An actual question my five year old asked me and I couldn’t answer, please help!
In: Physics
On top of all the replies you’ve already got, look up Betz’s law. Basically says that there is a limit to the efficiency of a wind turbine blade, only 59% of input energy can be recovered
Moving lots of blades is hard, so only 2 blades is the best. But, if you can’t make them big and strong enough, then you have to put more blades on the propeller.
“When a fan has fewer blades, there is less drag on the motor and it can go faster and move more air more [efficiently](https://www.thingzcontemporary.com/many-blades-ceiling-fan/). This results in much more airflow and will create a better wind chill effect, making your space feel cooler.”
Think of a fan like a shovel moving dirt. You can use one really large shovel and work slowly to move the same amount of dirt as you would with a tiny shovel working really quickly. The large shovel will require more effort for a single scoop, while the smaller shovel will require less effort, but repeated many times. Fan design is similar. Want to move more air? Add more blades or make your blades bigger.
Fans aren’t shovels though, so there are other factors that complicate things. As the fans spin, each blade is moving through the same body of air. One blade moves air, and the next blade comes immediately behind it. Air must move in to “fill” the space left by the last blade. As you increase the number of blades, you reduce the amount of time available for air to move in and fill this space.
Also, the tip of each blade creates disturbances in the air called turbulence. The effect is similar to dirt falling off the end of the shovel with each scoop. If you were to use many tiny shovels, you’d have proportionally more dirt falling off the tip of each scoop than you would if you used fewer large shovels. Likewise with fans, adding more, smaller blades will create more turbulence than simply making the existing blades larger. That’s why you don’t routinely see regular fans with more than about 5 blades; though they do exist.
It’s also worth noting that these design considerations are different for regular fans (called axial fans) and ducted fans (fans inside a housing). Ducted fans eliminate the effects of turbulence created by the blade tips by preventing the air from “spilling” off the tips of the blades. That’s why you’ll see ducted fans with many more blades. If you look at a jet engine, you’ll see a large fan that has nearly 100 blades.
Say for a moment that aerodynamics is all we care about. Two big blades can give you a lot of thrust, but it takes a lot to push them through the air. You can make them slimmer, which makes them slide through the air more easily, but then you lose thrust. So you make it four blades. Now you have a slightly more efficient propeller. You can keep doing this – there are other effects, like each blade has to go through air that the previous blade stirred up (interference) and you also have complicated effects because the farther you go out on the radius, the faster that blade is moving.
We’re going to ignore those for the moment. For our simplified case, the more blades you have, the more efficient your propeller is. If you had some magic material, you could add more and more blades, making them thinner and thinner until you had an infinite number of infinitesimally thin blades. At this point you have a mathematically perfect propeller – it would just look like a disk that moves air. Of course, you cannot do this. You design a propeller based on the material you have, the money you have, and what it is possible to manufacture. Those factors we ignored play big roles too, propeller design is very complicated.
It’s always a balance.
Number of blades:
* Fewer blades = More efficient shoveling of air because of turbulence (air swirls) created by other blades reduces efficiency. Usually simpler to make.
* More blades = More stable because the force is spread out over more blades and shovels more air compared to how long the propellers are.
Propeller tips breaking sound barrier is bad (because of lots of turbulence). The longer the propeller the faster the tips go compared to the center. But having too short blades means more loss of energy at the blade tips:
* Longer blades = Better at generating lift, shoveling more air at lower speeds. Longer propellers also less stable and vibrate more.
* Smaller blades = Allows higher top speeds since the propeller can go much faster without breaking the soundbarrier with the wingtips. More stable.
So basicly.WW1 airplane: We can’t make so good engines. So we’re gonna go with efficient short two-bladed propellers because that gives is the most thrust for our weak engines.
WW2 airplane: We gots a lot better engines now. But two-bladed propellers can’t shovel enough air to take our planes as fast as we’re going to go. So we’re going to go with 4 short blades!
Helicopter: We gotta generate lots of lift. So we’re going to go with longer and slower rotating blades!
Modern helicopter: Uh. Those blades aren’t generating enough lift. MORE BLADES! More blades is harder to make, but more stable too.
