A kite is a special sort of aircraft, attached to the ground by a string. When a kite is stopped from being blown backwards by the wind it will usually fly. The string we hold when flying a kite is what is used to stop the kite from flying away with the wind. Because this sort of aircraft has no engine it needs something else to make the kite move through the air. The power source for a kite is the wind. Without wind moving over the kite it won't fly. Some kites need lots of wind. Others need very little wind for them to fly.
What makes a kite fly?
Just like a normal aeroplane, or even a bird, there are four forces that affect a kite when it is flying. They are gravity, lift, thrust, and drag.
Gravity pulls everything towards the centre of the earth. Anything on the ground can't get pulled down any further than that, but things in the air can get pulled down to ground level. The heavier a kite is the harder it will be to fly. Kites are generally made from lightweight materials so that they will fly easily.
If gravity is a force that pulls everything towards the centre of the earth, why don't kites get pulled down to earth? The force that stops a kite from falling down is called lift. Lift is the force pushing the kite away from the surface of the earth. It is produced by air moving over the top of the kite at a faster speed than the air that is moving over the bottom of the kite. Daniel Bernoulli, a scientist in the eighteenth century, discovered that the pressure of air becomes lower when it is moving. The faster the air is moving the lower the pressure becomes. A kite is shaped so that air will be slowed down if it is travelling under the lower surface of a kite and will speed up if it is travelling over the upper surface of the kite.
The faster the air moves over the upper surface of the kite the lower the pressure of that air becomes. As the air pressure above the kite becomes lower, the air pressure below the kite pushes the kite up in order to equalize the pressures above and below the wing. The greater the difference between the low pressure above the wing and the higher pressure below the wing, the greater is the amount of lift produced. In order for all this to happen, air needs to be moving over the kite. So what makes the air flow over the kite?
When we fly kites we use a string to hold on to the kite, and to prevent it from being blown away by the wind. Birds move through the air by flapping their wings, while powered aircraft use an engine to push the air towards the back of the plane fast enough to make the aeroplane move forward. The force that makes something move forward through the air is called thrust. Birds use muscle power to develop thrust, and except for gliders, aeroplanes use motors. A kite cannot produce its own thrust, and instead must rely on being held in place while the wind moves past it. So the thrust that acts on a kite is produced because the kite is being held in one place by its string while the wind flows around the kite. If there was no wind a kite would only fly if the person holding the kite string started running, making their own wind as air started flowing around the kite.
What stops a kite from flying straight over your head and into the ground on the other side? As the air flows over the surfaces of a kite the wind gets held back a little bit by the roughness of the fabric and the sticks. This friction is called drag. Any kite will always have some drag, and some kites are not able to fly without some extra drag being exerted on them. This additional drag is usually created by the tail of the kite, and allows us to make the kite point in the correct direction. Air flowing through the tail pulls the bottom of the kite in the direction that the wind is blowing. If more drag is caused by the tail than by the rest of the kite then the kite will always point into the wind. A single, long tail causes a certain amount of drag, but the same tail, cut into several lengths will produce much more drag, because the air is slowed down even more when it has to pass around several strips rather than just one. When the top of a kite turns to one side, the bottom of the kite pivots in the opposite direction. The drag caused by the tail tries to pull the bottom of the kite back to the centre again.
The drag caused by the tail of a kite stops the kite from turning too much to one side or the other. A tail gives a kite lateral stability. If a kite has too little tail it will not be laterally stable, and will move around a lot, and might even start spinning. If the kite has too much tail the kite will be stable, but may be hard to keep flying because of the extra weight caused by the excess tail.
Basic kite terms.
A spar is the name for any of the sticks which act as the "skeleton" of the kite. The spine is the spar that runs from the top to the bottom of the kite. The cross-spar is the spar that runs from wingtip to wingtip.
The leading edges of the kite are the edges which the wind flows over first. The trailing edges are the edges which the wind flows around last.
The bridle is the line that connects the kite with the flying line. It is usually connected to the kite in two places, but some kites use bridles that are attached to the kite in many places. The point where the flying line attaches to the bridle is called the bridling point. Each piece of the bridle running from the bridling point to the surface of the kite is called a leg. A bridle that is attached to the kite in two places is known as a two leg bridle. The bridle is used to control the angle of attack of a kite. This is the angle at which the kite meets the wind. A low angle of attack tends to reduce the amount of tension on the kite string. The kite will also fly closer to vertically above the kite flier. A high angle of attack normally increases the pull on the flying line. It also makes the kite fly at a lower angle, relative to the flier. The angle of attack can also be altered to make a kite fly in different wind conditions. A high angle of attack will enable the kite to "catch" more of the wind in light wind conditions, whereas a low angle of attack will allow more wind to be "spilt" by the kite in strong winds.
The cover of the kite is called the sail. The sail is pushed against the spine and the cross-spar, dividing the sail into four areas. The two small areas at the top of the kite are called pilot sails, while the two larger segments are known as driving sails. The pilot sails partly control the direction in which the kite moves and the driving sails provide most of the lift.
