The airfoil — the structure commonly known as the wing — is what allows an aircraft to fly (see the figure). Basically, the airfoils travel through the air at a given speed to enable lift to occur.

An airfoil with relative wind travel.

When an airfoil travels through the air, the air separates over and under the wing. Both streams of air must divide at point A and reach point B at the same time. As you can see in the figure, the upper curvature of the wing is greater than the lower curvature.

Therefore, the air traveling over the top of the wing must move at a faster speed than the air traveling under the lower portion to reach point B at the same time.

Bernoulli’s principle states that as the velocity of a fluid (in this case, air) increases, the pressure exerted by that fluid decreases. Simply put, the faster air traveling over the top of the airfoil causes a decrease in pressure, which results in a lifting force. That’s why a wing needs forward motion to gain lift. Without enough forward motion (airspeed), lift is reduced, and the airplane won’t fly.

But speed isn’t the only vital component of lift. The angle of attack is a reference line that represents the angle of the chord line (the line from the leading edge of an airfoil to the trailing edge) in relation to the forward airflow against the wing.

As you increase the angle of attack, lift increases to a point (usually around 17 degrees) called the critical angle of attack; going past that point disturbs the airflow, and you lose lift (also known as experiencing a stall of the wing).

A stall happens when you exceed the critical angle of attack. To recover from a stall and begin allowing the wing to develop lift, you must reduce the angle of attack to below the critical level.

Bernoulli’s principle and wing lift are critical in aviation. Here’s how the exam may test you on this subject:

Most of the lift on an aircraft’s wings is because of
• (A) A decrease in pressure on the bottom side of the wing

• (B) A decrease in pressure on the upper side of the wing

• (C) A vacuum created under the wing

• (D) An increase in pressure on the upper side of the wing

• (E) None of the above

The correct answer is Choice (B).