Modern turboprop: Too noisy! We’re making special 6 bladed propellers that are much quieter. And computer power and advanced materials allows us to make them special advanced shapes that generate even less noise and more power. So now they look more like ship propellers. But for air! Still kinda short blades because we gotta go fast!
Ship propellers: Water dense yo. So we gotta make blades short (or they’ll break!) but we make them much wider to shovel a lot of water backwards.
P.S: Jet engines work entirely differently, even if they do have fans at the front they’re for compressing air into the engine, not generating thrust.
P.P.S: For ceiling fan. You’re moving lots of air, but you want to do it slowly and silently. So lots of wide blades. How long the blades are depends on how mobile you want the fan to be. Big fan = more air silently. Small fan = Noisier, but more mobile.
A lot of great answers here, mostly talking about super high-powered propeller setups.
I would like to add one thing though that I haven’t seen mentioned, and that’s air pressure. Sometimes, you need a lot of air moved, but moving it too fast would be bad/wasteful. Static pressure is important for Radiators, where maintaining a temperature is more important than simply pushing air through. The shape of the fan blades will tell you a lot about the function. Short, wide, and far apart? That’s meant for air flow. Long, curved, and close together? That’s for Static pressure.
You can use the wrong fan and still be fine. It’s an efficiency thing.
Generally for fan and propellers:
more blades (or bigger blades) = more force generated = more costs = more weight (which normally means even more costs as the structure will need to be made strong enough to hold the weight)
So the question the engineers designing fans or propellers face is nearly always:
1. Can it do it’s job?
2. If 1. is yes, how do I then make that as cheap as possible
The answer to question 2 above nearly always gives you 3 blades as a good compromise on all factors. If costs is less of an issue (e.g. military) than you tend to get more diverse blade counts. Blade count may also be affected by other factors (more blades tend to be quieter, limited space for the blade, the strength of the material, where you are using the device/structure etc.)
Its a balance between a lot of factors
Each additional blade adds more weight and cost which requires stronger($$$) engine components to overcome the increased drag from additional blades trying to move through the air. They also disturb the air they pass through so having 8 blades instead of 4 won’t give you twice the power because the extra blades are now passing through disturbed air and not working as efficiently. Generally you get better results from increasing the length of the propeller but this requires a stronger propeller($$$)
Generally the better option is to have a second cheap engine/propeller set (think twin engine propeller planes or the Chinook), or to use more elaborate propeller setups like [Contra-Rotating propellers](https://en.wikipedia.org/wiki/Contra-rotating_propellers) which boosts propeller efficiency giving you more thrust with the same horsepower despite driving two different sets of blades. This was used on the later Spitfires to get the most power possible out of their limited propeller diameter.
Good answers here already. I will add that some fans don’t look like what you consider to be a fan at all, if you look online at pictures of industrial exhaust fans you’ll see what I mean, but here’s what I want to say about that:
As others have said, how much air a fan moves depends on a few things: diameter of the fan, the angle of the blades in relation to the fluid you want to move, number of blades, the horsepower of the motor that’s turning the fan, and how many rotations the fan makes per minute.
In a factory with, say, a very dusty process, if you want to control that dusty air by sucking it away from the work area, you would need one kind of fan. Generally speaking a very big one with a big motor, because sucking is very expensive (lol). Especially if you are sucking large volumes of air heavy with dust you want to move from one place to another.
Try blowing a small piece of paper across a table with a straw. Easy. Now try sucking the same piece of paper up from a distance of an inch or more. Not so easy.
So part of what determines how many blades a propellor or fan will have is, as others described, what’s it for? How much fluid does it have to displace, and in what timeframe? What is the density of the fluid? Air? Water? Wet concrete?
Think about/look at the motor on a celling fan. Then go outside and look at the outside unit for your air conditioner. It has a fan that you can see and hear up close. (There’s also a compressor that makes it’s own noise) Would you want that fan in your living room? Why not?
Big, loud, blows too much air.
Can you propel a 4000lb boat with a celling fan? No. Why? Well, the motor for one. But also the orientation of the blades relative to the fluid being displaced. A boat propeller is shaped more like a screw, or like an auger.
That’s about the direction you want to move, and is beyond what I can eli5.
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