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