Aircraft speed: Difference between revisions

*[http://en.wikipedia.org/wiki/Indicated_airspeed|'''Indicated''' airspeed (IAS)] is the number displayed on the airspeed indicator.  

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).

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.