Equivalent airspeed
Encyclopedia
Equivalent airspeed 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.
Where:
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.
Where:
is dynamic pressure.
(this equation requires a consistent system of measurement)
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 high altitude, EAS may be obtained from CAS by correcting for compressibility error.
A simplified formula can be used that allows calculation of CAS from EAS.
where:
Pressure ratio:
& 
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.
International Standard Atmosphere
The International Standard Atmosphere is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes. It has been established to provide a common reference for temperature and pressure and consists of tables of...
at which the dynamic pressure is the same as the dynamic pressure at the true airspeed
True airspeed
True airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...
(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
Airspeed indicator
The airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the craft's airspeed, typically in knots, to the pilot.- Use :...
with zero error. It is useful for predicting aircraft handling, aerodynamic loads, stalling etc.
Where:
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.
Where:
is dynamic pressure.(this equation requires a consistent system of measurement)
EAS can also be obtained from the aircraft mach number
Mach number
Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...
and static pressure
Static pressure
In fluid mechanics the term static pressure has several uses:* In the design and operation of aircraft, static pressure is the air pressure in the aircraft’s static pressure system....
.
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
Impact pressure
In compressible fluid dynamics, impact pressure is the difference between total pressure and static pressure. In aerodynamics notation, this quantity is denoted as q_c or Q_c....
and static pressure (valid for subsonic flow):
Where:
is impact pressure.At standard sea level EAS is the same as calibrated airspeed
Calibrated airspeed
Calibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...
(CAS) and true airspeed
True airspeed
True airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...
(TAS). At high altitude, EAS may be obtained from CAS by correcting for compressibility error.
A simplified formula can be used that allows calculation of CAS from EAS.
where:
Pressure ratio:
& the airspeeds in either knots, km/h, mph or any other appropriate unitAbove 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
- Position errorPosition errorPosition error is one of the errors affecting the systems in an aircraft for measuring airspeed and altitude. It is not practical or necessary for an aircraft to have an airspeed indicating system and an altitude indicating system that are exactly accurate...
- True airspeedTrue airspeedTrue airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...
- Indicated airspeedIndicated airspeedIndicated airspeed is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed , which is the IAS corrected for instrument and installation errors....
- Calibrated airspeedCalibrated airspeedCalibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...