The air-speed indicator: indicated and equivalent air speeds

A Pitot-static tube is commonly used to measure air speed both in the laboratory and on aircraft. There are, however, differences in the requirements for the two applica­tions. In the laboratory, liquid manometers provide a simple and direct method for

measuring pressure. These would be completely unsuitable for use on an aircraft where a pressure transducer is used that converts the pressure measurement into an electrical signal. Pressure transducers are also becoming more and more commonly used for laboratory measurements.

When the measured pressure difference is converted into air speed, the correct value for the air density should, of course, be used in Eqn (2.19). This is easy enough in the laboratory, although for accurate results the variation of density with the ambient atmospheric pressure in the laboratory should be taken into account. At one time it was more difficult to use the actual air density for flight measurements. This was because the air-speed indicator (the combination of Pitot-static tube and transducer) would have been calibrated on the assumption that the air density took the standard sea-level International Standard Atmosphere (ISA) value. The (incor­rect) value of air speed obtained from Eqn (2.19) using this standard value of pressure with a hypothetical perfect transducer is known as the equivalent air speed (EAS). A term that is still in use. The relationship between true and equivalent air speed can be derived as follows. Using the correct value of density, p, in Eqn (2.19) shows that the relationship between the measured pressure difference and true air speed, v, is

Ap = -pv2

whereas if the standard value of density, po = 1.226kg/m3, is used we find

(2.26)

where ve is the equivalent air speed. But the values of Ap in Eqns (2.25) and (2.26) are the same and therefore

(2.27)

or

ve = vy/p/po

If the relative density a = p/po is introduced, Eqn (2.28) can be written as

ve vyfa

The term indicated air speed (IAS) is used for the measurement made with an actual (imperfect) air-speed indicator. Owing to instrument error, the IAS will normally differ from the EAS.

The following definitions may therefore be stated: IAS is the uncorrected reading shown by an actual air-speed indicator. Equivalent air speed EAS is the uncorrected reading that would be shown by a hypothetical, error-free, air-speed indicator. True air speed (TAS) is the actual speed of the aircraft relative to the air. Only when (7=1 will true and equivalent air speeds be equal. Normally the EAS is less than the TAS.

Formerly, the aircraft navigator would have needed to calculate the TAS from the IAS. But in modem aircraft, the conversion is done electronically. The calibration of the air-speed indicator also makes an approximate correction for compressibihty.