Equivalent airspeed
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Equivalent airspeed (EAS) is calibrated airspeed (CAS) corrected for the compressibility of air at a nontrivial Mach number. It is also the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the true airspeed (TAS) and altitude at which the aircraft is flying.^{[1]}^{[2]} In lowspeed flight, it is the speed which would be shown by an airspeed indicator with zero error.^{[3]} It is useful for predicting aircraft handling, aerodynamic loads, stalling etc.
where:
is actual air density.
is standard sea level density (1.225 kg/m^{3} or 0.00237 slug/ft^{3}).
EAS is a function of dynamic pressure.
where:
is dynamic pressure
EAS can also be obtained from the aircraft Mach number and static pressure.
where:
is the standard speed of sound at 15 °C (661.47 knots)
is Mach number
is static pressure
is standard sea level pressure (1013.25 hPa)
Combining the above with the expression for Mach number gives EAS as a function of impact pressure and static pressure (valid for subsonic flow):
where:
is impact pressure.
At standard sea level, EAS is the same as calibrated airspeed (CAS) and true airspeed (TAS). At any other altitude, EAS may be obtained from CAS by correcting for compressibility error.
The following simplified formula allows calculation of CAS from EAS:
where:
pressure ratio:
and are airspeeds and can be measured in knots, km/h, mph or any other appropriate unit.
The above formula is accurate within 1% up to Mach 1.2 and useful with acceptable error up to Mach 1.5. The 4th order Mach term can be neglected for speeds below Mach 0.85.
See also
Notes
 ^ Clancy, L.J. (1975), Aerodynamics, Section 3.8, Pitman Publishing Limited, London. ISBN 0273011200
 ^ Anderson, John D. (2007), Fundamentals of Aerodynamics, p.215 (4th edition), McGrawHill, New York USA. ISBN 9780072950465
 ^ Houghton, E.L. and Carpenter, P.W. (1993), Aerodynamics for Engineering Students, Section 2.3.3, ButterworthHeinemann, Oxford UK. ISBN 0340548479
References
 Anderson, John D. (2007), Fundamentals of Aerodynamics, Section 3.4 (4th edition), McGrawHill, New York USA. ISBN 9780072950465
 Gracey, William (1980), "Measurement of Aircraft Speed and Altitude" (11 MB), NASA Reference Publication 1046.