Pressure coefficient
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The pressure coefficient is a dimensionless number used in aerodynamics. Most often in the design and analysis of an airfoil. The relationship between the coefficient and the dimensional number is:
<math>C_p={p-p_\infty \over \frac{1}{2}*\rho*V^2}<math>
where
- <math>p_\infty<math> is the free stream pressure
- <math>\rho<math> is the fluid density (sea level air is 1.225kg/m^3)
- Cp of zero corresponds to a the same as the free stream pressure
Pressure distribution
An airfoil at a given angle of attack will have what is called a pressure distribution. This is simply the pressure at all points around an airfoil. Typically graphs of these distributions are drawn so that negative numbers are higher on the graph, as the upper surface of the airfoil will usually have a more negative Cp and will hence be the top line on the graph.
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Naca_5510-64_cp_distrib_by_javafoil.png
Image:Naca_5510-64_cp_distrib_by_javafoil.png
Cl and Cp relationship
The coefficient of lift can be calculated from the coefficient of pressure distribution by integration, or calculating the area between the lines on the distribution.
<math>C_p=\int_{LE}^{TE}C_{p_l}(x)-C_{p_u}(x)\,dx<math>
When the lower surface Cp is higher(more negative) on the distribution it counts as a negative area as this will be producing down force rather than lift