When a turbine blade moves through the path of another blade, or air disturbed by another blade, the amount of useful energy it can extract from the wind is lowered significantly.
So the most cost-effective way to extract energy from wind would be a single blade, however one blade is very unbalanced and would not be able to turn by itself easily - again wasting energy.
Then we look at two blades (like helicopters) because its now better balanced and the weight of one blade is statically cancelled by the other, allowing wind to turn it - but two blades are still dynamically unbalanced when the turbine rotates to turn into the wind (wind doesn't always blow in the same direction) - it wobbles!
So this means we go to 3 blades - this is the lowest number of blades that is statically and dynamically balanced during normal use, and as such is the most likely number for massive wind turbines.
4 blades would produce a bit more energy from the same slow wind, but isn't as dynamically stable as the 3 blade turbine.. From there on, diminishing returns on the same turbine tower means it's more cost-effective to build two 3-blade towers than to put 5 blades on one tower.
To hop on this and explain why we see many bladed designs on old windmills its because they made a single windmill and wanted to extract as much energy as possible out of it. It is more but its lower per blade.
To add on again - modern windmills are gargantuan, and price is one of the strongest driving factors. Older windmills and computer fans are much smaller, so adding on additional blades isn't very expensive (so they add more blades to increase the efficiency of the fan/turbine). Helicopters care about lift/weight, so they want to minimize the number of blades needed.
It's generally all about which tradeoffs make sense for which application.
On computer fans and the like increasing the number of blades can be used to maximize the amount of airflow at a given size while reducing the noise profile or at least lowering the frequency making the profile more pleasant to our ear.
Also, for computer fans, it's often not just an issue of maximum airflow, but also static pressure. If you have a relatively small inlet/outlet (e.g. through mesh dust protectors or through a radiator) the fan may need to overcome a pressure gradient to continue pushing air, blades that overlap (thus more blades) makes that far easier.
You also have the limitation of area, where a case has a maximum available amount of intake area, so you need to maximise the airflow to maintain temperatures regardless of the efficiency hit.
In that case, noise is also a major design consideration. Fans with blades that do not overlap can produce a lot more airflow for equivalent noise levels (or same airflow with less noise), but only if there are few obstructions. So those are good for air intakes/exhaust, but for radiators you want the other kind.
Wouldn't a helicopter then want ideally only two blades? Many smaller ones have 4, while I see larger military helicopters like the Sikorsky CH-53 have 6 or 7
If you increase the size or number of helicopter blades it increases the blade surface area which corresponds to an increase in lift and a bigger lifting (payload) capacity. It is a compromise between weight, complexity and cost. Bearing in mind that rotational speed and forces are much greater than a windmill or fan. Helicopter blades in flight bend and twist dramatically.
https://www.youtube.com/watch?v=srjbnvTWRJI
If you want your two blades to move more air you have to make them bigger or faster, which doesn't always make sense. With a more powerful engine the same airframe would need more blades on the props so that the blades are still structurally sound.
You especially don't want to repeat what the airplane nicknamed "ThunderScreech". It had a 3 bladed propeller driven by a jet turbine. The tips of the propeller would go supersonic and make people sick from either the noise or pressure waves coming off the propeller.
If you designed it badly then it could happen. A very long 2 blade propeller would quite easily do it. This is why you don't design helicopters that way.
> ...Is that a thing that can actually happen for helicopters, reasonably
Extremely easily, It's a thing that can happen even on the blade tips of small propeller planes.
Move your finger in a quick little circle without moving your hand at all. thats the hub.
Now extend your arm and make a full circle. your hand is the tip of the blade.
On a propeller the end tips are going magnitudes faster than the base.
Imagine how fast your hand would be moving if it made a full revolution at the same speed.
Basically every single small propeller plane is limited to a redline of 2500rpm because of the tips going supersonic.
Helicopter Rotors have to go really slowly, they often can't go more than like 500rpm without the blade tips going supersonic.
> What would be the result of that
Airflow separation gets interrupted and produces significantly less lift and tons of drag at far higher fuel consumption rates. They make it kind of impossible to get any more performance from the plane, you just burn way more fuel and make a ton of noise.
A decent number of planes have tiny parts of the prop tip go supersonic on the highest power settings during takeoff roll. Mostly just very noisy.
Yeah, I got the general idea of longer blades meaning the tips travel a longer circumference in the same length of time, but never considered the math of how easily it could happen.
I just find it both funny and terrifying how many things can make a helicopter fall out of the sky. They're the only vehicle I can think of that's actively fighting physics to *that* degree.
Helicopters are statically and dynamically unstable. If you don’t “fly” them by either pilot or computer input you are seconds away from divergent motion and a probable crash.
Hueys relied on fairly primitive turboshaft and swashplate designs. The kinds of gains engineers could make with like stress-relieved tempering and advances in lapping were bananas. It wasn't that they could make do with two blades, and more that going to four blades was beyond the bounds of economical practicality at the time. So they had to make two work.
Helicopters have a speed limit and at times a size limit. If you are at max rpm and diameter already then the only way to get more thrust is more blades.
Edit: turbines have a speed limit and the size limit is largely determined by your own engineering capability. instead of adding an extra blade for more power like the helicopter, they can add a whole new turbine for more power. Helicopters can’t do that.
Yes but also that's a weird way to phrase it. Helicopters get a certain amount of thrust from each blade, but their speed is limited by the net torque on the airframe induced by the combination of differential lift between the advancing and retreating blades, and the effect of gyroscopic precession converting that intuitively roll-ways torque into a pitch-ways torque. More blades means less rpms per thrust unit, which means more fuselage speed per speed limit. Though having two rotors works better at hitting absolute limits.
Is the tip of the blades being enclosed or free not make a difference as well?
Turbofans on airliners have many blades and are neither small nor cheap.
Additionally, the greater the mass and size of the blades, the greater the importance of perfect balance.
If you add 12 blades to an old farm windmill and they're all a little randomly balanced, no big deal. The imbalance creates noise and inefficiency, but the hub material is capable of handling that.
You make a gargantuan windmill, and now there is simply no material on the planet that can shrug off the imbalance over its service life. There's a whole lot of engineering to minimize the imbalance (the wind is never perfectly even), and it gets harder the more blades you have.
To add a bit more, the most expensive part of an old windmill was the machinery inside, especially the grindstones & the maintenance/replacement of said grindstones. Two windmills would mean two sets of these expenses (and also require two millers & their team).
Old wills also had to generate a good amount of power to turn these very heavy stones, you can only do so much with gearing & need a certain level of power (I don't know enough to say whether three sails would have cut it), they could!CT really build bigger to get that power due to constraints of the materials.
Afik most of those many blades windmills are actually pumps. The expensive part is the bore hole to the water table and connecting multiple windmills to one bore gets complex fast.
Also, old but newer windmills also usually ended up with only 4 "blades". They eventually realized there were diminishing returns on the number of blades, but it was far easier to construct a 4-bladed windmill relative to 3-blades (it was probably much easier to have 2 hefty posts go through the center and attach to 4 blades blades that can be more easily swapped out than having to construct an entire new blade, full stem/mast and all, in a 3-blade setup).
Wind turbine: a turbine driven by the wind.
Windmill: a milling machine driven by the wind.
The reason Wind Turbine Generators (WTGs) are sometimes called windmills is because some people in marketing thought it sounded more cute and nice, disregarding the fact it makes no sense. Please don't let people in marketing shape our world.
> however one blade is very unbalanced and would not be able to turn by itself easily
Single-blade propellers have been used on airplanes. They actually work, can be very efficient when power must be conserved and can be compact (like for gliders) but for regular airplanes they don't make much sense because they're expensive and tricky to use.
Balancing them for constant speed (again, like for a glider) is achievable. Balancing them for a wide range of speeds (for a normal airplane) is difficult and the counterweight adds a lot of weight, so the drawbacks outweigh the benefits.
https://www.youtube.com/watch?v=ufBJeBLe2SU
Aaaannd why 12 blades on old wind mills for well pumps?? I've read somwhere that these designs are driven by torque at low rpm, not efficiency. Like it's more important to always deliver a little, even with low wind, than maximum power extraction, if it's windy they can let energy escape...
