The Art of the Helicopter John Watkinson - Karatunov.net

Engines and transmissions 201
the valve train. When the camshaft opens the valve, the force is transmitted through
the oil pressure. The drilling is too small to allow a significant amount of oil to be
forced back out of the tappet. Hydraulic tappets are self-adjusting and the periodic
maintenance is eliminated.
6.7 The ignition system
Ignition of the charge is achieved by the generation of a spark between a pair of electrodes
in the cylinder head. A very high voltage is needed to break down the insulation
of the compressed charge so that a spark can occur. The high voltages present in ignition
systems cause stress to the components, and as a result the ignition system is probably
the least reliable part of the engine. This is overcome in aircraft engines by having a
completely duplicated ignition system. There are even two spark plugs per cylinder,
one for each ignition system. This also allows a small increase in power because the
charge is ignited in two places at once.
In cars, the high ignition voltage is generated by an induction coil driven by the
electrical system. If the electrical supply is cut off the engine stops. This is not reliable
enough for aircraft, and these use an alternative system that generates its own electricity
directly from engine shaft power using a rotating magnet. This magneto-dynamic
ignition unit is invariably known by the abbreviation of magneto. In a magneto system
the engine will keep running even if the electrical system of the helicopter has totally
failed. A pair of magnetos are fitted on the back of an aircraft engine, driven by the
gear cluster that drives the camshaft and the oil and fuel pumps. When the aircraft
engine is mounted backwards in a helicopter, the magnetos can be found adjacent to
the cockpit firewall.
The principle of operation of the magneto is based on the same physics as electricity
generation. When current flows through a coil of wire, it generates a magnetic field. The
field can be amplified by winding the coil around a soft iron core. If the magnetic field
passing through a coil is constant, there is no effect, but if the field changes, a voltage
proportional to the rate of change of flux is induced. The two phenomena described
above combine to give a coil the property of inductance. An inductor is a sort of magnetic
flywheel. A mechanical flywheel stores kinetic energy proportional to the square
of the speed. An inductor stores magnetic energy proportional to the square of the
current. In the same way that a flywheel resists changes of speed, an inductor resists
changes of current.
Imagine an inductor with a current flowing through it. If some external resistance
attempts to reduce the current, the magnetic flux in the coil is reduced. The falling flux
induces a voltage in the coil that attempts to increase the current. On the other hand
if something attempts to increase the current, the magnetic flux will increase, and the
change of flux in the opposite direction will induce a voltage that opposes the current.
As a result the inductor tries to keep the current passing through constant.
Figure 6.7 shows the construction of a magneto. The input shaft turns at camshaft
speed and carries a powerful permanent magnet. As the magnet turns its magnetic field
passes through stationary poles in alternate directions. Around one of the poles is a
winding of thick copper wire. The ends of this coil can be shorted out by a pair of
contacts, known as points, operated by a cam on the input shaft. A capacitor is fitted
across the points that are closed when the magnet flux in the stationary pole is changing
from one direction to the other. The changing flux induces a voltage in the coil, but the
coil is shorted out so there is very little resistance. The coil builds up a high current at
a rate determined by the inductance.