Kites

Kites are another excellent source of interdisciplinary education projects.One of our coolest project was to attach a Spy Cam to a kite and take some aerial videos.Compared with airplanes or helicopters the kites are much cheaper and you can fly them in stronger winds. Also,you can fly them almost all year round and in any weather (watch for storms!).

You can start with the history of kites,study when were they invented and compare the difference between them among different geographical areas.You will be amazed by their variety as well as by the their usage in different cultures.

Try to imagine how the first kite might have been invented and explore the animal world to see how its flight principle was used in nature.The closest comparison are the gliders and there is a wide range of examples of excellent fliers who use this principle.

Kites are very simple flying objects but you will be surprised to learn about the complexity of mathematical formulas that explain this kind of flight. NASA has considered some of them as an option for landing its crafts.I have included her a link to an interesting building calculator.You can play here with different shapes and see the effects of different designs over the their flight characteristics.

This is a good example of a simple object that can be used to exemplify fundamental laws of physics.Play with the Modeler program and you will learn about lift,drag,tension,center of gravity,center of pressure and torque,just to name a few.

Have you ever thought about all these when you have build your first one.Probably not but your children will!

Don't worry,it will not spoil their joy but will add a new dimension to their experience.The only thing you have to be careful of is the "dosage" of the scientific data you are going to insert in this activity.If they are very young you can just mention some of these notions.If they are older you can try to encourage them to do more research on this.

Making kites is a fun activity and simple enough to be suitable for kids of all ages.Building and painting them encourage also children imagination and nurture their artistic skills.

Building a kite is a simple activity and the joy of flying your own creation will pay of the time invested by you and your child in this activity.If you want you can also chose to buy one from a retail store.

History:

Chronology

This is an extensive chronology of kites starting with 478 BC.Did you know that the Romans were flying decorated windsocks as military banners or that kites were used in 1232by the Chinese for the psychological warfare by dropping leaflets in the enemy camp?


Aerodynamics

In this section you can find some basic principles regarding the kite aerodynamics. As you cans see, even these simple flying devices can incorporate a lot of science.

Kites are creatures of the wind since they don’t have any kind of propulsion system. This is way is so important to learn how to “read” the wind for a successful flight.In 1806,British Navy Rear Admiral Sir Francis Beaufort ,based on observation of natural phenomena’s, developed a simple maritime wind scale to determine the wind speed. His scale range from calm (zero wind speed) to hurricane.Each kite performs different at different wind speeds,accordingly to many factors (construction style is the most important) but as a rule of thumb it is not recommended to fly when the wind exceeds 19mph.

Also, the moving of the hot air (thermals) has a great influence in flight.

There are some similarities between kites and airplanes as well as many fundamental differences:

-both kites and airplanes are heavier than air

-both fly according to the same aerodynamic principles

-the same forces(gravity, lift and drag) are at work on kites and airplanes in flight

-all airplanes (except gliders) have their own source of propulsion, making their own ‘wind”

-the only kite used to carry a pilot are the so called Delta Wings (Rogallo Wings).Some of these although can have their own propulsion system.

-the Newton third law of motion describes better the flight of a kite than the Bernoulli principle.

Lift

The lift of a kite is generated mainly by the wind striking the sail (kite face). This produces a zone of increased pressure on its face and of decreased pressure on its backside. The flying line (tether) holds the kite under an angle (angle of attack) with the wind so the air is deflected downward. This generates an opposing upward force that balances the force of gravity. The flying line has to be connected to the kite’s center of pressure. This is the point where all forces (lift, drag and gravity) coincide.



The air that strikes the face of the kite also spills over and around the kite to the backside creating here a partial vacuum. This lowered pressure “sucks” the kite upward, adding to the lifting force. This can be explained better by the Bernoulli principle and it is somehow similar with the cambered airfoil used to generate lift by the airplane wing. This additional force is very small compared with the lift generated as a reaction to the pressure on the face of the kite.

The kite’s angle of attack is so high that applied to a wing will generate a stall (completely lost of lift) of an airplane. The only exception of a kite that uses better the Bernoulli principle is the parafoil.Compared with most other kites that are pushed more than are pulled upwards, the parafoil is modeled after a cambered airplane wing and it is capable to fly using a low angle of attack.

Shape

A kite can be described by his “aspect ratio” which is the rapport between the with (span) to the length(chord).A square shape has an aspect ratio of one and the wider a kite is the higher aspect ratio it has. A low aspect ratio determine more stability in flight compared with a high aspect ratio design.

