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Aircraft speed: Difference between revisions
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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'. | ||
For (near-)supersonic planes the speed can be expressed in | For (near-)supersonic planes the speed can be expressed in Mach. | ||
==Expressing speed== | ==Expressing speed== | ||
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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. | ||
The GS can be calculated from TAS by correcting it for the wind. | The GS can be calculated from TAS by correcting it for the prevailing wind. | ||
=== True Airspeed === | === True Airspeed === | ||
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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 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, 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. | ||
At sea level, a IAS of 400 knot roughly corresponds to 400 knot TAS. 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). | At sea level, a IAS of 400 knot roughly corresponds to 400 knot TAS. 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). | ||