Information AboutAirspeed |
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| aviation terminology | |
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For the British aircraft manufacturer, see Airspeed Ltd . Airspeed refers to the speed of an Aircraft relative to the air. There are several different measures of airspeed viz. indicated airspeed, calibrated airspeed, equivalent airspeed and true airspeed. The different measures of airspeed and their derivation from the aircraft instrumentation are discussed below. INDICATED AIRSPEED Indicated Airspeed (IAS) is the Airspeed Indicator reading (ASIR) corrected only for instrument error. (Source BCAR Section D Aeroplanes) Outside of the former Soviet bloc, most airspeed indicators show the speed in Knot s i.e. nautical miles per hour. Some light aircraft have airspeed indicators showing speed in miles per hour. An Airspeed Indicator is a differential pressure gauge with the pressure reading expressed in units of speed, rather than pressure. The airspeed is derived from the difference between the Total Pressure i.e. static plus Dynamic Air Pressure and the Static Air Pressure . The total pressure is usually detected by a pitot tube which is mounted facing forward; the static pressure is usually detected at static ports on one or both sides of the aircraft. Sometimes both pressure sources are combined in a single probe, a Pitot-static Tube . The static pressure measurement is subject to error due to inability to place the static ports at positions where the pressure is true static pressure. The correction for this error is the Position Error correction (PEC). The airspeed indicator is shows the airspeed calculated as follows: ;Where : : is the indicated airspeed, : is the total air pressure sensed by the pitot tube, : is the static air pressure sensed at the static source, : is 1.225 kg/m³, the air density at sea level, ISA conditions. This expression is based on the form of Bernoulli's Equation applicable to a perfect, incompressible, gas. The air density used is that at SL, ISA i.e. the conditions under which airspeed indicators are calibrated. (Modern airspeed indicators are calibrated for compressible flow at sea level ISA, see link to Calibrated Airspeed ) CALIBRATED AIRSPEED Calibrated Airspeed (CAS) is indicated airspeed corrected for position error. EQUIVALENT AIRSPEED Equivalent Airspeed (EAS) is calibrated airspeed corrected for error due to air compressibility which arises at high altitudes and mach numbers. Under standard sea level conditions EAS is the same as CAS. Let represent the dynamic pressure . Then the relationship between the pressure difference sensed by a pitot-static system and the dynamic pressure is given by: ;Where : : is the Mach number, : is the true airspeed, : is the equivalent airspeed, : is the ratio of the specific heats of air and : is the air density. The ratio of the specific heats, , is 1.4 in air. Substituting this value gives: (This section needs editing due to confusion between V (TAS) and Vi (CAS) and ambiguity regarding ASI calibration - incompressible flow equation above or compressible flow equation under Calibrated Airspeed ? If the ASI is calibrated to the CAS calibration equation which (for subsonic speeds) eliminates compressibility error at standard sea level then the compressibility correction above is not valid. See also link to Equivalent Airspeed ) This approximation is valid up to about Mach 2.3. Source: Aerodynamics of a Compressible Fluid. Liepmann and Puckett 1947. Publishers John Wiley & Sons Inc. The difference between calibrated airspeed and equivalent airspeed is negligible at low Mach numbers rising to 3% at Mach 0.5 and 13% at Mach 1 depending on altitude. The significance of equivalent airspeed is that at Mach numbers below the onset of wave drag, all of the aerodynamic forces and moments on an aircraft scale with the square of the equivalent airspeed. The equivalent airspeed is closely related to the Indicated Airspeed speed shown by the Airspeed Indicator . Thus, the handling and 'feel' of an aircraft, and the aerodynamic loads upon it, at a given equivalent airspeed, are very nearly constant and equal to those at SL, ISA irrespective of the actual flight conditions. TRUE AIRSPEED True Airspeed (TAS) is the true speed of the aircraft relative to the air. It differs from the equivalent airspeed because the airspeed indicator is calibrated at SL,ISA conditions, where the air density is 1.225 kg/m³ , whereas the air density in flight normally differs from this value. : Thus : ;Where : : is the air density at the flight condition. The air density may be calculated from: : ;Where : : is the air pressure at the flight condition, : is the air pressure at sea level = 1013.2 hPa, : is the air temperature at the flight condition, : is the air temperature at sea level, ISA = 288.15 K. Source: Aerodynamics of a Compressible Fluid. Liepmann and Puckett 1947. Publishers John Wiley & Sons Inc. GROUNDSPEED Groundspeed is the speed of the aircraft relative to the ground. It is not possible to determine groundspeed directly from the basic flight instrumentation i.e. the altimeter, airspeed indicator an outside air temperature gauge. Some additional means to provide position information is required. This might be traditional navigation, radio aided position location, inertial navigation or GPS . EXTERNAL LINKS |
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