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and/or direction changes. When
convergence occurs in lower levels,
it is usually associated with
upward air motions.
CONVERGENCE ZONE—An
area of convergence, sometimes
several miles wide, at other times
sure remain constant, in order to
attain saturation with respect to
water.
DIHEDRAL—The angle at which
the wings are slanted upward from
the root to the tip.
DIURNAL EFFECTS—A daily
variation (may be in temperature,
moisture, wind, cloud cover, etc.)
especially pertaining to a cycle
completed within a 24-hour period,
and which recurs every 24 hours.
DOLPHIN FLIGHT—Straight
flight following speed-to-fly theory.
Glides can often be extended and
average cross-country speeds
increased by flying faster in sink and
slower in lift without stopping to circle.
DOWNBURST—A strong, concentrated
downdraft, often associated
with a thunderstorm. When
these reach the ground, they
spread out, leading to strong and
even damaging surface winds.
DRAG—The force that resists the
movement of the glider through
the air.
DRY ADIABAT—A line on a thermodynamic
chart representing a
rate of temperature change at the
dry adiabatic lapse rate.
DRY ADIABATIC LAPSE RATE
(DALR)—The rate of decrease of
temperature with height of unsaturated
air lifted adiabatically (not
heat exchange). Numerically the
value is 3°C or 5.4°F per 1,000
feet.
DUST DEVIL—A small vigorous
circulation that can pick up dust or
other debris near the surface to
form a column hundreds or even
thousands of feet deep. At the
ground, winds can be strong
enough to flip an unattended glider
over on its back. Dust devils mark
the location where a thermal is
leaving the ground.
very narrow. These zones often
provide organized lift for many
miles along the convergence
zone, for instance, a sea-breeze
front.
CRITICAL ANGLE OF
ATTACK—Angle of attack, typically
around 18°, beyond which a
stall occurs. The critical angle of
attack can be exceeded at any airspeed
and at any nose attitude.
CROSS COUNTRY—In soaring,
any flight out of gliding range of the
take-off airfield. Note that this is
different than the definitions in the
CFRs for meeting the experience
requirements for various pilot certificates
and/or ratings.
CUMULUS CONGESTUS—A
cumulus cloud of significant vertical
extent and usually displaying
sharp edges. In warm climates,
these sometimes produce precipitation.
Also called towering cumulus,
these clouds indicate that
thunderstorm activity may soon
occur.
CUMULONIMBUS (CB)—Also
called thunderclouds, these are
deep convective clouds with a cirrus
anvil and may contain any of
the characteristics of a thunderstorm:
thunder, lightning, heavy
rain, hail, strong winds, turbulence,
and even tornadoes.
DEAD RECKONING—Navigation
by computing a heading from
true airspeed and wind, then estimating
time needed to fly to a destination.
DENSITY ALTITUDE—Pressure
altitude corrected for nonstandard
temperature variations.
Performance charts for many older
gliders are based on this value.
DEWPOINT (OR DEWPOINT
TEMPERATURE)—The temperature
to which a sample of air must
be cooled, while the amount of
water vapor and barometric presG-
3
DYNAMIC STABILITY—A
glider’s motion and time required
for a response to static stability.
ELEVATOR—Attached to the
back of the horizontal stabilizer,
the elevator controls movement
around the lateral axis.
EMPENNAGE—The tail group of
the aircraft usually supporting the
vertical stabilizer and rudder, as
well as the horizontal stabilizer
and elevator, or on some aircraft,
the V-Tail.
FLAPS—Hinged portion of the
trailing edge between the ailerons
and fuselage. In some gliders
ailerons and flaps are interconnected
to produce full-span “flaperons.”
In either case, flaps
change the lift and drag on the
wing.
FLUTTER—Resonant condition
leading to rapid, unstable oscillations
of part of the glider structure
(e.g., the wing) or a control surface
(e.g., elevator or aileron). Flutter
usually occurs at high speeds and
can quickly lead to structural failure.
FORWARD SLIP—A slide used
to dissipate altitude without increasing
　
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