In order to understand how wings keep airplanes up in the air, it is important that we take a quick look at four basic aerodynamic forces: lift, weight, thrust and drag.

In order for an airplane to fly straight and level, the following relationships must be true: Thrust = Drag and Lift = Weight. If, for any reason, the amount of drag becomes larger than the amount of thrust, the plane will slow down.

If the thrust is increased so that it is greater than the drag, the plane will speed up. If the amount of lift drops below the weight of the airplane, the plane will descend. By increasing the lift, the person flying the plane can make the airplane climb.

Thrust is an aerodynamic force that must be created by an airplane in order to overcome the drag. Airplanes create thrust using propellers, jet engines or rockets.

The plane's propeller, acts like a very powerful fan, pulling air past the blades. Drag is an aerodynamic force that resists the motion of an object moving through a fluid. The pilot wants to make the aircraft as small as possible to reduce drag. The amount of drag produced by the landing gear of a jet is so great that, at cruising speeds, the gear would be ripped right off of the plane.

Every object on Earth has weight including air. A 747 can weigh up to 870,000 pounds and still manage to get off the runway. Lift is the aerodynamic force that holds an airplane in the air. On airplanes, most of the lift required to keep the plane aloft is created by the wings.

A principal concept in aerodynamics is the idea that air is a fluid. Like all gases, air flows and behaves in a similar manner to water and other liquids. In fact, basic aerodynamic tests are sometimes performed underwater.

Another important concept is the fact that lift can exist only in the presence of a moving fluid. This is also true for drag.

Consequently, neither lift nor drag can be created in space where there is no air. This explains why spacecraft don't have wings.The Space Shuttle is a good example of a spacecraft that spends most of its time in space, where there is no air that can be used to create lift. However, when the shuttle re-enters the earth's atmosphere, its small wings produce enough lift to allow the shuttle to glide to a safe landing.

In the late 1600s, Isaac Newton theorized that air molecules behave like individual particles, and that the air hitting the bottom surface of a wing behaves like shotgun pellets bouncing off a metal plate. Each individual particle bounces off the bottom surface of the wing and is deflected downward. As the particles strike the bottom surface of the wing, they impart some of their momentum to the wing, thus incrementally nudging the wing upward with every impact.

Air approaching the top surface of the wing is compressed into the air above it as it moves upward. Then, as the top surface curves downward and away from the airstream, a low-pressure area is developed and the air above is pulled downward toward the back of the wing.

Air approaching the bottom surface of the wing is slowed, compressed and redirected in a downward path. As the air nears the rear of the wing, its speed and pressure gradually match that of the air coming over the top. The overall pressure encountered on the bottom of the wing are generally less than those on the top of the wing.

When you add up all the pressures acting on the wing, you end up with a net force on the wing. A portion of this lift goes into lifting the wing and the rest goes into slowing the wing down. As the amount of airflow turned by a given wing is increased, the speed and pressure differences between the top and bottom surfaces become more pronounced, and this increases the lift. There are many ways to increase the lift of a wing, such as increasing the angle of attack or increasing the speed of the airflow.

In general, the wings on most planes are designed to provide an appropriate amount of lift, along with minimal drag, while the plane is operating at its cruising speed. However, when these airplanes are taking off or landing, their speeds can be reduced to less than 200 miles per hour. This dramatic change in the wing's working conditions means that a different wing shape would probably better serve the aircraft.

To accommodate both flight conditions, airplane wings have moveable sections called flaps. During takeoff and landing, the flaps are extended rearward and downward from the trailing edge of the wings. This alters the shape of the wing, allowing the wing to turn more air, and thus create more lift. The downside of this alteration is that the drag on the wings also increases, so the flaps are put away for the rest of the flight.

The most important parts of an airplane, after the wing, are the propellers and engines. The propellers or jets provide the thrust that moves the plane forward. A propeller is really just a special, spinning wing. If you looked at the cross section of a propeller, you'd find that a propeller has a wing shape and an angle of attack.

The landing gear is also essential during take-off and landing. Some small planes have fixed landing gear, but most larger planes have retractable landing gear to reduce drag while in flight.

The tail of the airplane has two small wings, called the horizontal and vertical stabilizers, that the pilot uses to control the direction of the plane. With the horizontal tail wing, the pilot can change the plane's angle of attack, and therefore control whether the plane goes up or down. With the vertical tail wing, the pilot can turn the plane left or right.