Transcript UCM Aviation - University of Central Missouri

The purpose of this presentation is to provide the new multiengine pilot with
common multiengine system operations. Studying this presentation will
provide you with an overview of multiengine systems; therefore, it is
necessary to review the systems for the specific aircraft you will be flying. You
should study the POH/AFM and the PA-44-180 Seminole packet provided by
your flight instructor.
Fixed-pitch propellers are designed for best efficiency at one speed of
rotation and forward speed. This type of propeller will provide suitable
performance in a narrow range of airspeeds; however, efficiency would suffer
considerably outside this range. To provide high propeller efficiency through
a wide range of operation, the propeller blade angle must be controllable.
The most convenient way of controlling the propeller blade angle is by means
of a constant-speed governing system, which will be discussed in greater
detail on the following slides.
Constant-Speed propellers maintain a specified RPM by varying the pitch of
the propeller blades. By changing the pitch of the propeller blades, greater
propeller efficiency can be realized through all phases of flight.
The propeller RPM/pitch angle is indirectly controlled by the propeller lever in
the cockpit. The propeller lever is connected to a governor which is sensitive
to changes in engine RPM and directs engine oil to or from the propeller hub
which changes the propeller blade angle and RPM.
The figure shows how a constant
speed propeller is more efficient
through a wide range of speeds
than fixed pitch propellers.
The following slides will provide more detail about a propeller governor.
Once the pilot selects the RPM setting for the propeller, the propeller
governor automatically adjusts the blade angle to maintain the selected RPM.
The governor does this by using oil pressure from the engine lubricating
system.
Governors work in two ways:
On single engine aircraft –
Oil is typically sent into the propeller hub to increase blade angle and
relieved from the hub to decrease propeller blade angle.
On multiengine aircraft –
Oil is sent into the propeller hub to decrease propeller blade angle and
relieved from the hub to increase propeller blade angle.
Designing multiengine aircraft propellers to increase pitch during a loss of
oil pressure provides better safety and efficiency during single engine flight.
If oil pressure is lost it is more aerodynamically efficient for the propellers
to be in a high pitch, low drag setting.
PARTS OF A CONSTANT-SPEED PROPELLER:
Twin engine
Speed Adjusting Lever
Cockpit controlled
Controls speed adjusting screw
Speed Adjusting Screw
Controlled by speed adjusting lever
Controls tension on speeder spring
Speed adjusting screw hits stops (High, Low)
Speeder Spring
Moves to increase or decrease flyweight position
Speeder spring moves down, flyweights move in (underspeed)
Speeder spring moves up, flyweights move out (overspeed)
Control Valve (Pilot Valve)
Moves in same direction as the speeder spring
Directs oil flow to or from the hub
Underspeed condition
Flyweights in, pilot valve down
Oil from gear boost pump allowed in
Oil flow from engine oil sump to propeller hub
Overspeed condition
Flyweights out, pilot valve up
High pressure oil flows to engine sump
OPERATIONALLY – WHAT YOU NEED TO KNOW
You’ll need to be able to describe the
operation of the governor and propeller
pitch from the propeller control knob in
the cockpit all the way to the propeller
pitch and RPM changing.
Basically, the governor allows oil to be
drained from the propeller hub or go into
the propeller hub to change propeller
blade pitch and RPM. Oil is directed by
the position of the pilot valve which
opens a port to the prop hub or the
engine sump, or blocks both ports so the
current amount of oil stays in the prop
hub to maintain RPM.
Constant speed propellers installed on most multiengine airplanes are full
feathering, counterweighted, oil-pressure-to-decrease-pitch designs.
In this design, increased oil pressure from the propeller governor drives the
blade angle towards low pitch, high RPM-away from the feather blade angle.
In effect, the only thing that keeps these propellers from feathering is a
constant supply of high pressure engine oil. This is a necessity to enable
propeller feathering in the event of a loss of oil pressure or a propeller
governor failure.
There are essentially four forces acting upon a constant-speed propeller. Two forces
are twisting the blades towards a low pitch, high RPM while the other two forces are
twisting the blades towards a high pitch, low RPM.
Low Pitch, High RPM Forces
1)
2)
Aerodynamic Forces
Oil pressure from the propeller governor
High Pitch, Low RPM Forces
1)
2)
Counterweights – using inertia from the rotating propeller
Nitrogen Pressure or Spring Force in the propeller hub
Featherable Propellers
Purpose – To minimize drag in the event
of an engine failure.
To feather a propeller is to stop engine
rotation, with the propeller blades
streamlined with the airplane’s relative
wind, minimizing drag.
Feathering is necessary because of the
change in parasite drag with propeller
blade angle. When the propeller blade
angle is in the feathered position, the
change in parasite drag is at a minimum.
At the smaller blade angles near the flat
pitch position, the drag added by the
propeller is very large. At these small
blade angles, the propeller windmilling at
high RPM can create such a tremendous
amount of drag that the airplane may be
uncontrollable.
If an engine fails, you’ll want to feather the propeller of the inoperative
engine to reduce parasite drag.
