How Kites Fly

Recently the National Air and Space Museum hosted Kites of Asia Family Day.  It featured lots of kite activities, cultural crafts, indoor kite flyers, and Japanese kite masters.  All of the incredible kites and amazing activities made me wonder how many people actually understand how kites fly. To understand how a kites flies, you need to define what a kite is.  A kite is a heavier-than-air object that flies… just like an airplane.  Most kites have three main components: the kite body (which comes in many different shapes and sizes), the bridle (or harness), and the control line (or tether).  The kite body is made up of a framework and outer covering.  The framework is usually made from a lightweight material like wood or plastic.  Paper, fabric, or plastic is then stretched over the framework, turning it into a sort of wing.  The bridle and the control line help the kite flyer control the kite.  In flight, the kite is connected to the kite flyer by the control line, which is connected to the kite by the bridle.  The kite pivots and dives about the point where the bridle connects to the control line.

 

The four forces of flight (i.e. Lift, Weight, Drag, and Thrust) affect kites in the same way they affect airplanes, and anything else that flies.  Lift is the upward force that pushes a kite into the air.  Lift is generated by differences in air pressure, which are created by air in motion over the body of the kite.  Kites are shaped and angled so that the air moving over the top moves faster than the air moving over the bottom. Daniel Bernoulli, an 18th century Swiss mathematician, discovered that the pressure of a fluid (like air) decreases as the fluid speeds up.  Since the speed of the air above the kite is greater than the speed of air below, the pressure above is less than the pressure below and the kite is pushed into the air and — Tada — lift!  Weight is the downward force generated by the gravitational attraction of the Earth on the kite.  The force of weight pulls the kite toward the center of the Earth.  Thrust is the forward force that propels a kite in the direction of motion.  An airplane generates thrust with its engines, but a kite must rely on tension from the string and moving air created by the wind or the forward motion of the kite flyer to generate thrust.  Drag is the backward force that acts opposite to the direction of motion.  Drag is caused by the difference in air pressure between the front and back of the kite and the friction of the air moving over the surface of the kite.  To launch a kite into the air the force of lift must be greater than the force of weight.  To keep a kite flying steady the four forces must be in balance.  Lift must be equal to weight and thrust must be equal to drag.

Wind is obviously a big part of kite flying.  But what do you do if you don’t have any wind or you’re trying to fly your kite inside?  Check out the video of this national champion indoor kite flyer from the family day.  There obviously wasn’t any wind inside, so how was he able to fly kites in the middle of the Space Race gallery?  The kite flyers create lift, drag, and thrust with various walking patterns, arm movements, and spinning to make the indoor kite flying experience like a dance.  Whether inside or out it doesn’t matter whether the wind moves over the surface of the kite or the kite is pulled through the air — lift must overcome weight and thrust must overcome drag to keep the kite soaring. To learn more about the four forces of flight visit the How Things Fly website.  And to learn more about the aerodynamics of kites and experiment with different kites, visit the NASA website.