# Lift Pressure Distribution [closed]

• If the pressure goes always from high to low, we should have a compression, due the lower pressure of the air stream over the upper surface, and an expansion in the airstream in the lower surface.

• So What causes the "suction" on the upper surface of an airfoil and the "pushing" in the lower one?

• I undestand why there is a region of high pressure and a low pressure around the airfoil. But I am a little bit confused about the effects of the external pressure (Free stream) on the airfoil stream.

• Welcome to aviation.SE! We have a lot of questions on this site about lift. This one and this one might be helpful places to start (and check the related questions list for each of them). If those don't help then maybe you could add some more detail to your question about what you find confusing? Jan 4 at 17:59
• Lift is not just about suction and pushing. Thats just a VERY simplified way of explaining to laymen how lift works. A good place to start to get an idea of some of the mechanism involved is here: youtube.com/watch?v=PF22LM8AbII Jan 18 at 15:24

The simplest example will be if you place a plate with an angle of attack in the subsonic flow: the streamlines will get curved on the both plate sides as the flow turns. Therefore, even a flat plate will generate a lift once its angle of attack is different from zero. As the streamlines are turning on the upper surface, the pressure will get reduced from the free-stream to the airfoil surface. In other words, moving away from the airfoil, the pressure will increase - in the direction of the radius of curvature of the streamlines. Important to note is, that this pressure drop is normal to the streamlines. This change in pressure is simply an effect of turning the flow, i.e. changing its direction. Mathematically, the pressure gradient normal to the streamline, dp/dn, can be expressed as $$\frac{dp}{dn} = \rho \times \frac{V^2}{R} ,$$ where R is the radius of curvature the streamline. Derivation of this equation can be obtained if writing the momentum equations in the natural coordinates.