30mm fan motor selection -- 3 cell 7000 kv v 2 cell 14000 - RC Groups
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Aug 24, 2017, 12:12 AM
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P_J_Glor's Avatar
Discussion

30mm fan motor selection -- 3 cell 7000 kv v 2 cell 14000


It has been quite a while since I was really following EDFs closely, as after buying a couple of kits I realized I would really have to up my reflexes and flying skills from 'guided glider' escapement days. Didn't want to start recycling balsa & foam into trash before giving the models an honest chance. I also had a couple of flyaways in the 'old days', as well as early 're-training,' and don't need that to be repeated.

Since that time, edf's, as well as r/c systems and batteries have gotten smaller and lighter, so I have been thinking about using 30 mm fans to replace some HiLine units from the early days, and for equipping the "stick & tissue" Comet jets.

When I started searching for 30 mm fan units, I see that they typically come with either a 7,000 kv motor for 3 cells or a 14,000 kv motor for 2 cells. The listings I saw did not provide thrust, which would have answered a lot of questions. So with higher rpm but reduced power input, am I correct in assuming that the 14000 kv units are best suited for lighter construction -- such as Dave Rees's A-10?

I also have a Hawker Hunter designed for free flight that looks like a candidate for an upgrade to a brushless 30mm fan and lightweight r/c gear -- but again, which of the two motors would be the better choice?

Your input will be greatly appreciated. Thanks!

Pete G.
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Aug 24, 2017, 07:02 PM
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AntiArf's Avatar
Lower kv should be more efficient. If you can build reasonably light I would use the lower kv version versus the higher kv version, even if the higher kv is more powerful as is sometimes the case. Building a twin currently that uses 12mm inrunner 5600kv 30mm fans that were pirated from an A10 build from years ago, around 22" span, that was faster than my ME262 with the same setup. Either of the fans I believe you are referring to should be considerably more powerful than the ones I used. I think part of the reason people use the 2s fans such as the 14000kv, is that they're afraid to run smaller 0.8A-1A linear BECs on 3s. I've used 3 servos in the 2.5 gram range before on 3s with 6-7A ESCs without issue, as of yet. A number of small subjects can be built with 2 servos, and if using 3 servos on 3s, then test and verify that the BEC stays cool.
Aug 25, 2017, 02:39 AM
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P_J_Glor's Avatar
Thanks. I pretty much grew up on Comet kits, then Guillows when they became available, so I like that type of construction.

I know that iighter models are affected more by the wind, but as I was re-learning to fly, the air was reasonably still in the local park before work.

Your A-10 looks great. I have been thinking two servos, so am taking your recommendation to heart. As I said, I do have the Dave Reese A-10 plan. Also a Chris Starleaf Fokker F-100 plan that is of similar construction. So many planes and so little time! 😊

Pete G
Aug 25, 2017, 03:09 PM
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AntiArf's Avatar
Those are nice designs. I was looking at the Starlear F100 when I built the Lear23. Spent a good deal of time at Mike's Flying Pages, where some of the Starleaf builds are at: http://www.ffscale.co.uk/star.htm
Aug 25, 2017, 05:28 PM
yank and bank!!
It really depends on many things..... lower KV motors draw less current but need higher voltage to get a given speed and thrust and that higher voltage costs significant weight.


The fun will REALLY begin if the promises of solid state lipos pan out...
Aug 25, 2017, 08:56 PM
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AntiArf's Avatar
Quote:
Originally Posted by failboat
It really depends on many things..... lower KV motors draw less current but need higher voltage to get a given speed and thrust and that higher voltage costs significant weight.


The fun will REALLY begin if the promises of solid state lipos pan out...
HV doesn't cost added weight and is actually less.
Battery weight for equivalent watt-hours (real world usable power) weighs essentially the same outside of some possible and trivial hair splitting, irregardless of voltage.
HV will simply have more cells, but smaller.

The advantages of HV, besides efficiency which can vary, are lower ESC and harnessing weight. Higher voltage can use a lighter ESC (less amps per equivalent power) with lighter harness weight due to reduced wiring gauge, in addition to using a battery with smaller gauge wiring. Those are significant weight savings.
Aug 25, 2017, 09:42 PM
yank and bank!!
edited because I'm cranky

I like HV too, but in my experience with really good brands of batteries, once battery capcity is lower than 850mAh, voltage sag at full throttle eats away the equity of lower current and higher voltage (I would like to try small hyperion cells but I don't have suitable small connectors on my wattmeter to measure


I would get the 7000 kV also, but I know I can't get a battery with capcity and weight commensurate to the theorhetical lower current suitable for a plane that can use a 30mm fan... it'll simply need a bit larger battery
Last edited by failboat; Aug 25, 2017 at 09:55 PM.
Aug 25, 2017, 11:54 PM
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AntiArf's Avatar
Not sure what you mean by the "once battery capacity is lower than 850mAh, voltage sag at full throttle eats away the equity of lower current and higher voltage" statement.

Lipos decrease voltage linearly for all practical intent and purpose, during their discharge cycle. Quite different from many other types used before them, such as NiMh, NiCd, and lead-acid.

