The same three basic forces that act on full-sized passenger airplanes during flight also act on hand-launched gliders: gravity, lift, and drag. The force of gravity is determined by the weight of an object. How the weight is distributed along a glider is even more significant than its total weight. The point at which a glider will balance on your finger is called its center of gravity.

When air pressure acts on the wings and pushes the glider upward during flight, we call this force lift. In order to fly, the wings of a glider must produce enough lift to overcome the force of gravity acting on the aircraft. The amount of lift is influenced by the surface area of the wings, their shape, and how they are angled. Wings with greater surface area are subjected to a larger volume of air pushing up on them. An airplane's wings are generally curved on the top and flat on the bottom. Air flowing over the wings travels a longer distance than air passing under the wings, and, as a result, it flows faster. When air flows faster, pressure drops. Thus, the air pressure under the wings is greater than over them. With more pressure pushing up on the wings than pushing down, the plane stays aloft. Fixing the angle of a flat wing -- like that used for a paper glider -- so that it rises slightly toward the front produces a similar effect.

Drag, also referred to as air resistance, is the force that causes a glider to slow down in flight. A powered aircraft has engine thrust, which is the force that propels to overcome drag, but a glider does not, and so it must be designed in a way that minimizes drag. You should use wing material that is smooth and thin so that the surface offers little resistance to the air. You can also increase the lift-to-drag ratio -- which improves aerodynamic efficiency and increases range -- by designing a glider with longer wings. Because paper is not very strong, you might choose a stiffer material that can span a long distance without drooping. Alternately, you can build a frame for your wings from wooden dowels and stretch lightweight material such plastic wrap around them.