what are advantage/ disadvantage of many fan blades vs 3 or 4 blades…? and why does my desk fans usually have 3 blades as oppose to many small blades like a turbo fan engine fan disk?
In: Engineering
More blades have more surface area if they’re the same size, and unless there’s some weird air current physics I don’t know about, they would therefore have more air resistance and drag. Because of the higher resistance, it takes more power to spin at the same rate, but a greater surface area would also be able to push more air.
A regular propeller is just pushing air backwards. A jet engine is not only pushing air but also also gathering and compressing it, to be able to burn fuel in that denser air and push hot burned gas out the back at much higher speed or spin an engine shaft. The higher pressure needs more blades.
Oddly, in aircraft, it’s common to use one of these jet engines to power a propeller (turboprop) but it’s only practical there because there’s so much air available to the engine intakes. You couldn’t do this in a car for example to power the wheels. (There are some weird jet concept car exceptions but they weren’t very efficient).
One key constraint is that the blade tip speed should not get close to the speed of sound. (Basically the propeller’s efficiency is greatly reduced near the speed of sound).
Recall that at any given RPM, a longer propeller means faster blade tip speed.
So now we have some tradeoffs:
* We can have just a few blades but long ones; or many blades but short ones.
* The more blades we have, the more they interfere with each other (due to proximity)
For a turbo fan it’s impractical to have just a few blades because to generate equivalent thrust those few blades must be spun extremely fast, but doing so will exceed supersonic tip limitations. Hence turbo fans will use many shorter blades instead.
Conversely for regular propeller airplane, we don’t want to have too many blades, because they may negatively interfere with each other. So we use fewer but longer blades.
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