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Old Jan 19, 2014, 09:26 PM
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Exhaust outlet diameter for high blade count fans versus low blade count fans?

With the upswing in popularity among high blade count EDF units, and the increase of usage of these EDF units in PNP jets these days, I'd like to start a discussion regarding the difference in exhaust outlet size (whether that be an optimum size for speed or thrust) in comparison to low fan blade count EDF units. It seems there is a lot of discrepancy in the information between the differences in fans and the optimum outlet diameter. One side says the exhaust outlet diameter with high fan blade counts need a larger diameter outlet compared to the low fan blade count fans, and another side says the complete opposite. So, calling all in the "know" and any engineers are more than welcome so we can get down to the science of it and a valid answer. I thought of this when creating my thrust tubes, and I'd love to know more about the differences considering I also own both types of EDF units.
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Old Jan 20, 2014, 12:24 AM
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I can tell you that my ds-94 needs a condsiderably larger thrust tube diameter than the Dynamax ( but both are high blade count?? ). I also have found that the jetfan 80 needs a larger diameter than the elite 80mm.
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Old Jan 20, 2014, 11:40 AM
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It depends on the rotor design. Wemotec optimized their multi blade EVO fans for the same outlet diameters as the classic series. The blades have a different aspect ratio then the fans you mention needing larger outlets
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Old Jan 21, 2014, 08:51 AM
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Quote:
Originally Posted by bri6672 View Post
I can tell you that my ds-94 needs a condsiderably larger thrust tube diameter than the Dynamax ( but both are high blade count?? ). I also have found that the jetfan 80 needs a larger diameter than the elite 80mm.
Hmm, that is interesting, and I wonder if it's because of the reason Mr.Blobby described, the rotor design.

Quote:
Originally Posted by MrBlobby View Post
It depends on the rotor design. Wemotec optimized their multi blade EVO fans for the same outlet diameters as the classic series. The blades have a different aspect ratio then the fans you mention needing larger outlets
I would imagine that would be the primary factor as well because that's the primary variable that would be altered from fan to fan. It's interesting though because with low blade count fans, the outlet size recommended when creating a thrust tube is always the same regardless of the blade pitch, etc. However, with high blade count fans, all the information we usually see is something along the lines of, "A larger exhaust outlet is needed with high blade count fans".

Can we be absolutely positive that the exhaust exit needs to be larger on high blade count fans compared to low blade count fans, or is it the other way around, and how much larger or smaller?

This would be interesting to see in testing.
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Old Jan 21, 2014, 09:46 AM
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I think it has alot to do with pitch...a dynamax fan can run crazy small tail pipes...but it has relatively low pitch...the cs fans have crazy pitch.. Thats alot of load...by squeezibg the pipe too much the load goes way up and the blade stalls easier than a blade with less pitch...justlike a wing
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Old Jan 21, 2014, 10:49 AM
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Quote:
Originally Posted by corsair nut View Post
I think it has alot to do with pitch...a dynamax fan can run crazy small tail pipes...but it has relatively low pitch...the cs fans have crazy pitch.. Thats alot of load...by squeezibg the pipe too much the load goes way up and the blade stalls easier than a blade with less pitch...justlike a wing
Awe, that makes plenty of sense actually. Thanks man, I was hoping you were one of the people that would chime in . Maybe on my 90mm fan I just bought, I'll be conservative with the thrust tube size, and then work my way down from there. I have never tried any of the 90mm Dr. Mad Thurst/Lander 12 blade alloy combos http://www.hobbyking.com/hobbyking/s...200w_10s_.html. They aren't getting very good reviews so far, but I'll give it a whirl (even though this unit is heavy as hell, but maybe it'll help with CG when using 10S upfront). I just don't know what diameter exit I should try first on the Stinger 90. If this fan sucks, I'll just get a Tam Jets unit. I usually calculate 80% FSA for each fan I get, but that doesn't include taking into account for the higher blade counts. I usually am concerned with maximizing speed more than anything else.
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Old Jan 21, 2014, 11:43 AM
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I recently tried a 5 blade in my Habu 2 with 5lbs of thrust on a 4lbs AUW plane... climbed to ceiling WOT...

On a 6lbs thrust 12 blade 70mm WOT it would sink to floor...

Changed the shroud on the 6lbs 12 blade 70mm to a 4 stator shroud and it would climb out of my hand... it performed REALLY nice while on a low cell count too.

Seems like stators have a lot to do with static thrust... Dynamax can squeeze thrust tube tight and still get some good static thrust out of it.

Also, when I use long thrust tubes on some fans, save dynamax and smaller shallower pitched, there was just a loss in energy...

IE

LOWER static thrust

and

NO increase in efflux numbers...

I presume that would mean lower CFM too...

I'm sure the energy went somewhere but wasn't where I wanted it to go....

I've thought of just using a shorter thrust tube and put fan further back in fuse using shorter thrust tube and higher gauge wire to transport electrons..

For bigger planes the extra weight from the wire gauge isn't going to kill anything ... well see how this works out in thrust to watts here soon
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Old Jan 21, 2014, 11:47 AM
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Quote:
Originally Posted by corsair nut View Post
I think it has alot to do with pitch...a dynamax fan can run crazy small tail pipes...but it has relatively low pitch...the cs fans have crazy pitch.. Thats alot of load...by squeezibg the pipe too much the load goes way up and the blade stalls easier than a blade with less pitch...justlike a wing
Makes a lot of sense...
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Old Jan 21, 2014, 11:48 AM
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Interesting about the shroud.

Dont let the wires get too long...starts to get hard on the esc if its too long
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Old Jan 21, 2014, 12:00 PM
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Originally Posted by corsair nut View Post
Interesting about the shroud.

