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Equivalent airspeed (EAS) is 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. In low-speed flight, it is the speed which would be shown by an airspeed indicator with zero error. It is useful for predicting aircraft handling, aerodynamic loads, stalling etc.
is actual air density.
is standard sea level density (1.225 kg/m³ -or- 0.00237 slugs/ft³).
EAS is a function of dynamic pressure.
is dynamic pressure.
(this equation requires a consistent system of measurement)
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):
is impact pressure.
A simplified formula can be used that allows calculation of CAS from EAS.
& the airspeeds in either knots, km/h, mph or any other appropriate unit
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.
- Clancy, L.J. (1975), Aerodynamics, Section 3.8, Pitman Publishing Limited, London. ISBN 0-273-01120-0
- Anderson, John D. (2007), Fundamentals of Aerodynamics, p.215 (4th edition), McGraw-Hill, New York USA. ISBN 978-0-07-295046-5
- Houghton, E.L. and Carpenter, P.W. (1993), Aerodynamics for Engineering Students, Section 2.3.3, Butterworth-Heinemann, Oxford UK. ISBN 0-340-54847-9
- Anderson, John D. (2007), Fundamentals of Aerodynamics, Section 3.4 (4th edition), McGraw-Hill, New York USA. ISBN 978-0-07-295046-5
- Gracey, William (1980), "Measurement of Speed and Altitude", NASA Reference Publication 1046.