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.
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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.
More blades means more torque in low wind speed. Moreover, an odd number of blades is smoother than an even number in certain applications because of the loss of lift caused by passing in front of structural elements.
Think of a submarine with 4 “fins” just in front of the big spinning propeller. Having a seven bladed propeller means that at any time, only one blade is in front of one of the fins. This reduces vibration and provides smoother propulsion.
Why waste time use 4 blades when 3 blades do trick?
The top answer does leave out one of the most important aspects of Windmills: the [tip-speed ratio](https://en.m.wikipedia.org/wiki/Tip-speed_ratio)
It determines the ratio between the tangential speed of the tip of the windmill wing and the wind speed.
Generally the higher the tip-speed ratio, [the less Blades a Wind Turbine has](https://upload.wikimedia.org/wikipedia/de/9/98/Schnelllaufzahl.png)
Windmills heaving three blades is a balance between rotational Speed, noise, cost, efficiency, etc. (Slower Windmills with more Blades would need bigger gearboxes –> more cost; faster Windmills would be louder and more instable –> more expensive materials and more difficulty with regulations.)
One single answer is often not possible with such complex Topics.
Edit: The size of the Turbine is also important because the tip-speed is dependent on it, effecting the tip-speed ratio (older Windmills are smaller than Modern Wind Turbines –> less blades on modern Turbines)
I think because back in time, crossing two very long logs to create four blades was much easier than trying to secure three blades.
There are already a few good answers, but there’s one overarching truth to why modern windmills usually have 3 blades: optimization.
There are many tradeoffs, and 3 blades just happens to be how the math works out for balancing the design against size, materials, structure, generator power limits, design complexity, geometry, expected wind speed, etc and your optimization spits out 3 blades as the best for the typical sizes of power generating windmills.
They’re far, far higher-performance than the old 4-bladed cloth and frame windmills, which are just much easier to build and balance than 3-bladed fans.
If the fans were a different size, or made of different materials, or exposed to different wind speeds, or any number of other major factors, the optimal design might have been 5 blades, or 7, or 9.
For examples of when 3 blades are not optimal, look at many helicopters, which range from 2 to 9 blades depending on size, power, noise and other requirements. Computer cooling fans are also often highly optimized and have varying numbers of blades for various applications.
It’s a big, giant “it depends,” so 3 may not be the magic number forever, and there’s no canonical, set answer.
As for why 4 blades on old windmills, though: it’s just easier, and the technology wasn’t there for engineering and building windmills like we do now.
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.
More blades means more torque in low wind speed. Moreover, an odd number of blades is smoother than an even number in certain applications because of the loss of lift caused by passing in front of structural elements.
Think of a submarine with 4 “fins” just in front of the big spinning propeller. Having a seven bladed propeller means that at any time, only one blade is in front of one of the fins. This reduces vibration and provides smoother propulsion.
Why waste time use 4 blades when 3 blades do trick?
The top answer does leave out one of the most important aspects of Windmills: the [tip-speed ratio](https://en.m.wikipedia.org/wiki/Tip-speed_ratio)
It determines the ratio between the tangential speed of the tip of the windmill wing and the wind speed.
Generally the higher the tip-speed ratio, [the less Blades a Wind Turbine has](https://upload.wikimedia.org/wikipedia/de/9/98/Schnelllaufzahl.png)
Windmills heaving three blades is a balance between rotational Speed, noise, cost, efficiency, etc. (Slower Windmills with more Blades would need bigger gearboxes –> more cost; faster Windmills would be louder and more instable –> more expensive materials and more difficulty with regulations.)
One single answer is often not possible with such complex Topics.
Edit: The size of the Turbine is also important because the tip-speed is dependent on it, effecting the tip-speed ratio (older Windmills are smaller than Modern Wind Turbines –> less blades on modern Turbines)
I think because back in time, crossing two very long logs to create four blades was much easier than trying to secure three blades.
There are already a few good answers, but there’s one overarching truth to why modern windmills usually have 3 blades: optimization.
There are many tradeoffs, and 3 blades just happens to be how the math works out for balancing the design against size, materials, structure, generator power limits, design complexity, geometry, expected wind speed, etc and your optimization spits out 3 blades as the best for the typical sizes of power generating windmills.
They’re far, far higher-performance than the old 4-bladed cloth and frame windmills, which are just much easier to build and balance than 3-bladed fans.
If the fans were a different size, or made of different materials, or exposed to different wind speeds, or any number of other major factors, the optimal design might have been 5 blades, or 7, or 9.
For examples of when 3 blades are not optimal, look at many helicopters, which range from 2 to 9 blades depending on size, power, noise and other requirements. Computer cooling fans are also often highly optimized and have varying numbers of blades for various applications.
It’s a big, giant “it depends,” so 3 may not be the magic number forever, and there’s no canonical, set answer.
As for why 4 blades on old windmills, though: it’s just easier, and the technology wasn’t there for engineering and building windmills like we do now.
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