An actual question my five year old asked me and I couldn’t answer, please help!
In: Physics
More blades, faster acceleration, more thrust. like legs, more legs, faster movement and better manouverabilitu
Right up my alley because I’m about to do this for a living.
There are two big reasons props have more or less blades. One is ground clearance and two, power dissipation.
You know how some cars can be slow, but some are really fast? The slow car has less power than the fast one. So the same goes for propellers. The slow ones have two blades and the fast have three, four or even five. The more powerful the engine, the more blades you can have.
The other has to do with how high the engine is off the ground. The lower the engine is to the ground, the smaller the propeller is.
The big reason you don’t see huge two blade propellers on aircraft is because they’d be too close to the ground and they’d break apart if they started turning.
Imagine you have an airplane, and you want to move it forward, so you tie a rope to the front of it and pull it. The plane rolls forward.
If you want to move it forward faster, you need to give the rope a harder pull.
But there’s a problem: The harder you want to pull the rope, the thicker of a rope you’ll need. The bigger the rope is, the bigger of a pull you can give your airplane to make it go faster. But as you can imagine, the harder you want to pull, the stronger you need to be, and the more effort it takes to pull.
This is (very basically) how a plane moves forward. (How it moves upwards is a different story). But instead of a rope, you have a propellor. A propellor takes the air in front of the plane and pushes it really hard behind the plane. This moves the plane forward.
The more air the propellor can move, the more it moves the plane. It can either do this by turning faster, or by being bigger, or both. Just like the rope though, if you want a bigger propellor, or to turn it faster, you need more energy.
One of the ways you can make a propellor larger is by having more blades. You can also keep the same number of blades but make them larger/longer. An aerospace engineer could weigh in more on how specifically the different changes to the propellor affect performance, but in general: bigger propellor = more air moved = bigger push.
all the other answers are wrong.
its actually quite complicated,
you can have 2, 3 or 4. sometimes 5, but never 1 (except in extreme cases)
2 or 3 are usually most efficient for anything from a small pedestal fan to a giant turbine.
the reason why a 10MW Turbine has 3 blades and not 2, is different than
the reason why a plane prop has 4 blades instead of 3.
or a outboard prop has 3 instead of 2
in the first case the big turbine blades are optimised to squeeze maximum efficiency
the second case has to do with lower tip speed and lower noise (keeping it under speed of sound)
TL:DR: It depends!
Basically fans with more blades tend to be quieter and more pleasing aesthetically unlike their industrial counterparts with 2 or a maximum of 3 blades that move more air, but are noisier.
Ceiling fans for residential spaces have 4 to 5 blades and produce a decent current of air without the noise because their multiple blades create more drag on the motors that move the blades.
So ignoring other technicalities like blade pitch, the design of the blade surface, material used, the rule of thumb is that more blades in a fan is quieter, suitable for the home, but doesn’t create currents used on movie sets lol. Lesser number of blades – 2 and 3 are for industrial or standing fans used in public places where noise is not a concern but they literally need wind blowing everywhere.
This is as simple as I could go without mentioning jargons, I hope it helps. BTW you might just have a mechanical engineer as a kid 🙂
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