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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|pitot tube]] | 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 an aircraft the speed is "measured" with a [[#Pitot tube|pitot tube]]. Together with the static pressure one can determine not the speed of the aircraft, but the speed of the air flowing around the aircraft, the [http://en.wikipedia.org/wiki/Airspeed '''airspeed''']. Thus the speed of the aircraft relative to the airmass it is flying in. | ||
The airspeed can be indicated in knots, km/h or even m/s. In general however, knots are used. But | The airspeed can be indicated in knots, km/h or even m/s. In general however, knots are used. But in some countries (like Russia and China), km/h is used. | ||
In older planes, notably German WW II fighter planes, the airspeed is indicated in | In older planes, notably German WW II fighter planes, the airspeed is indicated in kilometers 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 knot, sometimes a 'K' is put before the acronym, so KEAS stands for 'equivalent airspeed in knot'. | If the speed is indicated in knot, sometimes a 'K' is put before the acronym, so KEAS stands for 'equivalent airspeed in knot'. | ||
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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]. | ||
The Indicated Airspeed is determined with | The Indicated Airspeed is determined with Total Pressure (measured with a [[#Pitot tube]]) and Static Pressure. Because of measurement faults IAS usually has a failure. Without this failure you get 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 aerodynamic 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 aerodynamic 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. | ||
Per definition CAS | Per definition CAS = TAS in standard ISA conditions and sea level. At 80.000 feet (the cruising altitude of a [[SR-71]]), the IAS of 400 knot corresponds to a TAS in excess of 1600 knot (..that corresponds with about Mach 3 at that altitude). | ||
=== Calibrated airspeed === | === Calibrated airspeed === | ||
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| V<sub>LO</sub> || Maximum landing gear operating speed. | | V<sub>LO</sub> || Maximum landing gear operating speed. | ||
|- | |- | ||
| V<sub>MO</sub> || Maximum operating limit speed. | | V<sub>MO</sub> / M<sub>MO</sub> || Maximum operating limit speed (Turbine aircraft). Usually the same as V<sub>C</sub> and the redline on your airspeed indicator. | ||
|- | |- | ||
| V<sub>NE</sub> || Never exceed speed. | | V<sub>NE</sub> || Never exceed speed (Reciprocating aircraft). The red line on your airspeed indicator. | ||
|- | |- | ||
| 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 (Reciprocating aircraft). Usually the same as V<sub>C</sub>. the speed on your airspeed indicator where the green arc changes to yellow. | ||
|- valign="top" | |- valign="top" | ||
| V<sub>R</sub> || Nose-wheel take off speed. | | V<sub>R</sub> || Nose-wheel take off speed. | ||
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== Pitot tube == | == Pitot tube == | ||
The [http://en.wikipedia.org/wiki/Pitot_tube pitot tube] is the tool to measure the | The [http://en.wikipedia.org/wiki/Pitot_tube pitot tube] is the tool to measure the Total Pressure. 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 indicated airspeed is determined with the impact pressure which is per definition: total pressure - static pressure. The impact pressure is NOT the dynamic pressure as impact pressure incorporates compressibility effects. Bigger aircraft have two (or even three) pitot tubes. | ||
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. | ||