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Aircraft speed: Difference between revisions
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Speed combines two factors, the distance travelled in a certain amount of time. In aviation speed is most often expressed in | Speed combines two factors, the distance travelled in a certain amount of time. In aviation speed is most often expressed in Knot (kt). One knot is one nautical mile per hour. In aircraft the speed is "measured" with a [[#Pitot tube]], the result is not the speed of the aircraft, it is the speed of the air flowing around the aircraft, the [http://en.wikipedia.org/wiki/Airspeed|'''airspeed''']. | ||
In older planes, notably German WW II fighter planes, the airspeed is indicated in kilometres per hour (km/h), which is still used in present-day European glider planes. The conversion factor is 1.852, i.e. you can roughly divide a reading in km/h by two in order to get the value in knot. | In older planes, notably German WW II fighter planes, the airspeed is indicated in kilometres per hour (km/h), which is still used in present-day European glider planes. The conversion factor is 1.852, i.e. you can roughly divide a reading in km/h by two in order to get the value in knot. | ||
If the speed is indicated in | If the speed is indicated in knot, sometimes a 'K' is put before the acronym, so KEAS stands for 'equivalent airspeed in knot'. | ||
For (near-)supersonic planes the speed can be expressed in '''Mach'''. | For (near-)supersonic planes the speed can be expressed in '''Mach'''. | ||
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GS is the velocity in the horizontal direction of the aircraft. I.e. in a steep dive, the aircraft can move very fast, but because the motion is chiefly vertical, the ground-speed can be very small at the same time. This is where the GS differs from the ground-speed of a car. | GS is the velocity in the horizontal direction of the aircraft. I.e. in a steep dive, the aircraft can move very fast, but because the motion is chiefly vertical, the ground-speed can be very small at the same time. This is where the GS differs from the ground-speed of a car. | ||
The GS can be calculated from TAS by correcting it for the wind. | |||
=== True Airspeed === | === True Airspeed === | ||
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Knowing TAS during flight is surprisingly useless - for navigation, ground speed is needed, and aerodynamic limits do not depend on TAS but rather IAS. The chief value of TAS is as a measure of aircraft performance and in pre-flight planning before the wind effect is taken into account. | Knowing TAS during flight is surprisingly useless - for navigation, ground speed is needed, and aerodynamic limits do not depend on TAS but rather IAS. The chief value of TAS is as a measure of aircraft performance and in pre-flight planning before the wind effect is taken into account. | ||
The TAS can be calculated from CAS, air temperature and [[Altitude#Pressure altitude|pressure altitude]]. | |||
Often TAS and GS are assumed (confused) to be the same, they are not. | Often TAS and GS are assumed (confused) to be the same, they are not. | ||
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The same IAS will correspond to a vastly different TAS when the plane starts climbing to high altitude. | The same IAS will correspond to a vastly different TAS when the plane starts climbing to high altitude. | ||
At sea level, a IAS of 400 knot roughly corresponds to 400 knot TAS. At 80.000 feet (the cruising altitude of | At sea level, a IAS of 400 knot roughly corresponds to 400 knot TAS. At 80.000 feet (the cruising altitude of a [[SR-71]]), the IAS of 400 knot corresponds to a TAS in excess of 1600 knot (..that corresponds with about Mach 3 at that altitude). | ||
=== Calibrated Airspeed === | === Calibrated Airspeed === | ||
*[http://en.wikipedia.org/wiki/Calibrated_airspeed|'''Calibrated''' airspeed (CAS)] corresponds to IAS corrected for the measurement error. | *[http://en.wikipedia.org/wiki/Calibrated_airspeed|'''Calibrated''' airspeed (CAS)] corresponds to IAS corrected for the measurement error. | ||
For various reasons, aircraft do not carry 'perfect' sensors as they would be used in a scientific experiment, so usually there is some discrepancy between the actual reading of the gauge and the reading a perfect instrument would show. CAS takes into account | For various reasons, aircraft do not carry 'perfect' sensors as they would be used in a scientific experiment, so usually there is some discrepancy between the actual reading of the gauge and the reading a perfect instrument would show. CAS takes into account these errors and can be calculated from the IAS. | ||
=== Equivalent Airspeed === | === Equivalent Airspeed === | ||
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*[http://en.wikipedia.org/wiki/Equivalent_airspeed|'''Equivalent''' airspeed (EAS)] takes into account another correction (above [[#Calibrated airspeed]], this time having to do with air properties rather than sensor errors. | *[http://en.wikipedia.org/wiki/Equivalent_airspeed|'''Equivalent''' airspeed (EAS)] takes into account another correction (above [[#Calibrated airspeed]], this time having to do with air properties rather than sensor errors. | ||
At high altitude, the compressibility of air changes, so even CAS becomes more and more unreliable. For the SR-71 Blackbird with a ceiling of 85.000 feet, the CAS becomes very unreliable and the plane has to be flown based on a KEAS. For more conventional aircraft, EAS is not used. Thus, EAS is what a perfect dynamic pressure sensor would show when properly calibrated for the air compressibility at the current altitude. | At high altitude, the compressibility of air changes, so even CAS becomes more and more unreliable. For the [[SR-71]] Blackbird with a ceiling of 85.000 feet, the CAS becomes very unreliable and the plane has to be flown based on a KEAS. For more conventional aircraft, EAS is not used. Thus, EAS is what a perfect dynamic pressure sensor would show when properly calibrated for the air compressibility at the current altitude. | ||
=== Mach number === | === Mach number === | ||