Windmill is a kind of power machine that can use wind energy without fuel. Early wind turbines were also called windmills. Modern wind turbines refer to wind turbines used for power generation, and some are also used for pumping irrigation.

Ancient windmills were developed from sails. It has 6 ~ 8 pairs of awnings like sailboats, which are distributed around a vertical axis. When the wind blows, it rotates around the axis like a lantern, which is called a lantern windmill. Because of its low efficiency, this kind of windmill is gradually replaced by other windmills, such as "vertical windmill" and "automatic rotor windmill".

China ancient windmill

The windmills in ancient China have obvious characteristics. Except for the horizontal axis, the sails are all sail-shaped. The sail is not installed in the radial position of the axle, but on eight columns around the axle frame. The sail is biased, that is, one side of the mast is narrow and the other side is wide, and it is tightened with ropes. As shown in figure 1, when the wind acts on a, the sail is in the downwind direction, and the sail is perpendicular to the wind direction (the maximum force is exerted), and it is tightened by the rope.

When turning to position c, the sail blows outward and the sail surface is parallel to the wind direction; Returning to the windward position at point E, it can use the wind or headwind by using the tightness of the rope and the deviation of the sail, just like in the sail.

The device makes the sail swing freely with the wind without special resistance, and the effective wind action range of the sail rotating around the circle exceeds 180 degrees. If in position G, the sail begins to turn with the wind, and the sail can also use some of the wind to do some work. The characteristics of this kind of sail windmill are unique to China.