To feather the propeller, the propeller control is brought fully aft. All oil
pressure is dumped from the governor, and the counterweights drive the
propeller blades towards feather. As centrifugal force acting on the
counterweights decays from decreasing RPM, additional forces are needed to
completely feather the blades. This additional force comes from either a
spring or high pressure air stored in the propeller dome, which forces the
blades into the feathered position. The entire process may take up to 10
seconds.
*Always follow the manufacturers direction for specific procedures to feather
the propeller.
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 RPM 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 RPM decays below approximately 800.
On engine shutdown after a flight, the anti-feathering pins prevent the propeller
from feathering. If the propellers were allowed to feather on shutdown, each
subsequent start would require the propellers to be moved out of the feather
position. This would cause excessive loads on the engine starter during the next
engine start.
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 RPM 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 RPM should be immediately
reduced until the engine has had several minutes to warm up; the pilot should
monitor cylinder head temperatures and oil temperatures.
Should the RPM, obtained with the starter, be insufficient to unfeather the
propeller, an increase in airspeed from a shallow dive will usually help, as this will
increase RPM. In any event, the AFM/POH procedures should be followed for the
exact unfeathering procedure.
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 RPM, low
pitch position, whereupon the propeller will usually begin to windmill. (On some
airplanes, an assist from the electric starter may be necessary to initiate rotation
and completely unfeather the propeller.) If fuel and ignition are present, the
engine will start and run. For airplanes used in training, this saves much electric
starter and battery wear. High oil pressure from the propeller governor recharges
the accumulator just moments after engine rotation begins.
When flying multiengine airplanes there can be an annoying rhythmic sound
that stems from the propellers being out of sync. The out of sync propellers
may sound like a washing machine, drumming or beat. To eliminate this
sound the pilot has three options for adjustment:
1)
Audible / manual adjustment
a)
2)
The pilot attempts to eliminate the sound by making small changes in one propellers RPM until the sound is
no longer heard.
Prop Sync
a)
b)
The pilot closely matches the RPMs of both propellers then engages the prop sync system, which matches the
propeller RPMs exactly.
Anytime an RPM adjustment is made, the pilot should disengage prop sync, make the RPM adjustment, then
re-engage prop sync.
On some twins there exists a small gauge that has a spinning disk inside, called a syncroscope.
This gauge is usually mounted near the tachometers. The pilot manually fine tunes the engine
RPM so as to stop disk rotation, thereby synchronizing the propellers.
Retractable landing gear systems improve aircraft performance by decreasing
drag. There are two types of retractable landing gear systems. They are
organized by how the system is operated, either electrical or hydraulic.
Electric –
An electrically driven motor drives creates a force through several
components which either extend or retract the landing gear and in some
cases the landing gear doors.
Hydraulic -
A hydraulic landing gear system utilizes a pump to pressurize hydraulic fluid
to actuate linkages to raise and lower the gear. The pump which
pressurizes the fluid in the system can be either engine drive or electrically
powered.
There are several types of safety and warning devices on a multiengine
aircraft. The safety and warning devices prevent and/or warn the pilot of an
unsafe situation, such as the aircraft is configured for landing but the gear is
not down or the gear is selected to the up position but the aircraft is still on
the ground.
Landing Configuration without Gear Extended:
A gear warning horn will sound when the airplane is configured for landing and the landing gear is
not down and locked. The horn is normally linked to the throttle or flap position.
Retraction of Gear while on the Ground:
Squat Switch:
Usually mounted in a bracket on one of the main gear shock struts. When the strut
is compressed by the weight of airplane, the switch opens the electrical circuit to the
motor or mechanism that powers retraction. In this way, if the landing gear switch in
the cockpit is placed in the RETRACT/UP position when weight is on the gear, the gear
will remain extended.
All multiengine airplanes have an emergency gear extension system.
Some aircraft designs use gravity to extend the gear, while others use
compressed gas or hydraulic systems.
If you experience a gear extension failure follow the manufacturers direction
found in the POH/AFM.
Fuel Crossfeed:
Fuel crossfeed allows an engine to draw fuel from a fuel tank(s) located in the
opposite wing. Normally, fuel tank(s) located on the left wing provide fuel to the left
engine and vice versa for fuel tanks on the right side.
Crossfeed operations are normally an emergency procedure only. You may use
crossfeed if one engine becomes inoperative and the operative engine fuel tank is
being depleted of fuel. By placing the appropriate selector on crossfeed you are
allowing the operative engine to use fuel from the opposing wing tank(s).
Combustion Heater:
A combustion heater is a small furnace that burns gasoline (supplied from
usually one of the fuel tanks) to produce heated air for occupant comfort
and windshield defogging.
Most combustion heaters have a maintenance interval and are equipped
with a separate hour meter.
When finished with the combustion heater, a cool down period is required.
Most heaters required that outside air be permitted to circulate through the
unit for at least 15 seconds in flight, or that the ventilation fan be operated
for at least 2 minutes on the ground. Failure to provide an adequate cool
down period will usually trip the thermal switch (protection device) and
render the heater inoperative until the switch is reset.