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engine rotation. To completely secure the engine, the
pilot must still turn off the fuel (mixture, electric boost
pump, and fuel selector), ignition, alternator/generator,
and close the cowl flaps. If the airplane is pressurized,
there may also be an air bleed to close for the failed
engine. Some airplanes are equipped with firewall
shutoff valves that secure several of these systems
with a single switch.
Completely securing a failed engine may not be necessary
or even desirable depending upon the failure
mode, altitude, and time available. The position of the
fuel controls, ignition, and alternator/generator
switches of the failed engine has no effect on aircraft
performance. There is always the distinct possibility
of manipulating the incorrect switch under conditions
of haste or pressure.
To unfeather a propeller, the engine must be rotated
so that oil pressure can be generated to move the
propeller blades from the feathered position. The
ignition is turned on prior to engine rotation with the
throttle at low idle and the mixture rich. With the
propeller control in a high r.p.m. position, the starter
is engaged. The engine will begin to windmill, start,
and run as oil pressure moves the blades out of
feather. As the engine starts, the propeller r.p.m.
should be immediately reduced until the engine has
had several minutes to warm up; the pilot should
monitor cylinder head and oil temperatures.
Should the r.p.m. obtained with the starter be insufficient
to unfeather the propeller, an increase in airspeed
Counterweight
Action
Aerodynamic Force
Hydraulic Force
High-pressure oil enters the cylinder through the center of
the propeller shaft and piston rod. The propeller control
regulates the flow of high-pressure oil from a governor.
A hydraulic piston in the hub of the propeller is connected
to each blade by a piston rod. This rod is attached to forks
that slide over the pitch-change pin mounted in the root of
each blade.
The oil pressure moves the piston toward the front of the
cylinder, moving the piston rod and forks forward.
The forks push the pitch-change pin of each blade
toward the front of the hub, causing the blades to twist
toward the low-pitch position.
A nitrogen pressure charge or mechanical spring in
the front of the hub opposes the oil pressure, and
causes the propeller to move toward high-pitch.
Counterweights also cause the blades to move toward
the high-pitch and feather positions. The counterweights
counteract the aerodynamic twisting force that
tries to move the blades toward a low-pitch angle.
Nitrogen Pressure or Spring
Force, and Counterweight Action
Figure 12-4. Pitch change forces.
12-4
Ch 12.qxd 5/7/04 9:54 AM Page 12-4
from a shallow dive will usually help. In any event, the
AFM/POH procedures should be followed for the
exact unfeathering procedure. Both feathering and
starting a feathered reciprocating engine on the ground
are strongly discouraged by manufacturers due to the
excessive stress and vibrations generated.
As just described, a loss of oil pressure from the propeller
governor allows the counterweights, spring
and/or dome charge to drive the blades to feather.
Logically then, the propeller blades should feather
every time an engine is shut down as oil pressure falls
to zero. Yet, this does not occur. Preventing this is a
small pin in the pitch changing mechanism of the
propeller hub that will not allow the propeller blades
to feather once r.p.m. drops below approximately
800. The pin senses a lack of centrifugal force from
propeller rotation and falls into place, preventing the
blades from feathering. Therefore, if a propeller is to
be feathered, it must be done before engine r.p.m.
decays below approximately 800. On one popular
model of turboprop engine, the propeller blades do,
in fact, feather with each shutdown. This propeller is
not equipped with such centrifugally-operated pins,
due to a unique engine design.
An unfeathering accumulator is an optional device that
permits starting a feathered engine in flight without the
use of the electric starter. An accumulator is any device
that stores a reserve of high pressure. On multiengine
airplanes, the unfeathering accumulator stores a small
reserve of engine oil under pressure from compressed
air or nitrogen. To start a feathered engine in flight,
the pilot moves the propeller control out of the
feather position to release the accumulator pressure.
The oil flows under pressure to the propeller hub and
drives the blades toward the high r.p.m., low pitch
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AIRPLANE FLYING HANDBOOK 飞机飞行手册下(25)
