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Old Sep 27, 2015, 01:23 AM
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EDF Figure of Merit - Watts/Thrust

Has anyone on RCG attempted to compile data of various fans and installations to check on their relative efficiencies?

For example: A system providing 100oz of static thrust and absorbing 2KW gives 20W/1oz thrust. The general goal would be to gather the data and some details of the system to see how the thrust can be maximised for the watts put in.

I've seen some scattered numbers in various threads but haven't been able to locate one thread that is focused on thrust efficiency.

I know there's more to EDF performance than simply static thrust but adding efflux to the data, if accurately captured, would be a welcome addition.

I'll start off with an observation that really opened my eyes on two extremes of power used to perform a very close performance. At the outset, this is only a rough observation but highlights what I'm interested in comparing.

A club member and myself have HSD Super Vipers powered by 12S LiPos. His is a factory stock 12S model, mine is much modified. Why mine is modified is a different subject, not for discussion at this time.

Performance seems very close for both models but my jet uses ~30% more energy than his. I'll have to bring a scale out to the field so we can compare thrusts but while we were 'formation flying' our models seemed closely matched.

His 105mm system draws close to 89A on a fresh charge, mine is 120A.
His motor is rated 700Kv and is coupled to a Change Sun, 12 blade impeller. Mine is 800 Kv and the same impeller but I've removed half the blades (No, not all on one side! ) and even trimmed away some of the remaining blades' trailing edges to bring my amps down to 120.
That's an enormous difference in efficiency as anyone can clearly see.

But regardless, there's two data points to start with. I'm not good at tabulating large quantity of data, assuming this thread draws many views or submissions, but it would be interesting to see what combo gives the best performance, much like the LiPo thread that focused on 6S, 5000mah packs. Fan sizes would define any categories and pack voltages sub categories. I'll have my own data to submit, including efflux if I can get a pitot sensor that can handle high enough air speed. My FrSky speed sensor is the slow one, only good to 167MPH but I also have one from Spektrum and Eagle Tree Systems. I'll look at their specs again as I've forgotten their maximums.

So let's have some numbers and I'll see how I can gather the data into something meaningful, okay? I'm open to suggestions and ideas too. If a similar thread exists, please point me in its direction and I'll not attempt to reinvent the wheel. Cool!
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Old Sep 27, 2015, 01:24 AM
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Old Sep 27, 2015, 01:25 AM
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Old Sep 27, 2015, 01:26 AM
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Old Sep 27, 2015, 01:26 AM
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Old Sep 27, 2015, 01:50 AM
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Comparing thrust efficiency is great if all were made with the same equipment and conditions. Unfortunately installed thrust will not tell the efficiency of any fan unless different fans were tested in the same airframe with the same voltage.
Good luck with collecting data that can be accurately compared. These comparisons will come with many biased opinions as some numbers come from vendors and others from modellers who are passionate about their favourite fans.
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Old Sep 27, 2015, 10:59 AM
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I realize there will be numbers 'all over the place' but let's see if there's some useful data that comes from it. Not every submitter is going to pad their numbers, right?
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Old Sep 27, 2015, 11:08 AM
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I wish you luck-- Since I've been on RCG, this has been tried multiply times.

And has never worked-- way to may variables .
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Old Sep 27, 2015, 11:10 AM
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Originally Posted by CoolerByTheLake View Post
I wish you luck-- Since I've been on RCG, this has been tried multiply times.

And has never worked-- way to may variables .
Were there threads dedicated? I couldn't locate any. Please link to one or two if you can. Thanks.
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Old Sep 27, 2015, 11:43 AM
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They were. But I don't have any links--sorry. The threads never lasted very long-- . Then the OP closed them for lack of interest, I would guess.

What 4stripes said-- is pretty dead on.. With all the different fans, and motor combos. add in all the choices of battery brands and C ratings. that's a ton right there. Now add in thrust tube length and exit diam.

As for me, I have both the Jf 90 and 120 fans, And now a few 127mm Dynamaxes. I know every thing I need/needed to know on them that matters to me. Once there installed in there respective air frame , I"m sure numbers will change. But on one of the Dynamax fans-- it will be all new territory. I'm having a custom built Neu motor made --not even sure of exact Kv yet? To go in a Y/A F-18 single, with bifurcated exhaust.
It should be interesting t see what I get on this one. And the info I do get, would be of little use to anyone but me.
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Old Sep 27, 2015, 11:57 AM
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Don't you love taking the lead when testing new stuff? I wish I had unlimited resources (not implying you do) to experiment and explore new territories.

