The Art of the Helicopter John Watkinson - Karatunov.net

In most cases the machine will have a gyroscopic direction indicator (see section 7.12)
set to the same heading as the compass during steady flight. The direction indicator is
used for turns because it does not suffer from acceleration errors so it is only necessary
to invoke the theory if the DI fails.
In everyday use there is very little that can go wrong with a compass. It should be
inspected for leaks of the liquid, for bubbles or for some obvious broken or cracked
component, and if the transparent housing is plastic, this may darken as the material
degrades with age and indicates that it is becoming brittle as well as difficult to see
through. The deviation card must be present and the machine must have been swung
on the appropriate occasions.
7.5 The flux gate compass
Figure 7.6(a) shows how a flux gate compass works. There are three radial pole assemblies
and the side view shows that these are split so that a single coil with a vertical axis
can be wound at the centre. There are a further three coils, one on each limb. The pole
pieces are made of a highly permeable material such as permalloy. The entire assembly
is suspended below a Hooke joint so that in steady flight it will hang level. An alternating
excitation signal, typically 400 Hz, is applied to the central coil. In the absence of
any external magnetic field, there would be no signal induced in the three sense coils
because they are wound around both poles and the flux from the excitation coil would
cancel out. However, in the presence of the earth’s magnetic field, the highly permeable
material is more prone to saturation in one direction than the other. Figure 7.6(b)
shows that the earth’s field adds to the excitation and shifts the operating point along
the characteristic curve of the magnetic material. It will be seen in Figure 7.6(b) that
the curvature of the magnetization characteristic has the effect of asymmetrically compressing
the flux waveform so that there is a resultant flux in the sensor coil. The relative
amplitudes of the three alternating signals induced in the sensor coils are a function of
the direction of the earth’s field.
The three signals are sent to any remote instrument needing magnetic heading information.
This could be a simple display or a tied gyroscopic DI. Figure 7.7 shows how
a remote display works. The three signals from the flux gate are supplied to three coils
arranged at 120 ◦ . There is also a second single coil that can rotate with respect to the
three input coils. The resultant of the flux produced by these three coils will have a
direction determined by the direction of the earth’s field at the sensor. As this is an
alternating flux it will induce a voltage in the secondary coil. However, if this coil is
exactly at right angles to the resultant flux, no voltage will be induced.
Figure 7.7 shows that the secondary coil signal is connected to an amplifier that
drives a motor to turn the secondary coil and the display card. The amplifier will drive
the motor until the secondary coil is transverse to the resultant field of the three input
coils. At this point the induced signal will disappear and the motor will stop. Thus as
the magnetic heading of the machine changes, the signals from the flux gate change the
resultant flux direction in the display. The secondary coil is driven until it once more is
in a null. In this way the motor and secondary coil follow the angle of the earth’s field
with respect to the flux gate.
The flux gate compass has no controls and can be mounted anywhere convenient. In
a fixed-wing aircraft the wing tip is a favoured location as it is well away from heavy
ferrous objects. In a helicopter the tail boom could be used. Some flux gate compasses
contain additional coils fed with carefully calibrated currents from a control box in
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