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→Stall speed and \(V_{NE}\)
==== Stall speed 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 aeroplane
*\(A\) is the wing area, which is fixed
*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.
Recall that an ASI actually measures the dynamic pressure, so we can mark a critical value 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. Therefore, the stall speed is defined as an indicated airspeed which is directly related to the dynamic pressure.
=== Altimeter ===