An actual question my five year old asked me and I couldn’t answer, please help!
In: Physics
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.
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