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Aircraft speed: Difference between revisions
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=== True Airspeed === | === True Airspeed === | ||
*'''True''' airspeed (TAS) is the speed in which the aircraft moves relative to the surrounding air. | *[http://en.wikipedia.org/wiki/True_airspeed|'''True''' airspeed (TAS)] is the speed in which the aircraft moves relative to the surrounding air. | ||
The difference between TAS and GS is that the air itself may move with respect to the ground (that's wind), and dependent on course relative to the wind direction a discrepancy between TAS and GS is induced. TAS can't really be measured directly but needs to be calculated. | The difference between TAS and GS is that the air itself may move with respect to the ground (that's wind), and dependent on course relative to the wind direction a discrepancy between TAS and GS is induced. TAS can't really be measured directly but needs to be calculated. | ||
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=== Indicated Airspeed === | === Indicated Airspeed === | ||
*'''Indicated''' airspeed (IAS) is the number displayed on the airspeed indicator. | *[http://en.wikipedia.org/wiki/Indicated_airspeed|'''Indicated''' airspeed (IAS)] is the number displayed on the airspeed indicator. | ||
Airspeed is usually measured with a [[#Pitot tube]] at the front of the aircraft. The ram pressure measured with the [[#Pitot tube]] is not the TAS since the pressure differs greatly with [[altitude]] (more specific the [[altitude#Density altitude| density altitude]]). The same IAS will correspond to a vastly different TAS when the plane starts climbing to high altitude. At sea level, a KIAS of 400 knot roughly corresponds to 400 knot TAS, at 80.000 feet (the cruising altitude of the SR-71), the same reading may indicate a TAS in excess of 1600 knot (is can be very difficult to reconcile an airspeed of 400 kt with a reading that one is flying in excess of Mach 3 when one doesn't know what the airspeed gauge shows). | Airspeed is usually measured with a [[#Pitot tube]] at the front of the aircraft. The ram pressure measured with the [[#Pitot tube]] is not the TAS since the pressure differs greatly with [[altitude]] (more specific the [[altitude#Density altitude| density altitude]]). The same IAS will correspond to a vastly different TAS when the plane starts climbing to high altitude. At sea level, a KIAS of 400 knot roughly corresponds to 400 knot TAS, at 80.000 feet (the cruising altitude of the SR-71), the same reading may indicate a TAS in excess of 1600 knot (is can be very difficult to reconcile an airspeed of 400 kt with a reading that one is flying in excess of Mach 3 when one doesn't know what the airspeed gauge shows). | ||
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=== Calibrated Airspeed === | === 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 this correction. | 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 this correction. | ||
=== Equivalent Airspeed === | === 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. | ||
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*The '''Mach number''' (M) is the ratio of an aircraft's TAS over the local speed of sound. | *The '''Mach number''' (M) is the ratio of an aircraft's TAS over the local speed of sound. | ||
A Mach number below 1 means that the plane moves | A Mach number below 1 means that the plane moves subsonic, a Mach number above 1 indicates supersonic flight. The Mach number is critical because a number of phenomena take place just around Mach 1 (transonic speed), for example a sudden increase in drag induced by shock-wave generation. However, since the speed of sound changes with the compressibility (and hence temperature) of air, the Mach number is dependent on [[altitude]] (as the air temperature drops at higher altitudes). This implies that Mach 2 at sea level corresponds to a faster TAS than Mach 2 at 30.000 ft. The precise relations between TAS, Mach number and altitude are rather complicated formulae and depend in essence on the local weather pattern determining the pressure and temperature gradients in the atmosphere. | ||
==V speeds== | ==V speeds== | ||