Sizing Fans and Electric Motors for Different Applications???

Are there any generic formulas for sizing fans and motors for certain levels of airflow or thrust?


1) For example, is there a basic aspect ratio/target RPM formula that would describe why an airplane propeller is a long and skinny while a cooling fan blade is much broader?

2) How could one determine analytically if there should be a gear reduction?

3) Are there a number of specific parameters that would describe the sweep/contour of a blade?

I realize that this can get very complicated. However, I need some basic info for a powerful DC fan (7"-8") needed to push a great deal of air (1000+ CFM). Currently, I am experimenting with ducted fans for RC electric airplanes. The noise level, unfortunately, is incredible.

There are a few high performance automobile radiator fans that are claiming

1000 CFM for 8" diameter blades. Any opinions?
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Your queries do not allow simple answers. Many books (good and bad!) have been written about this. I can help with your RC fan. When I do so, you will learn a lot about this topic!

NYC Doc wrote:

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Pat March

As I mentioned, I understand this is a complicated subject. However, if I could get references to a couple of formulas with an acceptible level of error, it would help.

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Hi, I answer briefly to the questions posed below: 1) There is actually a fast approach to work out the maximum speed limit that can be reached for a given blade, making simple fluid cinematic considerations: The helicoidal advance per blade turn, which is a geometrical parameter of the blade, multiplied by the radius of the blade will throw the maximum displaced volume per turn. Considering continuous axial fluid flow, perfect coupling between fluid and blades and no lateral fluid flow exists and also considering that the torque needed for spinning is available at turbine axle, this would be the air flow. For Some electrical turbine driven generator, the attack angle of the blades is continuously controlled and adjusted for extracting the demanded torque each moment. Unfortunately, this approach is far from being in accordance to reality for lateral air flow exist and the fluid will have no perfect coupling with the blade, and this data are generally available by means of empirical tests and the definition of drag-thrust coefficients affecting the former ideal approach. So have a look at blade manufacturer to have the numbers, which will depend on the blade and the fluid viscosity.

2) Once that you get the numbers from the 1st point, you will have a direct relationship between axle rpm and fluid flow, so gear reduction will be easy to calculate. 3) The main parameter of the blade contour will be the radius and the advance per turn. Being set this parameter, have a look at any math book on helicoidal profiles. Of course, blade manufacturers have specific and detailed knowledge on best profiles and material throwing maximum efficiency, based on empirical knowledge and specific hidrodinamyc considerations as turbulence assumptions and side effects. Best regards. Ignacio Simón Yarza. Mech & Electronic and control systems eng.

"NYC Doc" escribió en el mensaje news:DU2Re.21574$%

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Ignacio Simón Yarza

In point 1, replace "multiplied by the radius of the blade " by "multiplied by Pi*radius^2 of the blade" Best regards.

"Ignacio Simón Yarza" escribió en el mensaje news:df3uhl$po5$

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Ignacio Simón Yarza
1) What you wrote is correct, that is "power to be applied might be deemed as roughly proportional to the rpms powered to 3", assuming that there is a linear relationship between rpms and fluid linear speed and that there is no differential pressure between the inlet and the outlet. I explain it below:

Being: ro=fluid density v= average fluid speed at the outlet

Specific volumetric Fluid Energy before entering the blade area=0 Specific volumetric Fluid kinetical Energy after going through the blades=(0,5*ro*v^2)

flux volume going through the blades area per time unit (put minutes)= pi*radius^2*v

Power going through the blades area = (0,5*ro*v^2)*( pi*radius^2*v)=

6,28*ro*radius^2*v^3 Note that power is the energy per time unit

if v is assumed as proportional to rpm, it follows that power is proportional to rpm^3.

2) Centrifugal forces are proportional to rpm^2, so an increase of twice the rpm will increas the radial forces by 4 and so will do the forces on the ball bearings or sleeves. Furthermore, the speed increase will also make the pressure on the blades to grow. This latter growth will be just proportional to the speed, and deflection on the blades will also grow proportionally to speed in case they work on the elastic region of the material. As a conclusion, I wouldnt use blades rated for 1725 on a 3450 application.

Best regards. Ignacio Simón Yarza Mech&Electronic and control engineer.

----- Original Message ----- From: Newsgroups: Sent: Thursday, September 01, 2005 9:31 AM Subject: Re: 3450 rpm 24" 1 1/5hp fan blade?

"NYC Doc" escribió en el mensaje news:nY3Re.21581$%

Reply to
Ignacio Simón Yarza

Are there any basic formulas that incorporate the shape (sweep, pitch, aspect ratio) of the blades?



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