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When on an east or west heading, no error is apparent
while entering a turn to north or south. However, an
increase in airspeed or acceleration will cause the compass
to indicate a turn toward north; a decrease in airspeed
or acceleration will cause the compass to indicate
a turn toward south. On a north or south heading, no
error will be apparent because of acceleration or deceleration.
[Figure 4-18]
Figure 4-17. Compass correction card.
Figure 4-18. Acceleration error on a compass in the Northern Hemisphere.
4-14
TURNING ERROR
The turning error is directly related to magnetic dip; the
greater the dip, the greater the turning error. It is most
pronounced when you are turning to or from headings of
north or south. When you begin a turn from a heading of
north, the compass starts to turn in the opposite direction
and it lags behind the actual heading. As the turn continues,
the amount of lag decreases, then disappears, as the
glider reaches a heading of east or west.
When initiating a turn from a heading of east or west to
a heading of north, there is no error as you begin the
turn. As the heading approaches north, the compass
increasingly lags behind the glider’s actual heading.
When you turn from a heading of south, the compass initially
indicates a turn in the proper direction but at a
faster rate, and leads the glider’s actual heading. This
error also cancels out as the glider reaches a heading of
east or west. Turning from east or west to a heading of
south causes the compass to move correctly at the start
of a turn, but then it increasingly leads the actual heading
as the glider nears a southerly direction. [Figure 4-19]
The amount of lead or lag is approximately equal to the
latitude of the glider. For example, if you are turning
from a heading of south to a heading of west while flying
at 35º north latitude, the compass will rapidly turn
to a heading of 215º (180º+35º). At the midpoint of the
turn, the lead will decrease to approximately half
(17.5º), and upon reaching a heading of west, it will be
zero. The lead and lag errors discussed here are only
valid in the Northern Hemisphere. Lead and lag errors
in the Southern Hemisphere act in opposite directions.
YAW STRING
The most effective, yet least expensive, slip/skid
indicator is made from a piece of yarn mounted in the
free airstream in a place easily visible to the pilot as
shown in Figure 4-20. The yaw string helps you coor-
Figure 4-20. The yaw string and Inclinometer.
4-15
dinate rudder and aileron inputs. When the controls are
properly coordinated, the yarn points straight back,
aligned with the longitudinal axis of the glider. During
a slipping turn, the tail of the yaw string will be offset
toward the outside of the turn. To center the yaw string
in a slipping turn, add pressure to the rudder pedal that
is opposite the tail of the yaw string. During a skidding
turn, the tail of the yaw string will be offset toward the
inside of the turn. To center the yaw string in a skidding
turn, add pressure to the rudder pedal that is opposite
the tail of the yaw string.
INCLINOMETER
Another type of slip/skid indicator is the inclinometer.
Mounted in the bottom of a turn-and-bank indicator or
mounted separately in the instrument panel, the inclinometer
consists of a metal ball in an oil-filled, curved
glass tube. When the glider is flying in coordinated fashion,
the ball remains centered at the bottom of the glass
tube. The inclinometer differs from the yaw string during
uncoordinated flight. The ball moves to the inside of
the turn to indicate a slip and to the outside of the turn to
indicate a skid. If you remember the phrase "step on the
ball" in reference to the inclinometer, it will help you
coordinate the turn using rudder inputs.
GYROSCOPIC INSTRUMENTS
Gyroscopic instruments are found in virtually all modern
airplanes but are infrequently found in gliders.
This section is designed to provide you with a basic
understanding of how gyroscopic instruments function.
The three gyroscopic instruments found most frequently
in a glider are the heading indicator, the attitude
indicator, and the turn coordinator.
RIGIDITY IN SPACE
Rigidity in space and precession are the two fundamental
concepts that affect the operation of gyroscopic
instruments. Rigidity in space refers to the principle
that a gyroscope remains in a fixed position in the plane
in which it is spinning. By mounting this wheel, or
Figure 4-21. Regardless of the position of its base, a
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Glider Flying Handbook(41)
