27-07-2012, 03:17 PM
AIRCRAFT PROPELLER CONTROL AND OPERATION
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Propeller Control
Basic requirement: For flight operation, an engine is demanded to deliver power within a relatively narrow band of operating rotation speeds. During flight, the speed-sensitive governor of the propeller automatically controls the blade angle as required to maintain a constant RPM of the engine.
Three factors tend to vary the RPM of the engine during operation. These factors are power, airspeed, and air density. If the RPM is to maintain constant, the blade angle must vary directly with power, directly with airspeed, and inversely with air density. The speed-sensitive governor provides the means by which the propeller can adjust itself automatically to varying power and flight conditions while converting the power to thrust.
Fundamental Forces: Three fundamental forces are used to control blade angle. These forces are:
1. Centrifugal twisting moment, centrifugal force acting on a rotating blade which tends at all times to move the blade into low pitch.
2. Oil at engine pressure on the outboard piston side, which supplements the centrifugal twisting moment toward low pitch.
3. Propeller Governor oil on the inboard piston side, which balances the first two forces and move the blades toward high pitch
Counterweight assembly (this is only for counterweight propeller) which attached to the blades , the centrifugal forces of the counterweight will move the blades to high pitch setting.
Constant Speed, Counterweight Propellers
The Counterweight type propeller may be used to operate either as a controllable or constant speed propeller. The hydraulic counterweight propeller consists of a hub assembly, blade assembly, cylinder assembly, and counterweight assembly.
The counterweight assembly on the propeller is attached to the blades and moves with them. The centrifugal forces obtained from rotating counterweights move the blades to high angle setting. The centrifugal force of the counterweight assembly is depended on the rotational speed of the propellers RPM The propeller blades have a definite range of angular motion by an adjusting for high and low angle on the counterweight brackets.
Governor Operation Condition
On-Speed Condition
The on-speed condition exists when the propeller operation speed are constant. In this condition, the force of the flyweight (5) at the governor just balances the speeder spring (3) force on the pilot valve (10) and shutoff completely the line (13) connecting to the propeller , thus preventing the flow of oil to or from the propeller.
Over-Speed Condition
The over-speed condition which occurs when the aircraft altitude change or engine power is increased or engine RPM is tend to increase and the governor control is moved towards a lower RPM In this condition, the force of the flyweight (5) overcomes the speeder spring (3) force and raise the pilot valve (10) open the propeller line (13) to drain the oil from the cylinder (14). The counterweight (15) force in the propeller to turn the blades towards a higher pitch. With a higher pitch, more power is required to turn the propeller which in turn slows down the engine RPM As the speed is reduced, the flyweight force is reduced also and becomes equal to the speeder spring force. The pilot valve is lowered, and the governor resumes its on-speed condition which keeps the engine RPM constant.
Flight Operation
This is just only guide line for understanding. The engine or aircraft manufacturers' operating manual should be consulted for each particular aircraft.
Takeoff: Placing the governor control in the full forward position. This position is setting the propeller blades to low pitch angle Engine RPM will increase until it reaches the takeoff RPM for which the governor has been set. From this setting, the RPM will be held constant by the governor, which means that full power is available during takeoff and climb.
Cruising: Once the cruising RPM has been set , it will be held constant by the governor. All changes in attitude of the aircraft, altitude, and the engine power can be made without affecting the RPM as long as the blades do not contact the pitch limit stop.
Principles of Operation (Constant Speed with Counterweight Propellers)
The changes in the blades angle of a typical constant speed with counterweight propellers are accomplished by the action of two forces, one is hydraulic and the other is mechanical.
1. The cylinder is moved by oil flowing into it and opposed by centrifugal force of counterweight. This action moves the counterweight and the blades to rotate toward the low angle position.
2. When the oil is allowed to drain from the cylinder, the centrifugal force of counterweights take effect and the blades are turned toward the high angle position.
3. The constant speed control of the propeller is an engine driven governor of the flyweight type.