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### Coefficient of lift of an Airfoil

On a previous post I commented about airfoils and how they generate lift. Now let’s see how the lift varies as one increases the angle of attack. As mentioned on an earlier post, the angle of attack (α) is defined as the angle the airfoil makes with the horizontal as in the picture below:

On the previous post about this topic it was shown that increasing the angle of attack can increases the pressure difference between the bottom and the top of the airfoil resulting in an net increase of the lift force (Lf). It is usually convenient to define a non-dimensional lift force called the coefficient of lift (CL) which is defined as follow:

The coefficient of lift is defined mathematically as the lift force divided by the dynamic pressure of the incoming flow times the area of the airfoil. The most important concept to keep in mind is to realize that the coefficient of lift is a direct measurement of the lift force experienced by the airfoil. Now with this in mind let’s look at some experimental data gathered in a wind tunnel with an Clark Y-14 airfoil. (More information on the type of test facility and airfoil can be found here

Note from the graph above that the coefficient of lift is not zero when the angle of attack is zero. This just mean that the airfoil tested was not symmetric. As the angle of attack increases, the coefficient of lift increases remarkably until some where between 12° and 16° where a sudden decrease in lift is experienced. This sudden decrease in lift is a result of flow separation from the airfoil. Flow separation is usually not a good thing. If you are flying an aircraft and flow separation occurs, your plane will experience a sudden vertical drop until flow can re-attache to the wing. The data shown above was collected only using the airfoil with no Slat or Flap added. The Slat and Flap are additions to the airfoil the can increase the lift and delay separation from occurring.