Why can't the kite fly when you hold it? Don't blame the wind!

It's the annual kite-flying season again.

Speaking of kites, one has to think of Weifang's International Kite Festival. "In Weifang, there is nothing that cannot fly into the sky." The kites at the Kite Festival are so unique and bold in shape that people can't help but marvel, "Can this fly into the sky, too?"

Image source: Weifang Release

Part.1 How does a kite fly?

The principle of kite flying is actually not complicated and can be explained by simple force analysis.

First of all, what forces does a kite experience during its flight? The kite's own gravity, the tension of the kite string, and the thrust of the wind . The interaction of these three forces determines whether the kite will take off or land.

Gravity is downward vertically, and the pulling force can be decomposed into two forces, horizontal and vertical. The pulling force in the vertical direction is still downward. In order for a kite to fly, it also needs the upward thrust of the wind. Therefore, kite flying must rely on wind power, and the wind cannot be too small, as it needs to support its own gravity and part of the pulling force.

Compared to high altitude, the wind on the ground is often weaker, so the running start is crucial . When you run with the kite, the kite and the air will move relative to each other. When the kite reaches a certain height and the wind speed can provide enough lift, the kite will be able to fly.

Being able to keep the kite at an angle is also a crucial step in flying a kite. This angle is called the angle of attack , which is the angle between the plane of the kite and the relative wind flow. The angle of attack must exist for the kite to gain lift.

This is slightly different from airplanes. For airplanes, when the wing has an angle of attack, the airplane will gain lift, which is the same as a kite. However, some wings will also have lift when there is no angle of attack. Why?

Because this type of wing has a curvature, the air flow rate above and below is different, which causes a pressure difference. This pressure difference can also provide lift. This is the Bernoulli principle.

Part.2 Let out the line when the wind is strong, and reel it in when the wind is weak. The principle is actually very simple.

When flying a kite, some experienced people often instruct novices to "let out the line when the wind is strong and reel it in when the wind is weak." Is there any scientific basis for this?

The string on the kite is usually tied to the central axis, and slightly above the center of gravity, in order to ensure that the kite has a constant angle of attack during flight.

So how do you control the angle of attack?

If the kite is flying, the only thing we can do is to adjust the tension. "When the wind is strong, let out the line, and when the wind is light, reel it in." In fact, it means changing the angle of attack by adjusting the tension , so that the kite can fly stably.

Generally, a kite has a long tail at the end. Some are made in the shape of a centipede, some are made in the shape of a dragon, and some kites have two tails.

This tail bar is not just for decoration, it is also a key part that enables the kite to fly stably.

The function of the tail bar is to lower the center of gravity and to increase the restoring torque , that is, to prevent the kite from rotating along the axis of rotation. As long as it rotates, the tail bar will be straightened. This is the significance of the tail bar.

A small kite has a lot of knowledge. As long as you pay attention, life is full of science.

The May Day holiday is approaching, the spring scenery is bright and beautiful, and the breeze is gentle. It is a good time to fly kites. Bring your family and friends to fly kites together!

Produced by: Science Popularization China

Author: Mommy Says Science Producer: China Science Expo

This article only represents the author's views and does not represent the position of China Science Expo

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