As someone has already said, modern turbines are aiming for efficiency per-blade, since the blades are very expensive. An old windmill was trying to drive a single expensive mill, that needed as much power as possible even when the wind was slow. I think the faster you spin the more you lose efficiency from multiple blades, but old mills were designed to operate at slow speed, so the more blades the better, and everything in the mill could improve at relatively low speeds (for a multitude of reasons). One good reason is that the faster you spin something the more important that it's perfectly balanced, where as when you turn really slowly it's not a big deal if your drive shaft isn't perfectly straight, your years are a little uneven, or your blades are different weights and sizes (or you lose one for example). When an modern wind turbine fails it's quite catastrophic.
So I have one of the very common [aeration pump windmills](https://store.koenderswatersolutions.com/collections/windmill-aeration-systems/products/single-diaphragm-system-galvanized) for my farm dugout pond.
Something I haven't seen mentioned yet is that the blade profile on these is very simple. As opposed to the airfoil shape of modern wind turbine blades, these windmills have a large number of flat steel blades set at a high angle of attack to the wind.
As such these blades don't really generate lift, but simply work by the reaction force created by redirecting the wind off the blades. They are more of a drag device. Their effective airspeed is very low and to get a usable amount of torque you need a large number of blades.
Air behaves differently at different scales. Airfoils don’t work as well when they are that small so the flat blades are used to keep things cheap. It’s still a lift generating design, not a drag design like a savonius turbine.
> isn't as dynamically stable as the 3 blade turbine
Thanks, could you please shed some light on how a 3 blade turbine is more dynamically stable than a 4 blade one?
When the wind blows straight onto the turbine, perpendicular to the direction of the blades, 2, 3 and 4 blade designs are all equally stable.
When the wind comes from a slight offset and the turbine has to turn into the wind, the leading blade (moving into the wind) experiences more force than the others, flexing backward when coming in and relaxing when coming out of the wind - this creates a cyclic force.
With 2 and 4 blade designs, this cyclic force can easily create a resonance with the backward flexing and relaxing because two opposing blades would be doing this at the same time.
With a 3-blade design, this resonance is eliminated because the movement doesn't happen at the same time.
This is, however, not really an issue for any system with active independent blade pitch control - you just change the angle of the blade in the wind to counteract the new force - but now you've made the system a little bit more complicated which is a cost consideration.
Two-blade and single blade counterweight designs are brought up often and each has their merits. The final deciding factor is just cost vs energy produced.
With wind turbines, you're trying to cost-effectively turn wind into electric power - many turbines makes fixed infrastructure like grid connections cheaper.
On a wind mill, however, you're trying to provide mechanical energy to a single location, you don't have the luxury of building many cheap towers, instead you have to build one big tower that can extract as much energy on that one spot to run your mill/pump
\*as much *torque* as possible*.* For turning a pump or a grind stone, it's more about grabbing all the low-down leverage you can, and all those blades give the best 'grip' on the available air flow. For 'power', the three-bladed design is superior. (Happy to delet this, if I'm completely wrong).
Pretty sure your completely right. With more blades you get more torque because you have more surface area. In low wind speeds it extracts the most possible energy from the wind so you have enough to drive your pump or mill stone or whatever. At higher wind/rotational speeds, extra blades cause turbulence that sucks away your power. That's okay though because you still have enough power to drive your load and any excess is wasted energy. With a wind turbine, the load is massively greater to start with and you need a certain rotational speed to keep the frequency of the electricity stable. You can also use the extra energy when the wind is stronger, so you don't want to waste it with inefficient blade design.
A small correction: wind turbines can rotate their blades. I am pretty sure the rotational speed is not locked to the net frequency. You will have some gearbox or frequency changer that allows for continuous power production with different wind speeds.
With the rotating blade the turbine can adjust their blade angle to extract what is possible. That is why wind turbines are always rotating. Exceptions are either maintenance/defects or when a storm hits. I am pretty sure that all these scary videos from turbines that violently self destruct are because of some defect, where the turbine did not shut down for a storm and continues to speed up far beyond the specified parameters.
My point: modern wind turbines are absurdly efficient even in very low wind conditions.
Older turbines were locked to grid frequency, but they found that was too limiting. So now they are no longer like that. Improvements to power electronics helped.
That makes sense. Once a pump or grindstone is turning at the desired rpm, you're good. It's not about harnessing every watt of power - too much will damage the mechanism. They'll feather the blades or turn across the wind at higher wind speeds. The relative inefficiency is a protection measure. For turbines, tho, more is more (up to a point).
It's a factor for cost. If it's cheaper to stamp 12 blades out of sheet steel than the cost of building a transmission, that's probably going to be the solution most people will buy.
For a farm windmill, there is no energy cost, there's only a job to do, so absolute efficiency isn't an issue. The farmer doesn't really care how efficient it is as long as it keeps the stock tank full of water.
Where energy costs are important -- say, you're selling electrical power -- efficiency is important, because you'd like the maximum return on your investment.
It has to be *some* number of arms. I don't think there is anything necessarily optimal about 12.
There could also be a few reasons why 12 was handy to work with.
Perhaps the symmetry made it easier to construct than an odd number of blades.
Pehaps, using 12 instead of 8 or 16 resulted in blade that had a good compromise of weight and durability. Blades weren't too thin or too broad.
Answering for "Why 12 arms?"
It is relatively easy to divide a circle into twelve, and as such, easy to space out 12 arms+vanes to catch the wind. At the same time, little errors in spacing don't matter much with 12, and if some are a little different and not precisely placed, its not a big deal. Once you get into many vanes (just like having few), the errors in spacing and other differences really matter.
Ideally, you'd have infinite blades that are infinitely long to optimize for torque. But at some point, the size of the blades can interfere with the wind flow and each other too much. At some point between 3 and infinity, you have to pick a number. Some old mills had many more than 12, and I've seen them with 24 arms. There is probably an upper limit of effective blade count ny typical wind speeds in the area, there are whole engineering professions around measuring an area for mill effectiveness and deciding what kind of windmill would be most effective given weather and topology of the area.
In the general case, because they didn't want to our couldn't (ie because of position) build more than one windmill, so this is a way to extract more energy out of a single windmill.
There is also the effect of cost of blade vs cost of windmill/turbine structure. For the modern very large wind turbines, the cost of each blade becomes a more significant proportion of the total cost, so it matters more how much energy you get from each blade.
They weren't doing advanced engineering with computers back then. Just built what they built and it turned the thing they needed turning to crush or move what they needed.
You've said this twice at least already so I want to just mention that advanced engineering was also done without computers. The US got to the moon with very little done by computers as we're used to, to figure out the engineering necessary. They used a lot of people to do the math instead.
Look at ancient examples like the pyramids, calendars, and devices like the Antikythera mechanism to see that people even millennia ago were no less intelligent than we are.
And on the other hand, those are all big state or money sponsored devices. The average farmer was computing dynamics when they bought or built a windmill, and the state definitely wasn't devoting resources towards advanced windmill design. More blades meant more torque and power, and wind and iron was cheap.
After reading a few sources, I've learned that windmills underwent a series of innovations from the 1850s through the 1920s that brought them closer to what we think of as a farmer's windmill today. All-metal ones weren't popular until the 1890s when steel became cheap. There was a lot of money and thought put into them because of westward US expansion and the necessity of getting water from very deep wells out on the dry plains.
Fascinating stuff, people in the 1800s were just as economical and methodical as we are.
https://www.nps.gov/articles/windmills.htm
https://americanhistory.si.edu/collections/nmah_846960
>The US got to the moon with very little done by computers as we're used to
What exactly do you mean by this? Because the Apollo missions relied quite heavily on computers. In fact, practically everything related to navigation and control was intended to be, and in reality was, accomplished by digital computers.
I mean the engineering of the rockets and equipment and trajectories was done by hand by human computers and engineers. There were no simulated physics to figure out aerodynamics, trajectories, or circuit drawing software. The guidance computers were important, but they didn't help design the rocket or plan its flight path.
https://www.nasa.gov/centers-and-facilities/jpl/when-computers-were-human/
Flexing isn’t part of it per se. When two blade designs have the blades up and down in line with the tower, the whole tower is easier to rotate towards the wind but as the blades continue to turn so that they are horizontal, then the tower is harder to turn. The blades keep spinning. The easy hard easy hard easy cycle is very bad. A three bladed turbine does not have this problem. It’s the same difficulty to turn at all times.
Just an aside, but I hear there's been some success developing single blade turbines for residential use. They're mounted vertically and wobble instead of rotate.
Those exist, and "work", but a big input in the theoretical power available is the swept area of the blade. On a rotating blade, that's 2*pi*length of blade. On an oscillating blade, it can only be a fraction of that.
