Welcome back to our Word of the Week!

Every week, we look together at a word that is either interesting, funny or mysterious and that appeals to all of us in the aviation community.

Today we’re going to look at one of the instruments we have in our instrument panel.

Yes, it says RPM. No, the name of the instrument is not “RPM”. Instruments are not like cheese, what is written on them is not necessarily their name…

tachometer (n.)

UK /tækˈɒm.ɪ.tər/ US /tækˈɑː.mə.t̬ɚ/

Informal: ‘Rev meter’

A tachometer is a device for indicating the angular (rotary) speed of a rotating shaft or disc.

The term is usually restricted to mechanical or electrical instruments that indicate instantaneous values of speed in revolutions per minute, rather than devices that count the number of revolutions in a measured time interval and indicate only average values for the interval.

A tachometer is literally a “speed-measurer” and comes from Greek τάχος (táchos “speed”) and μέτρον (métron “measure”).

Since the speed that an tachometer measures is speed of rotation of the crankshaft, the numbers it reports are revolutions per minute, or RPM’s. And that is where the label you see in the instrument comes from.

It typically has markings indicating a safe range of rotation speeds.

Mechanical tachometers utilize the fact that the centrifugal force on a rotating mass depends on the speed of rotation and can be used to stretch or compress a mechanical spring. A resonance, or vibrating-reed, tachometer uses a series of consecutively tuned reeds to determine engine speed by indicating the vibration frequency of the machine.

It is important to realize that, although the RPM value may be in certain cases directly proportional to other engine parameters, such as manifold absolute pressure, our tachometer does one thing, and one thing only: It displays the speed at which the crankshaft is rotating.