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→Stall speed and \(V_{NE}\)
A rule of thumb for calculating TAS corrections is, for every 1000ft above mean sea level, the TAS is 2% higher than the IAS. For example, if you fly at QNH 5000ft, your TAS will be 10% higher than your IAS.
==== Stall speed , performance airspeed and \(V_{NE}\) ====
Just like the altimeter, it is possible to correct for the density variations with altitude in an ASI, but this is not done for a very important reason: the stall speed (\(V_S\)).
'''An aeroplane stalls when a critical angle of attack is reached ''' (''See also: [[Aerofoils and Wings]]''). There is a one to one mapping between the angle of attack (\(\alpha\)) and the lift coefficient (\(C_L\)), which is defined as:
\[C_L=\frac{L}{\frac{1}{2} \rho V^2 A} =f(\alpha)\]
Where:
*\(L\) is the lift force, usually equal to the weight of the aeroplanewhen flying unaccelerated*\(A\) is the wing area, which is fixedunless devices such as flaps are in the process of deployment
*Note that \(\frac{1}{2} \rho V^2\) is the dynamic pressure
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. This amount depends on the cockpit weight which is significant for a glider.
Recall that an ASI actually measures the dynamic pressure, so we it can mark a critical value (this marking is best fixed) on be used to indicate the ASI at which point the aeroplane stallsof stall, known as the stall speedi.e. It it is '''very important''' possible on each occasion to understand that calculate the aeroplane stalls at a critical dynamic pressureand mark its value on the ASI. In practice the markings on the ASI are typically based on the maximum all-up weight.
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. In other words, the ASI shows the stall margin correctly.
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 indicated airspeeds. In other words, the '''polar''' of the glider is invariant when expressed in terms of IAS. It, therefore, makes a lot of sense that the aeroplane keeps track of its indicated airspeed 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 displayed real-time for navigational reference.
However, the never exceed speed (\(V_{NE}\)) has nothing to 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 difference between 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. The same applies to glider pilots who wish to fly at high altitudes: you must remember that the airspeed window in which you can fly is reduced, and, by flying higher, the red mark on the ASI must gradually move inwards. Such a feature is available on a jet airliner in the 1960s: there is a \(V_{NE}\) flag on the ASI which is driven by the air data computer.
=== Altimeter ===
