Equivalent airspeed
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aviation Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' include fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air aircraft such as h ...
, equivalent airspeed (EAS) is calibrated airspeed (CAS) corrected for the compressibility of air at a non-trivial
Mach number The Mach number (M or Ma), often only Mach, (; ) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a boundary to the local speed of sound. It is named after the Austrian physicist and philosopher Erns ...
. It is also the
airspeed In aviation, airspeed is the speed of an aircraft relative to the air it is flying through (which itself is usually moving relative to the ground due to wind). In contrast, the ground speed is the speed of an aircraft with respect to the sur ...
at sea level in the
International Standard Atmosphere The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. It has been established to provide ...
at which the dynamic pressure is the same as the dynamic pressure at the
true airspeed The true airspeed (TAS; also KTAS, for ''knots true airspeed'') of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Tra ...
(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 The airspeed indicator (ASI) or airspeed gauge is a flight instrument indicating the airspeed of an aircraft in kilometres per hour (km/h), knots (kn or kt), miles per hour (MPH) and/or metres per second (m/s). The recommendation by ICAO is to u ...
with zero error.Houghton, E.L. and Carpenter, P.W. (1993), ''Aerodynamics for Engineering Students'', Section 2.3.3, Butterworth-Heinemann, Oxford UK. It is useful for predicting aircraft handling, aerodynamic loads, stalling etc. \mathrm = \mathrm \times \sqrt where is actual
air density The density of air or atmospheric density, denoted '' ρ'', is the mass per unit volume of Earth's atmosphere at a given point and time. Air density, like air pressure, decreases with increasing altitude. It also changes with variations in atmosph ...
and is standard sea level density (1.225 kg/m3 or 0.00237 slug/ft3). is a function of dynamic pressure: \mathrm = \sqrt where is the dynamic pressure q = \tfrac12\, \rho\, v^. EAS can also be obtained from the aircraft
Mach number The Mach number (M or Ma), often only Mach, (; ) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a boundary to the local speed of sound. It is named after the Austrian physicist and philosopher Erns ...
and
static pressure In fluid mechanics the term static pressure refers to a term in Bernoulli's equation written words as ''static pressure + dynamic pressure = total pressure''. Since pressure measurements at any single point in a fluid always give the static pres ...
. \mathrm = M \sqrt where is (the standard speed of sound at 15 °C), is the Mach number, is static pressure, and 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): \mathrm = a_0 \sqrt where is impact pressure. At standard sea level, EAS is the same as calibrated airspeed (CAS) and
true airspeed The true airspeed (TAS; also KTAS, for ''knots true airspeed'') of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Tra ...
(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: \mathrm = where the pressure ratio \delta = \tfrac, 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

*
Acronyms and abbreviations in avionics Below are abbreviations used in aviation, avionics, aerospace, and aeronautics. A B C D E F G H I J K L M N N numbers (turbines) O P Q R S T U V V speeds W X Y Z See also * List of avia ...
* ICAO recommendations on use of the International System of Units * Calibrated airspeed *
Flight instruments Flight instruments are the instruments in the cockpit of an aircraft that provide the pilot with data about the flight situation of that aircraft, such as altitude, airspeed, vertical speed, heading and much more other crucial information in f ...
*
Global Positioning System The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide ge ...
*
Indicated airspeed Indicated airspeed (IAS) is the airspeed of an aircraft as measured by its pitot-static system and displayed by the airspeed indicator (ASI). This is the pilots' primary airspeed reference. This value is not corrected for installation error, ...
*
Position error Position 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 ...
*
True airspeed The true airspeed (TAS; also KTAS, for ''knots true airspeed'') of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Tra ...


References


Bibliography

* Anderson, John D. (2007), ''Fundamentals of Aerodynamics'', Section 3.4 (4th edition), McGraw-Hill, New York USA. * Gracey, William (1980)
"Measurement of Aircraft Speed and Altitude"
{{Webarchive, url=https://web.archive.org/web/20210926222531/https://apps.dtic.mil/dtic/tr/fulltext/u2/a280006.pdf , date=2021-09-26 (11 MB), NASA Reference Publication 1046.


External links


Equivalent airspeed calculator
Airspeed