I have a 127mm (5") fan too. Stuck a 630Kv Scorpion 4035 heli motor in the unit. The 7-blade fan is a Haoye purchased dirt-cheap but have no idea what model it could propel. That Hornet sounds like a good subject. Good luck.
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Old Sep 27, 2015, 12:06 PM
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Would that be the Dyn-E-Max /Scorpion one? I have one of those to , and an XPS Dyanamax /Hacker640Kv .. which is why I'm building this new fan -- it can only run on 11s. I need to get my butt in gear and post it in the classifieds. No need for 3 Dynamax fans.
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Old Sep 27, 2015, 12:15 PM
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Watts per pound: many caveats...

Direct comparison (Watts per pound of thrust) only makes sense for:

- same size exhaust diameter

. same thrust value

Reason being that efflux velocity is costly, roughly because the kinetic energy in the moving air is growing with the square of the velocity

So, for example, if some fan is producing 400g of thrust instead of 200g with identical exhaust, then you should expect the power requirement to be more than twice as much even if the overall efficiency stays the same.
Likewise, if you get the same thrust with a smaller exhaust, this means that the efflux velocity will be higher, and it's not a fair comparison
(going for a smaller exhaust therefore only makes sense if the fan works in a model which is fast enough to actually need the higher efflux velocity, otherwise you have a less efficient propulsion system and no benefit).

BTW you don't have to measure efflux velocity: you can think of the air as having a uninform velocity, so if you know the thrust and the exhaust area you can calculate the efflux velocity (I got that from the EDF experts).

Now there is a formula which takes the non-linear rise in energy into account so you can compare different thrust values (at the same exhaust diameter) in terms of real efficiency (of turning mechanical energy into moving air), not thrust per Watt. I remember that I saw the formula in a post by the "Don".

For illustration purposes I attach a measurement sheet by WeMoTec giving Watt, thrust, efflux and efficiency figures for some Midi Evo setups.
You might just look at the second page in the PDF: conveniently, the efficiency (eta) of the HET 650-68-1130 is the same for 10s and 11s: 59%, and we can therefore directly compare how the thrust and the efflux change with power when the efficiency stays the same (assuming that the efficiency of the motor itself doesn't change - we are always talking Watts into the motor, not Watts on the rotor).
Here are the figures given (I assume that efflux is calculated using the exhaust area. There is a value given: 49.4 square centimeters (which is not the area of the fan shroud but 100% FSA (fan swept area) and, incidentallly, this is the reason why Schuebeler calls his 90mm fans: "51" size, and the 70mm fans "30" size etc.)
10s: 2128 Watts, 33.55 Newton thrust, 74,46 meters per second efflux
11s: 2646 Watts, 38.75 Newton thrust, 80.02 meters per second efflux
Watts are rising 24.34% whereas thrust is only growing 15.5% and efflux velocity rises only 7.46%, but comparing the cubes of the efflux velocities I get an increase of 24%, so I guess power is rising with the cube of the efflux velocity. BTW for turbulent drag the situation is the same, power requirement rising with the cube of the velocity, so if you want your plane to go twice as fast you need 8 times the power since drag rises with the square of the velocity, but since power is just force times velocity, you get the velocity in "once more"...

I hope someone can provide the correct correlation formula how Watts change with thrust (in otherwise identical circumstances and identical efficiency)

After all this I want to add that - for fair comparison - the fans should not only have the same exhaust tube, but also the same inlet cone or ring (or they should be tested in the same model). It has been found that some fans, like the Change Sun types, typically perform very well with inlet cone on a test stand (no thrust tube) but won't work so well in a model with restricted exhaust (and/or inlet).
Also, static tests only show part of the truth. A fan which is stalling (due to an exhaust too small for the rotor design) in a static test might be working fine at flying speed, a bit like a square pylon prop will not unload but "bite" in flight while the static thrust will be disappointing due to stall (and amps will be lower, so there is the opposite of unloading in flight).

cheers
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Old Sep 28, 2015, 05:03 PM
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Quote:
Originally Posted by 4stripes View Post
These comparisons will come with many biased opinions as some numbers come from vendors and others from modellers who are passionate about their favourite fans.
I find this to be quite insulting, only modelers are passionate about their favourite fans? And why would the numbers be biased? Its been proven time and time again over the last 10 years that people around the world can emulate a vendors setups and get the same readings, whether its on a commercially made thrust stand or a wooden sled on bearings with a luggage scale!

If everyone sticks to same tests methods, IE on a stand with an inlet lip, no exhaust and let your numbers settle, quote all the figures with the battery size used, then the amassed figures will be quite usable, and have been very much used in the past. They might not be 100% comparable, but they give a good base line that many many forum readers have used over the years to set up their models...
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Old Sep 28, 2015, 05:11 PM
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ERC,
Please don't take my comment personally. It was not directed specifically at you.
It was a generalization and is my opinion. This is a forum and people often give opinions from their perspective.
Have a nice day!
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