Airspeed - Wikipedia, The Free Encyclopedia

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An airspeed indicator is a flight

instrument that displays airspeed. This

airspeed indicator has standardizedmarkings for a multiengine airplane

Aircraft have pitot tubes for measuring

airspeed

AirspeedFrom Wikipedia, the free encyclopedia

Airspeed is the speed of an aircraft relative to the air. Among the

common conventions for qualifying airspeed are: indicated airspeed

("IAS"), calibrated airspeed ("CAS"), true airspeed ("TAS"), equivalent

airspeed ("EAS") and density airspeed.

The measurement and indication of airspeed is ordinarily accomplished

on board an aircraft by an airspeed indicator ("ASI") connected to a

pitot-static system. The pitot-static system comprises one or more pitot

probes (or tubes) facing the on-coming air flow to measure pitot pressure

(also called stagnation, total or ram pressure) and one or more static

ports to measure the static pressure in the air flow. These two pressures

are compared by the ASI to give an IAS reading.

Contents

1 Indicated airspeed

2 Calibrated airspeed

3 Equivalent airspeed

4 True airspeed

5 Groundspeed

6 See also

7 References

8 External links

Indicated airspeed

Main article: Indicated airspeed

Indicated airspeed (IAS) is the airspeed indicator reading (ASIR) uncorrected for instrument, position, and other

errors. From current EASA definitions: Indicated airspeed means the speed of an aircraft as shown on its pitot

static airspeed indicator calibrated to reflect standard atmosphere adiabatic compressible flow at sea level

uncorrected for airspeed system errors.[1]

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.

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 ram air pressure from the pitot

tube, or stagnation pressure, and the static pressure. The pitot tube is mounted facing forward; the static

pressure is frequently 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 at all airspeeds and

attitudes. The correction for this error is the position error correction (PEC) and varies for different aircraft and

eed - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/

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airspeeds. Further errors of 10% or more are common if the airplane is flown in uncoordinated flight.

Calibrated airspeed

Main article: Calibrated airspeed

Calibrated airspeed (CAS) is indicated airspeed corrected for instrument errors, position error (due to incorrect

pressure at the static port) and installation errors.

Calibrated airspeed values less than the speed of sound at standard sea level (661.4788 knots) are calculated as

follows:

minus position and installation error correction.

Where

is the calibrated airspeed,

is the impact 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.

Units other than knots and inches of mercury can be used, if used consistently.

This expression is based on the form of Bernoulli's equation applicable to a perfect, compressible gas. The

values for and are consistent with the ISA i.e. the conditions under which airspeed indicators are

calibrated.

Equivalent airspeed

Main article: Equivalent airspeed

Equivalent airspeed (EAS) is defined as the speed at sea level that would produce the same incompressible

dynamic pressure as the true airspeed at the altitude at which the vehicle is flying. An aircraft in forward flight is

subject to the effects of compressibility. Likewise, the calibrated airspeed is a function of the compressible

impact pressure. EAS, on the other hand, is a measure of airspeed that is a function of incompressible dynamic

pressure. Structural analysis is often in terms of incompressible dynamic pressure, so that equivalent airspeed is

a useful speed for structural testing. At standard sea level pressure, calibrated airspeed and equivalent airspeed

are equal. Up to about 200 knots CAS and 10,000 feet the difference is negligible, but at higher speeds and

altitudes CAS must be corrected for compressibility error to determine EAS. 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 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

standard sea level irrespective of the actual flight conditions.

True airspeed

Main article: True airspeed


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True airspeed (TAS) is the speed of the aircraft relative to the atmosphere. The true airspeed and heading of an

aircraft constitute its velocity relative to the atmosphere. The vector relationship between the true airspeed and

the speed with respect to the ground is:

where:

= Windspeed vector

Aircraft flight instruments, however, don't compute true airspeed as a function of groundspeed and windspeed.

They use impact and static pressures as well as a temperature input. True airspeed is equivalent airspeed that is

corrected for pressure altitude and temperature (which define density). The result is the true physical speed of

the aircraft plus or minus the wind component. True Airspeed is equal to calibrated airspeed and equivalent

airspeed at standard sea level conditions.

The simplest way to compute true airspeed is using a function of Mach number:

Where:

= Speed of sound at standard sea level (661.4788 knots)

= Mach number

= Temperature (kelvins)

= Standard sea level temperature (288.15 kelvins)

Or if Mach number is not known:

Where:

= Speed of sound at standard sea level (661.4788 knots)

= Impact pressure (inHg)

= Static pressure (inHg)

= Temperature (kelvins)

= Standard sea level temperature (288.15 kelvin)

The above equation is only for Mach numbers less than 1.0.

True airspeed differs from the equivalent airspeed because the airspeed indicator is calibrated at SL, ISA

conditions, where the air density is 1.225 kg/m, whereas the air density in flight normally differs from this

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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.

Groundspeed

Main article: Ground speed

Ground speed is the speed of the aircraft relative to the ground rather than through the air, which can itself be

moving.

See also

V speeds

Maneuvering speed

References

^ http://easa.europa.eu/ws_prod/g/doc/Agency_Mesures/Certification_Spec/decision_ED_2003_11_RM.pdf1.

Glauert H.,The Elements of Aerofoil and Airscrew Theory,

Chapter 2, Cambridge University Press, 1947Liepmann 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 (http://www.luizmonteiro.com/Altimetry.aspx)

Calculate Ground Speed and Wind Triangles (http://www.luizmonteiro.com/Wind.aspx)

True, Equivalent, and Calibrated Airspeed (http://www.mathpages.com/home/kmath282/kmath282.htm)

at MathPages


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Measurement of Aircraft Airspeed and Altitude (http://www.spaceagecontrol.com/pm/uploads

/Main.Freepubs/nasa-rp-1046.pdf) (NASA Publication 1046)

Newbyte airspeed converter (http://www.newbyte.co.il/calc.html)

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Categories: Aerodynamics Airspeed Aviation terminology

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Airspeed - Wikipedia, The Free Encyclopedia