Find out more about The Open University's Science courses and qualifications
How does a kite fly?
Most people associate kite flying with a strong wind to get the kite into the air but most kites are designed for light breezy conditions.
Although you do not need to know anything about aerodynamics to fly a kite, it helps you control your kite if you understand basic wind flow and lift. There are two main principles involved in the aerodynamics of a kite:
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.
The Bernoulli 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.
Another effect of a kite's flying aerodynamics is when the airflow is not just split along the upper and lower kite surfaces but when the split air vaults over the kite and doesn't meet up again right away. When this happens a lower air pressure is created directly behind the kite's flight pattern. The kite can be sucked into the area of low pressure and give your kite drag. If this effect is increased, the low-pressure area leaks onto the top surface of the kite reducing the lift and making it stall.
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.
You will find that the centre of pressure is best controlled by the positioning of your flying line. For example, in light winds you will achieve the best lift by lowering your towing line to the base of the kite. This may produce a slight wobble or bring about large circles in flight. If your bridle towing line is too high it may cause your kite to tip side to side and could flip over. Experiment with your line placement to get the most out of your kite flying experience.
Now that we understand the principles of aerodynamics, what is the best way of making a kite?
- Take two 90 cm long sticks, one slightly thicker than the other.
- Cut the thicker stick to 75 cm long (this will be your horizontal bar) and make pencil marks 2 cm in from each end.
- Using a small saw cut tiny, slanted notches on the marks.
- Mark the centre but don't put a notch in it.
- On the thinner stick make pencil marks 2 cm in from the ends and 20 cm from the bottom.
- Cut a V-notch at the top of the stick and slanted notches at the other marks. This will be your vertical bar.
- To attach the bars together, put the centre of the horizontal stick into the V-notch of the vertical stick and criss-cross a piece of string for a tight fit.
- Attach the end of another piece of string to notch A on the horizontal stick.
- Gently bend the stick down and wrap the string around the vertical stick at point B so the string is about 30 cm long.
- Bend the other side of the horizontal stick and attach the string at notch C, another 30 cm long.
- Then continue stringing the kite frame from point C to point D, from point D to notch A, from notch A to notch E and from E to point C.
Making the kite sail
- Once your frame is strung, lay it on a piece of tissue paper or wrapping paper or very thin material.
- Trace around the kite frame adding about 4 cm of border around the outside.
- Cut out the kite paper with scissors.
- Apply glue to all the inside edges and lay your frame on top of the paper and glue the paper over the string and horizontal stick.
Attaching the flying lines
- Gently punch two small holes on either side of the vertical stick about 10 cm from the top and 20 cm from the bottom of the kite.
- Cut a piece of string about 75 cm long.
- Thread the string through the holes at the top and knot it securely around the vertical stick.
- Thread the other end through the bottom holes and tie that too.
- Tie the rest of your ball of kite string about 1/3 of the way down from the top of the flying line you just made.
Your kite is now ready to fly. Try running along with the wind blowing against you. Let out the string as the kite goes higher.