14.34 Engine Instruments

14.34.2 Describe the function and principle of operation of the following instruments:

(a) tachometers (rpm) gauges (centrifugal and drag cup);

Tachometers indicate engine speed in revolutions per minute (RPM), allowing the pilot to monitor and control engine performance.

Centrifugal Tachometer (Mechanical)

Principle:

• Driven mechanically from the engine (usually via a cable)
• Uses centrifugal force acting on rotating weights

Operation:

• As engine RPM increases, rotating flyweights move outward due to centrifugal force
• This movement is transmitted through a mechanism to move the pointer on the gauge
• Higher RPM → greater outward movement → higher indication

Click here: Centrifugal Tacho

Drag Cup Tachometer (Eddy Current Type)

Principle:

• Based on electromagnetic induction (eddy currents)

Operation:

• A permanent magnet rotates with the engine
• Surrounding it is a non-magnetic aluminium cup (drag cup) attached to the pointer
• Rotation of the magnet induces eddy currents in the cup
• These currents create a magnetic field that drags the cup around
• The faster the rotation, the greater the drag → greater pointer deflection