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Engineers must use aerodynamic principles when designing all airplanes. It is also necessary for architects of sky scrapers and bridges to understand aerodynamic forces because air flowing past stationary structures also exerts pressure upon them. Aerodynamic forces are also important to designers of cars, submarines, pumps, automobile carburetors, and gas and water turbines because moving fluids affects each in their own way.
There are two basic aerodynamic forces: lift and drag. But these alone cannot produce flight. Movement is necessary; but it doesn't matter what moves. Lift enables an airplane, or other object, to climb into the air and remain aloft during flight. For example, an object held flat against a stream of air, is pushed backward. However, if the object is rotated forward toward the ground, the air can now push it up as well as back.
Lift is produced by the motion of an airfoil, or wing, moving through the air. The special shape, or camber (the curved upper surface and a straight or less curved under surface), of the airfoil is what produces a change in air pressure above the wing by deflecting or turning the air. As the airfoil moves through the air, air goes above and below.
Another way of understanding the deflection of air by an airfoil, is by applying Newton's Third Law of Motion. The airfoil deflects the air going over the upper surface downward as it leaves the trailing edge of the wing. When the wing is at an angle, this deflection is even greater. According to Newton's Law, for every action there is an equal, but opposite reaction. Therefore, if the airfoil deflects the air down, the resulting opposite reaction is an upward push. Deflection is an important source of lift. There are some planes that do not have cambered wings. Their wings are flat and are designed to fly at very high speeds. These planes get lift only by deflection. Planes flying upside down, even with cambered wings, can deflect enough air to get lift.
Drag is the second aerodynamic force. It resists the forward motion of any object: airplane, car truck, boat, fish, or bird. The shape of the object determines the amount of drag which is produced. Objects that are streamlined produce the least amount of drag. Streamlining has additional advantages of reduced engine power necessary of movement, less fuel required, and overall increased performance and maneuverability of objects in motion. However, not all drag is detrimental.
There are four kinds of drag. First is friction drag. It occurs next to the surface of an object. It affects all vehicles. The second type of drag is form drag. Air flowing past an object breaks away from the surface to form little swirling pockets of air called eddies. These take energy from the object and slow it down. It occurs with non-streamlined objects. The third kind of drag is induced drag. This type of drag affects only airplanes. The fourth kind of drag occurs only when planes fly faster than the speed of sound.
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Last modified: Tue Mar 24 13:18:32 PST 1998
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