Aircraft speed: Difference between revisions

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Aircraft speed (view source)

Revision as of 19:52, 7 October 2011

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*'''Indicated''' airspeed (IAS) is the number displayed on the airspeed indicator.  
*'''Indicated''' airspeed (IAS) is the number displayed on the airspeed indicator.  


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.
Airspeed is usually measured with a [[#Pitot tube]] at the front of the aircraft. The ram pressure measured with the [[#Pitot tube]] is not the TAS since the pressure differs greatly with [[altitude]] (more specific the [[altitude#Density altitude| density altitude]]). The same IAS will correspond to a vastly different TAS when the plane starts climbing to high altitude. At sea level, a KIAS of 400 knot roughly corresponds to 400 knot TAS, at 80.000 feet (the cruising altitude of the SR-71), the same reading may indicate a TAS in excess of 1600 knot (is can be very difficult to reconcile an airspeed of 400 kt with a reading that one is flying in excess of Mach 3 when one doesn't know what the airspeed gauge shows).
 
It becomes apparent that IAS is in fact not TAS once one takes into account that the ram pressure is not only a function of velocity relative to the air but also a function of the air density, i.e. it changes with altitude (or more precisely, with density altitude). The same IAS reading therefore may correspond to vastly different TAS when the plane starts climbing to high altitude. At sea level, a KIAS of 400 kt roughly corresponds to 400 kt TAS, at 80.000 ft (the cruising altitude of the SR-71), the same reading may indicate a TAS in excess of 1600 kt (is can be very difficult to reconcile an airspeed of 400 kt with a reading that one is flying in excess of Mach 3 when one doesn't know what the airspeed gauge shows).


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.
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