Aircraft speed: Difference between revisions

 
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=== True airspeed ===
=== True airspeed ===
* [http://en.wikipedia.org/wiki/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.  
[[File:C182s-ais-tas.png|thumb|Airspeed Indicator from the [[Cessna_182S]] showing difference between IAS (118 knots) and TAS (134 knots).Note the white TAS disc was rotated to match OAT and pressure altitude (+10° / 8000ft)]]


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, unless standing still on the ground where the TAS can be "seen" with the windbag.
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, unless standing still on the ground where the TAS can be "seen" with the windbag.
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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.


[[File:C182s-ais-tas.png|thumb|Airspeed Indicator from the [[Cessna_182S]] showing difference between IAS (118 knots) and TAS (134 knots).Note the white TAS disc was rotated to match OAT and pressure altitude (+10° / 8000ft)]]
Some planes Airspeed Indicators have a disc that can be rotated to obtain the TAS. Usually you need to set your Altimeter to pressure Altitude (29.92 inHg / 1013.25 hPa) and rotate the disc so the altitude and outside air temperature (OAT) align. The needle then will point to the TAS. The differences can get quite huge.
Some planes Airspeed Indicators have a disc that can be rotated to obtain the TAS. Usually you need to set your Altimeter to pressure Altitude (29.92 inHg / 1013.25 hPa) and rotate the disc so the altitude and outside air temperature (OAT) align. The needle then will point to the TAS. The differences can get quite huge.


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