Airplanes fly because they are able to generate a force called Lift which normally moves the airplane upward. Lift is generated by the forward motion of the airplane through the air. This motion is produced by the Thrust of the engine(s).
The figure below is a simple diagram of the four forces acting on an airplane – Thrust, Lift , Drag and Weight. Drag is the force produced by the resistance of the air to the forward motion of the airplane. Swish your hand rapidly side-to-side and you will feel that resistance on your hand.
Weight is the force created by the pull of gravity toward the center of the earth. You will feel the effect of this force if you jump up from the floor. Your weight will force you back down.When the Thrust produced by the engine(s) is greater than the force of Drag, the airplane moves forward. When the forward motion is enough to produce a force of Lift that is greater than the Weight, the airplane moves upward.While any part of the airplane can produce Lift, the most Lift comes from the wings.
How is Thrust Generated?
There are two basic types of airplanes - propeller driven planes and jet planes.
Propeller Driven Planes - Propeller driven airplanes use a propeller that is turned by some type of engine. Propellers are shaped just like the wings, and also generate lift, except that the lift is forward instead of up and is called thrust. Each propeller is made up of two or more blades. The first propellers were made of wood, but now most propellers now are made of metal. The F4U Corsair is a propeller driven aircraft.
Jet Planes - Jet planes do not have propellers. Instead, they have jet engines that move the airplane forward through another physical principal discovered by Sir Isaac Newton (1642 - 1727). This is Newton's Third Law of Motion - "For every action there is an equal and opposite reaction." You can think of a jet engine as a tube in which a liquid fuel (like the fuel diesel trucks use) is burned at high pressure with air from a compressor. The resulting heat forces the gases out of the back of the tube at high speed. In accordance with Newton's Law, an equal force is applied in the forward direction, moving the engine (and the plane it is attached to) forward.
Reference:
http://www.kids.ct.gov/kids/cwp/view.asp?a=2731&q=330926
