In a wind tunnel we see the way air flows over the wing in streamlines. Where the flow of air is faster and the pressure is less the streamlines are closer together.

Wind tunnel tests reveal how the shape of the wing increases the speed of air flowing over the top surface.

View the video on the right. Bernoulli's Law states that when the relative speed of a fluid increases the pressure drops. Pressure above the wing is lower than pressure below the wing. This difference in air pressure produces a net upwards force called lift which tends to push the wing upwards.



The wing area is important. A small pressure difference over each square metre of wing is enough to generate a considerable force if the wing area is large enough. Lift is geater when the surface area of the wing is large and/or the speed of the plane is high.

The more air that flows over the wing the greater the force of lift that is generated. Answer the following questions.

Why do slow moving gliders have large wing spans?



Why do fast jets have small wings?