. This provides a measure of
protection against PIOs. The additional speed also provides
a safer margin above stall airspeed. This is very
desirable on gusty days because variations in the headwind
component will have a considerable effect on
indicated airspeed.
VERTICAL GUSTS
DURING HIGH-SPEED CRUISE
Although PIOs occur most commonly during launch,
they can occur during cruising flight, even when cruising
at high speed. Turbulence usually plays a role in
this type of PIO, as does the elasticity and flexibility of
the glider structure. An example is an encounter with
an abrupt updraft during wings-level high-speed
cruise. The upward-blowing gust increases the angle
of attack of the main wings, which bend upward very
quickly, storing elastic energy in the wing spars. For a
moment, the G-loading in the cabin is significantly
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Glider Flying Handbook(89)
