17.4 Propeller Synchronising Flashcards
A propeller synchronising system is used to set all propellers, installed on a multi-engine aircraft, at exactly the same RPM, thereby eliminating what?
vibration and excess noise
This is achieved by adjusting the propeller governors which can be either mechanical or electrical.
The synchronisation system is normally used for all flight phases except when
take off and landing
A master engine (normally the left hand one) is used to establish the RPM to which all other engines (slaves) adjust.
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System Components
Tachometer or Frequency Generator
These are used with each engine to generate a signal proportional to the RPM of the engine. The tachometer is usually mounted on the accessory gearbox while a frequency generator can be part of the propeller governor.
If a tachometer is used, the signal voltage is directed to a differential motor to compare the master engine RPM and the slave engine RPM. The engine that generates the higher voltage determines the direction that the differential motor rotates and adjust the governor setting of the slave engine.
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System Components
Comparison Unit
The comparison unit is used to compare the RPM signal of the slave engine(s) to the RPM signal of the master engine.
If a frequency generator is used, the engine signals are sent to an electronic unit which compares the frequencies and sends a correcting signal to the slave engine governor control mechanism.
Comparison Units have a limited range of operation and the slave engines must be within approximately 100 RPM of the master engine RPM for synchronisation to occur.
System Components
Master Engine Selector
Used on some 4-engined aircraft to allow the pilot to select the master engine to be used (normally #2 or #3).
This provides an alternative engine master in the event that the engine being used as the master becomes inoperative.
system Components
Resynchronisation Button
This is used by some systems to interrupt the synchronisation system operation and allow the slave governor synchronisation mechanisms to centre, providing for full travel (100 RPM) towards the master engine RPM. This control is used if one or more slave engines are more than a 100 RPM different from the master engine without the need to operate individual toggle switches
System Operation
the system is not used during take off and landing. If the system were used, failure of the master engine would result in the slave engines trying to follow the master and thereby causing a loss of power by 100 RPM.
During normal operation, the slave engines are near the master engine RPM when the system is switched on. The comparison unit senses any differences in RPM and causes the slave engine governors to adjust to the same RPM as the master engine.
If a master control system is incorporated with the synchronisation system the master control can be used at any time to adjust the RPM of all engines. As the master control lever is moved, the synchronisation system is interrupted and the engines can go out of synchronisation for a few seconds. When the lever stops moving the system returns to synchronisation.
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Propeller Synchrophasing
Synchrophasing is a refinement of synchronisation and allows the pilot to set the blades of the slave engines a number of degrees in the rotation behind the blades of the master engine.
Synchrophasing is used to further reduce the noise created by the engines. The synchrophase angle can be varied by the pilot to adjust for different flight conditions and still achieve a minimum noise level.
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Synchrophaser with Mechanical Governing System
Through a synchrophaser the system receives a phase signal from the propellers in addition to the RPM signal through the RPM pickup or additional pick-ups.
The system then sets the slave propeller to the determined phase position. The phase position of the slave propeller is some degrees off that of the master propeller. T
his offsetting is programmed in such a way that vibrations in the cabin are kept to a minimum.
Thus the synchrophaser sets the slave propeller to a constant phase displacement.
Synchrophaser with Electronic Governing System
The RPM of the propeller is governed by the propeller electronic control units (PEC) with the left propeller being nominated as the master for synchrophasing. As the PECs control propeller RPM very exactly (+/- 0.2%), the difference in RPM is very small when synchrophasing
(Synchrophaser with Electronic Governing System)
The synchrophasing function is automatically activated to reduce propeller noise, if the power lever is at CLIMB or CRUISE, with the RPM difference less than 100 RPM, and the pilot selectable phase difference typically up to 40 degrees (depending upon type).
Synchrophasing acts to reduce the cabin noise by ensuring that the relative position, or phase difference, between the slave propeller and master propeller is controlled to a demanded angle. The phase angle is calculated by timing the differences between the master and slave propeller Magnetic Pickup Unit (MPU) signals over a complete propeller revolution. The phase demand is determined from either the condition lever angle (CLA) position or the output from the Active Noise and Vibration Suppression System (ANVS).
Magnetic Pickup Unit (MPU)
The purpose of the dual channel MPU is to provide the PEC with a Power Turbine speed (Np) indication and determine the phase of the propeller.
The speed and phase are detected by 6 of the 7 targets located on the de-icing slip-ring while the seventh is used as a master reference for balancing purposes.
