The Art of the Helicopter John Watkinson - Karatunov.net

210 The Art of the Helicopter
When the engine is at idle, the throttle will nearly be closed. The minimum throttle
opening is set by throttle stop screw that prevents the engine inadvertently being
stopped. With the throttle closed the airflow through the venturi is very small and
insufficient to draw fuel. As the throttle disc reaches the closed position it admits low
manifold pressure to a drilling in the side of the tract. This draws fuel from the float
chamber through the idle jet (5), and air from the idle mixture screw (6). In effect a
miniature carburettor is created to handle the small charge flow at idle.
As altitude increases, the air density falls but the fuel density doesn’t. The carburettor
tends to produce a richer mixture than necessary. This is overcome by fitting a
mixture control. There are two ways of controlling the mixture. In the first, shown in
Figure 6.9(a), a needle valve (8) is placed in series with the main jet and changes the
resistance to fuel flow. Placing the mixture control to idle cut-off stops the fuel flow
altogether and stops the engine. In the second method, shown in Figure 6.9(b), low
pressure from the inlet manifold is applied to the float chamber by a valve (9) to oppose
the venturi suction and reduce the fuel flow. Idle cut-off is achieved by allowing full
manifold pressure into the float chamber. There is no resultant pressure across the fuel
and flow ceases.
The carburettor is a simple device but it does suffer from one major problem. The
evaporation of fuel requires heat, and in the absence of any other source the heat is taken
from the incoming air and the body of the carburettor. In humid conditions this can
lead to the formation of ice in the carburettor venturi. The rapid flow around the edge of
a nearly closed throttle and subsequent expansion into the low pressure in the manifold
duplicates the mechanism of the domestic refrigerator and can cause throttle icing. This
is particularly dangerous because at low power on a descent the ice build-up will not
be noticed. When full power is required to arrest the descent it will not be available.
The engine will artificially be throttled by ice and the mixture will be unpredictable.
The traditional approach to icing is to supply hot air to the carburettor intake. This
is conveniently obtained by drawing air over the exhaust system. Unfortunately hot air
is less dense and reduces engine power so it cannot be used all the time. The pilot is
therefore provided with a control to select the carburettor heat. If there is the slightest
suspicion of icing, full heat should be applied. If engine smoothness and power improve,
there was icing. It may be necessary to apply heat periodically or continuously to control
icing. If power reduces, there was no ice and the engine is running on air that is too hot.
Heat control requires a disciplined approach because the effects of an incorrect decision
may not be evident until it is too late. Heat should be used for descents, but the
prolonged use of heat on the ground is to be avoided if the hot air is not filtered. Some
machines, such as the Robinson R-22, have a carburettor temperature gauge which
is useful for deciding if heat is necessary. Air temperature has less effect on the probability
of icing than does humidity. Carburettor heat has another problem which is
that it affects interpretation of the manifold pressure gauge. This will be considered in
section 6.14.
In comparison with modern automotive practice, in which electronic fuel injection
is used almost exclusively, the aviation carburettor is a museum piece.
6.11 Fuel injection
In a fuel injected engine there is no carburettor. The fuel is sprayed into the inlet
manifold just before the inlet valve by a fuel nozzle. There is one nozzle per cylinder.
The use of fuel injection has a number of advantages. Fuel vaporizing takes place in