The Art of the Helicopter John Watkinson - Karatunov.net

208 The Art of the Helicopter
away from the spark plug. Unfortunately when charge is compressed the temperature
will be raised and it can detonate instantly when ignited. This causes a rapid pressure
step that creates shock waves travelling through the engine. A metallic ringing noise
called ‘pinking’ or knock can be heard. If sustained, engine damage will result.
The ability of a fuel to resist detonation must be matched to the compression ratio of
the engine it will be used in. The detonation resistance of fuel is measured in a special
variable compression test engine. The fuel under test is used to drive the engine and
the compression is increased until detonation commences. The engine is then run on
iso-octane, a relatively detonation resistant hydrocarbon, and this is steadily diluted
with heptane, a detonation prone fuel, until detonation commences. The percentage
of octane at which this takes place is the octane number of the fuel. Since the test was
devised, more detonation resistant fuels have been developed, and the scale has been
extended above 100 by extrapolation.
Using fuel of inadequate octane number for the engine causes damage through detonation.
The machine carries mandatory placards adjacent to the fuel fillers stating the
octane number of the fuel that must be used.
A rich mixture helps to prevent detonation by reducing the temperature increase
during compression. Fuels are often specified by two octane numbers; the second is
for a rich mixture. For example, many piston helicopters burn 100/130 LL fuel. At
normal mixtures this is 100 octane, but at full rich mixture it is 130 octane. LL stands
for low lead. In the 1920s it was discovered that the addition of tetra-ethyl lead to
fuel increased the octane number. Decades later it was proved that the resultant lead
pollution was slowing the rate of brain development of children living near busy roads
and less damaging alternatives have had to be found.
Whilst fuel of inadequate octane rating causes damage, occasional use of higher
octane fuel is acceptable. Prolonged use of high octane fuel may result in spark plugs
being fouled by the octane-boosting additives. No power increase will be observed
because the power output is determined by the engine design, particularly the compression
ratio.
The carburettor has to mix the air and fuel in the chosen proportion. Figure 6.9 shows
that it is a fairly simple device. Engine power is controlled by throttle (3): a disc that
pivots in the intake tract. With the throttle wide open, the pressure in the inlet manifold
will be nearly atmospheric, whereas with the throttle closed down to the idle position,
the manifold pressure will be near zero. Manifold pressure is displayed on a gauge in the
cockpit as it indicates the proportion of maximum power the engine is producing. The
pilot needs to know how much reserve power is necessary before committing himself
to a manoeuvre needing higher power: a procedure called a power check.
The result of throttling the engine is to reduce the flow of air through the carburettor.
The fuel flow must remain proportional to the mass flow. This is done by the creation
of a small constriction or venturi in the inlet tract. The air must go faster through the
smaller cross-section. This increases the dynamic pressure of the air. As the static and
dynamic pressures sum to a constant, the static pressure must reduce, as described by
Bernoulli’s theory. The reduction in static pressure causes a pressure difference across
the fuel in the carburettor body making it flow through a metering orifice or jet (2) and
mix with the air. Clearly the depth of fuel could affect the pressure and the carburettor
has a float valve (1) that admits fuel as it is used to prevent the level falling.
The system is self-balancing, because greater airflow causes greater suction at the
venturi and increases the fuel flow. If the throttle is opened suddenly, the fuel inertia
prevents a rapid increase in fuel delivery and the mixture will weaken momentarily.
Weakening is prevented by the action of a small pump (4) that produces a jet of fuel
when the throttle is opened quickly.