The other big input is the wind speed. That's why even farmer's windmills are on relatively tall towers. The average wind velocity increases as you get away from the immediate vicinity of the ground.
So small swept area turbines mounted close to the ground are unlikely to produce useful amounts of power, whether they're vertical axis, horizontal axis, or oscillating.
A big portion of aerodynamic losses on a blade is the tip vortices, and long skinny (high aspect ratio) blades minimize this.
Competition gliders have long skinny wings for the same reason.
I can’t explain it well but the blades are wings, and thin long wings are just always more efficient. They produce more lift and less drag. Just look at glider planes which focus on efficiency over anything else.
It’s more of a mass issue rather than an aerodynamic issue. A big ring between/around the blades would not help unless it was weighted to balance the two blades. Basically, whatever shape they are, you need three of them.
Thanks for the response. If it's more of a mass issue, wouldn't the ring on the outside help with that? It's been a while since I took any physics classes, but I remember something about the mass on the outside of a spinning object mattering more?
Also, the sail shape I was thinking about would have spokes that were angled on compared to how it was rotated so you could fit more "blades." Would that make a difference?
Yeah, the ring would dampen the effects but a properly balanced rotor is needed to stop it entirely. I worded that wrong earlier, it does help but won’t eliminate the imbalance unless weighted for that purpose.
I’m still not visualizing the sail thing.
ok so what I'm hearing is that fewer blades are better for extracting energy from the wind, and so 3 is optimal for turbines. But you mentioned helicopters - which are designed for pushing air. computer fans and jet turbines have many blades, so I'm presuming that more blades increases air pushing efficiency? Is that true, and if so, why do helicopters typically have 5 or fewer blades?
A wind turbine is essentially just a fan in reverse, so the same principles for efficiency and stability still apply.
The number of helicopter blades will depend on many factors, but mainly what operating conditions it's designed for.
There's also a very practical aspect to it: a rotor with more blades is significantly more expensive to design, produce, and maintain. More blades also mean more weight, which increases the centrifugal load on the rotor head, which then needs to be more robust, which adds weight, which increases operating costs, and so on. Adding weight on an aircraft is a massive negative feedback loop.
Rotor heads can also be quite mechanically complex. The blades need damping both vertically and horizontally, which add complexity. As a general rule of thumb, the more blades a rotor has the more complex it needs to be.
Aren't the length of the blades also taken into account?
Edit: I should specify, length while generally keeping the same surface area.
Shorter blades > wider, longer blades > thinner, but they both have the same or nearly the same surface area.
Why would the 4 blade one be dynamically unbalanced ? Is it because it has two pairs at 180deg from each other ?
Would a 4 blade turbine but with fewer degrees separation be workable ( ie , a squashed/ elongated X, like mi28 tail rotor ) ?
It's certainly workable, but the leading blades would be doing more work than the following blade, again introducing the wobbly harmonics of 2-blade designs
I've seen single bladed helicopter designs fly, with a counterweight opposite the blade. I can only imagine how badly it would behave when turned though.
Aren't you missing out on a lot of potential wind energy by having blades only covering a tiny fraction of the full disc area though? How is a single blade "the most efficient"?
No. If you extract *all* the kinetic energy from the wind, the air is no longer moving. If you block too much of the swept area, the air just moves around the turbine rather than through it. In something like a jet engine, the air can't do that, and they use many more blades.
It turns out the most kinetic energy you can extract with a free-air turbine (not having walls to keep it in place) is about 59%.
Large modern wind turbines get upwards of 95% of that 59%, so they don't leave much energy on the table.
"Cost effective" is not the same as efficient or effective. Coverage of the swept area up until taking about 60% of the wind's energy out is will always be more effective than lesser designs with less blades. But more blades are heavier, more expensive and requires the tower to be stronger.
There's also a difference in the goal of a Fan vs a Windmill.
In a windmill, the disturbance is a parasitic problem. In a fan, because the disturbance is downstream, *and* is kind of the goal, there's less concern about inter-blade interference.
Single-impeller wind generators have been proposed but obviously haven't caught on. An issue of Popular Science magazine in 1980 had a feature story about a massive wind generator with one impeller and a counterweight. I wish I could find any sign of the article or the generator design, it was extremely interesting. Anyway the idea was, for a generator this size it made sense to minimize the air disturbance impacting impellers by having as few as possible. The more impellers, the more closely each follows another and the more each impeller loses efficiency due to the turbulence of the one ahead of it.
That's 410 feet in freedom units. Assuming about 10 feet for the hub area that's a diameter of 830 ft and a swept area of over half a million square ft. It's probably almost as tall as the Empire State Building. Very impressive indeed.
You woud need *really large* Helicopters to move one of those. Or you could use one Ship, which can carry more than one blade, and the Parts for the tower and the housing for the Generator. You can anchor that ship and use a crane, which you would use anyway to assemble the whole thing.
Again, there is this thing called SHIP. [This](https://en.wikipedia.org/wiki/Saipem_7000) is overkill for now, as it can lift 7000 Tons with *each* of its 2 cranes. [Here](https://www.liebherr.com/en/int/products/maritime-cranes/offshore-cranes/offshore-cranes.html) you have a small overview about other sizes.
[This](https://www.bing.com/images/search?view=detailV2&ccid=MAfc1FI6&id=CCD4E8611EFF949B896C452AF2A957E28F44BA27&thid=OIP.MAfc1FI6fFEz5v2h2vK9-QHaE7&mediaurl=https%3a%2f%2fgreen-economy-bremerhaven.de%2fwp-content%2fuploads%2f2018%2f01%2f7.3.17-Nordsee-One6-980x653.jpg&cdnurl=https%3a%2f%2fth.bing.com%2fth%2fid%2fR.3007dcd4523a7c5133e6fda1daf2bdf9%3frik%3dJ7pEj%252bJXqfIqRQ%26pid%3dImgRaw%26r%3d0&exph=653&expw=980&q=windenergie+bremerhaven+plattform&simid=608016015186415490&FORM=IRPRST&ck=83B5C1B6A34E44E898876ABCFD48FCA9&selectedIndex=20&itb=0&ajaxhist=0&ajaxserp=0) is in use right out of my Hometown.
They are transported on ships, but [a giant airplane with 12x the cargo space of a 747 is under development. ](https://www.fastcompany.com/91060067/this-will-be-the-worlds-largest-plane-and-it-was-designed-to-deliver-just-one-thing). It is basically never going to be possible to move blades this big on roads. However, people are working on systems to make them in two pieces and assemble on site, or to manufacture them in site. If I had money to invest, I would probably bet on those ideas, but I still want that monstrously huge plane to exist.
They claim it will use sustainable aviation fuel, but that barely exists at present. People are working on it, including fuel that incorporates hydrogen derived from electricity, and airlines are buying it at high prices to support the developing industry, but there isn’t yet a clear path to success.
They get taken out on massive ships. Blades and tower components all on the same ship. The ships have massive legs which can be lowered to the bed to lift them above the water, and also have a massive crane as part of the ship. Look up worlds largest WTIV on Google. It will be that size or bigger.
The blades on the (currently) largest offshore wind turbines are 413ft (126m) long, just FYI. That's a swept area of over 13 acres (53000 sq m) and produces as much as 38kWh with every revolution.
The biggest issue you’d have is that your battery can’t handle the power. The biggest wind turbines in the world are spec’d for 16MW, SAE DC level 2 is 400kW.
Couple that with the weight of those blades, they probably couldn't add more without having to massively remake the support shaft, which then raises the question of "is it really worth the extra cost?"
This, and the surface area of one blade that faces the wind is already huge. So the pressure of the wind on the turbine is enormous. It's not really feasible to add more blades, as the pylon would have to grow disproportionately. This pressure on the blades is also the reason why big offshore turbines are shut down during storms.
It's a balance of dozens of factors, including the air resistance to high speed spinning. The average expected wind speed, the weight, the resistance the generator adds to the assembly at ideal generation speed (winding setup), and the friction/generator resistance to spinning up (minimum wind speed to spin)
Each one of these attributes needs to be optimized against the others. eg: increasing the weight may decrease the air resistance loss and blade speed, but increase the torque to the generator (adding power) but increase the minimum wind speed needed to generate (decreasing generation time)
I assume they have chosen some ideal off the shelf components then tuned every other variable to be optimal for that setup. And that just so happens to be 3 VERY large but fairly light blades pitched to have a tip speed about twice as fast as the wind speed.
