An actual question my five year old asked me and I couldn’t answer, please help!
In: Physics
This is a complicated question! There are a few design considerations from the number of blades a fan will have:
**efficiency**: One blade spinning really fast is absolutely the most efficient way of moving air, as it doesn’t hit the disturbed of the blade in “front” of it, however monoprops are difficult to balance (although they do exist). Two blades are normally used on low power engines and fans that value fuel efficiency, however the next consideration is…
**blade loading**: The point of the blades is to “push” the air. If you are pushing a lot of air the blades may not be strong enough, so you can add more. This reduces efficiency, but allows you to move more air.
**rotational speed**: another way to move more air is to spin faster, however if the blades spin too fast the tip may go faster than the air can keep up with. This means you get shockwaves at the tip which makes the fan less efficient, and can damage the blades. You can make the blades shorter to avoid this, but then you would need to add more.
**Not quite ELI5**: The trade off between these factors is complex.
Big slow fans with few blades move air efficiently, but are hard to fit on the front of a plane. They are also harder to slow down and spin up. They also have a low exit velocity of air. If your plane is moving faster than the air can be pushed out the back this causes drag.
Small, fast fans are typically less efficient but can be put on the front of planes without hitting the ground. They can handle more power and are more controllable, and tend to produce less drag.
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