What leads to a decreased stall speed Vs (IAS)?
An aircraft stall results from a rapid decrease in lift caused by the separation of airflow from the wing’s surface brought on by exceeding the critical Angle of Attack (AoA).
Stall can occur at any pitch attitude or airspeed. Since the Lift Coefficient (CL) increases with an increase in AoA, at some point the CL peaks and then begins to drop off, at “CLmax”.
The stalling speed of a particular aircraft is not a fixed value for all flight situations, but a given aircraft always stalls at the same AoA regardless of airspeed, weight, Load Factor, or Density Altitude. This critical AoA varies from approximately 16° to 20° depending on the aircraft’s design.
Load factor is the ratio of the lift of an aircraft to its weight and represents a global measure of the stress "Load" to which the structure of the aircraft is subjected:
- n = L / W
Where:
- n: Load Factor
- L: Lift
- W: Weight
There are three flight situations in which the critical AoA is most frequently exceeded: low speed, high speed, and turning.
The lower the airspeed becomes, the more the AoA must be increased. When recovering from a dive with high speed, the AoA changes abruptly from quite low to very high, and the direction of the relative wind changes, so the aircraft reaches the stalling angle at a speed much greater than the normal stall speed.
Factors that decrease stall speed:
- Lowered Flaps.
- Lowered Weight.
- Lowered G Force/Load Factor.
Therefore, decreasing weight leads to a decreased stall speed Vs (IAS).
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