Airspeeds—IAS, CAS, EAS, TAS

First, it is essential to mention the speed of sound, which is given by

a = л/yRT (A. 10)

where g is the ratio of specific heat (with constant pressure to constant volume) and taken as 1.4, R is the gas constant with a value of 287 J/Kg K. For sea-level conditions, where temperature T is 288 K, the speed of sound is computed to be nearly 340 m/s.

It has already been mentioned that the pitot-static tube can be used to measure the airspeed. The (PT — Ps) measured by the pitot-static tube is passed on to an indicator, which is calibrated to standard sea-level conditions. The airspeed read from this instrument is called the indicated airspeed (IAS). IAS will be affected by instrument and position errors. Instrument errors could arise from mechanical inac­curacies while position errors are caused by the location of the pitot-static tube in the flow field, which is distorted because of the interference from fuselage or wing. Correction for these errors in IAS will yield what is known as calibrated airspeed (CAS). Modern aircraft have airspeed indicators that directly read CAS [1,2].

True airspeed (TAS) is the actual airspeed of the aircraft relative to the air. The relationship between TAS and ground speed is given by

TAS = Vg + Vw

where Vg is the speed with respect to the ground and Vw is the wind speed.

Equivalent airspeed (EAS) is defined as the speed at standard sea-level condi­tions, which produces the same dynamic pressure as the TAS, i. e.,

2 P0CEAS)2 = 2 p(TAS)2 or

EAS p

TAS =—where s = — (A.11)

Vs P0

Thus, TAS results when EAS is corrected for density altitude. When TAS is known, Mach number can be computed using the relation

M = = JgRr where a is the speed of sound (340 m/s at sea level).

In terms of total pressure PT and static pressure P at a given flight altitude, TAS and EAS for subsonic flight (M < 0.3) can be obtained using the relation:

2(PT — P)

V P0
where po is the air density at sea level. Interestingly, Mach number can be viewed as the ratio of inertia force to elastic force.

For high subsonic flight (M > 0.3), TAS and CAS are given by

(A. 13)

where a is the speed of sound at given altitude, a0 is the speed of sound at sea-level conditions, and P0 is the static pressure at sea level (taken as 1.01325 x 105 N/m2). In other words, the relationship between TAS and CAS can also be expressed as

Aircraft/speeds are classified as subsonic M < 1; transonic 0.8 < M < 1.2; super­sonic 1 < M < 5; and hypersonic M > 5.