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Aircraft speed: Difference between revisions
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== Indicated Airspeed == | == Indicated Airspeed == | ||
'''Indicated''' airspeed 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. | ||
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). | 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). | ||