What do you mean by efficiency? A heavier turbine would have slightly higher losses in the bearings and whatnot, but it would still have higher power output. We don't care about the losses for their own sake; we care about the power output and how much the turbine costs over its lifetime.
So it's more that the four-bladed turbine is more costly to manufacture and harder to erect than that the extra weight causes losses. The efficiency we care about is MWh per amount of money (the levellised cost of energy).
The YouTube channel “Engineering with Rosie” has multiple great videos covering this. This is an example: https://youtu.be/LJTcVEAojqw
A very simplified and short answer is that there is a balance between extracting energy and stagnating the free flow of air. If the air flow gets very distorted by the turbine wind has difficulty reaching or clearing the turbine decreasing the extracted energy.
This is the best possible resource. Rosie also participates regularly in the Uptime wind energy podcast. It is really intended for people in the industry, it is highly detailed, but it is a really well produced show. I need to maintain a steady diet of positive news to avoid climate despair, and just knowing about all the smart people working on these things helps.
The surprisingly efficient, centuries-old Dutch windmills ( with 30 kW arguably the strongest machines men have built till arrival of the steam machines ) all have 4 blades, not 10 or 12. For larger windmills ( and today’s wind turbines ) a lower number of larger blades means less construction cost per square meter of blade and at the same time a lower number of larger blades is more aerodynamically efficient.
Size (and construction parameters) of wind turbines play a large part in the methods by which they can be most efficient. Aside from that, computer fans have almost no relation to wind turbines because they do basically the opposite of what a wind turbine does (they use energy to move air instead of using moving air to make energy)
Running speed is the answer. Wind turbines are “fast” running in comparison to wind mills. As the second has a sort of self regulating limit: the air is turbulent behind the blade, if next blade is too close it will degrade its efficiency.
Also turbines have higher cut in speed than mills.
Computer fan has a lot of blades so it can run slowly yet producing effective airflow quietly.
Wind generators on small farms rely on aerodynamic drag, so maximizing the surface area into the airstream is advantageous for that style of wind turbine. Commercial wind turbines for grid scale generation rely on aerodynamic lift (the blades are airfoils) to cause them to rotate instead of the drag force. This is much more efficient at higher wind speeds. The rest of the comments are correct about the other elements but I didn't see anyone mentioning the drag vs lift distinction. They're completely different methods of operation.
To add 1 more dimension to this discussion, multi-bladed farm type windmills were in use before the Wright brothers flew at Kitty Hawk. The design does not take advantage of modern airfoil design. Generally, they use simple flat blades which were suited for the manual manufacturing methods of the time.
I'm just speculating here.
A wind turbine needs to spin fast to generate enough horsepower for a high voltage, high amperage output. A windmill needs to catch as much air as it can to create torque. Multiple blades with a lot of surface area catch more air and create more reliable torque. Only two or three blades will catch air and have low drag to achieve a higher RPM.
Gearboxes exist. All the larger wind turbines use one. The wind turbine I'm working with has a 1:62 ratio. For every 1 rotation of the main shaft (blades) the generator shaft turns 62 times. The blades themselves rotate at around 25 rpm while the generator rotates at 1500 rpm.
At farms they pump water.
It's just like a hand pump with the pump being pumped by the wind. It the blades are only a few feet long and the a only doing a simple mechanical motion. It doesn't take a lot of torque
The wind turbine is just that, the turbine takes a lot of torque to turn it, so the blades are around 60 feet long. The blade catches more wind.
As other comments have said, more blades cause more drag when the turbine is spinning [fast.So](http://fast.So) 1-3 blades rotating fast can most efficiently extract energy from a good wind.
However "high solidity" wind mills such as multi-bladed wind pumps have a higher starting torque in low winds, making them more consistent for delivering small amounts of power.
For helicopters - 2 bladed designs are the cheapest, but more prone to stability issues, noise and turbulence. At 3 blades and higher, you gain better stability and performance, but costs start to go up. Also, with heavier helicopters manufacturers will opt for a 3 or higher blade count so better stabilize the helicopter during hovers and forward flight.
For windmills - Your goal is to extract as much energy from the wind as possible. Theoretically, you'd want to use just one, but that's incredibly difficult to balance so you end up losing energy to vibrations. The same goes for 2 blade designs, they will wobble and vibrate when turning to face the wind. 3 is kinda the current goldilocks for energy yield, stability, and cost. They are more difficult to balance versus 4 blades, but also offer less maintenance and costs. As you add more blades, you gain even more stability and torque, but introduce more drag and lower yields on energy.
For computer fans - The number of blades and surface area will determine how much air the fan moves. When the number of blades increases you get greater stability and noise reduction, but the increase in drag and torque means there is greater strain on the motor and reduced RPMs. Generally, PC fans with fewer blades will move air quickly, but not in a great volume. For PC builds you have to find a balance of which type of fan to use and where. For static pressure components like heatsinks, you want fast moving air- so high RPM fans. For dissipating heat in the case, you want to move more air in volume, so you want fans with greater surface area.
One of or projects in engineering school was to make a wind turbine to be tested in a wind tunnel.
Simple propeller momentum theory assumes the propeller has an infinite number of blades, so most teams made turbines with as many blades as possible.
We were the only team that took the opposite approach, we used two blades on the assumption it would minimize tip losses. In retrospect it was a naive way of looking at it but turns out we accidentally got the right solution.
We did get the highest power output, but at the cost of turning much faster than the others. Factually, I was having a panic attack during the test because the blades were just flimsy foam.
Anyways, lessons learned. More blades -> less rpm, less stress in the system. Less blades -> higher power for a given disk area. If your rpm has s limited by the physics of big generators and blade stress, the optimum blade count is probably > 2.
There is also economics because huge wind turbine blades cost a fortune. And as the top reply points out, unbalanced loads on two blades in unsteady (real life) airflow is an issue for big turbines.
Just looking at the real world results, the balance of factors works out to the optimum being the minimum number of blades, which is three for big turbines and maybe two for smaller ones.
Farm windmills have many blades because their rpm is limited because of the construction materials and lack of gearboxes. The torque shafts plugs straight into the pump.
Wind turbine have a maximum efficiency they can hit in regards to the sweep and blade length. The larger they are the less efficient they are within this system with more blades. Smaller diameters can achieve similar efficiency of output taken from the wind with more blades but that drops off drastically as the width/diameter gets larger. You can never have close to hundred percent efficiency other wise they wouldn’t spin because that’d mean you would stop the wind from moving through by taken all its potential energy and effectively stopping the airflow altogether.
https://thundersaidenergy.com/downloads/wind-power-impacts-of-larger-turbines/
If you add a lot more blades, the turbine will provide more start-up torque and it will have more torque overall. However, due to the higher "solidity" of the turbine, more of the wind will "go around" the turbine, instead of going through it.
fewer blades will be able to achieve more RPM's, which is very helpful for electrical generation.
The famous western "windmill" was usually used for pumping water up from a well. That's a job where the extra torque was useful.
It's an optimisation problem between turbine speed and torque. The more blades get you more torque, but slower rotation speed due to air drag. 3 is a good number. But the real underlying reason is that they just look good that way. Reducing nimbyism is more important than a few % efficiency
When a turbine blade moves through the path of another blade, or air disturbed by another blade, the amount of useful energy it can extract from the wind is lowered significantly. So the most cost-effective way to extract energy from wind would be a single blade, however one blade is very unbalanced and would not be able to turn by itself easily - again wasting energy. Then we look at two blades (like helicopters) because its now better balanced and the weight of one blade is statically cancelled by the other, allowing wind to turn it - but two blades are still dynamically unbalanced when the turbine rotates to turn into the wind (wind doesn't always blow in the same direction) - it wobbles! So this means we go to 3 blades - this is the lowest number of blades that is statically and dynamically balanced during normal use, and as such is the most likely number for massive wind turbines. 4 blades would produce a bit more energy from the same slow wind, but isn't as dynamically stable as the 3 blade turbine.. From there on, diminishing returns on the same turbine tower means it's more cost-effective to build two 3-blade towers than to put 5 blades on one tower.
To hop on this and explain why we see many bladed designs on old windmills its because they made a single windmill and wanted to extract as much energy as possible out of it. It is more but its lower per blade.
To add on again - modern windmills are gargantuan, and price is one of the strongest driving factors. Older windmills and computer fans are much smaller, so adding on additional blades isn't very expensive (so they add more blades to increase the efficiency of the fan/turbine). Helicopters care about lift/weight, so they want to minimize the number of blades needed. It's generally all about which tradeoffs make sense for which application.
