CFI Topics
Text

Formula

Lesson 4

The stall speed (Vs) is then determined by Weight, Maximum coefficient of lift, Wing area and Air density. The formula would be:

Stall Speed (Vs) = Air density times maximum coefficient of lift times wing area divided by Weight.

Based on this equation, it is obvious that increasing the maximum coefficient of lift, increasing the air density (flying from higher to lower altitude) and the total wing area (Deploying the flaps will increase the wing area) will decrease the stall speed of the aircraft while increasing the denominator (weight) will increase the stall speed.

In demonstrating the Vs, a pilot manipulates the control column aft to the stops. This increases the AOA (angle of attack) and thus increases the drag and decreases the airspeed (with no or low power). The stall occurs when the airflow over the wing separates and lift no longer overcomes weight. This leads to an increased “sinking” feeling, something akin to when your heart gets tangled up into your throat as you flare a little higher over the runway then you wanted. The next event leads to a chill down your spine and an extra bit of dental work to protect your smile. Not all aircraft mush down – some will have a sudden pitch down effect as the lift is lost. If you keep the pitch attitude stable and the rudder and the ailerons neutral the aircraft will demonstrate the “falling leaf” maneuver. Please don’t try this without an experienced instructor, and never at low altitude. A much maligned secondary stall is essentially a stall followed by a temporary lift followed by another stall. A normal stall occurs when the wing loading is at or near 1G.

It is important to remember that the aircraft attitude may not always be nose high (pitch-up). The loss of lift can occur at any attitude and at any altitude and at any airspeed. The airflow that matters is the relative wind. So, sudden changes in wind velocity or shift in direction can create the same scenario as sometimes occurs in strong wind shears.

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