# From theoretical airflow speed of rotating propeller to a realistic value?

1. Jul 18, 2007

### ash01

Hey,

Yet another propeller question on this board.
I have done my research, both here & on google, but I cannot seem to find an answer.

Lets say I have a motor on wich I mount a propeller. This motor is static (not moving like on a flying airplane), so say its a small ventilator standing on my desk. What I want to know is the volume flow of air its generating at a certain motor rpm. This is what I do:

Prop pitch = 6"
Prop dia. = 9"
Motor rpm = 2000rpm

For 1 rotation, the prop moves the air 6"
For 2000 rotations, this means 12000". So the theoretical airflow speed would be 1000ft/min.
The disk area works out to 0,44 square foot.
So this would mean a volume flow of 440 CFM

Now my question; in how far is this value representative for the real flow? Does there exist a coefficient which takes this into account?
If someone could tell me a rough value this would also be good for giving me an idea..

2. Jul 19, 2007

### kach22i

Good question, I assume that if the propeller is standing mounted on your desk that more power is consumed (for equal amount of thrust) to feed or draw in the air.

Perhaps there is a "ram air" factor which must be added, or a feed air equation which must be subtracted.

If you get your question answered, maybe I can get a related answer for A, B & C.

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