Why Do Wind Turbines Have Three Blades?

Follow Real Engineering on facebook at:

For allowing me to use his sound record of wind turbines

References
[1] Page 340 Wind Energy Handbook – Tony Burton

[RealEngineering]

I plan to remake this video at some point. It's a really shallow explanation. I simply did not have the skill and experience to make a longer video when this was released. I could crank this video out in a single day now. The subject definitely deserves a more in-depth explanation.

[robbstark]

hey, engineering student of renewable energies here, the centrifugal force is not a major reason why the turbines usually have 3 instead of 2 blades. It is due to the fact that in different heights you have different wind speeds and having an even number of blades gives you periodic stress on the system, when the upper blade encouters the highest wind speeds the lower blade is receiving relatively slow winds additional to passing in front of the tower which disturbs additionally. this periodic torque stress will wear down the machine pretty quickly, thus we use 3. plus your notion about the the wrenches is entirely wrong, doubling the blades wont double the torque, adding more blades will decrease the speed needed to extract the energy from the wind. unbelievable you didn't caught that contradiction, those principal are mutually exclusive. be a good man of science and remake the video instead of just putting a half-arsed remark in the notes about it being just the "basics". It's not that you left out additional information it's that you are factually wrong about your explaination. your channel looks pretty promising if you are able to take constructive critizism seriously. pm me if you want some references or advice, cheers

[LukasDolezal]

quite shallow explaination why not 4 blades. did not explain why the additional power is small, you just stated it. but thats the important part no?

[markmark]

Ok, here's some real info for you - note I live in one of the world's largest wind turbine producing cities in China and do consultancy engineering for a few of the companies associated.

A one bladed WT is actually the most efficient, this is due to the air having time to recover by the time the next sweep comes around, having done some aircraft wing turbulence stories you may be able to visualize that. Problem for one blade is balance and harmonics, but that's another story.

2 blade is still more efficient than 3 for exactly the same reason, But in the real world, 2 blades are also hard to balance and align exactly opposing each other and a big harmonic problem when they "shade" - shading is when the lower blade sweeps past the tower where there is no air flow. The loss of pressure when shading makes the blade flick forwards as the wind pressure on the blade stops momentarily.

So simply put, one more blade, 3, is the minimal suitable for these factors. They are less disturbed by balance and shading issues which also helps to control harmonics.


Note that 4 would suffer shading issues like 2 does, so 5 would be the next choice, and note that many car radiator cooling fans have 5 so they can mass produce thousands with larger tolerances.

I like your channel a lot, but this video misses the spot way too far. I have the strong impression sure you just read the english wikipedia page, which, for whatever reason, does not cover the root of the problem in comparsion to other wikipedia languages and masses of other articles on the web about the topic.

Do you remember how three-phase electricity works and why it's magic? If the load is identical on all phases the fourth wire has zero current the other three equal out completely.
The same principle applies to a three-blade turbine similarly: There is zero unbalance and much less vibration. The forces on the central axis are always straight and therefore three-blade turbines last longer both in the gearing as well as in the tower construction, are less noisy and have a smaller tendency to soar into big fatal movements like in the crash you show in the video (they're not fully resistant either as you video shows :-) )

PS: Initial construction cost-wise a two-blade design seems to be the real sweet spot and if you research a bit int cheap turbines for underdeveloped rural regions you will find a lot of two-blade designs. But in the long run three-blade designs are more cost-efficient due to the fundamental mathematical priciple (and not because of the sheer cost structure)

[Wendoverproductions]

Great video. Looking forward to the next!

[sjoormen]

Actualy that isn't the case. Two bladed turbines have problems with vibrations when pivoting when wind is changing course.

[CrazySteTV]

I would have preferred to have a more Technical Explanation of what the Advantages would be on 4 or even 6 Blade Turbine rather than just writing it off with Cost... i know Costs count, but the Technical aspect should be the primary aspect of the video.

That said: will you be doing a Video on the "Vertical Axis Wind Turbines" soon?
because in theory, they are a lot more efficient and cost effective than the classic Horizontal Axis Turbines, yet you don't see too many around yet... WHY???

[HolgerBrandsmeier]

I really like your channel but was rather disappointed about this particular video. When you change the number of blades many other variables can also be changed to compensate for a fixed say energy output. You only gave one argument each but didn't properly argue why everything else stays the same. For the four blade you say the cost increases at a marginal increase in the performance, but you could also make each blade smaller and in particular all your arguments for the two blade variant apply (less speed and less noise) which would even make the four blade variant look better.

[eKriZZLe]

At least the comment section was informative, so the video wasn't a complete waste of time.

[rogersanchez]

because it looks like the Mercedes logo and it looks cool

[Sketch]

More blades mean more "Solidity" (a factor of wind turbines). It is the area of the inscribed circle of the blades length divided by the frontal area of the blades as experienced from the axis of rotation. More blades would mean more solidity and this would block a large part of the airflow, lowering the effectiveness of the next turbine in the row. Also the effectiveness is more affected by the length of each wing as there is little to no torque at the base of each wing, while adding an extra wing would increase the rotational mass, the cost of the construction when a similar increase in weight and cost invested in wing length would have better performance!
PS: Two blades have balancing issues as the height of the turbine oscillates, so will the torque from the wind and to make matter worst when one blade of a two blade turbine is at the top position the other blade is in front of the mast having no air flow at all, so both the rotating assembly would have a knock like imbalance when a blade passes the mast, AND the whole turbine would have a great oscillation from the oscillation of the torque transmitted to the mast!
You're welcome!

[MSE]

a nice simple video, but you eliminated a 4-blade design based on "will only provide marginally more performance"
any reference for that?
also, if sound is the issue, does a 2-blade design make sense in off-shore wind farms?

[jarnomikkola]

Are you sure it wouldn't be better to have 3.14... blades ? How, I don't care...

[samuelrj]

This all makes sense but you could make the exact same argument comparing 3, 4, and 5 blade designs and conclude that 4 is the best.

[VidarrKerr]

I passed one of these blades on the highway once. It took a long time to pass it. These things are so huge; when you are right next to it, it is unbelievable.

[mk-jlzd]

Curiosity says:Theoretically, a one-bladed turbine is the most aerodynamically efficient configuration. However, it is not very practical because of stability problems. Turbines with two blades offer the next best design, but are affected by a wobbling phenomenon similar to gyroscopic precession.

Since a wind turbine must always face into the wind, the blades will have to change their direction vertically when there is a shift in wind direction. This is referred to as yawing. In the case of a two-bladed system, when the blades are vertical (i.e., in line with the tower and the axis of rotation) there is very little resistance to the yawing motion.

But when the two blades are in the horizontal position, the blades span a greater distance from the axis of rotation and so experience maximum resistance to yawing (notice how a spinning figure skater slows down when they bring their arms away from their body. As a result, the yawing motion starts and stops twice per revolution, and this leads to stress on the turbine due to blade chattering.

On the other hand, a turbine with three blades has very little vibration or chatter. This is because when one blade is in the horizontal position, its resistance to the yaw force is counter-balanced by the two other blades. So, a three-bladed turbine represents the best combination of high rotational speed and minimum stress.

[edwardgriffin]

Q: Why Do Wind Turbines Have Three Blades ?
A: They all love Mercedes Benz cars.

[sukhoy]

I don't get why people complain so much about engineering videos on internet, taking everything down to the "wrong" details. This is not a college lesson, it's meant for people with no background to take some interest, and to actually learn something in an easy way. I think the way he explains the basics in 3 minutes is perfect.

[DaveMcIroy]

Aaaand another video where the question isn't answered.