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UCM Aviation. Multi-Engine Training Systems. Required Materials.
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UCM Aviation Multi-Engine Training Systems
Required Materials 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.
Propeller 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 Propeller 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 following slides will provide more detail about a propeller governor. The figure shows how a constant speed propeller is more efficient through a wide range of speeds than fixed pitch propellers.
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 angleand 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 Propeller Governor 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
Operational Understanding Propeller Governor 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.
Multiengine Propeller Specifics 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.
Forces on the PropellerMultiengine 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 • Aerodynamic Forces • Oil pressure from the propeller governor High Pitch, Low RPM Forces • Counterweights – using inertia from the rotating propeller • Nitrogen Pressure or Spring Force in the propeller hub
Propeller Feathering1 of 2 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.
Propeller Feathering 2 of 2 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.
Anti-Feathering Pins 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.
Unfeathering 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.
Unfeathering Accumulator 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.
Propeller Synchronization 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: • Audible / manual adjustment • The pilot attempts to eliminate the sound by making small changes in one propellers RPM until the sound is no longer heard. • Prop Sync • 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 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.
Safety Devices 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.
Safety Devices – Failure to Extend 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 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).
Environmental 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.
