Air is a Fluid
First, we must clarify that air does not fly over an airfoil as smoothly and as straight as is commonly depicted. Let’s take a little detour to look at air as a fluid to explain how the shape of an airfoil diverts air down. When a moving fluid such as air or water comes in contact with a curved object such as an airfoil, it tends to follow the curve. This is known as the Coanda effect. As long as the curve is not too drastic, the molecules of air try to follow the airfoil shape. This is because of a property that fluids have called viscosity, which is a resistance to flow. Those molecules closest to the surface have a very small velocity. The difference in velocity of the air molecules causes the air to be pulled downwards towards the wing surface, thus to follow the surface of the airfoil shape.
The Coanda effect: The tendency to follow the curve for a fluid (left) is a result of its viscosity. (www.rz.uni-frankfurt.de/~weltner/Mis6/Image69.gif) |
