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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 '''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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*[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. | *[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. | 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. The GS can be calculated from TAS by correcting it for the prevailing wind at altitude or by measuring the time between passing two points on the ground [[radio beacons]] 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. | 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 === | ||
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*[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]. | *[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 IAS 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 IAS can be the CAS. The IAS 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. | ||
In spite of this dependence on [[altitude]], IAS is a very useful quantity in flight. Many aerodynamical properties, for example drag, lift, 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 speed. The stall speed of an aircraft at sea level is very different from the stall speed (in TAS) at 30.000 ft - but they correspond to the same IAS reading. | In spite of this dependence on [[altitude]], IAS is a very useful quantity in flight. Many aerodynamical properties, for example drag, lift, 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 speed. The stall speed of an aircraft at sea level is very different from the stall speed (in TAS) at 30.000 ft - but they correspond to the same IAS reading. | ||
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=== Calibrated Airspeed === | === Calibrated Airspeed === | ||
*[http://en.wikipedia.org/wiki/Calibrated_airspeed '''Calibrated''' airspeed (CAS)] is calculated from the | *[http://en.wikipedia.org/wiki/Calibrated_airspeed '''Calibrated''' airspeed (CAS)] is calculated from the [[#Pitot tube]] measurement and correcting it for standard errors. | ||
Modern equipment can most often can indicate the CAS. For navigation the CAS is the first step to calculate the GS. | Modern equipment can most often can indicate the CAS. For navigation the CAS is the first step to calculate the GS. | ||
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|V<sub>NO</sub> || Maximum structural cruising speed or maximum speed for normal operations. | |V<sub>NO</sub> || Maximum structural cruising speed or maximum speed for normal operations. | ||
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*Not knowing the (complete list of) V speeds has caused dramatic accidents. It has occurred that the pilot and co-pilot were not aware of the minimal speed of an aircraft during landing with one engine | *Not knowing the (complete list of) V speeds has caused dramatic accidents. It has occurred that the pilot and co-pilot were not aware of the minimal speed of an aircraft during landing with one engine damaged causing loss of control just before touch-down (the pilot gave full throttle hoping to gain speed expecting to get back control causing the left-over engine push the aircraft to one side). | ||
==Pitot tube== | ==Pitot tube== | ||