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
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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