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Old Sep 30, 2012, 05:30 AM
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frank40's Avatar
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How to compeare fans

I have been reading a lot of the threads regarding fan's on this forum, and one thing strikes me... it's all about thrust. Are we looking at this from a wrong point a view?

Thrust is good but with out velocity, it has no vault to us. Maybe I and wrong but I think we should have another factor, lets call it K …. K is thrust multiplied by velocity, this way it is easier to comparer the fan.

I will try to explain what I mean, fist you need to know that watt is the amount of work been done.....

You have two piles of sand that you want to move 10 feet, you have 1 minute to move each pile. The first pile you get a shovel and on the second one you get a teaspoon. In order to do both jobs i one minute, the teaspoon will have to take less sand but at faster paste (velocity).

From this example you can see the work that has been done is the same, but if you meausere it you will get two diferent result.

Am I making any seens
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Old Sep 30, 2012, 07:20 AM
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Kevin Cox's Avatar
St. Louis Intl, Missouri, United States
Joined Jan 1997
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You are making sense...
Thrust is a bench mark that everyone can measure...velocity...not so much. What you are describing sounds a little like horsepower (work)...or CFM.

My opinion below only (disclaimer)

Thrust is important but so is velocity. Back in the old days we didn't focus on thrust we stuck to the belief that a certain performance could be expected for a given wattage. Static thrust is really important if you plan to build a VTOL. I used static thrust reading only to verify my internal ducting efficiency and these were based on the "Kress" papers..i.e. one could expect a loss of up to 20% of static thrust with a bifurcated intake. This didn't mean it would fly, just slow to accelerate.

I, to this day, am more interested in the electrical parameters than the thrust. If I had to pick between getting a thrust number or a velocity number for a particular fan, ideally you would want both, I would prefer the velocity.

To be honest there are too many variables to deal with when it comes to EDFs, to get consistent results from multiple people at different parts of the world with different equipment and processes.

Great topic though!!!
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Old Sep 30, 2012, 07:29 AM
Life begins at transition
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Efficiency is the easiest one.

From the change in momentum through the system:
Pout=sqrt(thrust^3 / 2.rho.A)

where Pout is the work done on the air (good), thrust is self explanitory, rho is air density (assumed incompressible) and A is the exhaust area.

Pin could be shaft or electical. If we used shaft, it would remove the motor from the comparison. Again, not hard, just need to know Io, Kv and Rm for the motor in use.

eta = Pout / Pin.

This is where the g^3/w^2 comparison comes from. Even if working out eta is too hard, g^3/w^2 isn't!
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Old Sep 30, 2012, 10:16 AM
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Quote:
Originally Posted by frank40 View Post
I have been reading a lot of the threads regarding fan's on this forum, and one thing strikes me... it's all about thrust. Are we looking at this from a wrong point a view?

Thrust is good but with out velocity, it has no vault to us. Maybe I and wrong but I think we should have another factor, lets call it K . K is thrust multiplied by velocity, this way it is easier to comparer the fan.

I will try to explain what I mean, fist you need to know that watt is the amount of work been done.....

You have two piles of sand that you want to move 10 feet, you have 1 minute to move each pile. The first pile you get a shovel and on the second one you get a teaspoon. In order to do both jobs i one minute, the teaspoon will have to take less sand but at faster paste (velocity).

From this example you can see the work that has been done is the same, but if you meausere it you will get two diferent result.

Am I making any seens
I use "K" as the value for Kash = $ and the most I can get out of a reliable product with a bit of help from the guys here in RCG. And modifications.

Steve
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Old Sep 30, 2012, 12:09 PM
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The way to compare 2 fans is by computing g/W^(2/3). You can also use g/W, but this is not very helpful when comparing fans at differing power levels, since the value drops off dramatically at higher power settings. The reason it does so, is that much of the energy is being converted into efflux, which is obviously not being measured.

g/W^(2/3) actually gets around this issue and the value remains almost constant irrespective of power level and is therefore useful to compare different fans.

