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If I have a motor use in 2 cases, the 1st case I use standard blade and 20% longer blade in the 2nd case.

People told me that longer blade will create higher CFM. However, I am not totally sure. First, I don't know any equation to relate blades with CFM change. Second, I think from the power point of view, longer blade will have larger surface area but the acceleration of the air particle will decrease. It means more air pass the blade area, but the air might move slower. I am really confused. Please help.

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fan pitch is just as important as diameter. more pitch = more cfm as well as more pitch needs more hp to operate efficently. hope this helps – user15976 Nov 14 '12 at 16:12

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In the simplest case the volume of air moved is just the volume of the blades swept out in a rotation. Then the air flow/minute is this multiplied by the revolutions/minute.

Beyond this you would need CFD modelling of the aerodynamics of the blade and it's effect on the air pressure infront and behind it.

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