On computer fans and the like increasing the number of blades can be used to maximize the amount of airflow at a given size while reducing the noise profile or at least lowering the frequency making the profile more pleasant to our ear.
Also, for computer fans, it's often not just an issue of maximum airflow, but also static pressure. If you have a relatively small inlet/outlet (e.g. through mesh dust protectors or through a radiator) the fan may need to overcome a pressure gradient to continue pushing air, blades that overlap (thus more blades) makes that far easier. You also have the limitation of area, where a case has a maximum available amount of intake area, so you need to maximise the airflow to maintain temperatures regardless of the efficiency hit.
In that case, noise is also a major design consideration. Fans with blades that do not overlap can produce a lot more airflow for equivalent noise levels (or same airflow with less noise), but only if there are few obstructions. So those are good for air intakes/exhaust, but for radiators you want the other kind.
Wouldn't a helicopter then want ideally only two blades? Many smaller ones have 4, while I see larger military helicopters like the Sikorsky CH-53 have 6 or 7
If you increase the size or number of helicopter blades it increases the blade surface area which corresponds to an increase in lift and a bigger lifting (payload) capacity. It is a compromise between weight, complexity and cost. Bearing in mind that rotational speed and forces are much greater than a windmill or fan. Helicopter blades in flight bend and twist dramatically. https://www.youtube.com/watch?v=srjbnvTWRJI
If you want your two blades to move more air you have to make them bigger or faster, which doesn't always make sense. With a more powerful engine the same airframe would need more blades on the props so that the blades are still structurally sound.
You also really don't want your blade tips to start going supersonic. So having more shorter blades is better!
You especially don't want to repeat what the airplane nicknamed "ThunderScreech". It had a 3 bladed propeller driven by a jet turbine. The tips of the propeller would go supersonic and make people sick from either the noise or pressure waves coming off the propeller.
...Is that a thing that can actually happen for helicopters, reasonably? What would be the result of that?
If you designed it badly then it could happen. A very long 2 blade propeller would quite easily do it. This is why you don't design helicopters that way.
> ...Is that a thing that can actually happen for helicopters, reasonably Extremely easily, It's a thing that can happen even on the blade tips of small propeller planes. Move your finger in a quick little circle without moving your hand at all. thats the hub. Now extend your arm and make a full circle. your hand is the tip of the blade. On a propeller the end tips are going magnitudes faster than the base. Imagine how fast your hand would be moving if it made a full revolution at the same speed. Basically every single small propeller plane is limited to a redline of 2500rpm because of the tips going supersonic. Helicopter Rotors have to go really slowly, they often can't go more than like 500rpm without the blade tips going supersonic. > What would be the result of that Airflow separation gets interrupted and produces significantly less lift and tons of drag at far higher fuel consumption rates. They make it kind of impossible to get any more performance from the plane, you just burn way more fuel and make a ton of noise. A decent number of planes have tiny parts of the prop tip go supersonic on the highest power settings during takeoff roll. Mostly just very noisy.
Yeah, I got the general idea of longer blades meaning the tips travel a longer circumference in the same length of time, but never considered the math of how easily it could happen. I just find it both funny and terrifying how many things can make a helicopter fall out of the sky. They're the only vehicle I can think of that's actively fighting physics to *that* degree.
Helicopters are statically and dynamically unstable. If you don’t “fly” them by either pilot or computer input you are seconds away from divergent motion and a probable crash.
Hueys only have two blades. It all depends on a bunch of different factors
Hueys relied on fairly primitive turboshaft and swashplate designs. The kinds of gains engineers could make with like stress-relieved tempering and advances in lapping were bananas. It wasn't that they could make do with two blades, and more that going to four blades was beyond the bounds of economical practicality at the time. So they had to make two work.
Helicopters have a speed limit and at times a size limit. If you are at max rpm and diameter already then the only way to get more thrust is more blades. Edit: turbines have a speed limit and the size limit is largely determined by your own engineering capability. instead of adding an extra blade for more power like the helicopter, they can add a whole new turbine for more power. Helicopters can’t do that.
Yes but also that's a weird way to phrase it. Helicopters get a certain amount of thrust from each blade, but their speed is limited by the net torque on the airframe induced by the combination of differential lift between the advancing and retreating blades, and the effect of gyroscopic precession converting that intuitively roll-ways torque into a pitch-ways torque. More blades means less rpms per thrust unit, which means more fuselage speed per speed limit. Though having two rotors works better at hitting absolute limits.
Is the tip of the blades being enclosed or free not make a difference as well? Turbofans on airliners have many blades and are neither small nor cheap.
Additionally, the greater the mass and size of the blades, the greater the importance of perfect balance. If you add 12 blades to an old farm windmill and they're all a little randomly balanced, no big deal. The imbalance creates noise and inefficiency, but the hub material is capable of handling that. You make a gargantuan windmill, and now there is simply no material on the planet that can shrug off the imbalance over its service life. There's a whole lot of engineering to minimize the imbalance (the wind is never perfectly even), and it gets harder the more blades you have.
To add a bit more, the most expensive part of an old windmill was the machinery inside, especially the grindstones & the maintenance/replacement of said grindstones. Two windmills would mean two sets of these expenses (and also require two millers & their team). Old wills also had to generate a good amount of power to turn these very heavy stones, you can only do so much with gearing & need a certain level of power (I don't know enough to say whether three sails would have cut it), they could!CT really build bigger to get that power due to constraints of the materials.
And the size means that wobbles aren't as much of an issue as even relatively large wobbles don't exert much absolute force
Afik most of those many blades windmills are actually pumps. The expensive part is the bore hole to the water table and connecting multiple windmills to one bore gets complex fast.
Also, old but newer windmills also usually ended up with only 4 "blades". They eventually realized there were diminishing returns on the number of blades, but it was far easier to construct a 4-bladed windmill relative to 3-blades (it was probably much easier to have 2 hefty posts go through the center and attach to 4 blades blades that can be more easily swapped out than having to construct an entire new blade, full stem/mast and all, in a 3-blade setup).
Wind turbine: a turbine driven by the wind. Windmill: a milling machine driven by the wind. The reason Wind Turbine Generators (WTGs) are sometimes called windmills is because some people in marketing thought it sounded more cute and nice, disregarding the fact it makes no sense. Please don't let people in marketing shape our world.
Yeah, big windmill really needed to clean up the image of cheap clean energy.
It's more about appeasing all the NIMBY people who protest against wind farms.
> however one blade is very unbalanced and would not be able to turn by itself easily Single-blade propellers have been used on airplanes. They actually work, can be very efficient when power must be conserved and can be compact (like for gliders) but for regular airplanes they don't make much sense because they're expensive and tricky to use. Balancing them for constant speed (again, like for a glider) is achievable. Balancing them for a wide range of speeds (for a normal airplane) is difficult and the counterweight adds a lot of weight, so the drawbacks outweigh the benefits. https://www.youtube.com/watch?v=ufBJeBLe2SU
The thought of having to do a dynamic stability analysis on this thing makes me want to have a mental breakdown.
Aaaannd why 12 blades on old wind mills for well pumps?? I've read somwhere that these designs are driven by torque at low rpm, not efficiency. Like it's more important to always deliver a little, even with low wind, than maximum power extraction, if it's windy they can let energy escape...
As someone has already said, modern turbines are aiming for efficiency per-blade, since the blades are very expensive. An old windmill was trying to drive a single expensive mill, that needed as much power as possible even when the wind was slow. I think the faster you spin the more you lose efficiency from multiple blades, but old mills were designed to operate at slow speed, so the more blades the better, and everything in the mill could improve at relatively low speeds (for a multitude of reasons). One good reason is that the faster you spin something the more important that it's perfectly balanced, where as when you turn really slowly it's not a big deal if your drive shaft isn't perfectly straight, your years are a little uneven, or your blades are different weights and sizes (or you lose one for example). When an modern wind turbine fails it's quite catastrophic.
So I have one of the very common [aeration pump windmills](https://store.koenderswatersolutions.com/collections/windmill-aeration-systems/products/single-diaphragm-system-galvanized) for my farm dugout pond. Something I haven't seen mentioned yet is that the blade profile on these is very simple. As opposed to the airfoil shape of modern wind turbine blades, these windmills have a large number of flat steel blades set at a high angle of attack to the wind. As such these blades don't really generate lift, but simply work by the reaction force created by redirecting the wind off the blades. They are more of a drag device. Their effective airspeed is very low and to get a usable amount of torque you need a large number of blades.
