Wilson, Grady W. Raspet Flight Research Laboratory, Mississippi State University, Starkville, Mississippi.
Last reviewed:January 2020
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An aircraft instrument that provides an indication of the vertical change of the aircraft position within the air mass. It is more commonly known as the vertical-velocity or vertical-speed indicator. Contained within a sealed case, it is connected to the aircraft static pressure source, the so-called at-rest air pressure outside the aircraft, through a calibrated leak (see illustration). Although the instrument operates from a static pressure source, it is a differential pressure indicator. The differential pressure is established between the static pressure in the diaphragm or pressure capsule and the trapped static pressure within the case. When the aircraft changes vertical position, the static pressure in the diaphragm changes immediately but, because of the metering action of the calibrated leak, the case pressure will remain at its prior value and cause the needle to show a change in vertical speed. The needle is usually calibrated in feet per minute but may be calibrated in any appropriate unit of length over time. Because of the calibrated leak in the system, a finite time (usually 6–9 s) is required for the pressure inside the instrument to equalize and a reliable rate indication to become available.
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