Propellers Flashcards
How does a propeller function?
A propeller is essentially a rotating wing. As the engine turns it, the air moving past the curved forward surface of the propeller causes a low pressure when compared to the area on the aft side of the propeller, which is relatively flat. As in a wing, the difference in pressure causes a reactive force in the direction of the lesser pressure. On a wing, this force is upward and is called lift. On a propeller, this force is forward and is called thrust. It is this force that moves the aircraft.
Why are constant speed propellers used?
To obtain maximum efficiency from the propeller by rotating it at a constant speed and to keep the engine RPM constant while adjusting the power output with the throttle lever.
What are the components of a propeller governor and how does it operate?
A typical propeller governor consists of a drive gear to engage the engine, an oil pump, a pilot valve controlled by flyweights, a relief valve system, a piston connected mechanically to the blades, a speeder spring, and an adjusting rack for control from the flight deck. The governor senses the RPM of the engine/propeller assembly via the flyweights. The force of the flyweights is counterbalanced against the force of the speeder spring that is set via the adjusting rack. If the speed of the engine and propeller increase, the flyweight force increases and they move outward. This opens the pilot valve and oil is pumped against the piston. The piston motion transmits
the force to the blades which increase pitch angle. At a higher pitch angle, the force of the air striking the blades increases. This increases the load on the engine and serves to suppress engine RPM and keep it constant.
What is a test club propeller? When and why is it used?
A test club propeller is a four-bladed, fixed pitch propeller used during ground testing or break-in of a reciprocating engine. The blades are short and designed to put the correct amount of load on the engine during the test break-in period. The multi-blade design also provides additional cooling airflow.
What is the maximum interval between lubrication of a propeller and where does the technician find the proper procedures for lubrication of a particular propeller?
Propellers must be lubricated every 100 hours or at 12 calendar months, whichever occurs first. If annual operation is significantly less than 100 hours, calendar lubrication intervals should be reduced 6 months. Also, if the propeller is exposed to adverse atmospheric conditions, such as high humidity and, salt air, the calendar interval should be shortened to 6 months. The manufacturer’s instructions should be consulted for the proper propeller lubrication procedures as well as oil and grease specifications.
How is the angle of a propeller blade measured while the propeller is mounted on the engine?
Propeller blade angle is measure at a blade station(s) specified by the manufacturer. Blade stations are measured in inches from the base of the blade toward the tip. Using a propeller or universal protract placed at the correct blade station, follow the manufacturer’s instruction. The angle between the plane of rotation and the propeller blade face at the specified station is what is being measured.
In general, what is the procedure for removing a propeller?
Always follow manufacturer’s instruction. In general, remove the spinner dome. Then cut and remove the safety wire on the propeller mounting studs. Support the propeller assembly with a sling. Make an alignment mark on the hub and engine flange to maintain dynamic balance during reinstallation. Unscrew the four mounting bolts from the engine bushings. Unscrew the two mounting nuts and the attached studs from the engine bushings. If the propeller is removed between overhauls, mounting studs, nuts, and washers may be reused if not damaged or corroded. Using care and supporting the weight of the propeller assembly with the sling, remove the propeller from the mounting flange.
What is the function of a typical propeller synchronization system and how does it operate?
Propeller synchronization is used to eliminate the unpleasant beat produced by unsynchronized propeller operation. A typical synchrophasing system is an electronic system. It functions to match the RPM of both engines and establish a blade phase relationship between the left and right propellers to reduce cabin noise. A switch on the flight deck controls the system. When in the “ON” position, pick-ups on each propeller send a signal to a control box. The control box sends a command signal to an RPM trimming coil on the propeller governor of the slow engine to adjust the RPM to equal that of the other propeller.
Explain why is ice a problem for propeller operation. Then, name a means for anti-icing and a means of deicing aircraft propellers.
Ice formation on a propeller blade in effect produces a distorted blade airfoil section and makes the propeller inefficient. Ice collects asymmetrically and produces propeller unbalance resulting in destructive vibration and increased blade weight. A typical propeller anti-icing system includes an on-board reservoir of anti-icing fluid that is pumped to a slinger ring mounted on the rear of the assembly and distributed to the blades by centrifugal force. A typical propeller de-icing system includes electric heating elements adhered to the leading edge of each blade. Power is transferred to the propeller elements through brushes and a slip ring. A timer cycles the elements on and off in sequence or this can be controlled by the pilot.