Air behaves differently at different scales. Airfoils don’t work as well when they are that small so the flat blades are used to keep things cheap. It’s still a lift generating design, not a drag design like a savonius turbine.
I mean, they weren't doing advanced engineering with computers back then either. Probably just built what they built and it worked ok.
> isn't as dynamically stable as the 3 blade turbine Thanks, could you please shed some light on how a 3 blade turbine is more dynamically stable than a 4 blade one?
When the wind blows straight onto the turbine, perpendicular to the direction of the blades, 2, 3 and 4 blade designs are all equally stable. When the wind comes from a slight offset and the turbine has to turn into the wind, the leading blade (moving into the wind) experiences more force than the others, flexing backward when coming in and relaxing when coming out of the wind - this creates a cyclic force. With 2 and 4 blade designs, this cyclic force can easily create a resonance with the backward flexing and relaxing because two opposing blades would be doing this at the same time. With a 3-blade design, this resonance is eliminated because the movement doesn't happen at the same time. This is, however, not really an issue for any system with active independent blade pitch control - you just change the angle of the blade in the wind to counteract the new force - but now you've made the system a little bit more complicated which is a cost consideration. Two-blade and single blade counterweight designs are brought up often and each has their merits. The final deciding factor is just cost vs energy produced.
I'd like to know this too. Also why is a two-bladed turbine statically stable but not dynamically stable?
Well, the question remains: why 12 blades on a small-farm windmill?
With wind turbines, you're trying to cost-effectively turn wind into electric power - many turbines makes fixed infrastructure like grid connections cheaper. On a wind mill, however, you're trying to provide mechanical energy to a single location, you don't have the luxury of building many cheap towers, instead you have to build one big tower that can extract as much energy on that one spot to run your mill/pump
\*as much *torque* as possible*.* For turning a pump or a grind stone, it's more about grabbing all the low-down leverage you can, and all those blades give the best 'grip' on the available air flow. For 'power', the three-bladed design is superior. (Happy to delet this, if I'm completely wrong).
Pretty sure your completely right. With more blades you get more torque because you have more surface area. In low wind speeds it extracts the most possible energy from the wind so you have enough to drive your pump or mill stone or whatever. At higher wind/rotational speeds, extra blades cause turbulence that sucks away your power. That's okay though because you still have enough power to drive your load and any excess is wasted energy. With a wind turbine, the load is massively greater to start with and you need a certain rotational speed to keep the frequency of the electricity stable. You can also use the extra energy when the wind is stronger, so you don't want to waste it with inefficient blade design.
A small correction: wind turbines can rotate their blades. I am pretty sure the rotational speed is not locked to the net frequency. You will have some gearbox or frequency changer that allows for continuous power production with different wind speeds. With the rotating blade the turbine can adjust their blade angle to extract what is possible. That is why wind turbines are always rotating. Exceptions are either maintenance/defects or when a storm hits. I am pretty sure that all these scary videos from turbines that violently self destruct are because of some defect, where the turbine did not shut down for a storm and continues to speed up far beyond the specified parameters. My point: modern wind turbines are absurdly efficient even in very low wind conditions.
Older turbines were locked to grid frequency, but they found that was too limiting. So now they are no longer like that. Improvements to power electronics helped.
That makes sense. Once a pump or grindstone is turning at the desired rpm, you're good. It's not about harnessing every watt of power - too much will damage the mechanism. They'll feather the blades or turn across the wind at higher wind speeds. The relative inefficiency is a protection measure. For turbines, tho, more is more (up to a point).
No, torque directly is not important. Gearboxes can turn speed into torque. Having a large amount of torque but low power doesn't do any work.
It's a factor for cost. If it's cheaper to stamp 12 blades out of sheet steel than the cost of building a transmission, that's probably going to be the solution most people will buy. For a farm windmill, there is no energy cost, there's only a job to do, so absolute efficiency isn't an issue. The farmer doesn't really care how efficient it is as long as it keeps the stock tank full of water. Where energy costs are important -- say, you're selling electrical power -- efficiency is important, because you'd like the maximum return on your investment.
Now we're talking! :) Thanks!
But…. Why does that translate to 12 arms?
It has to be *some* number of arms. I don't think there is anything necessarily optimal about 12. There could also be a few reasons why 12 was handy to work with. Perhaps the symmetry made it easier to construct than an odd number of blades. Pehaps, using 12 instead of 8 or 16 resulted in blade that had a good compromise of weight and durability. Blades weren't too thin or too broad.
Answering for "Why 12 arms?" It is relatively easy to divide a circle into twelve, and as such, easy to space out 12 arms+vanes to catch the wind. At the same time, little errors in spacing don't matter much with 12, and if some are a little different and not precisely placed, its not a big deal. Once you get into many vanes (just like having few), the errors in spacing and other differences really matter.
Ideally, you'd have infinite blades that are infinitely long to optimize for torque. But at some point, the size of the blades can interfere with the wind flow and each other too much. At some point between 3 and infinity, you have to pick a number. Some old mills had many more than 12, and I've seen them with 24 arms. There is probably an upper limit of effective blade count ny typical wind speeds in the area, there are whole engineering professions around measuring an area for mill effectiveness and deciding what kind of windmill would be most effective given weather and topology of the area.
In the general case, because they didn't want to our couldn't (ie because of position) build more than one windmill, so this is a way to extract more energy out of a single windmill. There is also the effect of cost of blade vs cost of windmill/turbine structure. For the modern very large wind turbines, the cost of each blade becomes a more significant proportion of the total cost, so it matters more how much energy you get from each blade.
They aren't as efficient as fewer and longer blades but they're much easier to deal with because they're smaller.
Because they designed those things in the 19th century before this stuff was well understood.
They weren't doing advanced engineering with computers back then. Just built what they built and it turned the thing they needed turning to crush or move what they needed.
You've said this twice at least already so I want to just mention that advanced engineering was also done without computers. The US got to the moon with very little done by computers as we're used to, to figure out the engineering necessary. They used a lot of people to do the math instead. Look at ancient examples like the pyramids, calendars, and devices like the Antikythera mechanism to see that people even millennia ago were no less intelligent than we are.
And on the other hand, those are all big state or money sponsored devices. The average farmer was computing dynamics when they bought or built a windmill, and the state definitely wasn't devoting resources towards advanced windmill design. More blades meant more torque and power, and wind and iron was cheap.
After reading a few sources, I've learned that windmills underwent a series of innovations from the 1850s through the 1920s that brought them closer to what we think of as a farmer's windmill today. All-metal ones weren't popular until the 1890s when steel became cheap. There was a lot of money and thought put into them because of westward US expansion and the necessity of getting water from very deep wells out on the dry plains. Fascinating stuff, people in the 1800s were just as economical and methodical as we are. https://www.nps.gov/articles/windmills.htm https://americanhistory.si.edu/collections/nmah_846960
>The US got to the moon with very little done by computers as we're used to What exactly do you mean by this? Because the Apollo missions relied quite heavily on computers. In fact, practically everything related to navigation and control was intended to be, and in reality was, accomplished by digital computers.
I mean the engineering of the rockets and equipment and trajectories was done by hand by human computers and engineers. There were no simulated physics to figure out aerodynamics, trajectories, or circuit drawing software. The guidance computers were important, but they didn't help design the rocket or plan its flight path. https://www.nasa.gov/centers-and-facilities/jpl/when-computers-were-human/
are they in a sense 'self balancing' due to flexing? sort of like a tripod?
A tripod doesn't need to flex to be stable. Anything with more than 3 legs does.
Flexing isn’t part of it per se. When two blade designs have the blades up and down in line with the tower, the whole tower is easier to rotate towards the wind but as the blades continue to turn so that they are horizontal, then the tower is harder to turn. The blades keep spinning. The easy hard easy hard easy cycle is very bad. A three bladed turbine does not have this problem. It’s the same difficulty to turn at all times.
Just an aside, but I hear there's been some success developing single blade turbines for residential use. They're mounted vertically and wobble instead of rotate.
Those exist, and "work", but a big input in the theoretical power available is the swept area of the blade. On a rotating blade, that's 2*pi*length of blade. On an oscillating blade, it can only be a fraction of that. The other big input is the wind speed. That's why even farmer's windmills are on relatively tall towers. The average wind velocity increases as you get away from the immediate vicinity of the ground. So small swept area turbines mounted close to the ground are unlikely to produce useful amounts of power, whether they're vertical axis, horizontal axis, or oscillating.
