The top and bottom halves of the first airfoil are identical - this is referred to as "symmetric". The airfoil on the right is not symmetric. Its top and bottom halves are not the same. The underside of the airfoil is curved or "cambered".

If you arm wrestle someone, who wins? The person with the most force. If both people push with the same force no one's arm moves! Lift on airplane wings is somewhat similar. As air flows over the top and bottom of the wing the pressure and velocity of the air change. These changes create lift. But on a symmetric wing the changes on the top half match the bottom half. If the forces on the top and bottom of the wing are the same there is no lift. Because the cambered airfoil is not symmetric the forces on the top and bottom are different - in this case lift is generated.

Can you ever have lift on a symmetric airfoil? Yes! What if you raised the nose of the airplane. This would place the wing at an angle. Engineers call this the "angle of attack". Now the forces on the top and bottom of the wing will create lift.

The greater the angle of attack the more lift - up to a limit. If the angle is too high the air will not flow over the airfoil so that lift can be created and the wing will "stall". It will lose lift.

Coefficient of Lift Graph for a Symmetric Airfoil

Aerospace engineers use wind tunnels to determine the lift and drag curves. Here are lift and drag curves for symmetric and cambered airfoils. Look at the lift curve. Across the bottom (x-axis) is the angle of attack. On the vertical (y-axis) is the coefficient of lift ( a parameter that describes the lift). You can see that as the angle of attack increases the C_L increases - there is more lift! What happens at 17⁰? The C_L starts to drop. Oops we're losing lift and are headed for a stall.

Coefficient of Lift Graph for a Cambered Airfoil

Look at both charts for 0⁰ angle of attack. What is the C_L for the symmetric airfoil and what is the C_L for the cambered airfoil? The C_L for the symmetric airfoil is 0! For the cambered airfoil it is 0.35. Remember what we discussed earlier? A symmetric airfoil at 0⁰ angle of attack has no lift. A cambered airfoil can have lift at 0⁰ angle of attack.

In wind tunnels engineers often blow smoke into the air and over the model. This way they can see how the air is flowing. They can see if it flows smoothly over a wing. They can see how much drag an airfoil has by the size of the wake (an area with no airflow at the trailing edge of the airfoil) and they can also see stalls when the air has separated from the wing and actually starts to flow towards the front the airfoil.