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Handbook of Aeronautical Knowledge for basic propeller
principles.
4-9
Full
Idle
100%
Power
Needle Jet
Main Jet
Idle Jet
50%
Figure 4-10. Throttle position and jetting system used.
Pilot (or idle) Jet
Idle Air
Pilot Air Screw Adjustment
Air Bleed Holes
Pilot Hole
Throttle Valve
Bypass Hole
Figure 4-11. Pilot or idle jet system.
Ground Adjustable-Pitch Propeller
Adjustable-pitch propellers for WSC aircraft can be adjusted
only on the ground with hand tools. If an engine is overrevving,
more pitch can be added to the propeller. If the
engine is not developing the full recommended rpm during
fl ight, then some pitch can be taken out of the blades. This
should be done according to the WSC aircraft’s POH and by
a qualifi ed technician.
Induction Systems
The induction system brings air in from the atmosphere,
mixes it with fuel, and delivers the fuel/air mixture (fuel/oil/
air mixture for two stroke engines) to the engine intake and
to the cylinders where combustion occurs. Outside air enters
the induction system through an air fi lter on the engine. The
air fi lter inhibits the entry of dust and other foreign objects.
Two types of induction systems are used in WSC engines:
1. The carburetor system is most common. It mixes the
fuel and air in the carburetor before this mixture enters
the engine intake.
2. The fuel injection system injects the fuel into the air
just before entry into each cylinder.
Carburetor Systems
WSC aircraft use fl oat-type carburetors. The “fl oat-type
carburetor” acquires its name from a fl oat that rests on fuel
within the carburetor fl oat chamber, commonly known as
the fuel bowls. The fl oat maintains the fuel level in the
fuel bowls. As fuel is used by the engine, the fuel and fl oat
levels drop, opening the valve letting more fuel into the
fuel bowls until the proper level of fuel in the fuel bowls
is achieved and the valve is closed. Reference the Pilot’s
Handbook of Aeronautical Knowledge for basic information
on fl oat carburetor operation. Modern two- and four-stroke
carburetors operate with three separate jetting systems
depending on engine power. [Figure 4-10]
When the throttle is closed for engine idling, the throttle
valve is closed and the fuel/air mixture is supplied through
the idle (pilot) jet and idle (pilot) air passage. The fuel/air
mixture is supplied to the cylinders through the bypass hole.
[Figure 4-11]
As the throttle is advanced and the throttle valve is raised,
the fuel is sucked up through the main jet but is controlled
by the opening and taper of the jet needle and needle jet.
This is effective throughout most of the midrange operation.
About half throttle, the main jet size starts to infl uence the
amount of fuel mixed with the air and this effect continues
until it is the main infl uence at the highest throttle settings.
[Figures 4-10 and 4-12]
4-10
Figure 4-12. Jet needle/needle jet and main jet system.
Main Jet
Air Inlet
Needle Jet
Jet Needle
Throttle Slide
Cutaway
Figure 4-13. Typical two-stroke carburetor.
This condition may occur at high elevation airports and
during climbs or cruise fl ight at high altitudes. To maintain
the correct fuel/oil/air mixture, the main jets are usually
changed for smaller jets based on the density altitude of
the base airport. Operating from low altitude airports and
climbing to altitude where the mixture becomes rich for short
periods is acceptable.
Operating an aircraft at a lower altitude airport with the jets
set for higher altitudes will create too lean of a mixture, heat
up the engine, and cause the engine to seize. The pilot must
be aware of the jetting for the machine to adjust the mixture.
Consult your POH for specifi c procedures for setting jets at
different density altitudes.
Four-Stroke Mixture Settings
Four-stroke engines typically have automatic mixture control
for higher altitudes or a mixture control that can be operated
by the pilot.
Carburetor Icing
One disadvantage of the carburetor system versus the fuel
injected system is its icing tendency. Carburetor ice occurs
due to the effect of fuel vaporization and the decrease in air
pressure in the venturi, which causes a sharp temperature
drop in the carburetor. If water vapor in the air condenses
when the carburetor temperature is at or below freezing, ice
may form on internal surfaces of the carburetor, including
the throttle valve.
Ice generally forms in the vicinity of the venturi throat. This
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Weight-Shift Control Aircraft Flying Handbook(43)
