Principles of Flight

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