Air and other gases are fluids, just like a liquid. Fluids have the ability to flow and fill containers as well as having the ability to fill an available volume of containers. With that being said, we need to understand the properties of fluids (air) to understand the principles of flight.
Viscosity
Viscosity is when a fluid begins to resist flowing, in another words the thicker the fluid is the more resist to flowing it becomes.
An example of this is oil and water. Water flows freely in one direction while the oil flow much slower in the other direction.
Friction
Fluids create friction when they flow around an object. As air flows around the aircraft, it has friction even though the surface appears very smooth.
Friction plays another factor on the flow of fluids over and around an object. When any two materials come in to contact with each other, friction exists. If an object or surface is rough and the other is smooth, the flow differs significantly.
A good example is the surface of an aircraft wing. If you look at the surface of a wing with the naked eye or rub your hand along the surface, it appears to be smooth. However, if you look under a microscope magnified to 1,500x the surface of the metal wing is rough which causes resistance in the air flowing over the wing.
Air molecules will adhere to the rough surface of the wing, causing even more resists in the airflow over the wing. When these two factors occur, that is known as drag.
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