Dont let the wires get too long...starts to get hard on the esc if its too long
Yeap

I'm going to test with 2, 4 and 6 gauge wire and stiffing caps at the end with a slow up and down lag to prevent big burst...

The thing I think about with long wires and current is my ground pounding days... a stiffening capacitor really took care of a lot of transients

I even fly with a slight lag in throttle now, it sounds pretty realistic and my packs come down around 10 - 20% cooler...
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Old Jan 21, 2014, 12:54 PM
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On the JF 90, Rainer suggests this for exit dia.
http://www.rcgroups.com/forums/showp...postcount=1225

" Then i realized that the outlet was around 78-79mm "

That puts outlet diameter at 100% "FSA
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Old Jan 21, 2014, 01:54 PM
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Reason multi blade rotors are not suitable for high speed applications is identical to multi blade propellors: propwash. Single blade props were/are known for insane speeds at control line models where 3-4 blade props show poor performance vs a 2 blader.
For this reason the thrusttube cannot be reduced to much or the blades will stall in it's wash. Low pitched multiblades should be able to cope with small dia as CN showed and high pitched rotors stall early.

For this reason multi blade rotor is not an option for high speed record projects because they are more suitable for the lower speed range in favour for more static thrust (power=force*speed, Watts=N*m/s). For identical power multiblades needs less rpm (lower kV or less voltage) or in reversed: multiblades draw more current at same Voltage and rpm. With identical power vs low blade rotor and 80% fsa the efflux speed will be reduced in favour for thrust (in theory, because none have really thought about where behind the thrusttube is thrust generated and I bet it's not in the centre of the woosh.)

Not bashing the multis but there are limmited reasons to chose them in relation to the nature of small electric motors which are not very torqy by nature.
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Old Jan 21, 2014, 02:13 PM
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Ps,
P=F*V shows more power results in more thrust and efflux. For high efflux speed projects it is very clear the thrust/efflux ratio is far from desired but it shows oncovered potential.
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Old Jan 21, 2014, 02:52 PM
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Quote:
Originally Posted by erh7771 View Post
I recently tried a 5 blade in my Habu 2 with 5lbs of thrust on a 4lbs AUW plane... climbed to ceiling WOT...

On a 6lbs thrust 12 blade 70mm WOT it would sink to floor...

Changed the shroud on the 6lbs 12 blade 70mm to a 4 stator shroud and it would climb out of my hand... it performed REALLY nice while on a low cell count too.

Seems like stators have a lot to do with static thrust... Dynamax can squeeze thrust tube tight and still get some good static thrust out of it.

Also, when I use long thrust tubes on some fans, save dynamax and smaller shallower pitched, there was just a loss in energy...

IE

LOWER static thrust

and

NO increase in efflux numbers...

I presume that would mean lower CFM too...

I'm sure the energy went somewhere but wasn't where I wanted it to go....

I've thought of just using a shorter thrust tube and put fan further back in fuse using shorter thrust tube and higher gauge wire to transport electrons..

For bigger planes the extra weight from the wire gauge isn't going to kill anything ... well see how this works out in thrust to watts here soon
Excellent, thank you for posting this. I am definitely interested to see your findings, and would love to see more when the time comes.

Quote:
Originally Posted by CoolerByTheLake View Post
On the JF 90, Rainer suggests this for exit dia.
http://www.rcgroups.com/forums/showp...postcount=1225

" Then i realized that the outlet was around 78-79mm "

That puts outlet diameter at 100% "FSA
Wow, no kidding. Hmm, that definitely changes things a bit, as that was much larger of a recommended FSA than I thought it would be. Now that I see this, keeping the stock outlet size of the stinger 90 may be more beneficial, and if it's altered it would be ever so slightly.

Quote:
Originally Posted by Erik v. Schaik View Post
Reason multi blade rotors are not suitable for high speed applications is identical to multi blade propellors: propwash. Single blade props were/are known for insane speeds at control line models where 3-4 blade props show poor performance vs a 2 blader.
For this reason the thrusttube cannot be reduced to much or the blades will stall in it's wash. Low pitched multiblades should be able to cope with small dia as CN showed and high pitched rotors stall early.

For this reason multi blade rotor is not an option for high speed record projects because they are more suitable for the lower speed range in favour for more static thrust (power=force*speed, Watts=N*m/s). For identical power multiblades needs less rpm (lower kV or less voltage) or in reversed: multiblades draw more current at same Voltage and rpm. With identical power vs low blade rotor and 80% fsa the efflux speed will be reduced in favour for thrust (in theory, because none have really thought about where behind the thrusttube is thrust generated and I bet it's not in the centre of the woosh.)

Not bashing the multis but there are limmited reasons to chose them in relation to the nature of small electric motors which are not very torqy by nature.
Absolutely amazing answer. Thank you for typing this and your contribution to the thread. That all makes 100% perfect sense. Looks like I goofed on choosing the 12 blade 90mm EDF unit that I did for some more speed on the Stinger 90, because I should be going with a 4-5 blade unit on 10S. I would rather go for speed than thrust. The fuse on that plane is thick though, so it's doubtful it would ever reach high top speeds, but I wanted more than the stock set-up by a good deal. I intended to go 10S on it as well for give or take 4000watts. It looks like I should have a good increase in thrust compared to the stock unit (even though that's a 12 blade as well, but of a much lower power and on 6S). I may have to return the unit, and just go with a 4-5 blade unit.
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Old Jan 21, 2014, 03:23 PM
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exsactly, don't for get drag, not all jets are equal. so one set up with more thrust may actuly go a little faster due to the high drag of the modle you are flying.
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