- True airspeed
True airspeed (TAS) is the speed of an aircraft relative to the airmass in which it flies, i.e. the magnitude of the vector difference of the velocity of the aircraft and the velocity of the air. Under zero wind conditions and in horizontal flight, this is equal to the speed over the ground. Under wind conditions an estimation of the wind is used to make a windspeed vector calculation that computes an estimated ground speed from the true air speed and a wind correction angle to maintain the desired ground track.
Aircraft display an
indicated airspeed on an instrument called anairspeed indicator . Indicated airspeed will differ from true airspeed at air densities other than some reference density. Air density is affected by temperature, moisture content, and altitude. Indicated airspeed is used in aircraft "operation" as the aircraft stalling speed and structural limiting speeds are dependent on indicated airspeed, irrespective of true airspeed. However, proper navigation viadead reckoning (without constant ground reference) requires the use of true airspeed and wind corrections.TAS can be calculated as a function of
Mach number and static air temperature:;Where : true airspeed: is the standard speed of sound at 15 °C (661.47 knots): is Mach number,: is static air temperature in
kelvin s,: is standard sea level temperature (288.15 K)Combining the above with the expression for Mach number under subsonic compresible flow gives an expression for TAS as a function of impact pressure (
pitot tube ), static pressure and static air temperature:;Where : is impact pressure: is static pressure
Electronic Flight Instrument System s (EFIS) contain anair data computer with inputs of impact pressure, static pressure andtotal air temperature . In order to compute TAS the air data computer must convert total air temperature to static air temperature. This is a function of Mach number:;Where: total air temperature
For manual calculation of TAS in knots where Mach number and static air temperature are known the expression may be simplified to:
remembering that temperature is in kelvins.
In simple aircraft, without an air data computer or
Machmeter , true airspeed can be calculated as a function ofcalibrated airspeed and local air density (or static air temperature and pressure altitude which determine density). Some airspeed indicators incorporate aslide rule mechanism to perform this calculation. Otherwise, it can be performed using [http://www.flightplan.za.net/trueAirspeed.php this java applet] or a device such as theE6B (a handheld circularslide rule ).ee also
*
Equivalent airspeed
*Calibrated airspeed
*Flight planning References
External links
* [http://www.mathpages.com/home/kmath282/kmath282.htm True, Equivalent, and Calibrated Airspeed] at MathPages
* [http://www.newbyte.co.il/calc.html Newbyte airspeed converter]
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