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Aircraft speed: Difference between revisions
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reworking page, part one, more will come ....
(pitot tubes. knot. small corrections. This article has a weird name btw.) |
(reworking page, part one, more will come ....) |
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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. | ||
==Expressing speed== | |||
== Ground Speed == | === Ground Speed === | ||
'''Ground''' speed (GS) is the velocity with 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 gauge. 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>. | '''Ground''' speed (GS) is the velocity with 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 gauge. 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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It is perhaps also worth noting that GS measures only the horizontal component of the aircraft velocity - in a steep dive, the aircraft can move very fast, but because the motion is chiefly vertical, the groundspeed can be very small at the same time. | It is perhaps also worth noting that GS measures only the horizontal component of the aircraft velocity - in a steep dive, the aircraft can move very fast, but because the motion is chiefly vertical, the groundspeed can be very small at the same time. | ||
== True Airspeed == | === True Airspeed === | ||
'''True''' airspeed (TAS) is the velocity with 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. | '''True''' airspeed (TAS) is the velocity with 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. | ||
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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. | ||
== Indicated Airspeed == | === Indicated Airspeed === | ||
'''Indicated''' airspeed (IAS) is (usually) the number displayed on the airspeed gauge. 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. | '''Indicated''' airspeed (IAS) is (usually) the number displayed on the airspeed gauge. 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. | ||
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In spite of this dependence on density altitude, IAS is a very useful quantity in flight. Many aerodynamical properties, for example drag, the stress on the airframe, stall speed or the forces on control surfaces depend on the dynamic pressure generated by the airstream, not on the actual aircraft velocity. Thus, the actual stall speed of an aircraft at sea level is very different from the stall speed at 30.000 ft - but they correspond to the same IAS reading. | In spite of this dependence on density altitude, IAS is a very useful quantity in flight. Many aerodynamical properties, for example drag, the stress on the airframe, stall speed or the forces on control surfaces depend on the dynamic pressure generated by the airstream, not on the actual aircraft velocity. Thus, the actual stall speed of an aircraft at sea level is very different from the stall speed at 30.000 ft - but they correspond to the same IAS reading. | ||
== Calibrated Airspeed == | === Calibrated Airspeed === | ||
'''Calibrated''' airspeed (CAS) corresponds to IAS corrected for the measurement error. For various reasons, airplanes 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. | '''Calibrated''' airspeed (CAS) corresponds to IAS corrected for the measurement error. For various reasons, airplanes 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 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''' (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 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. | ||
==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). V speeds are used to compare aircraft performance and will be mentioned in the aircraft flight manual (AFM). | |||
<!-- please add to this list when needed --> | |||
{| | |||
|- valign="top" | |||
|V<sub>1</sub> || Takeoff decision speed & Critical engine failure recognition speed. | |||
During take-off the speed at which the aircraft safely can take-off even when one of more engines fail. The co-pilot will call out V<sub>1</sub> during take-off. | |||
|- valign="top" | |||
|V<sub>r</sub> ||Nosewheel take off speed. | |||
The speed at which the nosewheel leaves the ground. As the speed increases the yokes will be pulled at V<sub>r</sub>. It is also the speed at which the aircraft still can be stopped if there is a critical failure. The co-pilot will call out "rotate" during take off. V<sub>r</sub> is very similar to V<sub>rot</sub> and V<sub>ref</sub>. | |||
|- | |||
|V<sub>3</sub> || Flap retraction speed. | |||
|- | |||
|V<sub>C</sub> || Design cruising speed, also known as the optimum cruise speed, is the most efficient speed in terms of distance, speed and fuel usage. | |||
|- valign="top" | |||
|V<sub>S</sub> ||Stall speed or minimum steady flight speed for which the aircraft is still controllable. | |||
|- valign="top" | |||
|V<sub>S<sub>0</sub></sub> || Stall speed or minimum flight speed in landing configuration. | |||
|- | |||
|V<sub>S<sub>R</sub></sub>|| Reference stall speed. | |||
|- | |||
|V<sub>MO</sub> || Maximum operating limit speed. | |||
|- | |||
|V<sub>NE</sub> || Never exceed speed. | |||
|- | |||
|V<sub>NO</sub> || Maximum structural cruising speed or maximum speed for normal operations. | |||
|} | |||
==Pitot tube== | ==Pitot tube== | ||