Textbook

How Air Moves

Aerodynamics is the study of the motions and forces of gases on an object. Understanding these forces aids in the design of airplanes, sailboats, race cars, and sports equipment moving through the air as well as buildings, bridges, and windmills affected by the wind moving past them.

Sometimes this is difficult to picture. When we walk through air, we don't really feel anything, unless there is a wind! Then we feel the force that the air makes on us. Yet, even when there is no wind and we can not feel the air, it does exert a force on objects.

When air flows over an object at very high speeds, like over fighter aircraft, or goes through jet engines with very high temperatures, the normal rules of aerodynamics sometimes don't apply. Gas dynamics expands the rules and laws of aerodynamics to include these special cases of high speed flows and high temperature flows.

Sometimes, if an aircraft flies very high and very fast, even the rules of gas dynamics break down. At high altitudes air molecules are very far apart and temperatures around the plane can be so high that they cause chemical reactions among the air molecules. Hypersonics is the study of the air motion in these conditions and applies to vehicles like the space shuttle that travel extremely fast.

Sports equipment like our tennis ball travels at "subsonic speeds", meaning less than the speed of sound (about 760 MPH).

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