Another factor that determines flight efficiency is the lift to drag ratio (L/D). A low L/D results in a kite flying low on the horizon while a high L/D ratio will determine a flight well above the horizon. The highest L/D ratio is present in the delta kites. They can rise fast and fly directly overhead at a 90-degree angle to the flyer.

Stability

In flight, all kites are relatively unstable and they have a tendency to roll along their longitudinal axis, yaw along the vertical axis and pitch along the lateral axis.

To solutions to increase this stability is to adjust the angle of attack, a dihedral angle profile, a convex leading edge, and adding a keel and/or a long tail.

As I have mentioned before, the angle of attack is the angle formed by the wind blowing on the face of the kite and his longitudinal axis. An increase in the wind speed generates drag increases that determine the kite to slide down its arc. This has as a consequence an increase of the angle of attack and turbulence behind the kite generating very tight lateral spins. Since there are no ways you can reduce the angle of attack during the flight the only solution is to change the tension in the flying line accordingly to the wind condition. The general rule is low wind speed-high angle of attack (tight the line), high wind speed –low angle of attack (lose the line).

Another way to increase the stability is to build the frame under a dihedral angle. This design is also used for some airplane wings and gives the kite a natural tendency to right itself up when upset.

The keel is a vertical surface that split the flowing air stream in two so an equal pressure is exerted on both sides of the kite keeping it facing the wind. This stabilizing design is similar to the one used by sailboats. The disadvantage of a kell is that it can affect the efficiency.A small keel works better at lower wind speeds while a large keel might decrease the stability because it can catch every guts of wind, blowing the kite back and forth.

A tail increase the drag and offsets its yaw and pitch by swinging in the opposite directions, in proportion with the upsetting tendency. The only thing you have to make sure of is that the tail is not too long because it will not sway rhythmically enough to neutralize the destabilizing forces. Contrary, a short tail will not be able to compensate these forces. As a general rule the tail should be about seven times the length of the kite spine. Also you can adjust the tail length accordingly to the wind speed. The stronger the wind, the longer the tail.

An alternative to the tail is the drogue, which is a simple conical fabric tube with the large end facing the wind like a windsock. The drogue is attached in the same place as a tail and it has a lever effect in flight because it concentrates the drag within the cone. The longer the drogue line the greater the stabilizing effect. Some kites use a spinner as a drogue. The spinner cone consists of a number of fabric triangles collected at the central point. The escaping air sets the device spinning during the flight.

The aerodynamic considerations presented so far can be applied to almost all kites. The exceptions are the parafoil and the rotor kite.

Designed for the first time by Domina (Dom) C.Jalbert in 1964 the parafoil has no rigid structure. Its wing-shape form is maintained in flight through the internal pressure of air in a series of pocket-type cells. It’s aerodynamic is very similar to the cambered airfoil used by most airplane wings.

In this short video you can see a parafoil with a spinner.



The rotor kite is also very interesting.In this case the lift is generated through an auto rotation action induced by the motion of the surrounding currents.

Construction:

You can build your own kite or you can buy one from a store.If you chose the first option (I highly recommend it)you can use simple materials that can be found around the house.For a starter bamboo sticks, a plastic sheet and rope will be good enough.You can also order special materials like Tyvek plastic sheets or Mylar film for the sails,aluminum,fiberglass-reinforced plastic,epoxy tubes or graphite rods for the frame as well as hard-PVC,Ertalon, PUR and rubber connecting parts.

Make sure you chose very carefully the flying line.You don't want to use your creation during a stronger wind.You can chose between nylon lines,Kevlar lines or Spectra lines.

Kite Modeler

This a free software developed by NASA will help you to study the physics and math which describe the flight of a kite. You can choose from several types of kites and change the shape, size, and materials to produce your own design. You can change the values of different variables which affect the design and immediately see the new flight characteristics. With this version of the program, you can even test how your kite would fly on Mars, or off the top of a mountain. The program tells you if your design is stable or not and also computes a prediction of how high your kite will fly.


Sport:

Did you thought that kites are only for kids? Have you heard about Kitesurfing, Landboarding and Snowkiting ? Watch this video:



If this is too extreme for you there is another option to practice kite sports:



Kite flying is for all ages.Watch this old gentleman flying simultaneously three of them.



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