True airspeed: Difference between revisions

The '''true [[airspeed]]''' ('''TAS'''; also '''KTAS''', for ''knots true airspeed'') of an [[aircraft]] is the [[speed]] of the aircraft relative to the airmass[[air inmass]] through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Traditionally it is measured using an analogue [[airspeed indicator|TAS indicator]], but as the [[Global Positioning System|GPS]] has become available for civilian use, the importance of such analogueair-measuring instruments has decreased. Since ''indicated'', as opposed to ''true'', airspeed is a better indicator of powermargin usedabove andthe lift[[Stall available(fluid dynamics)|stall]], Truetrue airspeed is not used for controlling the aircraft during taxiing, takeoff, climb, descent, approach or landing; for these purposes the [[Indicatedindicated airspeed]] – IAS or KIAS (knots indicated airspeed) – is used. However, since indicated airspeed only shows true speed through the air at standard sea level pressure and temperature, a TAS meter is necessary for navigation purposes at cruising altitude in less dense air. The IAS meter reads very nearly the TAS at lower altitude and at lower speed. On jet airliners the TAS meter is usually hidden at speeds below {{convert|200|kn|km/h}}. Neither provides for accurate [[Ground speed|speed over the ground]], since surface winds or winds aloft are not taken into account.

The [[airspeed indicator]] (ASI), driven by ram air into a [[Pitotpitot tube]] and still air into a barometric static port, and shows what is called [[indicated airspeed]] (IAS). The differential pressure is affected by [[air density]]. The ratio between the two measurements is temperature-dependent and pressure-dependent, according to the [[ideal gas law]].

At sea level in the [[International Standard Atmosphere]] (ISA) and at low speeds where air compressibility is negligible (i.e., assuming a constant air density), IAS corresponds to TAS. When the air density or temperature around the aircraft differs from standard sea level conditions, IAS will no longer correspond to TAS, thus it will no longer reflect aircraft performance. The ASI will indicate less than TAS when the air density decreases due to a change in altitude or air temperature. For this reason, TAS cannot be measured directly. In flight, it can be calculated either by using an [[E6B]] flight calculator or its equivalent.

For low speeds, the data required are [[outside air temperature|static air temperature]], pressure altitude and IAS (or [[Calibrated_airspeedCalibrated airspeed|CAS]] for more precision). Above approximately {{convert|100|kn|km/h}}, the compressibility error rises significantly and TAS must be calculated by the Mach speed. Mach incorporates the above data including the compressibility factor. Modern aircraft instrumentation use an ''air data computer'' to perform this calculation in real time and display the TAS reading directly on the [[electronic flight instrument system]].

Since temperature variations are of a smaller influence, the ASI error can be roughly estimated as indicating about 2% less than TAS per {{convert|1000|ft|m|sigfig=1}} of altitude above sea level. For example, an aircraft flying at {{convert|15000|ft|m}} in the international standard atmosphere with an IAS of {{convert|100|kn|km/h}}, is actually flying at {{convert|126|kn|km/h}} TAS.

To maintain a desired [[ground track]] whilstwhile flying in the moving airmass, the pilot of an aircraft must use knowledge of wind speed, wind direction, and true air speed to determine the required heading. See also [[wind triangle]].

At low speeds and altitudes, IAS and CAS are close to [[equivalent airspeed]] (EAS). TAS can be calculated as a function of EAS and air density:<ref>Clancy, L. J., ''Aerodynamics'', Section 3.8.</ref>

<math>\mathrm{TAS}rho_0 (EAS)^2 = \mathrm{EAS}\sqrt{\frac{\rho_0}{\rho}}, (TAS)^2</math>

In simple aircraft, without an air data computer or [[Machmetermachmeter]], true airspeed can be calculated as a function of [[calibrated airspeed]] and local air density (or static air temperature and pressure altitude, which determine density). Some airspeed indicators incorporate a [[slide rule]] mechanism to perform this calculation. Otherwise, it can be performed using [http://www.newbyte.co.il/calc.html this applet] or a device such as the [[E6B]] (a handheld circular [[slide rule]]).

* [http://www.newbyte.co.il/calccalculator/index.htmlphp Newbyte airspeed converter], [https://archive.is/20121209064246/http://market.android.com/details?id=appinventor.ai_barkan86.AtmosCalculatorFree Android Version]