How to calculate air velocity in a fan and what data should I know ?

So if I have a fan motor, fan blade and a battery, what data should I get from these items ?

I think I will need to know the number of turns per second. How can I determine it ?

  • $\begingroup$ If you can find the power generated by the motor, you can use $W=\Delta k$ to find the velocity. Unfortunately I don't know of a way to find the power generated by the motor. $\endgroup$ – udiboy1209 Jul 30 '13 at 12:27
  • $\begingroup$ It's an engineering problem with many variables. The subject you need to look up is propeller fans. $\endgroup$ – Mike Dunlavey Jul 30 '13 at 21:11
  • $\begingroup$ The term for "number of turns per second" in physics is frequency. $\endgroup$ – Constantine Aug 3 '13 at 19:17
  • 1
    $\begingroup$ If you actually have the fan you could use a Anemometer $\endgroup$ – Michal Jan 26 '14 at 19:42

Measure the "thickness" of the fan blade. That is, the distance from the front to back of the twist in the blade. Now find how many times per second the blade rotates.

Multiply that blade thickness (distance) by the number of times per second the blade moves through that position. It will give you an upper limit on air velocity.


You can calculate air velocity as v = f/A where f=volumetric air flow and A = cross-sectional area of the air passage. Also, you have m=p*f, where m=mass flow rate and p=air density. Here 1 it says that "By conservation of energy, the energy consumed in rotating the fan is the same as the energy required to deliver the air: Pfan ∝ υ"


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