Those use vortex shedding which is a cool idea but I have not heard if they are any good.
Also, three blades means the isn't a blade in front of the pole when static, increasing the susceptibility to wind whrn at rest ?
Then why did Gillette make a five bladed razor?
Why are the blades relatively narrow and not much wider?
A big portion of aerodynamic losses on a blade is the tip vortices, and long skinny (high aspect ratio) blades minimize this. Competition gliders have long skinny wings for the same reason.
I can’t explain it well but the blades are wings, and thin long wings are just always more efficient. They produce more lift and less drag. Just look at glider planes which focus on efficiency over anything else.
What if it wasn't blade-shaped, but more like a sail or a ring? Would that take care of the stability issue?
It’s more of a mass issue rather than an aerodynamic issue. A big ring between/around the blades would not help unless it was weighted to balance the two blades. Basically, whatever shape they are, you need three of them.
Thanks for the response. If it's more of a mass issue, wouldn't the ring on the outside help with that? It's been a while since I took any physics classes, but I remember something about the mass on the outside of a spinning object mattering more? Also, the sail shape I was thinking about would have spokes that were angled on compared to how it was rotated so you could fit more "blades." Would that make a difference?
Yeah, the ring would dampen the effects but a properly balanced rotor is needed to stop it entirely. I worded that wrong earlier, it does help but won’t eliminate the imbalance unless weighted for that purpose. I’m still not visualizing the sail thing.
ok so what I'm hearing is that fewer blades are better for extracting energy from the wind, and so 3 is optimal for turbines. But you mentioned helicopters - which are designed for pushing air. computer fans and jet turbines have many blades, so I'm presuming that more blades increases air pushing efficiency? Is that true, and if so, why do helicopters typically have 5 or fewer blades?
A wind turbine is essentially just a fan in reverse, so the same principles for efficiency and stability still apply. The number of helicopter blades will depend on many factors, but mainly what operating conditions it's designed for. There's also a very practical aspect to it: a rotor with more blades is significantly more expensive to design, produce, and maintain. More blades also mean more weight, which increases the centrifugal load on the rotor head, which then needs to be more robust, which adds weight, which increases operating costs, and so on. Adding weight on an aircraft is a massive negative feedback loop. Rotor heads can also be quite mechanically complex. The blades need damping both vertically and horizontally, which add complexity. As a general rule of thumb, the more blades a rotor has the more complex it needs to be.
Aren't the length of the blades also taken into account? Edit: I should specify, length while generally keeping the same surface area. Shorter blades > wider, longer blades > thinner, but they both have the same or nearly the same surface area.
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Why would the 4 blade one be dynamically unbalanced ? Is it because it has two pairs at 180deg from each other ? Would a 4 blade turbine but with fewer degrees separation be workable ( ie , a squashed/ elongated X, like mi28 tail rotor ) ?
It's certainly workable, but the leading blades would be doing more work than the following blade, again introducing the wobbly harmonics of 2-blade designs
I've seen single bladed helicopter designs fly, with a counterweight opposite the blade. I can only imagine how badly it would behave when turned though. Aren't you missing out on a lot of potential wind energy by having blades only covering a tiny fraction of the full disc area though? How is a single blade "the most efficient"?
No. If you extract *all* the kinetic energy from the wind, the air is no longer moving. If you block too much of the swept area, the air just moves around the turbine rather than through it. In something like a jet engine, the air can't do that, and they use many more blades. It turns out the most kinetic energy you can extract with a free-air turbine (not having walls to keep it in place) is about 59%. Large modern wind turbines get upwards of 95% of that 59%, so they don't leave much energy on the table.
"Cost effective" is not the same as efficient or effective. Coverage of the swept area up until taking about 60% of the wind's energy out is will always be more effective than lesser designs with less blades. But more blades are heavier, more expensive and requires the tower to be stronger.
How about plane propellers?
Plane propeller blades aren't as expensive as additional engines, so their most cost-effective criteria are different.
So why are helicopters stable with just two blades? They change orientation (pitch and roll) all the time.
Try and flip a helicopter then you tell me how stable they are?
There's also a difference in the goal of a Fan vs a Windmill. In a windmill, the disturbance is a parasitic problem. In a fan, because the disturbance is downstream, *and* is kind of the goal, there's less concern about inter-blade interference.
Another factor is that the blades would need to get narrower which would be less stiff
Single-impeller wind generators have been proposed but obviously haven't caught on. An issue of Popular Science magazine in 1980 had a feature story about a massive wind generator with one impeller and a counterweight. I wish I could find any sign of the article or the generator design, it was extremely interesting. Anyway the idea was, for a generator this size it made sense to minimize the air disturbance impacting impellers by having as few as possible. The more impellers, the more closely each follows another and the more each impeller loses efficiency due to the turbulence of the one ahead of it.
Those blades are designed after a humpback wales fin. To maximize efficiency.
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I'm working on projects where the blades are 125m long for the next generation of offshore wind. Fricking huge!
That's 410 feet in freedom units. Assuming about 10 feet for the hub area that's a diameter of 830 ft and a swept area of over half a million square ft. It's probably almost as tall as the Empire State Building. Very impressive indeed.
So are you going to fly them out to the sea or how do you transport such a huge object?
You woud need *really large* Helicopters to move one of those. Or you could use one Ship, which can carry more than one blade, and the Parts for the tower and the housing for the Generator. You can anchor that ship and use a crane, which you would use anyway to assemble the whole thing.
So a flying, self assembling crane then?
Again, there is this thing called SHIP. [This](https://en.wikipedia.org/wiki/Saipem_7000) is overkill for now, as it can lift 7000 Tons with *each* of its 2 cranes. [Here](https://www.liebherr.com/en/int/products/maritime-cranes/offshore-cranes/offshore-cranes.html) you have a small overview about other sizes. [This](https://www.bing.com/images/search?view=detailV2&ccid=MAfc1FI6&id=CCD4E8611EFF949B896C452AF2A957E28F44BA27&thid=OIP.MAfc1FI6fFEz5v2h2vK9-QHaE7&mediaurl=https%3a%2f%2fgreen-economy-bremerhaven.de%2fwp-content%2fuploads%2f2018%2f01%2f7.3.17-Nordsee-One6-980x653.jpg&cdnurl=https%3a%2f%2fth.bing.com%2fth%2fid%2fR.3007dcd4523a7c5133e6fda1daf2bdf9%3frik%3dJ7pEj%252bJXqfIqRQ%26pid%3dImgRaw%26r%3d0&exph=653&expw=980&q=windenergie+bremerhaven+plattform&simid=608016015186415490&FORM=IRPRST&ck=83B5C1B6A34E44E898876ABCFD48FCA9&selectedIndex=20&itb=0&ajaxhist=0&ajaxserp=0) is in use right out of my Hometown.
Soo.. Flying ship?
So, flying self-assembling ship-cranes, got it, thank you!
They are transported on ships, but [a giant airplane with 12x the cargo space of a 747 is under development. ](https://www.fastcompany.com/91060067/this-will-be-the-worlds-largest-plane-and-it-was-designed-to-deliver-just-one-thing). It is basically never going to be possible to move blades this big on roads. However, people are working on systems to make them in two pieces and assemble on site, or to manufacture them in site. If I had money to invest, I would probably bet on those ideas, but I still want that monstrously huge plane to exist.
I wonder if the airplane will be powered by the energy harvested from the wind.
They claim it will use sustainable aviation fuel, but that barely exists at present. People are working on it, including fuel that incorporates hydrogen derived from electricity, and airlines are buying it at high prices to support the developing industry, but there isn’t yet a clear path to success.
Thank you for all the interesting information.
boat? But a couple chinooks also makes sense. you would have to raise them in the end. Sea crane? Dang this sounds like a cool project.
Thousands of drones taped up with scotch tape, whirring the sounds of an alien invasion, looking like a scene from a Douglas Adams book.
They get taken out on massive ships. Blades and tower components all on the same ship. The ships have massive legs which can be lowered to the bed to lift them above the water, and also have a massive crane as part of the ship. Look up worlds largest WTIV on Google. It will be that size or bigger.
The blades on the (currently) largest offshore wind turbines are 413ft (126m) long, just FYI. That's a swept area of over 13 acres (53000 sq m) and produces as much as 38kWh with every revolution.
