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the card rotates toward south.
Oscillation Error
Oscillation is a combination of all of the other errors, and it
results in the compass card swinging back and forth around
the heading being flown. When setting the gyroscopic
heading indicator to agree with the magnetic compass, use
the average indication between the swings.
The Vertical Card Magnetic Compass
The floating magnet type of compass not only has all the
errors just described, but also lends itself to confused reading.
It is easy to begin a turn in the wrong direction because its card
appears backward. East is on what the pilot would expect to be
the west side. The vertical card magnetic compass eliminates
some of the errors and confusion. The dial of this compass
is graduated with letters representing the cardinal directions,
numbers every 30°, and marks every 5°. The dial is rotated by
a set of gears from the shaft-mounted magnet, and the nose
of the symbolic airplane on the instrument glass represents
the lubber line for reading the heading of the aircraft from
the dial. Eddy currents induced into an aluminum-damping
cup damp oscillation of the magnet. [Figure 3-22]
The Flux Gate Compass System
As mentioned earlier, the lines of flux in the Earth’s magnetic
field have two basic characteristics: a magnet aligns with
these lines, and an electrical current is induced, or generated,
in any wire crossed by them.
3-15
Figure 3-22. Vertical Card Magnetic Compass.
Figure 3-23. The soft iron frame of the flux valve accepts the flux
from the Earth’s magnetic field each time the current in the center
coil reverses. This flux causes current to flow in the three pickup
coils.
Figure 3-24. The current in each of the three pickup coils changes
with the heading of the aircraft.
Figure 3-25. Pictorial Navigation Indicator (HSI Top), Slaving
Control and Compensator Unit.
The flux gate compass that drives slaved gyros uses the
characteristic of current induction. The flux valve is a small,
segmented ring, like the one in Figure 3-23, made of soft
iron that readily accepts lines of magnetic flux. An electrical
coil is wound around each of the three legs to accept the
current induced in this ring by the Earth’s magnetic field. A
coil wound around the iron spacer in the center of the frame
has 400-Hz alternating current (A.C.) flowing through it.
During the times when this current reaches its peak, twice
during each cycle, there is so much magnetism produced by
this coil that the frame cannot accept the lines of flux from
the Earth’s field.
But as the current reverses between the peaks, it demagnetizes
the frame so it can accept the flux from the Earth’s field. As
this flux cuts across the windings in the three coils, it causes
current to flow in them. These three coils are connected in
such a way that the current flowing in them changes as the
heading of the aircraft changes. [Figure 3-24]
The three coils are connected to three similar but smaller coils
in a synchro inside the instrument case. The synchro rotates
the dial of a radio magnetic indicator (RMI) or a horizontal
situation indicator (HSI).
Remote Indicating Compass
Remote indicating compasses were developed to compensate
for the errors and limitations of the older type of heading
indicators. The two panel-mounted components of a typical
system are the pictorial navigation indicator and the slaving
control and compensator unit. [Figure 3-25] The pictorial
navigation indicator is commonly referred to as a HSI.
3-16
Figure 3-26. Driven by signals from a flux valve, the compass card
in this RMI indicates the heading of the aircraft opposite the upper
center index mark. The green pointer is driven by the ADF.
The slaving control and compensator unit has a pushbutton
that provides a means of selecting either the “slaved gyro”
or “free gyro” mode. This unit also has a slaving meter
and two manual heading-drive buttons. The slaving meter
indicates the difference between the displayed heading and
the magnetic heading. A right deflection indicates a clockwise
error of the compass card; a left deflection indicates a
counterclockwise error. Whenever the aircraft is in a turn
and the card rotates, the slaving meter shows a full deflection
to one side or the other. When the system is in “free gyro”
mode, the compass card may be adjusted by depressing the
appropriate heading-drive button.
A separate unit, the magnetic slaving transmitter is mounted
remotely; usually in a wingtip to eliminate the possibility of
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