It would seem like a simple question. Most will answer, "Take 3 shovels and do not giggle," and the question "Why so?", All the same will give an answer, saying that large windmills with three blades are not stupid, the same people do.

And let's take a closer look at this issue.

The main operation of the windmill is due to the deceleration of the air flow, the larger the area of ​​the wind wheel (the larger the number of blades), the greater the slowing down of the flow. "Eureka", shout some, we take a lot of blades and stop the wind. This is exactly what they do in the desert regions of the US and Australia. They use a multi-blade (about 24 blades) windmills to lift the water.

For wind turbines, such windmills are not suitable because they have a low speed of about 1.

Speed $\mathit{Z}$ - wind generator, is the ratio of the circumferential velocity of the end of the blade ($\mathit{\omega }\mathit{R}$) to the speed of the incoming air stream ($\mathit{v}$):

$\mathit{Z}\mathit{=}\frac{\mathit{\omega }\mathit{R}}{\mathit{v}}$

The slow-moving effect is due to the fact that at a certain wind speed, the blades seem to interfere with each other due to their turbulence, changing the direction of air flow. And at certain speeds such windmill slows itself. From here we see that many bladed windmills do not give much spin, so these windmills are used where you do not need high speed.

In turn, small bladed windmills have a high speed, reaching 12 units in single-blade wind turbines.

Of course, speed depends not only on the number of blades, but also on the blade profile and its performance. So, for example, a full-profile blade made without flaws has a higher speed ratio than a semi-profile blade, or a blade with a rough surface.

So why not use a single-blade windmill for wind turbines? Here comes another parameter called Gyroscopic torque.

Wind is not a constant value not only in its speed but also in the direction, which makes the wind turbine practically always works in dynamic mode constantly changing its direction. At this time, the screw rotates, and it is known that any rapidly rotating body seeks, like a whirlwind, to keep the direction of its axis of rotation unchanged, and counteracts any attempt to reject this axis. If you force the end of the axis to deviate in either direction, it will resist, and its end will go in the direction perpendicular to the impact. This is called gyroscopic torque, its value depends on the mass, diameter and rotation of the screw. I will remind, the mass of blades also grows with increase of turns at the expense of centrifugal force.

And as the power of the screw depends a little on speed, then choose such configuration of blades at which gyroscopic now not so much, and turns, at it's not that small.

Three wind blades are such a compromise for wind turbines. The speed ratio is about 6 to 7 units on average.