stalling an airplane mid-flight

Thanks to my brother, I recently had the opportunity to fly a small Cessna aircraft under supervision of a flight instructor. The instructor took off and landed, but gave me the controls during flight. During this time we went through a few instructive maneuvers, including stalling the plane mid-flight. Here I explain how stalling an airplane works and how to get out of a stall once it has occurred.

Consider the following sketch showing the forces acting on an airfoil during steady-state flight:

four_forces_sans_equation

If the weight of the airplane exceeds the lift being generated by the airfoil, the plane will start falling.

Lift is generated by a pressure difference between the bottom and top of the airfoil:

presure_gradient

v_high_and_low

Bernoulli’s principle tells us that when air flows more quickly over the top of the airfoil than the bottom, the pressure on the bottom of the airfoil exceeds the pressure at the top of the airfoil. The thrust of the airplane (from the propeller) forces the air across the airfoil. Because the shape of the airfoil mildly obstructs flow over the top of the airfoil versus the bottom, the air on top is forced to move faster. The resulting pressure differential generates lift.

Stalling an airplane means configuring it mid-flight in such a way that lift fails. This is done by first lowering the propeller speed, thereby reducing airflow across the airfoil, and then angling the plane upward so that the remaining airflow across the airfoil fails to produce the required pressure difference for lift:

no_pressure_diff

Once this configuration occurs, the forces of weight and drag in the vertical direction exceed the force of thrust in the vertical direction, and the airplane starts falling:

stall_position

To get out of the stall, one increases the propeller speed to force airflow across the airfoil; and then pushes the yoke forward to bring the elevators down, which reduces the angle between the airfoil and the earth’s surface, thereby placing the airfoil back into a configuration that allows it to generate lift.

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