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A tachometer (usually abbreviated to tacho) is a device for measuring how fast an object is rotating in revolutions per minute; for helicopters this is usually the main rotor.

Four types of tacho are available:

  • Prop tacho
This uses a light sensor to detect the passing of the blades between it and a light source (for example, the sky or sun). The revolutions are counted, and the measured RPM is displayed. Often there is a button to tell the tacho how many blades the prop has, and it divides the counted interruptions by that number.
As a prop tacho needs to be placed quite close to the spinning blades, they are of limited use on helicopters. They can be tied to a stake in the ground, or tied to the tail boom, but the risk of injury holding them under the blades by hand is relatively high.
  • Optical tacho
Also called a heli tacho, this contains a rotating disk that interrupts light coming through a viewfinder. The user looks through the viewfinder and adjusts the speed of rotation of the disk until the main rotor appears stationary. The speed of the main rotor can then be read off the tacho.
Optical tachos require two people to use, but are much safer as they do not require anyone to get near the running helicopter. Optical tachos can also be used to check RPM during flight, to help reduce bogging and setting up the throttle curve.
A datalogger measures rotor RPM (or sometimes engine RPM, which will translate exactly except when in autorotation) using either an optical or magnetic pickup. This along with many other parameters is stored in the datalogger's memory and can be reviewed later as graphs on a computer.
  • Audio tacho
Finally, the rarely seen audio tachometer captures the sound of the blades moving air, and analyzes this to try to determine the likely head speed.

A governor reduces the need for a tacho, but one is still useful for initially setting the correct RPM, and also for setting up a throttle curve to fall back to in case the governor fails (for example, if the magnet falls off or the sensor stops working).

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