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of the engine until the blades become adjusted to
absorb the increased power. If an emergency
demanding full power should arise during approach,
the sudden advancing of the throttle will cause
momentary overspeeding of the engine beyond the
r.p.m. for which the governor is adjusted. This
temporary increase in engine speed acts as an
emergency power reserve.
Some important points to remember concerning
constant-speed propeller operation are:
• The red line on the tachometer not only indicates
maximum allowable r.p.m.; it also indicates
the r.p.m. required to obtain the engine’s
rated horsepower.
• Amomentary propeller overspeed may occur
when the throttle is advanced rapidly for takeoff.
This is usually not serious if the rated r.p.m. is
not exceeded by 10 percent for more than
3 seconds.
• The green arc on the tachometer indicates the
normal operating range. When developing
Ch 11.qxd 5/7/04 8:50 AM Page 11-6
11-7
power in this range, the engine drives the propeller.
Below the green arc, however, it is usually
the windmilling propeller that powers the
engine. Prolonged operation below the green arc
can be detrimental to the engine.
• On takeoffs from low elevation airports, the
manifold pressure in inches of mercury may
exceed the r.p.m. This is normal in most cases.
The pilot should consult the AFM/POH
for limitations.
• All power changes should be made smoothly
and slowly to avoid overboosting and/or
overspeeding.
TURBOCHARGING
The turbocharged engine allows the pilot to maintain
sufficient cruise power at high altitudes where there is
less drag, which means faster true airspeeds and
increased range with fuel economy. At the same time,
the powerplant has flexibility and can be flown at a low
altitude without the increased fuel consumption of a
turbine engine. When attached to the standard
powerplant, the turbocharger does not take any
horsepower from the powerplant to operate; it is
relatively simple mechanically, and some models can
pressurize the cabin as well.
The turbocharger is an exhaust-driven device, which
raises the pressure and density of the induction air
delivered to the engine. It consists of two separate
components: a compressor and a turbine connected by
a common shaft. The compressor supplies pressurized
air to the engine for high altitude operation. The
compressor and its housing are between the ambient
air intake and the induction air manifold. The turbine
and its housing are part of the exhaust system and
utilize the flow of exhaust gases to drive the
compressor. [Figure 11-5]
The turbine has the capability of producing manifold
pressure in excess of the maximum allowable for the
particular engine. In order not to exceed the maximum
allowable manifold pressure, a bypass or waste gate is
used so that some of the exhaust will be diverted
overboard before it passes through the turbine.
The position of the waste gate regulates the output of
the turbine and therefore, the compressed air available
to the engine. When the waste gate is closed, all of the
exhaust gases pass through and drive the turbine. As
the waste gate opens, some of the exhaust gases are
routed around the turbine, through the exhaust bypass
and overboard through the exhaust pipe.
The waste gate actuator is a spring-loaded piston,
operated by engine oil pressure. The actuator, which
adjusts the waste gate position, is connected to the
waste gate by a mechanical linkage.
The control center of the turbocharger system is
the pressure controller. This device simplifies
turbocharging to one control: the throttle. Once the
pilot has set the desired manifold pressure, virtually no
throttle adjustment is required with changes in altitude.
The controller senses compressor discharge
requirements for various altitudes and controls the oil
pressure to the waste gate actuator which adjusts the
waste gate accordingly. Thus the turbocharger
maintains only the manifold pressure called for by the
throttle setting.
GROUND BOOSTING VS. ALTITUDE
TURBOCHARGING
Altitude turbocharging (sometimes called “normalizing”)
is accomplished by using a turbocharger that
will maintain maximum allowable sea level manifold
pressure (normally 29 – 30 inches Hg) up to a certain
altitude. This altitude is specified by the airplane
manufacturer and is referred to as the airplane’s
critical altitude. Above the critical altitude,
EXHAUST GAS
DISCHARGE
WASTE GATE
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