Why do windmills typically have 4 blades, yet all modern wind turbines have 3?

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Why do windmills typically have 4 blades, yet all modern wind turbines have 3?

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Wind turbines have three blades for a balance of stability and efficiency. It’s the fewest number of blades you can have while still keeping the structure from shaking itself apart from gyroscopic forces. Since more blades means less efficiency, three is the best we can do.

Windmills were made in a time when precision engineering and machining weren’t nearly as advanced as they are now. It follows the same principle – fewer blades is more efficient – but four blades is a *whole* lot easier to manually balance than three is.

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For people asking why two blades isn’t stable:

Two blades is only stable if the turbine doesn’t rotate laterally. Because an efficient turbine needs to rotate to maximize its angle to the wind, three is much better.

Two-bladed (and even one-bladed) turbines *do* exist. The problem is that because of their instability, they produce a lot more wear on their components and are more prone to failure.

There is still research being done into making two-bladed turbines more viable, and if a good solution to their instability is found, They may well end up becoming the standard. For now, though, three blades is all-around the best option.

The old farm multi-blade wind mills for pumping water were designed for a high start up torque to get lift the column of water up the well pipe, with a bit of simplicity thrown in for the realities of maintenance and repairs in the rural field. That’s why they had a lot of “rotor solidity” – looking straight on at the rotor, the disc it sweeps had a large proportion of blade area to the total swept area. That gave them the high start up torque in low wind speeds.

The modern electrical generation wind turbine is designed for efficiently extracting as much as it can from the available wind, which means high torque at typical operating wind speeds. In reality though, there isn’t much power to be extracted at low wind speeds, since the power available is related to the square of the wind speed. If you have too many blades, you’re beginning to slow down the wind, which lowers the available power. Theoretically, a two bladed turbine would be more efficient, but a three bladed design isn’t that bad and has fewer balancing and stability issues.

See [this website](https://interestingengineering.com/the-scientific-reason-why-wind-turbines-have-3-blades) for a bit more discussion of the issues.

Right angles are really easy to make with pre-industrial tools. Since a mill needs to balance pressure on all the blades (or they are more likely to break in heavy wind) the easiest way to balance is two beams crossed at right angles.

[removed]

3 blades done properly works best

4 blades that form a cross is easy to put together

People who built windmills were more inclined to go for the easy to build version, as they weren’t doing precision engineering.

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0

Why do windmills typically have 4 blades, yet all modern wind turbines have 3?

In: 9124

Wind turbines have three blades for a balance of stability and efficiency. It’s the fewest number of blades you can have while still keeping the structure from shaking itself apart from gyroscopic forces. Since more blades means less efficiency, three is the best we can do.

Windmills were made in a time when precision engineering and machining weren’t nearly as advanced as they are now. It follows the same principle – fewer blades is more efficient – but four blades is a *whole* lot easier to manually balance than three is.

—-

For people asking why two blades isn’t stable:

Two blades is only stable if the turbine doesn’t rotate laterally. Because an efficient turbine needs to rotate to maximize its angle to the wind, three is much better.

Two-bladed (and even one-bladed) turbines *do* exist. The problem is that because of their instability, they produce a lot more wear on their components and are more prone to failure.

There is still research being done into making two-bladed turbines more viable, and if a good solution to their instability is found, They may well end up becoming the standard. For now, though, three blades is all-around the best option.

The old farm multi-blade wind mills for pumping water were designed for a high start up torque to get lift the column of water up the well pipe, with a bit of simplicity thrown in for the realities of maintenance and repairs in the rural field. That’s why they had a lot of “rotor solidity” – looking straight on at the rotor, the disc it sweeps had a large proportion of blade area to the total swept area. That gave them the high start up torque in low wind speeds.

The modern electrical generation wind turbine is designed for efficiently extracting as much as it can from the available wind, which means high torque at typical operating wind speeds. In reality though, there isn’t much power to be extracted at low wind speeds, since the power available is related to the square of the wind speed. If you have too many blades, you’re beginning to slow down the wind, which lowers the available power. Theoretically, a two bladed turbine would be more efficient, but a three bladed design isn’t that bad and has fewer balancing and stability issues.

See [this website](https://interestingengineering.com/the-scientific-reason-why-wind-turbines-have-3-blades) for a bit more discussion of the issues.

Right angles are really easy to make with pre-industrial tools. Since a mill needs to balance pressure on all the blades (or they are more likely to break in heavy wind) the easiest way to balance is two beams crossed at right angles.

[removed]

3 blades done properly works best

4 blades that form a cross is easy to put together

People who built windmills were more inclined to go for the easy to build version, as they weren’t doing precision engineering.