Also, bear in mind that momentum theory is our friend - it implies that thrust and efflux are directly related for any given outlet size or shroud diameter (in the bench scenario). Many people posting in this forum seem to think that, for example, fan A can produce more efflux than fan B, but at the same thrust. This is simply not possible if they are both the same diameter fans - if my 90mm fan pushes more thrust than yours, then I have more efflux too!

Jacques
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Old Sep 30, 2012, 03:52 PM
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You seem to forget the exit size .
when I run my wm 400 mk 2 on 80 % FSA exit and on 60% (48MM DIA) the thrust is more or less the same but a hell of difference in airspeed. And in the air the difference is even bigger.
Claus
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Old Sep 30, 2012, 04:59 PM
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Brentwood, California
Joined Jul 2007
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I still think watts per pound gives me the best idea how a plane will fly...
I check my amps, voltage, watts.... do a static pull and if all looks good. I fly it...after it's flying great I never look back and don't care about the numbers and enjoy the plane
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Old Oct 01, 2012, 01:52 AM
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Quote:
Originally Posted by SCALECRAFT View Post
I use "K" as the value for Kash = $ and the most I can get out of a reliable product with a bit of help from the guys here in RCG. And modifications.

Steve
lol, I'm in the SAME category...

Also, if I'm goin to pay 4 times the price of a cheepy anything I'd like to get at least twice the everything else...

Static thrust is a good way to measure performance without adding a ton of other variants IMHO
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Old Oct 01, 2012, 04:08 AM
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Static thrust is only useful at a certain power level, with a certain exhaust diameter... Hence the need for a more robust comparison.
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Old Oct 01, 2012, 04:41 AM
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Originally Posted by Odysis View Post
Static thrust is only useful at a certain power level, with a certain exhaust diameter... Hence the need for a more robust comparison.
Then total CFM comes into play but that's REALLY high power...efflux becomes more important then..

Stuff like Knife is doing....few folk are going to force 4000 watts through 70mm fans...

How would one practically measure the rest?

A simple vertical test stand, weight scale and watt meter seems to take care of the 95% of how to compare fans.
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Old Oct 01, 2012, 04:44 AM
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Yeah, but how do you compare someone running 1.2kw against someone running 1.5? Or someone using an 85% FSA exhaust vs a 90%?
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Old Oct 01, 2012, 05:32 AM
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Originally Posted by Odysis View Post
Yeah, but how do you compare someone running 1.2kw against someone running 1.5? Or someone using an 85% FSA exhaust vs a 90%?
Practically?

I was thinking, a big balloon...

How fast it takes to fill up or lift something off the ground could be a good indicator of how much CFM a fan is putting out that uses a thrust tube.

The thing with static thrust is I can do with it what I'd like,...few fans GAIN static thrust with a thrust tube
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Old Oct 01, 2012, 07:21 AM
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Now you're talking volume erh, I wanted a formula some time ago to calculate an EDF's potential.
You could have a million mile per hour velocity but the amount of air volume dictates how much load said velocity/thrust can push.. IMHO
My experience says its not the motor or the fan but the builders correctly informed choice of fan design and motor power for the application (airframe)
Where ever you go its always going to be different depending on altitude, density and humidity...
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Old Oct 01, 2012, 07:32 AM
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To work out CFM (or mass flow):

Mass flow = rho * exhaust area * efflux velocity

efflux velocity = sqrt (thrust/ (2*rho*area) )

Everything to do with fans has been done to death in the turbine (especially turbofan) or HVAC fields.
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Old Oct 01, 2012, 09:21 AM
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Originally Posted by Odysis View Post
To work out CFM (or mass flow):

Mass flow = rho * exhaust area * efflux velocity

efflux velocity = sqrt (thrust/ (2*rho*area) )

Everything to do with fans has been done to death in the turbine (especially turbofan) or HVAC fields.
Please give an example using a 90mm for us that are math symbol resistant

Thx in advance for any input

Also, do you think CFM is what we should be lookin at as the ultimate tell tale?
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