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Aircraft speed: Difference between revisions
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Speed combines two factors, the distance travelled in a certain amount of time. In aviation speed is most often expressed in Knots (kt). One knot is one nautical mile per hour. In aircraft the speed is "measured" with a [[#Pitot tube]], the result is not the speed of the aircraft, it is the speed of the air flowing around the aircraft, the '''airspeed'''. | |||
In older planes, notably German WW II fighter planes, the airspeed is indicated in kilometres per hour (km/h), which is still used in present-day European glider planes. The conversion factor is 1.852, i.e. you can roughly divide a reading in km/h by two in order to get the value in knot. | |||
If the speed is indicated in knots, sometimes a 'K' is put before the acronym, so KEAS stands for 'equivalent airspeed in knots'. | |||
For (near-)supersonic planes the speed can be expressed in '''Mach'''. | |||
==Expressing speed== | ==Expressing speed== | ||
=== Ground Speed === | === Ground Speed === | ||
*'''Ground''' speed (GS) is the horizontal speed in which the aircraft moves relative to a fixed point on the ground. | |||
One needs to know ground speed in order to see how long a flight from A to B actually takes. Nowadays GS can be directly measured using a GPS system, and some aircraft equipped with such a system have a groundspeed indicator. Without a GPS, GS has to be calculated from airspeed and the local wind pattern or estimated by measuring the time between passing two points on the ground with a known distance, but in Flightgear you can always cheat and get it from the property browser under <tt>velocities/groundspeed-kt</tt>. | |||
GS is the velocity in the horizontal direction of the aircraft. I.e. in a steep dive, the aircraft can move very fast, but because the motion is chiefly vertical, the ground-speed can be very small at the same time. This is where the GS differs from the ground-speed of a car. | |||
=== True Airspeed === | === 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. | |||
Knowing TAS during flight is surprisingly useless - for navigation, ground speed is needed, and aerodynamic limits do not depend on TAS but rather IAS. The chief value of TAS is as a measure of aircraft performance and in pre-flight planning before the wind effect is taken into account. | Knowing TAS during flight is surprisingly useless - for navigation, ground speed is needed, and aerodynamic limits do not depend on TAS but rather IAS. The chief value of TAS is as a measure of aircraft performance and in pre-flight planning before the wind effect is taken into account. | ||
Often TAS and GS are assumed (confused) to be the same, they are not. | |||
=== Indicated Airspeed === | === Indicated Airspeed === | ||
'''Indicated''' airspeed (IAS) is | *'''Indicated''' airspeed (IAS) is the number displayed on the airspeed indicator. | ||
Airspeed is usually measured by means of a [[#Pitot tube]] at the front of the aircraft which is exposed to the airstream and hence registers not only the ambient air pressure but also the dynamical ram pressure (created by the plane motion ramming air into the tube). This dynamic pressure component is a measure for the aircraft velocity, and airspeed gauges can be thought of as dynamic pressure gauges with funny labels in terms of velocities. | |||
It becomes apparent that IAS is in fact not TAS once one takes into account that the ram pressure is not only a function of velocity relative to the air but also a function of the air density, i.e. it changes with altitude (or more precisely, with density altitude). The same IAS reading therefore may correspond to vastly different TAS when the plane starts climbing to high altitude. At sea level, a KIAS of 400 kt roughly corresponds to 400 kt TAS, at 80.000 ft (the cruising altitude of the SR-71), the same reading may indicate a TAS in excess of 1600 kt (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). | It becomes apparent that IAS is in fact not TAS once one takes into account that the ram pressure is not only a function of velocity relative to the air but also a function of the air density, i.e. it changes with altitude (or more precisely, with density altitude). The same IAS reading therefore may correspond to vastly different TAS when the plane starts climbing to high altitude. At sea level, a KIAS of 400 kt roughly corresponds to 400 kt TAS, at 80.000 ft (the cruising altitude of the SR-71), the same reading may indicate a TAS in excess of 1600 kt (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. For various reasons, | *'''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. | |||
=== Equivalent Airspeed === | === Equivalent Airspeed === | ||
'''Equivalent''' airspeed (EAS) takes into account | *'''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 foot, 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. | |||
=== Mach number === | === Mach number === | ||
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 | *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 sub-sonically, a Mach number above 1 indicates supersonic flight. The Mach number is critical because a number of phenomena take place just around Mach 1, 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== | ||
For complete V speed "definitions" list please visit [http://en.wikipedia.org/wiki/V_speeds Wikipedia]. Here a small abstract. Note that V speed definitions can depend of local [[Flight rules]]. Most V speeds depend on the aircraft configuration (how much it weights etc.) so must be calculated forehand and must be included in the flight-plan. V speeds are used to compare aircraft performance and will be mentioned in the aircraft flight manual (AFM). | For the complete V speed "definitions" list please visit [http://en.wikipedia.org/wiki/V_speeds Wikipedia]. Here a small abstract. Note that V speed definitions can depend of local [[Flight rules]]. Most V speeds depend on the aircraft configuration (how much it weights etc.) so must be calculated forehand and must be included in the flight-plan. V speeds are used to compare aircraft performance and will be mentioned in the aircraft flight manual (AFM). | ||
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Ice is a known cause of blockage of the pitot tube hence there are pitot heaters preventing forming of ice. Another cause of blockage are insects. Blockage of pitot tubes is a known cause of some very dramatic accidents and every pilot should learn how to deal with strange behaving speed indicators and autopilots. | Ice is a known cause of blockage of the pitot tube hence there are pitot heaters preventing forming of ice. Another cause of blockage are insects. Blockage of pitot tubes is a known cause of some very dramatic accidents and every pilot should learn how to deal with strange behaving speed indicators and autopilots. | ||
==Additional information== | |||
*Velocity: A vector combining speed and (angle of) direction. Often used as synonym of speed. | |||
[[Category:Aviation]] | [[Category:Aviation]] | ||