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.
Building a Stable Craft
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.