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Aircraft speed: Difference between revisions
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pitot tubes. knot. small corrections. This article has a weird name btw.
(pitot tubes. knot. small corrections. This article has a weird name btw.) |
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The '''speed''' of an aircraft is the amount of distance the aircraft travels in relation to a reference point per unit time. As with any velocity measurement, the questions to ask in order to understand any given velocity measure are: What is the reference point? and How is the measurement done? | The '''speed''' of an aircraft is the amount of distance the aircraft travels in relation to a reference point per unit time. As with any velocity measurement, the questions to ask in order to understand any given velocity measure are: What is the reference point? and How is the measurement done? In aviation there are several different ways to indicate speed: | ||
* '''ground''' speed (GS) | * '''ground''' speed (GS) | ||
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* '''indicated''' airspeed (IAS) | * '''indicated''' airspeed (IAS) | ||
In modern aviation, speed is usually measured in units of Knots (kt). | In modern aviation, speed is usually measured in units of Knots (kt). The Knot is one Nautical Mile per Hour. In older planes, notably German WW II fighter planes, airspeed gauges are 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 kt. If the speed is measured in kt, sometimes a 'K' is put before the acronym, so KEAS stands for 'equivalent airspeed measured in kt'. | ||
Finally, for supersonic planes, the '''Mach number''' is often used to describe aircraft velocity. | Finally, for supersonic planes, the '''Mach number''' is often used to describe aircraft velocity. | ||
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== Equivalent Airspeed == | == Equivalent Airspeed == | ||
'''Equivalent''' airspeed takes into account yet another correction, 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 ceiling of 85.000 ft, that actually is an effect and the plane is flown based on a KEAS velocity measure. For more conventional airplanes, EAS is not much used. Thus, EAS is what a perfect dynamic pressure sensor would show when properly calibrated for the air compressibility at the current altitude. | '''Equivalent''' airspeed (EAS) takes into account yet another correction, 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 ceiling of 85.000 ft, that actually is an effect and the plane is flown based on a KEAS velocity measure. For more conventional airplanes, EAS is not much 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''' is the ratio of an aircraft's TAS over the local speed of sound. A Mach number below 1 means that the plane moves subsonically, a Mach number above 1 indicates supersonic flight. The Mach number is interesting because a number of phenomena take place just around Mach 1, for example a sudden increase in drag induced by shockwave 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. | The '''Mach number''' is the ratio of an aircraft's TAS over the local speed of sound. A Mach number below 1 means that the plane moves subsonically, a Mach number above 1 indicates supersonic flight. The Mach number is interesting because a number of phenomena take place just around Mach 1, for example a sudden increase in drag induced by shockwave 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. | ||
==Pitot tube== | |||
The pitot tube is the tool to measure the airspeed. It is a tube directed forwards. The air pushes inwards and the pressure is measured. The measured pressure is corrected towards the indicated airspeed. Bigger aircraft have two pitot tubes and the indicator displays the average of the two. However, most often only one pitot tube is used for 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. | |||
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. | |||
[[Category:Aviation]] | [[Category:Aviation]] | ||