Which statement best describes Bernoulli's principle as it relates to lift?

• A. Air traveling faster over the curved upper surface of an airfoil causes lower pressure on the top surface.
• B. Air traveling slower over the upper surface causes higher pressure on the bottom surface.
• C. Pressure on both surfaces is equal and lift is due to weight.
• D. Lift is produced by thrust acting on the wing.

Answer: (A)

Lift comes from a pressure difference created by how air moves around the wing. Bernoulli’s principle tells us that faster moving air has lower static pressure. As the wing’s curved top surface and angle of attack steer air to flow faster over the top, the top experiences lower pressure. The air flowing beneath the wing moves more slowly and stays at higher pressure. That higher pressure underneath pushes upward more than the lower pressure on top pushes downward, producing the upward lift.

The idea that lift is due to equal pressure on both surfaces is incorrect, because equal pressures wouldn’t create an upward force. And lift isn’t produced by thrust; thrust pushes the aircraft forward, while lift arises from the pressure difference on the wing.