Stalling Speed
We know that the stall always occurs at the same angle of attack, not airspeed. So where does our flight manual stall speed come from and why does it change?
What is the Stalling Speed?
The stall speed given in our aircraft manual is the stall speed in level flight at maximum weight. As we know, the lift required to maintain level flight is constant so we can use the lift formula to find the IAS we must fly at for any angle of attack (AoA).
Lift = CL ½ ρ V2 S
We can now simplify the formula for the lift required in straight and level flight into:
Lift Required = AoA x minimum IAS
We know the lift created increases as AoA increases up to a certain point, known as the critical angle of attack. If we increase the AoA beyond the critical AoA, the wing stalls and lift dramatically decreases.
So the aircraft always stalls when it exceeds the critical angle of attack – the stalling speed is the IAS in straight and level flight at this angle of attack.
To put it another way, the stall speed is the minimum IAS the aircraft must fly in order to generate enough lift to remain straight & level at the critical angle of attack.
Why does Stall Speed Change?
Weight (Wing Loading)
As weight increases, the lift required increases. So higher weight requires a higher airspeed at the same angle of attack in order to generate sufficient lift. The term wing loading is sometimes used, which is a way of expressing how much weight the wings are supporting. Wing Loading is calculated by dividing the total weight by wing area: Wing Loading = Total Weight / Wing Area
Power
Since the stall occurs at a high angle of attack, the thrust vector is angled upwards, above the flight path. There is a component of thrust acting upwards, helping to support the weight of the aircraft and reducing the lift required. With less lift required, any given angle of attack can be flown at a slower speed while still maintaining straight and level flight.
Flaps
Flaps increase the lift coefficient (CL) and surface area (S) by changing the shape of the aerofoil. This means the wing creates more lift so less airspeed is required to continue flying straight and level.
Load Factor (Maneuvering)
All maneuvers that increase the load factor will increase the stall speed. This is covered in more detail in the next lesson.
Icing
Ice, frost, damage (say from a bird strike), or anything else that negatively changes the aerofoil shape and prevents the smooth flow of air over the wing will reduce the lift created by the wing. This leads to a higher stall IAS and the wing can be expected to stall at a lower angle of attack due to the poor airflow around the aerofoil section.
Altitude
Changes in altitude DO NOT cause a change in the indicated stall speed. This is because the reducing air pressure at higher altitudes has the same effect on both the wing and the airspeed indicator (remember: our ASI measures dynamic pressure, NOT speed). This means the airspeed indicator will read the same stall speed at any altitude.