How do Variometers Work?

Unlike birds and various flying animals that regularly travel the sky, human beings are unable to innately sense the rate at which they sink and rise in altitude. Having such information is crucial for the safety of piloting an aircraft as operators will need to know their positioning, rate of change, and altitude at all times for a safe and efficient flight. With a flight instrument known as a variometer or vertical speed indicator, pilots can track their rate of descent or climb in a measurement of meters, feet, or knots depending on the country and the aircraft. As the variometer may be used for many procedures such as gliding, having an understanding of how such instruments work and how they may be used is paramount for any pilot.

The standard variometer consists of a housing and dial, and a diaphragm or other components are placed within the housing chamber with a connection to the static port located on a relatively undisturbed location of the fuselage. The static port is tasked with collecting static pressure, that of which decreases as an aircraft increases its altitude. As static air pressure enters the variometer air chamber, air will expand as static air pressure decreases. As a result of this expansion, the variometer will measure the amount of air leaving the air chamber and convey a reading to the pilot. In general, airflow from the chamber can be measured in a variety of ways, though the most common methods are using mechanical measures or a heat-sensitive electrical resistor.

A variometer or vertical speed indicator may vary in its construction and design depending on when the aircraft was assembled and when the variometer was created. For the most simplistic variometers, a thermos bottle can be used as a reservoir to increase the storage capacity of the instrument. For electronic types, self-heating thermistors may be used to detect air flow that cools down the components. As thermistors reach a difference in resistances from cooling, the resulting voltage difference can be used to provide readings for the pilot. In more modern variometer designs, pressure sensors can be used in lieu of air flow measurements in order to detect changes in altitude. As such designs do not require air bottles, they are smaller and are more reliable. When the variometer is installed in a powered aircraft, it is most often referred to as a vertical speed indicator rather than a variometer.

For applications such as gliding, knowing the total change in energy for altitude and speed is important outside of just altitude rate changes. With a total energy variometer, the movement of air mass surrounding the aircraft can be measured with a small probe placed at the tail fin of the assembly. Connected to the static outlet of the variometer, the probe provides suction that decreases and offsets pressure at the static port during climbs and increases suction during descents. As such, air mass movement can be measured with ease.


December 23, 2020

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