All You Need to Know about Tachometers and Their Types

Posted on May 2, 2022 Jerold Perkins Aviation

The word “tachometer” is derived from the Greek words “tachos,” which means speed, and “metron,” which means “measure.” However, contrary to what the name may suggest, tachometers do not necessarily track the speed of an aircraft, but rather, they measure an engine’s operating speed in revolutions per minute (RPM). Tachometers indicate the mechanical motion of either the entire aircraft or one of its components. They measure the rotational speed and the angular (aka rotary) velocity of either the disc or the shaft of the rotating body.

Tachometers work better when aircraft are in motion rather than when they are resting or idling. They primarily aid in averting excessive wear and tear while maintaining an optimum speed of the engine. However, due to improvements and additions that surpass previous reading systems, the usage of advanced tachometers has risen sharply. With this information in mind, we shall discuss tachometers and the different types available in aviation markets today.

Tachometers and Their Principle of Operation

As previously mentioned, tachometers (or tachs, for short) are used to determine the angular speed of a rotating body by employing the principle of relative motion. Their working principle is built off the premise that a motor, such as a generator motor, produces a voltage directly proportional to the shaft’s angular velocity. The proportionality constant represents the relation between the voltage and its corresponding mechanical motion has values typically ranging from 1 to 30 for every 1000 RPM.

Tachometers are usually mounted on the output shaft of AC or DC inverter motors that require precise speed control. In such applications, they take advantage of “closed-loop feedback,” where signals sent by the tachometer allow you to adjust the output to the DC or AC inverter motor, as required. Conventionally, airplanes require separate tachometers for every engine, which means that aircraft typically function by averaging similar engines in weight, speed, and size.

What Is Tach Time?

Aircraft tachs measure time in six-minute increments or every 1/10th of an hour. Though, airplanes must continuously have their tach time measured and documented to determine when 50-hour and 100-hour inspections are required. However, today, a majority of aircraft engineers are opting for the 25-hour tach time inspection which implies that, with fewer tach hours inspected in the hangar, the plane would obtain additional time airborne. Thus, the tach time serves as a rough estimate of an airplane’s time spent flying.

Furthermore, pilots are encouraged to routinely inspect recorded tach times on their aircraft log sheet to detect any possible oil requirements, a complete overhaul of the engine, or to schedule sessions for upcoming aircraft maintenance. To suit various aircraft, there is typically a 10-20% tach time difference among different aircraft types, like air taxis.

Types of Tachometers

Today, tachometers can be classified into different types based on their intended application  and function, as categorized below:

  1. Electronic Tachometers - Electronic tachometers convert rotating angular velocity into a corresponding electronic voltage. Next, they are further subpided into DC and AC tachometer generators, where each works to convert the engine’s angular speed into DC or AC voltage, respectively. To achieve this, electrical voltage generated in the conductor is caused by the magnetic field’s perpendicular motion relative to the conductor. Furthermore, the conductor’s angular velocity directly determines the magnitude of voltage running through.

  2. Time-Measuring Tachometer - Suitable for recording low-speed rotation with high accuracy, this tachometer calculates angular velocity by measuring the interval between two pulses.

  3. Frequency-Measuring Tachometer - Frequency measuring tachometers calculate the angular velocity of a rotating body by measuring the frequency or the rate per unit of all incoming pulses.

  4. Contact Tachometers - As their name implies, a contact tachometer is used to measure a device’s rotational speed by coming in physical contact with an apparatus to collect precise measurements. It consists of a free-spinning wheel that makes contact with a rotating shaft. Furthermore, depending on the type of tachometer model used, a magnetic sensor or an optical encoder can be employed to display readings.

  5. Non-Contact Tachometers - Non-contact tachometers measure RPM speeds without making direct contact with a rotating body, unlike contact tachs. Non-contact tachs contain a laser beam which, when projected onto a rotating body, provides a value for the rotating object’s RPM on a corresponding digital screen.
To Conclude

Aerospace Exchange is your reliable source for procuring all types of aircraft tachometers, as well as other new, obsolete, and hard-to-find civil and defense aviation parts. Furthermore, we at Aerospace Exchange work around-the-clock to effectively remove all your part procurement worries through the implementation of our user-friendly interface and dedicated customer service assistance to provide help at any time. Get started on the procurement process by filling out an Instant RFQ form, and someone from our team will respond to your request within 15 minutes or less to continue the purchasing process!

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