The tail of a kite refers to the strips of paper, plastic or fabric which are attached to the bottom of the kite in order to increase the drag of the kite. Tails can be made in a number of different ways, and can also be added to the wingtips of kites.
Kites can be bowed, or bent, in order to give the cross-spar a dihedral angle. When this happens the cross-spar is bent so that the wingtips are at a slight backwards angle to the spine. This helps make the kite stable. If the kite starts to roll to one side the wing on that side presents a greater surface area to the wind and the wing on the opposite side appears to reduce in surface area. The greater pressure being exerted on the wing with more surface area being exposed to the wind forces it to return to a normal attitude again.
There are seven distinct groups of kites. All kites will fit into one of these categories.
Flat kites include all kites that are not bowed in some way. They don't have to be any particular shape, as long as they are flat, or planar. All flat kites need tails in order to fly.
Bowed kites are those kites that have a dihedral angle. Because they are bowed they do not need tails.
Parafoils are special sort of kites which are shaped very much like an aeroplane wing. They rely on the wind to hold them open as they generally have no spars. These kites normally do not need tails.
Box kites are also called cellular kites. They have many surfaces, some of which normally lie vertically, while others lie horizontally. Because of these surfaces, which act in a similar way to the dihedral angle on bowed kites, this sort of kite does not need a tail. They are normally a strong wind kite.
Compound box kites are basically box kites with wings attached to them. They fly in lighter winds than normal box kites and can pull much harder because of the increased surface area being presented to the wind.
Delta wings are the best kites for very light winds. They tend to be pushed up and forward so that they lie nearly parallel to the wind. Because of this they have a very low angle of attack. Consequently they don't pull very hard at all. The sail forms a billow, and this, combined with a keel in place of a bridle, allows deltas to fly without a tail. This kite was named the delta because it looked like the letter "delta" in the Greek alphabet, which corresponds to our letter "d".
The sled is similar to the parafoil in that it relies on the wind to hold it open. It has several spines running the length of the kite, but no cross-spars. Sleds often have vents cut in the sail, near the bottom of the kite, instead of using a tail. They pull very hard and fly with a high angle of attack.
Safety rules about kite flying.
Never fly a kite in a thunderstorm or while it is raining. The kite, or the flying line, might be hit by lightning.
Never fly a kite with wire, or anything that could conduct electricity through the flying line to you, and don't use a wet flying line.
Never fly a kite near power lines, antennae or transmission towers. If your kite does get caught in power lines, do not attempt to retrieve it. Call the S. E. C. or Fire Brigade. Never fly a kite above crowds, near public streets, highways, airports or helicopter pads, or other areas where your kite might be a hazard to other people. In Australia kites are legally allowed to be flown up to three hundred feet above ground level, and may not be flown within five kilometers of an airport.
Don't climb trees, buildings or power poles to retrieve your kite. Ask for help from an adult, or make another kite.
If your kite pulls strongly, wear gloves while flying it, to prevent the line cutting into your hands or causing a friction burn.
Don't use monofilament fishing line to fly your kite. It is hard to see, and it can cut into skin very easily when it is pulled taut by a flying kite.
Choosing a flying site.
Allow lots of room to fly a kite. Always choose a site with plenty of space and no trees or power lines. Try to keep out of the "wind shadow" of trees or buildings. If you are standing with your back to the wind, and there is a building behind you, it is important to know that any obstacle to the flow of the wind will create turbulence. If the building is ten metres high then the air will still be turbulent up to thirty metres downwind of the base of the building.
Beaches are often good flying sites, but keep away from other people.
Some people like to fly kites from hilltops, but it is normally better lower down the slope that faces into the wind. This is because of the turbulence caused by the wind shadow of the hill.
Try to choose a location with even ground. If you are watching your kite you can't see your feet, and you may stumble and fall if the ground is rough. Keep an eye out for park benches, rubbish bins, goal posts, and any other obstructions you might run into while launching your kite.
Launching your kite.
Have someone hold the kite up, facing into the wind while you wind out about thirty metres of line. You should be facing your kite, and the wind should be blowing from behind you. Rather than getting your helper to throw the kite into the air, have your helper hold the kite lightly, so that you can pull it out of their hands when you are ready. If the wind is blowing strongly all that you will need to do is to wind in a few metres of line rapidly and the kite will rise into the sky. If the wind is only light you may need to run a few steps into the wind in order to get the kite to an altitude where there is more wind. The wind is often stronger at higher altitudes than it is on the ground.
Sometimes the wind might be so light that this technique doesn't work. In that case, you may need to let out as much line as you possibly can and then pull the line in sharply. The kite will start to rise. Keep pulling in line until the kite stops climbing towards vertical. Then let out line. The kite will start gliding downwind, taking out line in the process. Keep repeating this procedure until the kite reaches stronger winds, and doesn't start sinking back when you stop pulling in line.
Remember that in Australia the maximum height above ground level that kites are allowed to fly is 300 feet. That works out to just over 91 metres. Aircraft still use feet for measuring distance from the ground, so all laws relating to aircraft still use imperial measurements for altitude.