The Physics of Kites!

by Brianne Estes, Robyn Sauls, and Kathleen McDonald

sbestes@email.unc.edu, srobyn@email.unc.edu, kmmcdona@email.unc.edu

Essential Aspects of a Kite

Forces on a Kite

Even though there are many different types of kites, the forces acting on each kite is exactly the same. These same forces along with thrust are what act on any airplane. An important fact about kites is that they are heavier then air and rely on forces called aerodynamics in order to fly. Kites are constructed of a solid frame made of wood or plastic that is covered with some type of material. This material is necessary in order to generate the lift that is necessary to overcome the weight of the kite.

Although flying kites may seem like a childhood activity, kite flying is actually a great example of the application of aerodynamic forces.  There are many different types of kites, some of which can be seen in the Types of Kites Diagram.  Forces are involved in everyday actions such as walking, and kite flying is no different.  Forces applied to the kite are what keep the flight in the air and in motion.  Also, the forces act according to Newton’s Laws of Motions (See Newton’s Laws of Motion Diagram).

Aerodynamic Forces

First, in order to adequately understand the physics of kite flying, it is essential to know what aerodynamic forces are.  A force occurs at wherever two objects meet.  However, something different happens when solids interact with fluids (liquids or gasses).  The point of contact for an object surrounded by a fluid is actually the entire surface, meaning force occurs everywhere on the surface of the object.  Pressure is the medium through which the force is transmitted and as seen in the diagram Aerodynamic Forces, and it acts perpendicular to the surface of the object.  Moreover, aerodynamic forces occur when the object is moving through the fluid or the fluid is moving past the object.  For a moving fluid, pressure varies because velocity varies.

More definitions that will aid in understanding Aerodynamics are “lift” and “drag.”  These are simply separate components that combine to form aerodynamic forces.  Lift is the component of the total force that is perpendicular to the direction of movement and drag is the component along the direction of movement.

Conclusively, aerodynamic forces are created when an object moves through a fluid, and specifically with the kite moves through the air.

Kite Lauch and Flight

Because a kite is a solid object on Earth and heavier than air, gravity causes a downward force on it that is typically referred to as weight.  In order to counteract gravity and launch the kite (A on Kite Launch and Flight Diagram), the kite flyer must create a lift force that is greater than the kite’s weight.  Lots of different elements come into play when determining the size of the lift force, but most important is the velocity of the air or wind blowing past the kite.  More specifically, the relative velocity is the most important part of creating the lift.  Choosing a fixed reference point such as the ground, is essential in determining relative velocity between the kite and the air.

One way to gain relative velocity from the wind is to face the kite with the wind on the posterior side of the kite flyer’s body.  Windy days are great days to fly kites because the force of the wind combined with the force of pulling slightly on the line creates a lift.  Running and/or moving backwards are ways to create this relative velocity when there is not enough wind.

Because the velocity of the wind typically increases while increasing altitude, standing still while the kite rises during the launch (B on Kite Launch and Flight Diagram) is normally not a problem.  The boundary layer (See Boundary Layer Diagram) is what causes the change in velocity.  The boundary layer is a slim layer of fluid adjacent to the surface where the velocity changes from zero at the surface to a free stream number away from the surface.  Variations in the collisions and sticking of molecules create this boundary layer.  If enough altitude is reached, the velocity and the force generated by the lift will remain fairly the constant, even though velocity is usually low and unsteady inside the boundary layer.

After the kite is launched it will cruise (located at point C) at an altitude in which all forces and torques on the kite are balanced. If the forces acting on the kite are changed the kite will move around until they are balanced once again. For example if you pull on the line of a kite you will increase the velocity of the kite. Because of this increased velocity the lift of the kite will increase which causes the kite to climb (located at point D).

Using the http://www.grc.nasa.gov/WWW/K-12/airplane/kiteprog.html (KiteModeler) you can design your own kite and solve the equations necessary to determine the flight characteristics of your specific design.