Power Consumption of Ducted Fans?

[hul]

Can anyone point me in the direction of the formula Motocalc uses to calculate the power absorbed by a fan?

This is an example of Motocalc's constants off their website:
WeMoTec Mini Fan
Diam = 2.70
Pitch = 3.66
Hub = 1.30
PCoeff = 0.506
TCoeff = 0.807

I tried the propeller formula (power = Kp x diameter^4 x pitch x speed^3) with PCoeff in place of Kp, but it doesn't give the right results.

Thanks, Hans

[Gary Warner]

No responses… I’ll give it a shot.

I don't know for sure, but these Pcoeff and Tcoeff (I assume that’s power coefficient and thrust coefficient) may be internal coefficient adjustment values for the MotoCalc's math. They may not have real-world meanings.

For example:

Power coefficient for a normal prop of the same size is "X". This Pcoeff of .506 might just be X = X * .506

Just a guess. I don't really know.

Gary
--

[hul]

Thanks, Gary.
The propeller formula gives about 60% of actual power input (at least for the one fan I was looking at). The correction factor would have to about 1.6.

Oh well, the formula seems to be Motocalc's secret.

Hans

[Bruce Abbott]

A ducted fan usually has many blades, stators, and an obstruction in the centre. Therefore the standard prop formula doesn't work very well.

Here are some simple formulae which accept Motocalc's fan constants. Corrections for non-standard elevation, air pressure and temperature are NOT included!
Efflux_Velocity = rpm * Fan_Pitch / 1056
mph inches

area = ((PI * (Fan_Diameter / 2) ^ 2) - (PI * (Hub_Diameter / 2) ^ 2)) / 144
ft in in

Thrust = Efflux_Velocity ^ 2 * area * Thrust_Coeff / 12.4
oz mph sqft

power = Efflux_Velocity ^ 3 * area * Power_Coeff / 100
Watts mph sqft

[hul]

thanks a lot

Hans