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on the rate of change of static pressure. When flying through
a rising mass of air (like a thermal), the needle on the variom-
eter will jump (and usually beep to notify the pilot) before any
change on the altimeter is even noticeable.
Detecting Yaw
The glider is yawing when it is not pointing exactly in the
direction it is flying (relative to the air around it). Instead the
glider is angled sideways and is “slipping” or “skidding”
through the air. The string on the windshield indicates whether
the glider is flying straight (string straight) or whether it is
yawing (string left or right). The glider produces the least drag
when it flies straight through the air. When it is yawing, the
drag increases -- so in general, glider pilots try to keep the
string straight.
Why gliders carry ballast
A plane’s lift, drag and glide ratio characteristics are governed
solely by its construction, and are predetermined at takeoff.
Without thrust, the only other characteristic that the pilot has
control over (besides normal control surfaces) is the weight of
the plane.
A heavier glider will sink faster that a light glider. The glide
ratio is not affected by weight because while a heavier glider
will sink faster, it will do so at a higher airspeed. The plane
will come down faster, but will cover the same distance (at a
higher speed) as a lighter glider with the same glide ratio and
starting altitude. In order to help them fly faster, some gliders
have tanks that can hold up to 500 pounds of water. Higher
speeds are desirable for cross-country flying.
The downsides of heavier sailplanes include
reduced climb rates in a lifting environ-
ment (such as a thermal)
and, possibly, shorter flight duration if suitable lift cannot be
located. To prevent this, the water ballast can be jettisoned at
any time through dump valves, allowing the pilots to reduce
the weight of the plane to increase climb rates, or to reduce
speed as they come in for a landing.
Landing
Landing a glider is much like landing a conventional plane,
except there is usually a single small wheel located directly
under the pilot. The wings on gliders are very strong, and the
tips are reinforced to prevent damage in case they scrape
along the ground during a landing. Even so, pilots can usually
manage to keep both wing tips off the ground until the plane
has slowed sufficiently (kind of like riding a fast bike down the
runway). Glider tails typically have a tiny wheel that prevents
the tail from scraping while on the ground.
When landing the glider, the pilot needs to be able to control
the glide path (the rate of descent relative to distance traveled)
in order to bring the glider down in the right location. The pilot
has to be able to reduce the amount of lift produced by the
wings without changing the speed or attitude of the glider. He
does this by deploying spoilers on each wing. The spoilers
disrupt the airflow over the wing, drastically reducing the lift it
produces and
also increasing the drag.