If you're talking about the seemingly higher voltage sag of a 3s lipo versus a 2s during discharge, then that's all relative and equates to the same thing, in terms of overall power. Sure the 2s has a lower "total" voltage sag, but it has to supply proportionately more current to the higher kv motor by means of linear mathematical relationships I=V/R and power=VI, given equal power levels. That implies that the potential to deliver power that degrades during discharge is the same for both setups assuming equal efficiency, given both batteries have equal watt/hour ratings and are supplying equal power motors but with different kvs.
Last edited by AntiArf; Aug 26, 2017 at 12:00 AM.
Aug 26, 2017, 07:38 AM
yank and bank!!
I was speaking about using a smaller battery in general. And yes, I understand wattage and the fact that a 2S power system draws about 50% more current than a 3S system for the same input power.


I have several 450mAh 3S batteries from several of the better manufacturers. All of them sag much much much worse than somewhat larger batteries. So my point is I'd go with the 7000kV fan, but I also know that in many instance it's likely that I won't be able to use a 33% lighter battery because C rating becomes even more of a lie for sub-600mAh sized batteries. Been there, measured that.
Aug 26, 2017, 05:54 PM
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AntiArf's Avatar
My point was to make a real 2s vs 3s comparison, you have to have an apples to apples comparison, or at least as equal as possible. That implies that both setups have fans/motors with similar efficiency and power rating, and where both batteries using the same manufacturer's brand of cells have equal total watt/hour rating. They will essentially weigh the same amount, while one is 2s and the other is 3s. At this point we're no longer concerned with whatever C ratings they slap on a label, as the actual cells are made up of the same material with the same properties. Again gotta be apples to apples.

What happens is that the HV lipo has smaller cells, but due to operating at lower current levels the cells (while smaller) are not taxed for as much current as the lower voltage setup demands from it's fewer but larger cells. When all points are made equal, the lower voltage setup will not have an advantage, since the voltage will end up sagging proportionally more due to higher current demand. Again the voltage drop may look less, but you have to look at it in terms of actual power, or V x I. Sure someone can use a heaver battery on the lower voltage setup, but if you make the total battery weights equal from the same manufacturer's cells, you won't have an advantage from the lower voltage battery.

As for C ratings in general, they often are a total joke. Usually though you'll find much of the inconsistency comes when people look at the mAh rating and C rating, without taking into account the actual weight of the battery and it's individual cells. Some of the best small lipos I've had over time are older TP lipos. Around 15C versus some today that are over 20, but when you compare actual weight, they become closer. The newer lipos play games such as derating the mAh, so that they can give a higher C rating. Again to make all things equal, actual weight has to be taken into account.
Last edited by AntiArf; Aug 26, 2017 at 06:12 PM.
Aug 26, 2017, 06:57 PM
yank and bank!!
I agree with the vast majority of your contentions; I just meant that the smaller the battery capacity is, the less likely that it will even be able to produce the Whr in some applications...... I would be willing to bet money a 350mAh 3S battery cannot produce the same Whr as a 500mAh 2S battery from the same MFR in challenging applications because there is a limit on real world energy density for particularly small battery capacities.

Now are the fans we're talking about challenging enough to bring this issue to a head? testing with instrumentation and flight tests are the only way to know. 7 amps is a lot less current demand than some of the hv experiments I have tried.... right around 16C for a 450mAh battery (yes I know C ratings are crap but the terminology can be used to conveniently speak about battery loading).

I am impressed with the thrust results others have gotten with the 7000kV version of this fan and may get one
Aug 26, 2017, 07:04 PM
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Bare's Avatar
I now have 3 of the 7000 kv 30's None exceed 8 amp at WOT, on 3s...in my basic testings
My 750 mah 70 gm 3s batteries exhibit zero issues delivering those 8 amps.
Sure these are heavier than my 17 gm 300 mah 2S which would likely have issues at 4 amps.
Hardly news there.
Performance uses energy.. period. Going slower works, for a lower energy cost, if that's the primary criteria.
We all have differing world views.. mercifully.
Can only suggest to Build according to one's hopes.
Aug 26, 2017, 07:23 PM
yank and bank!!
How is that aluminum tape holding up? How long have you run these fans now?
Aug 26, 2017, 07:49 PM
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Bare's Avatar
Only did the ali tape on the motor bell on the First one. Got brave or lazy.. with the others
Run times ? about 10 mins so far. Not a lot, but long enough to check for problems.
Happily No explosions.. so far.
Bad rotor setups though:
.. the motor to rotor ali rod/tube adaptor MUST (imo) be ali tape wrapped/shimmed as the rotors' bore to the adaptor diameter
is a problematically sloppy. fit. It can be rocked in the hole., so concentricity is down to the grip of the teeny tiny retention screw.. ie: barely
Not a one off either Mine are All exactly the same.
Someone was clearly napping on the CNC machine setup.
Aug 26, 2017, 11:10 PM
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AntiArf's Avatar
If you ran smooth for 10 minutes with no vibration, I'd be amazed if there are any future problems. Just the rotor fit issue that you mentioned and corrected could be enough to cause major problems. I haven't ran mine yet, but will make sure to check and correct the adapter issue that you found. Thought about a small ring of heat shrink tube on the bell, if I have a size that would shrink down. Seems the aluminum tape would also provide the added bit of strength for assurance also.


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