Equivalent airspeed
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Equivalent airspeed (EAS) is the airspeed which represents the same dynamic pressure at altitude that would be generated by flying at the corresponding true airspeed (TAS) at sea level. It is useful for predicting aircraft handling, aerodynamic loads, stalling etc.
<math>EAS = TAS \times \sqrt{\frac{actual\ air\ density}{standard\ air\ density}}</math>
- Where
- standard air density is 1.225 kg/m³.
EAS can also be obtained from the aircraft mach number and static air pressure.
<math>EAS ={a_{sl}} M_a \sqrt{P\over P_{sl}}</math>
- Where
- <math>{a_{sl}}</math> is the standard speed of sound at 15 °C (661.47 knots)
- <math>M_a</math> is Mach number,
- <math>P</math> is static air pressure,
- <math>P_{sl}</math> is standard sea level pressure (1013.25 hPa)
Combining the above with the expression for Mach number as a function of impact and static pressures gives, for subsonic compresible flow:
<math>EAS={a_{sl}}\sqrt{{5P\over P_{sl}}[(\frac{q_c}{P}+1)^\frac{2}{7}-1]}</math>
- Where
- <math>{q_c}</math> is impact pressure
At sea level EAS is the same as calibrated airspeed (CAS). At high altitude, EAS may be obtained from CAS by correcting for compressibility error.
