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airspeed indicator[¦er‚spēd ‚in·də‚kād·ər]
an instrument used in aviation to measure the flight speed of an aircraft (airplane or helicopter) relative to the ambient air. The determination of the airspeed V is necessary in handling an airplane (since the lift from the wing is proportional to the square of the airspeed), as well as for navigational purposes—for example, to calculate the flight path of the airplane.
An airspeed indicator has three main parts: a Pitot-static tube, a duct, and a needle gauge. The Pitot tube measures the static pressure ps and the dynamic (total) pressure Pd. The difference between them equals the dynamic pressure—that is, 0.5 ρ V2, where ρ is the density of the air. The scale is graduated in units of airspeed to correspond to the deformation of the sensing element—the manometric (aneroid) capsule—of the airspeed indicator. When measuring flying speeds above 800 km/hr, a correction is made to take into account the compressibility of air.
The readings of the instrument are directly proportional to the value of ρ, which depends on the pressure p and the temperature T of the ambient air. If they are assumed to be constant(p = 101,325 newtons per sq m = 760 mm of mercury; T = 288° K), then the instrument will show the indicated (instrument) airspeed. If, however, the readings are corrected for their variation with altitude (this is done automatically by a device within the mechanical linkage between the sensing element and the needle), the instrument will show the true airspeed. In practice, airspeed indicators have two needles (combined airspeed indicators), one of which shows instrument readings, and the other the true airspeed.
A. L. GORELIK