Air is a Fluid  the viscosity of air. However, since air is a fluid and has  viscosity properties, it resists flow around any object to  some extent.  Another factor at work when a fluid flows over or around  an object is called friction. Friction is the resistance that one  surface or object encounters when moving over another.  Friction exists between any two materials that contact each  other.  When most people hear the word “fluid,” they usually think  of liquid. However, gasses, like air, are also fluids. Fluids  take on the shape of their containers. Fluids generally do not  resist deformation when even the smallest stress is applied,  or they resist it only slightly. We call this slight resistance  viscosity. Fluids also have the ability to flow. Just as a liquid  flows and fills a container, air will expand to fill the available  volume of its container. Both liquids and gasses display these  unique fluid properties, even though they differ greatly in  density. Understanding the fluid properties of air is essential  to understanding the principles of flight.  Viscosity is the property of a fluid that causes it to resist  flowing. The way individual molecules of the fluid tend to  adhere, or stick, to each other determines how much a fluid  resists flow. High-viscosity fluids are “thick” and resist flow;  low-viscosity fluids are “thin” and flow easily. Air has a low  viscosity and flows easily.  The effects of friction can be demonstrated using a similar  example as before. If identical fluids are poured down two  identical ramps, they flow in the same manner and at the  same speed. If the surface of one ramp is rough, and the other  smooth, the flow down the two ramps differs significantly.  The rough surface ramp impedes the flow of the fluid due  to resistance from the surface (friction). It is important to  remember that all surfaces, no matter how smooth they  appear, are not smooth on a microscopic level and impede  the flow of a fluid.  Using two liquids as an example, similar amounts of oil and  water poured down two identical ramps will flow at different  rates due to their different viscosity. The water seems to flow  freely while the oil flows much more slowly.  The surface of a wing, like any other surface, has a certain  roughness at the microscopic level. The surface roughness  causes resistance and slows the velocity of the air flowing  over the wing.  [Figure 4-1]  Molecules of air pass over the surface of the wing and actually  adhere (stick, or cling) to the surface because of friction. Air  molecules near the surface of the wing resist motion and have  a relative velocity near zero. The roughness of the surface  impedes their motion. The layer of molecules that adhere to  the wing surface is referred to as the boundary layer.  As another example, different types of similar liquids will  display different behaviors because of different viscosities.  Grease is very viscous. Given time, grease will flow, even  though the flow rate will be slow. Motor oil is less viscous  than grease and flows much more easily, but it is more viscous  and flows more slowly than gasoline.  All fluids are viscous and have a resistance to flow, whether  or not we observe this resistance. We cannot easily observe  Viscosity  Friction  Figure 4-1.  Microscopic surface of a wing.  4-2  Leading edge of wing under 1,500x magnification 