Airspeed is the speed of an aircraft relative to the air. There are several different measures of airspeed: indicated airspeed, calibrated airspeed, equivalent airspeed and true airspeed. An Airbus A380, currently the worlds largest airliner An aircraft is any vehicle or craft capable of atmospheric flight. ...

Indicated airspeed

Indicated airspeed (IAS) is the airspeed indicator reading (ASIR) corrected only for instrument error. (Source BCAR Section D Aeroplanes) Aircraft display an Indicated Airspeed (abbreviated IAS) on an instrument called an airspeed indicator. ... Airspeed Indicator The airspeed indicator is an instrument used in an aircraft to display the crafts airspeed, typically in knots, to the pilot. ...

Outside of the former Soviet bloc, most airspeed indicators show the speed in knots i.e. nautical miles per hour. Some light aircraft have airspeed indicators showing speed in miles per hour. A knot is a unit of speed, abbreviated kt or kn. ...

An airspeed indicator is a differential pressure gauge with the pressure reading expressed in units of speed, rather than pressure. The airspeed is derived from the difference between the total pressure and the static air pressure. The total pressure is usually detected by a pitot tube which is mounted facing forward; the static pressure is usually detected at static ports on one or both sides of the aircraft. Sometimes both pressure sources are combined in a single probe, a pitot-static tube. The static pressure measurement is subject to error due to inability to place the static ports at positions where the pressure is true static pressure. The correction for this error is the position error correction (PEC) and varies for different aircraft. Airspeed Indicator The airspeed indicator is an instrument used in an aircraft to display the crafts airspeed, typically in knots, to the pilot. ... Static pressure is a term used in ventilation engineering, airspeed indication, fluid statics, hydraulics and flow measurement. ... A pitot-static system is an avionics component which directs ram air pressure and static air pressure lines into the appropriate flight instruments. ... This article belongs in one or more categories. ...

Indicated airspeed values less than the speed of sound at standard sea level (661.4788 knots) are calculated as follows: The speed of sound is a term used to describe the speed of sound waves passing through an elastic medium. ...

minus PEC

Where 

is the indicated airspeed,

is the differential air pressure (inches Hg) sensed by the pitot tube,

is 29.92126 inches Hg; static air pressure at standard sea level,

is 661.4788 knots;, speed of sound at standard sea level.

This expression is based on the form of Bernoulli's equation applicable to a perfect, incompressible gas. The values for P_o and A_o are consistent with the ISA i.e. the conditions under which airspeed indicators are calibrated. In fluid dynamics, Bernoullis equation, derived by Daniel Bernoulli, describes the behavior of a fluid moving along a streamline. ... Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ...

(Modern airspeed indicators are calibrated for incompressible flow at sea level ISA, see link to calibrated airspeed) Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ... Calibrated airspeed (CAS) is indicated airspeed, corrected for instrument error and position error. ...

Calibrated airspeed

Calibrated airspeed (CAS) is indicated airspeed corrected for position error and installation errors. Calibrated airspeed (CAS) is indicated airspeed, corrected for instrument error and position error. ...

Equivalent airspeed

Equivalent airspeed (EAS) is calibrated airspeed corrected for error due to air compressibility which arises at high altitudes and mach numbers. Under standard sea level conditions EAS is the same as CAS. 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. ... Fluid Dynamics Compressibility (physics) is a measure of the relative volume change of fluid or solid as a response to a pressure (or mean stress) change: . For a gas the magnitude of the compressibility depends strongly on whether the process is adiabatic or isothermal, while this difference is small in... An F/A-18 Hornet approaching the sound barrier. ...

Let represent the dynamic pressure .

Then the relationship between the pressure difference sensed by a pitot-static system and the dynamic pressure is given by:

Where 

is the Mach number,

is the true airspeed,

is the equivalent airspeed,

is the ratio of the specific heats of air and

is the air density.

The ratio of the specific heats, , is 1.4 in air. Substituting this value gives: An F/A-18 Hornet approaching the sound barrier. ...

(This section needs editing due to confusion between V (TAS) and Vi (CAS) and ambiguity regarding ASI calibration - incompressible flow equation above or compressible flow equation under calibrated airspeed? If the ASI is calibrated to the CAS calibration equation which (for subsonic speeds) eliminates compressibility error at standard sea level then the compressibility correction above is not valid. See also link to equivalent airspeed) Calibrated airspeed (CAS) is indicated airspeed, corrected for instrument error and position error. ... 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. ...

This approximation is valid up to about Mach 2.3.

Source: Aerodynamics of a Compressible Fluid. Liepmann and Puckett 1947. Publishers John Wiley & Sons Inc.

The difference between calibrated airspeed and equivalent airspeed is negligible at low Mach numbers rising to 3% at Mach 0.5 and 13% at Mach 1 depending on altitude.

The significance of equivalent airspeed is that at Mach numbers below the onset of wave drag, all of the aerodynamic forces and moments on an aircraft scale with the square of the equivalent airspeed. The equivalent airspeed is closely related to the Indicated airspeed speed shown by the airspeed indicator. Thus, the handling and 'feel' of an aircraft, and the aerodynamic loads upon it, at a given equivalent airspeed, are very nearly constant and equal to those at SL, ISA irrespective of the actual flight conditions. Aircraft display an Indicated Airspeed (abbreviated IAS) on an instrument called an airspeed indicator. ... Airspeed Indicator The airspeed indicator is an instrument used in an aircraft to display the crafts airspeed, typically in knots, to the pilot. ... Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ...

True airspeed

True airspeed (TAS) is the true speed of the aircraft relative to the air. It differs from the equivalent airspeed because the airspeed indicator is calibrated at SL,ISA conditions, where the air density is 1.225 kg/m³ , whereas the air density in flight normally differs from this value. True airspeed (TAS) is the speed of an aircraft relative to the airmass in which it flies, i. ... Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ...

Thus

Where 

is the air density at the flight condition.

The air density may be calculated from:

Where 

is the air pressure at the flight condition,

is the air pressure at sea level = 1013.2 hPa,

is the air temperature at the flight condition,

is the air temperature at sea level, ISA = 288.15 K.

Source: Aerodynamics of a Compressible Fluid. Liepmann and Puckett 1947. Publishers John Wiley & Sons Inc. Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ...

Groundspeed

Groundspeed is the speed of the aircraft relative to the ground rather than through the air, which can itself be moving. Ground speed is the speed of an aircraft compared to the ground without factoring in the wind or other weather conditions. ...

References

• Glauert H : The Elements of Aerofoil and Airscrew Theory, Chapter 2, Cambridge University Press, 1947
• Liepmann H W and A E Pucket : Introduction to Aerodynamics of a Compressible Fluid, John Wiley and Sons, Inc. 1947

External links

• Calculate True and Equivalent Airspeed
• Calculate Ground Speed and Wind Triangles