Propeller speed Flashcards
What is the purpose of a propeller tachometer in an aircraft?
The purpose of a propeller tachometer is to provide the pilot with a visual indication of the propeller’s Revolutions Per Minute (RPM), which is crucial for monitoring the efficiency and performance of the aircraft.
How is the propeller mounted in relation to the engine, and how does it affect RPM?
The propeller can be mounted directly on the engine crankshaft, making its RPM the same as the crankshaft RPM, or on a shaft geared to the crankshaft, making the propeller RPM different from the engine RPM.
Describe the operation of a mechanical tachometer.
A mechanical tachometer operates by using a flexible drive shaft connected to the engine. The shaft drives a flyweight assembly in the indicator, where centrifugal force varies with RPM, moving the pointer on the tachometer to indicate engine RPM.
What is the function of the tachometer in an aircraft with a fixed-pitch propeller?
In an aircraft with a fixed-pitch propeller, the tachometer also serves as an indicator of engine power.
How is a tachometer typically calibrated and marked?
A tachometer is calibrated in hundreds of RPM and marked with color codes, such as a green arc indicating the maximum continuous operating RPM. Additional markings reflect engine or propeller limitations.
How does a magnetic drag cup work in a mechanical tachometer?
In a mechanical tachometer, a magnetic drag cup is rotated by the engine via a flex drive cable. As the magnet turns, it induces eddy currents in the aluminum cup, creating a magnetic field that interacts with the rotating magnet’s field, causing the cup and attached pointer to rotate and indicate RPM.
Why are electric tachometers used instead of mechanical ones in certain aircraft?
Electric tachometers are used in aircraft where engines are not mounted near the instrument panel, as they provide greater accuracy and lower maintenance compared to mechanical linkages.
What principle do electric tachometers operate on?
Electric tachometers operate on the principle of relative motion between a magnetic field and the shaft of the coupled device, inducing Electromotive Force (EMF) in a coil, which is proportional to the shaft’s speed.
What are the main components of an electric tachometer system?
An electric tachometer system consists of a tachometer generator attached to the engine or propeller driveshaft, and a tachometer near the pilot. The generator’s frequency output is proportional to the engine speed and is mirrored by a synchronous motor in the indicator.
What are the two types of tachometer generators based on induced voltage?
The two types of tachometer generators are DC tachometer generators and AC tachometer generators.
How does a DC tachometer generator work?
A DC tachometer generator works by using a permanent magnet and an armature that rotates in the magnetic field, inducing EMF proportional to shaft speed. A commutator converts the alternating current into direct current, and a voltmeter measures the induced EMF to indicate RPM.
What are the disadvantages of DC tachometer generators, and how does the AC tachometer generator address them?
DC tachometer generators have disadvantages such as wear and tear on the commutator and brushes. The AC tachometer generator eliminates these issues by using a stationary armature and a rotating magnetic field, inducing EMF in the stationary coil without the need for a commutator or brushes.
How does an AC tachometer generator measure shaft speed?
An AC tachometer generator measures shaft speed by inducing EMF in a stationary coil due to a rotating magnetic field. The amplitude and frequency of the induced EMF are proportional to the shaft speed, and the signal is rectified and smoothed for measurement.
