The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 7.7 The remote indicator for a flux gate compass works by aligning its rotatable coil at right angles to the
resultant flux generate by the three signals from the compass. At all other angles a voltage will be induced in
the coil and this will be amplified to drive the motor. The coil also carries the direction indicator.
order to cancel out deviation. In this case during swinging of the compass the goal is
to find the correct values for these currents.
It is also possible to remove the variation in the earth’s magnetic field and obtain a
true heading rather than a magnetic heading. This requires a device called a differential
transmitter. This contains two sets of windings at 120 ◦ , but one set can be turned by an
operator control. If the two sets of windings are aligned, the relative amplitudes of the
signals coming out will be the same as those entering, but if the secondary winding is
turned, the output signals are effectively rotated. Thus if the differential transmitter is
set to the amount of local magnetic variation, the remote display will read true heading.
7.6 Pressure instruments
The instruments operated by pressure are the altimeter, the vertical speed indicator
(VSI) and the airspeed indicator (ASI). Of these, the first two measure static pressure
piped to the instruments from the static vents, whereas the ASI measures the dynamic
pressure due to motion sensed as the pressure difference between the pitot head and the
static vents. The necessary pipework is shown in Figure 7.8. The static vents are usually
installed one each side of the fuselage in order to cancel any effect due to asymmetrical
airflow. Each carries a mandatory placard stating that the vent is to be kept clear. The
static pipes initially run upwards from the vents to prevent the entry of water.
The pitot head is installed facing forward at some point where the airflow is reasonably
undisturbed. It may be heated to prevent icing. The pitot should be checked for blockage
as part of the pre-flight checks. It is good practice to cap the pitot with a suitable cover
when the machine is on the ground, in which case the checks will include the removal
and stowage of the cover.
7.7 The altimeter
The altimeter is to all intents and purposes an aneroid barometer. The mechanism is
shown in Figure 7.9. It contains a sealed capsule that is corrugated to allow it to flex.
The capsule is evacuated, hence the term aneroid, and so ambient pressure attempts to
crush it. The stiffness of the corrugations and a separate spring oppose the pressure.
The capsule will expand as ambient pressure falls with rising altitude, and a system of
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