The Art of the Helicopter John Watkinson - Karatunov.net

204 The Art of the Helicopter
To take an example, if on switching from ‘both’ to ‘left’ there is no drop, either the
right magneto was not working or it has not been switched off due to a broken wire.
Switching to ‘right’ will show which. If the engine cuts, the right magneto is inoperative.
If a drop is obtained, the right magneto is permanently on due to a wiring fault. This
can be proved by switching off the ignition. If the engine continues to run, the wire to
the ignition switch is broken. The engine can be stopped by the idle cut-off as normal.
If the drops are not identical, one ignition system is working better than the other for
some reason. If only an RPM loss is noted, the different drop could be due to a timing
difference between the magnetos. If one magneto gives a larger drop and a misfire,
consideration should be given to the possibility of a failed spark plug or capacitor or
a fractured HT lead.
The cylinders are numbered from what would be the front in an aircraft, which in a
helicopter corresponds to being numbered away from the drive pulley. In a Lycoming
engine mounted back to front in a helicopter, the odd numbered cylinders are on the
port side. The sequence of power strokes is designed to reduce torsional stress in the
crankshaft by firing cylinder pairs alternately. This gives the firing order of 1-3-2-4.
6.8 The starter
The starter is basically a powerful electric motor used to turn the engine. A reduction
gearing system is needed so that the starter can develop enough torque to overcome
the engine compression. A small pinion on the starter engages with a large ring gear
on the engine flywheel. The gears are only engaged during starting. This can be done
by a helical thread mechanism that screws the pinion along the motor shaft or by a
solenoid that slides it. In the latter type a one-way clutch prevents the engine driving
the starter. Recently geared starter motors have become available. These have a high
speed electric motor and a reduction gearbox and are lighter than conventional starters
and consume less current. The weight saving may be compounded because they allow
a smaller battery and thinner wiring to be used.
The motor must be quite powerful, and it will require a current of several hundred
amperes to operate it. In order to minimize power loss in the cables, these run direct
from the battery to the motor. This current is beyond the capability of a switch, and a
relay is used. The starter button on the collective lever switches a small current through
a solenoid coil, and this attracts the solenoid armature, sliding the starter gear into
engagement with the flywheel and pulling the main motor contacts closed. Normally
the starter motor gear is driven through a one-way clutch so that it can run at high
speed when the engine fires without damaging the starter motor. A warning light is
fitted which should go out if the starter button is released. If it does not the starter may
still be engaged.
If the starter fails to operate, this could indicate a faulty solenoid. It is generally
possible to hear the solenoid operating. If the solenoid chatters or vibrates this indicates
a loose battery connection or a discharged battery.
The starter motor dissipates a lot of heat when starting, and if the engine does not
start promptly because it is flooded or vapour locked, prolonged cranking will flatten
the battery and overheat the starter motor. It is better to allow a cooling period between
starting attempts than to burn out the starter motor. The starter motor should never
be operated when the engine is running as engaging the starter pinion with the fast
turning gear on the flywheel could remove teeth. The engine must be allowed to come
to a complete halt before making a further starting attempt.