Kite Science Why a Kite Flies?

Newton’s Third Law Newton's Third Law states that for every action force, there exists a reaction force that is equal in magnitude but opposite in direction. The kite is kept on the earth by the tension in the string and the force of gravity. The kite is kept in the air by lift. To raise and lower the kite in the air we can use lift by increasing and reducing the tension in the string.

Bernoulli’s Theorem Bernoulli's Theorem explains why the kite has lift. The kite creates an obstacle to the normal airflow that causes the air to change direction and speed. If you hold the kite out flat to the wind like a wing, beneath the kite the wind is slowed down and therefore the pressure of the wind increases to compensate. Above the kite, the air is not slowed down and sometimes is sped up so that pressure either stays the same or decreases. The pressure difference between the kite's bottom and top surfaces causes the lift force.

Lift Wind moving across the sail of a kite creates pressure.  Lift results from this wind pressure being deflected along the face of the kite. In other words, the wind pushes up on the kite. Think of wind pressure like a hand, pushing the kite up into the sky and holding it there. If the hand is removed, the kite will fall. At the same time, wind passing over the top of the kite creates an area of low pressure, like a vacuum, along the back of the kite. This creates a pull from behind.

Drag, Gravity, Thrust Drag is created by wind resistance on the kite's surface (and tail). Drag can also result from turbulence behind the kite.  Gravity is the downward force created by the weight of the kite.  Thrust is the power of the wind which creates lift. To fly, a kite needs to have enough lift to overcome gravity and drag.

Lift and Drag Lift and drag are important to remember in the performance of your kite. For your kite to fly stationary in the sky the lift and drag must be equal and opposite to the gravity and tension forces pulling it down. When the wind dies, so does the kite. Without wind there is no resistance for Newton's Third Law and without air moving across the surface of the kite Bernoulli's Theorem of lift is also gone. When this happens you've got two options:  a) Run like mad and create your own wind, or  b) Watch helplessly as your prize kite flutters to the ground. 

Kite String All of these forces - lift, drag, gravity, and the thrust of the wind come together in the kite at a place called the center of pressure. And not surprisingly, that's where you tie your kite string. 

Adjusting for Different Types of Wind Most kites can be adjusted for different types of wind. Adjustments are made by changing the point where your flying line attaches to the kite. Usually this "tow-point" is about one third from the top. By moving the tow-point slightly, you change the angle that the kite leans into the wind. Leaning more forward will decrease pull and allow more stable flying in stronger winds. Leaning less forward will allow the sail to catch more breeze when winds are light. If you lean the kite too far forward, the top will flip under and crash. And if you lean the kite too far back, it won't even try to fly.

Tails Many kite designs come with tails. By adding drag at the bottom of the kite, they keep the nose pointed to the sky and add stability. So if your kite is unstable, try adding tail. If your kite is getting dragged back to the ground, try reducing the length of your tail. Remember to keep things properly balanced. Put tails in the center of the kite. If you add them in more then one place, make sure the tails are equal in length and equal distance from the center of the kite.

Bows Another important part of some kite designs is the bow. A bow is a string that goes from side to side and keeps the edges of your kite bent back. Scientists call this bend or curve a "dihedral". Remember that the bow goes behind the kite, not in front. Like a tail, the dihedral helps keep your kite balanced in the sky. The wind will go around the curved surface and push the kite straight up. That's why curved kites usually fly better than flat ones.

When the wind gets stronger, try increasing the amount of bend by tightening the bow string. In lighter winds, loosen the bow. But always be careful when making adjustments not to break the sticks in your kite by bending too much.

Kite won’t fly? Not every flight goes well. If your kite isn't flying right, maybe you have one of these problems: Lousy Wind: There may not be enough wind. Or maybe there is too much. The amount of wind you need to fly easily depends on the design of your kite. If your kite uses a tail, try adding or reducing the tail's length in different winds. Turbulence: Are you trying to fly behind a big tree or building? The wind is going to be really bad there. Tuned Out: Remember that you can adjust the towpoint on most kites for different winds. This is called "tuning". If your tow-point is too high or too low, your kite won't fly. Try setting it about 1/3 from the top of the kite for starters. Loopy: If your kite loops around in circles, try adding tail, adjusting the tow-point, or tightening the bow line. Dragging: If your kite won't lift, try reducing tail, adjusting the tow-point, or loosening the bow line. Is your flying line wet or too heavy? Is the sail of the kite too loose to catch the wind? Make adjustments to lighten the load and increase efficiency. Unstable: Winds close to the ground aren't as good as the wind up fifty feet or so. Get a good launch and fly up into smoother winds. Technical Difficulties: When all else fails, make sure your kite is put together right.