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→Stall speed and \(V_{NE}\)
If the angle of attack is to reach a critical value, the lift coefficient is also to reach a critical value. Because the weight of the aeroplane (equal to the lift) and the size of the wings are fixed, we conclude that the aeroplane needs a minimum amount of dynamic pressure to fly: any less and the aeroplane stalls.
Recall that an ASI actually measures the dynamic pressure, so we can mark a critical value (this marking is best fixed) on the ASI at which point the aeroplane stalls, known as the stall speed. It is '''very important''' to understand that the aeroplane stalls at a critical dynamic pressure.
We want this stall speed to be a well defined value that the pilot can easily compare a cockpit reading to. In other words, the stall speed should be a function of the critical dynamic pressure and nothing else. Therefore, the stall speed defined for an aeroplane is an indicated airspeed. If the ASI does not correct for the density variations and read the IAS all the time, the pilot can conveniently compare his flying to the stall speed. Note that the reasoning above applies to other flying conditions apart from stalling: the mapping between the angle of attack to a wide range of aerodynamic performances is one to one. Therefore, other speeds such as the speed of minimum sink (best glide) are also best defined as an indicated airspeeds. It, therefore, makes a lot of sense that the aeroplane keeps track of its indicated airspeed which even if, with the aid of modern computers, calculating the TAS is a piece of cake. On larger aeroplanes with sophisticated avionics, the TAS is directly related displayed real-time for navigational reference. However, the never exceed speed (\(V_{NE}\)) has nothing to the do with angle of attack or dynamic pressure: it is the speed that, when exceeded, the aeroplane may fail structurally. The failure of an airframe is dominated by aeroelastic effects, the most notable one being the flutter of the wings (there are videos on YouTube that shows this phenomenon). These horrible things occur when the '''TAS''' reaches a critical value. Recall that, at high altitudes, the TAS is higher than the IAS. Therefore, as you fly higher, '''your \(V_{NE}\), expressed in terms of IAS, will reduce.''' Failing to understand this can lead to serious consequences of overspeeding. The point where the stall speed (IAS) corresponds to the never exceed speed (TAS) because of a decrease of density gives the theoretical ceiling. This is the theoretical maximum altitude at which the aeroplane can fly. If you fly at this altitude, you must fly at this speed precisely, or you will either stall or overspeed. The Lockheed U-2, which flies at very high altitudes, have very notable problems when the ceiling is reached. For a U-2 in cruise, the stall speed and the never exceed speed is less than 10 knots apart on the ASI. This calls for very accurate handling by the pilot.
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