Explaining the Airfoil Phenomenon: Lower Pressure on Top, Higher Pressure on Bottom

The air pressure above the airfoil is lower than on the bottom, a principle that is crucial in understanding how aircraft generate lift. This phenomenon is explained by Bernoulli's principle, which states that faster-moving air has lower pressure compared to slower-moving air.

Principle of Bernoulli and Airfoils

As air flows over an airfoil, it moves faster over the top due to the curvature of the wing's shape. According to Bernoulli's principle, since this air moves faster, it experiences lower pressure. Conversely, the air moves more slowly on the bottom surface, resulting in higher pressure. This pressure difference creates an upward force, the lift, which allows the aircraft to ascend.

Why Pressure Below is Higher Than Atmospheric

While the air pressure on the bottom of an airfoil is generally higher than on the top, it does not necessarily need to be higher than atmospheric pressure. The principle still holds: faster-moving air over the top will have lower pressure, even if below the surrounding air pressure. The overall effect is that the pressure difference between the top and bottom of the wing generates lift.

Visualizing the Pressure Difference

To visualize this, consider the following figures:

Figure 3

Scroll down to Figure 3, which shows the shape of an airfoil. Observe the curvature that causes air to move faster on the top.

Figure 14

Further down, click on Figure 14 to examine the details. This is a wind tunnel experiment that measures pressure coefficients along the chord of the airfoil.

Zero Angle of Attack

When the airfoil is at zero angle of attack, the air moves faster over the top surface. The pressure coefficient (C_P) on the top is lower, indicating lower pressure. The pressure on the bottom is also generally lower, below atmospheric pressure, especially towards the trailing edge.

Pressure Coefficient Plot

The pressure coefficient plot (Figure 3 in the reference) shows a negative value for the top of the airfoil near the trailing edge, indicating lower pressure. The bottom of the airfoil has more positive pressure coefficients, meaning the pressure is lower than the surrounding air, closer to the bottom of the airfoil.

Summary of the Phenomenon

The lower pressure on the top of an airfoil is a result of faster-moving air, which is facilitated by the airfoil's shape. This lower pressure, combined with the higher pressure on the bottom, generates the lift necessary for flight. The pressure difference is a key component of how airfoils and airplanes create lift, even when the airfoil is at zero angle of attack.