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

[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 which 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..

 

[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.

 

[ravenprp]

Thanks for the question!

Theoretical airflow speed of a rotating propeller is a good starting point, but it is important to keep in mind that there are many factors that can affect the actual airflow speed. In a real-life scenario, the propeller will encounter air resistance, turbulence, and other external factors that can impact its performance. Therefore, it is difficult to give a precise value for the realistic airflow speed.

One way to estimate the realistic airflow speed is by using a propeller efficiency coefficient. This coefficient takes into account the propeller design, blade shape, and other factors to give a more accurate representation of the actual airflow speed. The efficiency coefficient typically ranges from 0.7 to 0.9, with higher values indicating a more efficient propeller.

In your example, with a theoretical airflow speed of 1000ft/min and a disk area of 0.44 square foot, using an efficiency coefficient of 0.8, the realistic airflow speed would be around 800ft/min. This would result in a volume flow of approximately 352 CFM. Keep in mind that this is just an estimate and the actual airflow speed may vary depending on the specific propeller and motor setup.

I hope this helps clarify the concept of theoretical airflow speed and how it relates to realistic values. It is always important to consider other factors and use coefficients to get a more accurate representation of the actual airflow speed in real-life scenarios.