I am sitting here looking at the box fan in my window.
I notice the blades are narrower at the ends than they are at the center hub.
Years ago they were wider. Clover leaf shapes.
I am wondering, why the change? I would think wider blades would push more air.
They do.
Perhaps smaller blades encounter less resistance, therefore require less electricy?
Yes.
But do they push as much air?
Only if they spin faster.
Related: Why are wind generator blades so narrow, and only three?
Fewer is better. Three is as low as you can go without causing more problems than you solve.
The faster a fan spins, the fewer blades it needs, and the narrower the blades need to be, to get maxmum efficiency - air moved per unit of power expended.
Electric motors are getting faster and cheaper (largely because of improvements in permanent magnets, which are now far stronger for a given size, being made from rare-earths rather than just steel); So fan blades are getting faster and narrower.
A fan blade develops most of its drag at the tips, and the tips of the blade move faster than the hub, so thinning the blades towards the tips is generally beneficial.
Wind turbines have similar efficiency and speed realted design issues.
An efficient electricity generating wind turbine will ideally maintain a high speed, and having a large blade area increases torque at the expense of speed. So the blades need to be as thin and as few in number as possible.
Three is the smallest practical number for a stable, steerable turbine (even number blade turbines suffer higher gyroscopic stresses when steering to face the wind, so two blades are typically less good than three; One blade turbines wear rapidly due to lack of balance).
For applications where torque is more important than speed or efficiency (such as raising water from a well, where getting
some water on low wind days is better than getting the most water possible over a year, but having more days with no water at all) a turbine with more, wider, blades that are optimised for low speed by widening towards the tips, is more appropriate, which is why farm wind turbines used for that purpose look very different from electricity generating turbines.