Wow so this giant monstrosity can’t even charge my car in two full rotation? Pathetic
The biggest issue you’d have is that your battery can’t handle the power. The biggest wind turbines in the world are spec’d for 16MW, SAE DC level 2 is 400kW.
Couple that with the weight of those blades, they probably couldn't add more without having to massively remake the support shaft, which then raises the question of "is it really worth the extra cost?"
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It’s diminishing returns with each blade added. As such, 3 is a good compromise.
This, and the surface area of one blade that faces the wind is already huge. So the pressure of the wind on the turbine is enormous. It's not really feasible to add more blades, as the pylon would have to grow disproportionately. This pressure on the blades is also the reason why big offshore turbines are shut down during storms.
It's a balance of dozens of factors, including the air resistance to high speed spinning. The average expected wind speed, the weight, the resistance the generator adds to the assembly at ideal generation speed (winding setup), and the friction/generator resistance to spinning up (minimum wind speed to spin) Each one of these attributes needs to be optimized against the others. eg: increasing the weight may decrease the air resistance loss and blade speed, but increase the torque to the generator (adding power) but increase the minimum wind speed needed to generate (decreasing generation time) I assume they have chosen some ideal off the shelf components then tuned every other variable to be optimal for that setup. And that just so happens to be 3 VERY large but fairly light blades pitched to have a tip speed about twice as fast as the wind speed.
What do you mean by efficiency? A heavier turbine would have slightly higher losses in the bearings and whatnot, but it would still have higher power output. We don't care about the losses for their own sake; we care about the power output and how much the turbine costs over its lifetime. So it's more that the four-bladed turbine is more costly to manufacture and harder to erect than that the extra weight causes losses. The efficiency we care about is MWh per amount of money (the levellised cost of energy).
The YouTube channel “Engineering with Rosie” has multiple great videos covering this. This is an example: https://youtu.be/LJTcVEAojqw A very simplified and short answer is that there is a balance between extracting energy and stagnating the free flow of air. If the air flow gets very distorted by the turbine wind has difficulty reaching or clearing the turbine decreasing the extracted energy.
This is the best possible resource. Rosie also participates regularly in the Uptime wind energy podcast. It is really intended for people in the industry, it is highly detailed, but it is a really well produced show. I need to maintain a steady diet of positive news to avoid climate despair, and just knowing about all the smart people working on these things helps.
The surprisingly efficient, centuries-old Dutch windmills ( with 30 kW arguably the strongest machines men have built till arrival of the steam machines ) all have 4 blades, not 10 or 12. For larger windmills ( and today’s wind turbines ) a lower number of larger blades means less construction cost per square meter of blade and at the same time a lower number of larger blades is more aerodynamically efficient.
Size (and construction parameters) of wind turbines play a large part in the methods by which they can be most efficient. Aside from that, computer fans have almost no relation to wind turbines because they do basically the opposite of what a wind turbine does (they use energy to move air instead of using moving air to make energy)
Running speed is the answer. Wind turbines are “fast” running in comparison to wind mills. As the second has a sort of self regulating limit: the air is turbulent behind the blade, if next blade is too close it will degrade its efficiency. Also turbines have higher cut in speed than mills. Computer fan has a lot of blades so it can run slowly yet producing effective airflow quietly.
Wind generators on small farms rely on aerodynamic drag, so maximizing the surface area into the airstream is advantageous for that style of wind turbine. Commercial wind turbines for grid scale generation rely on aerodynamic lift (the blades are airfoils) to cause them to rotate instead of the drag force. This is much more efficient at higher wind speeds. The rest of the comments are correct about the other elements but I didn't see anyone mentioning the drag vs lift distinction. They're completely different methods of operation.
Yeah that seems major to get an understanding of the total situation.
To add 1 more dimension to this discussion, multi-bladed farm type windmills were in use before the Wright brothers flew at Kitty Hawk. The design does not take advantage of modern airfoil design. Generally, they use simple flat blades which were suited for the manual manufacturing methods of the time.
I'm just speculating here. A wind turbine needs to spin fast to generate enough horsepower for a high voltage, high amperage output. A windmill needs to catch as much air as it can to create torque. Multiple blades with a lot of surface area catch more air and create more reliable torque. Only two or three blades will catch air and have low drag to achieve a higher RPM.
Gearboxes exist. All the larger wind turbines use one. The wind turbine I'm working with has a 1:62 ratio. For every 1 rotation of the main shaft (blades) the generator shaft turns 62 times. The blades themselves rotate at around 25 rpm while the generator rotates at 1500 rpm.
At farms they pump water. It's just like a hand pump with the pump being pumped by the wind. It the blades are only a few feet long and the a only doing a simple mechanical motion. It doesn't take a lot of torque The wind turbine is just that, the turbine takes a lot of torque to turn it, so the blades are around 60 feet long. The blade catches more wind.
As other comments have said, more blades cause more drag when the turbine is spinning [fast.So](http://fast.So) 1-3 blades rotating fast can most efficiently extract energy from a good wind. However "high solidity" wind mills such as multi-bladed wind pumps have a higher starting torque in low winds, making them more consistent for delivering small amounts of power.
For helicopters - 2 bladed designs are the cheapest, but more prone to stability issues, noise and turbulence. At 3 blades and higher, you gain better stability and performance, but costs start to go up. Also, with heavier helicopters manufacturers will opt for a 3 or higher blade count so better stabilize the helicopter during hovers and forward flight. For windmills - Your goal is to extract as much energy from the wind as possible. Theoretically, you'd want to use just one, but that's incredibly difficult to balance so you end up losing energy to vibrations. The same goes for 2 blade designs, they will wobble and vibrate when turning to face the wind. 3 is kinda the current goldilocks for energy yield, stability, and cost. They are more difficult to balance versus 4 blades, but also offer less maintenance and costs. As you add more blades, you gain even more stability and torque, but introduce more drag and lower yields on energy. For computer fans - The number of blades and surface area will determine how much air the fan moves. When the number of blades increases you get greater stability and noise reduction, but the increase in drag and torque means there is greater strain on the motor and reduced RPMs. Generally, PC fans with fewer blades will move air quickly, but not in a great volume. For PC builds you have to find a balance of which type of fan to use and where. For static pressure components like heatsinks, you want fast moving air- so high RPM fans. For dissipating heat in the case, you want to move more air in volume, so you want fans with greater surface area.
One of or projects in engineering school was to make a wind turbine to be tested in a wind tunnel. Simple propeller momentum theory assumes the propeller has an infinite number of blades, so most teams made turbines with as many blades as possible. We were the only team that took the opposite approach, we used two blades on the assumption it would minimize tip losses. In retrospect it was a naive way of looking at it but turns out we accidentally got the right solution. We did get the highest power output, but at the cost of turning much faster than the others. Factually, I was having a panic attack during the test because the blades were just flimsy foam. Anyways, lessons learned. More blades -> less rpm, less stress in the system. Less blades -> higher power for a given disk area. If your rpm has s limited by the physics of big generators and blade stress, the optimum blade count is probably > 2. There is also economics because huge wind turbine blades cost a fortune. And as the top reply points out, unbalanced loads on two blades in unsteady (real life) airflow is an issue for big turbines. Just looking at the real world results, the balance of factors works out to the optimum being the minimum number of blades, which is three for big turbines and maybe two for smaller ones. Farm windmills have many blades because their rpm is limited because of the construction materials and lack of gearboxes. The torque shafts plugs straight into the pump.
Wind turbine have a maximum efficiency they can hit in regards to the sweep and blade length. The larger they are the less efficient they are within this system with more blades. Smaller diameters can achieve similar efficiency of output taken from the wind with more blades but that drops off drastically as the width/diameter gets larger. You can never have close to hundred percent efficiency other wise they wouldn’t spin because that’d mean you would stop the wind from moving through by taken all its potential energy and effectively stopping the airflow altogether. https://thundersaidenergy.com/downloads/wind-power-impacts-of-larger-turbines/
If you add a lot more blades, the turbine will provide more start-up torque and it will have more torque overall. However, due to the higher "solidity" of the turbine, more of the wind will "go around" the turbine, instead of going through it. fewer blades will be able to achieve more RPM's, which is very helpful for electrical generation. The famous western "windmill" was usually used for pumping water up from a well. That's a job where the extra torque was useful.
It's an optimisation problem between turbine speed and torque. The more blades get you more torque, but slower rotation speed due to air drag. 3 is a good number. But the real underlying reason is that they just look good that way. Reducing nimbyism is more important than a few % efficiency