Paper folded and creased into the shapes of airplanes.
Human experimentation with flying did not begin with the Wright brothers. People have been fascinated by flight since ancient times. The first flying devices made from paper were kites, constructed by the Chinese around 1 c.e. Even Leonardo da Vinci tried to devise a way for humans to fly. It is said he used parchment folded into winged flyers during his experiments. At the beginning of the twenty-first century, paper airplanes were used as a common technique to study aerodynamics. During World War I, flying paper airplanes became a popular activity with children. In the 1940’s, the General Mills Company offered a series of fourteen paper model warplanes.
The wings of an airplane share a shape with those of insects, bats, and birds, called an airfoil. An airfoil is curved on top and flat on the bottom. Air rushing over the wing travels faster than the current going under the flat bottom of the plane. The eighteenth century Swiss scientist Daniel Bernoulli discovered that when air speeds up, its pressure is reduced. When air slows down, its pressure is increased. Therefore, the slower air going over the wing pushes down, which is known as weight, or gravity. The faster air under the wing pushes upward. This tug-of-war between opposing forces is what causes lift. During level flight, lift and weight pull equally. If lift pulls harder, the plane will rise. If weight pulls more, the plane will fall.
The center of lift on a paper airplane is the point at which lift seems to be working. The center of gravity is the balance point of the plane, the point at which gravity seems to be working. On paper airplanes, the center of gravity needs to coincide with the center of lift. If the center of lift is in front of or behind the center of gravity, the nose of the plane will pitch up or down accordingly.
Another set of opposing forces present during flight are drag and thrust. These two forces are what pull the plane forward or back. Real planes get their thrust from a propeller or engine. Paper airplanes get their thrust from being launched or thrown by a person. A throw gives a plane its initial speed, and gravity pulls it along.
When a plane flies level, drag is what pulls it back. Most of drag comes from air resistance. As a plane flies, air sticks to it, creating turbulence, or resistance to motion. If the nose of a plane points down, gravity will add thrust and the plane will crash. Any surface not parallel to the flow of air adds drag. Sharp creases and accurate folding will reduce drag and increase time aloft. Lift also contributes to drag by pulling up and a little back. A typical paper airplane’s drag is one fifth of its weight.
Differences in wing loading, the specific amount of weight a standard size area of the wing lifts in flight, will create difference in speeds. Wing loading is how many pounds per square foot the wing is lifting. The larger the wing area, the less wing loading and more slowly the plane will glide.
Another factor that affects flight is stability, which helps an airplane return to steady flight after a bad throw or a strong gust of wind. There are three basic types of stability: pitch, directional, and spiral.
Pitch stability keeps the airplane flying at a constant speed. If the nose of a plane pitches up, the plane will slow down. If it pitches down, the speed will increase. There is a small distance along the length of a plane where it must balance to provide optimum pitch stability. On a paper airplane, this distance is less than one inch long. If the balance point is too far forward, the plane will dive; too far back, and it will spin out of control.
Directional stability can be maintained by creating a fin on the back of the plane to counteract the tendency to spin. On most paper airplanes, the body acts as the fin. If most of the plane’s body is behind the balance point, it will be directionally stable. Bending the wing tips up will add to its stability.
Spiral stability is when the plane flies straight and smooth. A spirally unstable plane will circle, turning tighter and tighter, until it spins into a dive. To correct a spirally unstable plane, the wings, as viewed from the nose, should be bent up slightly so that they make a Y shape with the body.
Getting a good flight out of a paper airplane requires a good throw. To get the most out of a throw, the plane should be held on the bottom near the front, using the forefinger and thumb. How a plane is thrown depends on the type of flying intended. The types of flying include slow flight, fast flight, and high (world record) flight.
Slow flight is achieved by holding the plane in front of the shoulder and pushing the plane forward and slightly downward. For fast flying, the plane should be held in front of the shoulder for short flights and above the shoulder for long flights. The high throw is mainly used for competition and is achieved by throwing the plane straight up as hard as possible. If done properly, the plane will spiral up, level off, and glide slowly forward.
Blackburn, Ken, and Jeff Lammers. The World Record Paper Airplane Book. New York: Workman, 1994. Informative source of theory of flight. Contains black-and-white sketches as well as color models of paper airplanes. Also discusses flight contests. One of the authors is a Guinness World Record holder for paper airplane time aloft. Botermans, Jack. Paper Flight. New York: Henry Holt, 1983. Contains folding and flying instructions for paper airplanes. Discusses the origins of paper airplanes. Collins, John M. The Gliding Flight. Berkley, Calif.: Ten Speed Press, 1989. Provides tips and techniques on folding and flying paper airplanes. Discusses the theory of paper airplane flight. Kenneway, Eric. Complete Origami. New York: St. Martin’s Press, 1987. Provides probable history of paper airplanes. Offers tips and techniques on folding paper airplanes and other origami.
Forces of flight
This paper airplane, constructed in 1992, broke the world record for size.