The study of forces that affect an object moving through air is known as aero-dynamics. The same forces that allow kites and gliders to fly apply to paper airplanes and real airplanes as well. In fact, all objects are affected as they move through air or air moves past them. Designers or airplanes, sailboats, race cars and even buildings rely on the same principles of aerodynamics.
For airplanes, we consider 4 basic forces:
Now, if your interest in airplanes only goes as far a booking your holidays though Fly.com or Expedia, you might not be interested in the details, but it is always good to have a basic understanding. If you are an avid paper plane-maker or you plan on getting a pilot's license then you might want to make understanding aerodynamics a priority! Let's start with gravity. Gravity is a constant force that pulls the plane toward the ground. Thrust usually comes from an airplane's engine but paper airplanes get thrust from you throwing them. Drag is the opposing force to thrust and is caused by friction of the plane's skin with air. Drag slows the plance reducing the wing's ability to generate lift.
Lift is created when moving air above a wing creates lower pressure. A Swiss mathematician, Daniel Bernoulli, discovered this effect which you can demonstrate. Tape a piece of paper to a table edge and blow across it at lower pressure that the still air below it. This slight pressure difference causes the paper to rise.
The same principle applies to curved wings found on real airplanes. Air moving over the top of the wing has to travel slighty farther (and so a little faster) than the air beneath it. This causes slighty lower pressure above the wing which creates lift. The best wing shape for lift depends on many things and usually is designed using a computer and a wind tunnel.
Paper airplanes don't have curved wings so how do they fly? They use the angle of attack of their flat wings to create lift. Even though the paper wings are flat, you will notice that air movig across the top surface has to travel slightly farther (and faster). Lift is generated from the same low pressure (Bernoulli effect) as with a curved wing although not nearly as much.
The total amount of lift also depends on air speed and wing size. Airplanes can fly only when total lift is large enough to counteract their weight. Since all the airplanes in this program weigh about the same, those with smaller wings fly faster. This happens because larger wings can generate enough lift even when they are flying more slowly.