11 - Propeller

ATA-61 PROPELLERS

2 hrs

Propeller Feathering

Feathering is the procedure to change the pitch angle of the propeller . Feathering of a propeller blade is necessary when an engine fails or whenever it is necessary to shut off one engine. A pressure of air on the face & back of the feather blade is equal and propeller will stop rotating. If not feathered the propeller incase of failed engine will windmill & will create an excessive drag which is dangerous to the flight. A feathered propeller creates less resistance (less drag) and disturbances in the flow of  air over the wings & the tail. It also eliminates vibration which might damage the structure very badly. Feathering also prevents further damage to the failed engine. It is to be seen that a feathered blade is in an approximately in line of the flight position (streamline with lines of  flight). Aerofoil:

An aerofoil is a surface designed to obtain a desirable reaction from the air through which it is moves. Thus we can say that any part of the A/c which converts its air resistance into a force useful for a flight profile. The blades of a propeller so designed that when they rotate, their shape & position causes a higher pressure to be built in behind them than in front of them so that they pull the A/c forward. Auto feathering:

This system is used to automatically feather the failed engines propeller which will present excess drag from the failed engine during a flight region. This is normally used during takeoff and landing. If the engine torque falls below as certain value the torque pressure s/w through an arming relay provides an electrical supply through a dump valve. Dump valve is mounted on a propeller over speed governor & will bypass the governor oil pressure through the propeller if the system is activated. This will cause the propeller to feather to the desired angle. Because of engine failure it disables the other engines auto feathering cct that means it cannot be auto feathered.

When the system is armed or activated through the power lever s/w's with 90% of the engine speed. The s/w is made & provides a supply to 400 Ib ft torque s/w. If the right engine fails its arming relay i s armed as the engine torque drops below 200 Ib f t. torque s/w is made to complete an electrical ground to the dump valve cct. Then the dump valve roots away the governor oil & the propeller gets feathered at the same time the left engine auto feathering system is disabled & the indicator lights goes out. Propeller Synchrophaser System:

This system used in some twin engine a/c. If the RPM of both propellers are same then it is known as propeller synchronizing. When the rpm and phase angle of the propeller is the same then it is known as synchrophasing. This system reduces vibration and eliminates unpleasant beat. This is achieved by utilizing the speed governor of one propeller as master unit & the governor of the second propeller as slave unit. Both governors have magnetic pickups which supply electrical pulses to a control unit which detects the difference in frequency of the pulses. The resultant o/p of the control unit is fed to a stepping type of motor actuator which is called as u corrector motor mounts on the slave governor. Which is consequently trimmed to maintain its propeller RPM to same value valu e as the master governor unit with a limited range.

The limiting range of operation is built into the synchronize system to prevent the slave governor from loosing more than a fixed amount of propeller RPM in the event of master engine propeller being feathered. Before the system is activated the RPM of each propeller is manually synchronized as close as possible. When this is done the system is activated. The maximum synchronizing RPM range is typically is +/- 67 achieved. In synchrophasing system the magnetic pickups picks the RPM and phase angle of the master and slave engine in the form of pulses are given to the control box. The control box controls the slave engine’s RPM and phase angle as described above with respect to master

engine.