Relating Airfoil Geometry and Pressures



Before discussing in detail the methods used to predict airfoil pressure distributions let's consider, more intuitively the relationship between airfoil geometry and airfoil pressure distributions.

We first look at the effect of changes in surface curvature (Click on figure to look in more detail.)


The figure below shows how the airfoil pressures vary with angle of attack. Note that the "nose peak" becomes more extreme as the angle increases.


To make this a bit more clear, you may use the small java program below to change the angle of attack and see its effect on Cp, Cl, and Cm. Click on the upper half of the plot to increase the angle of attack, alpha, and on the lower portion to decrease it.


Let's consider, in more detail the relationship between airfoil geometry and airfoil pressure distributions. The next few examples show some of the effects of changes in camber, leading edge radius, trailing edge angle, and local distortions in the airfoil surface.

A reflexed airfoil section has reduced camber over the aft section producing less lift over this region. and therefore less nose-down pitching moment. In this case the aft section is actually pushing downward and Cm at zero lift is positive.



A natural laminar flow section has a thickness distribution that leads to a favorable pressure gradient over a portion of the airfoil. In this case, the rather sharp nose leads to favorable gradients over 50% of the section.



This is a symmetrical section at 4° angle of attack.
Note the pressure peak near the nose. A thicker section would have a less prominent peak.



Here is a thicker section at 0°. Only one line is shown on the plot because at zero lift, the upper and lower surface pressure coincide.



A conventional cambered section.



An aft-loaded section, the opposite of a reflexed airfoil carries more lift over the aft part of the airfoil. Supercritical airfoil sections look a bit like this.


The best way to develop a feel for the effect of the airfoil geometry on pressures is to interactively modify the section and watch how the pressures change. A Program for ANalysis and Design of Airfoils (PANDA) does just this and is available from Desktop Aeronautics. A very simple version of this program, is built into this text and allows you to vary airfoil shape to see the effects on pressures. (Go to Interactive Airfoil Analysis page by clicking here.) The full version of PANDA permits arbitrary airfoil shapes, permits finer adjustment to the shape, includes compressibility, and computes boundary layer properties.