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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 Knot (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 [http://en.wikipedia.org/wiki/Airspeed | Speed combines two factors, the distance travelled in a certain amount of time. In aviation speed is most often expressed in Knot (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 [http://en.wikipedia.org/wiki/Airspeed '''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. | 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. | ||
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==Expressing speed== | ==Expressing speed== | ||
=== Ground Speed === | === Ground Speed === | ||
*[http://en.wikipedia.org/wiki/Ground_speed | *[http://en.wikipedia.org/wiki/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 the GS 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 GS 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>. | One needs to know the GS 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 GS 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>. | ||
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=== True Airspeed === | === True Airspeed === | ||
*[http://en.wikipedia.org/wiki/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. | ||
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. | 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. | ||
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=== Indicated Airspeed === | === Indicated Airspeed === | ||
*[http://en.wikipedia.org/wiki/Indicated_airspeed | *[http://en.wikipedia.org/wiki/Indicated_airspeed '''Indicated''' airspeed (IAS)] is the number displayed on the [http://en.wikipedia.org/wiki/Airspeed_indicator airspeed indicator]. | ||
Airspeed is usually measured with a [[#Pitot tube]] at the front of the aircraft. The ram pressure measured with the [[#Pitot tube]] is not the TAS since the pressure differs greatly with [[altitude]] (more specific the density of the air). The higher the [[altitude]] the lower the IAS while flying the same TAS. | Airspeed is usually measured with a [[#Pitot tube]] at the front of the aircraft. The ram pressure measured with the [[#Pitot tube]] is not the TAS since the pressure differs greatly with [[altitude]] (more specific the density of the air). The higher the [[altitude]] the lower the IAS while flying the same TAS. | ||
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=== Calibrated Airspeed === | === Calibrated Airspeed === | ||
*[http://en.wikipedia.org/wiki/Calibrated_airspeed | *[http://en.wikipedia.org/wiki/Calibrated_airspeed '''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 these errors and can be calculated from the IAS. | 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 these errors and can be calculated from the IAS. | ||
=== Equivalent Airspeed === | === Equivalent Airspeed === | ||
*[http://en.wikipedia.org/wiki/Equivalent_airspeed | *[http://en.wikipedia.org/wiki/Equivalent_airspeed '''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 feet, 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. | 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 feet, 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. | ||
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=== Mach number === | === Mach number === | ||
*The [http://en.wikipedia.org/wiki/Mach_number | *The [http://en.wikipedia.org/wiki/Mach_number '''Mach number''' (M)] is the speed of the aircraft divided by the speed of sound (at that altitude). It is a calculated number without a unit. | ||
The aircraft's behaviour at Mach 1 at sea level is about the same as the behaviour of the aircraft at an altitude of 60000 feet. | The aircraft's behaviour at Mach 1 at sea level is about the same as the behaviour of the aircraft at an altitude of 60000 feet. | ||
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==Pitot tube== | ==Pitot tube== | ||
The pitot tube is the tool to measure the airspeed. It is a tube directed forwards, exposed to the airstream. The air is being pushed inwards (rammed) by the motion of the aircraft and the (ram) pressure is measured. The measured pressure is corrected indicating the airspeed. The ram pressure is also called the dynamic pressure opposite the static pressure that us used to indicate [[altitude]]. Bigger aircraft have two pitot tubes and the indicator displays the average of the two. However, most often only one pitot tube is used to control the autopilot, even when the indicator is connected with two. | The [http://en.wikipedia.org/wiki/Pitot_tube pitot tube] is the tool to measure the airspeed. It is a tube directed forwards, exposed to the airstream. The air is being pushed inwards (rammed) by the motion of the aircraft and the (ram) pressure is measured. The measured pressure is corrected indicating the airspeed. The ram pressure is also called the dynamic pressure opposite the static pressure that us used to indicate [[altitude]]. Bigger aircraft have two pitot tubes and the indicator displays the average of the two. However, most often only one pitot tube is used to control the autopilot, even when the indicator is connected with two. | ||
The pitot tube can be blocked easy, once blocked, or worse, partially blocked the IAS will have no relation with the speed of the aircraft. This situation is enhanced if the pitot tube controlling the autopilot is blocked. | The pitot tube can be blocked easy, once blocked, or worse, partially blocked the IAS will have no relation with the speed of the aircraft. This situation is enhanced if the pitot tube controlling the autopilot is blocked. | ||