Airfoil Pressure Distributions

The aerodynamic performance of airfoil sections can be studied most easily by reference to the distribution of pressure over the airfoil. This distribution is usually expressed in terms of the pressure coefficient:

Cp is the difference between local static pressure and freestream static pressure, nondimensionalized by the freestream dynamic pressure.

What does an airfoil pressure distribution look like? We generally plot Cp vs. x/c.

x/c varies from 0 at the leading edge to 1.0 at the trailing edge. Cp is plotted "upside-down" with negative values (suction), higher on the plot. (This is done so that the upper surface of a conventional lifting airfoil corresponds to the upper curve.)

The Cp starts from about 1.0 at the stagnation point near the leading edge...

It rises rapidly (pressure decreases) on both the upper and lower surfaces...

...and finally recovers to a small positive value of Cp near the trailing edge.

Various parts of the pressure distribution are depicted in the figure below and are described in the following sections.

We can get a more intuitive picture of the pressure distribution by looking at some examples and this is done in some of the following sections in this chapter.