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motor upgrade help please

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richard cohen31/07/2016 17:54:29
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i'm a dyed in the wool i.c flyer but decided to join the 21st century with a HK skipper and its a proper hoot but flying for over a year off water has taken its toll on the motor bearings and i need to replace it. There are no markings on the motor to pattern a replacement. model specs are:

Specs:
Wingspan: 700mm
Length: 970mm
Wing Loading: 25g/dm²
Flying Weight: 575g
Motor Thrust: 850g

its running a 6x4 prop and motor size is approx 29mm dia x 30mm length powered by a 3s 11.1 1500 lipo

I have purchased a 1400kv A2212 2826 10T as it a similar size and specs indicate it will provide around 135watts on a 6x5.5 prop but having fitted the motor, its power is marginal at best on the 6x4 or the 6x5.5. it wont take off from grass like it used to and required a handlaunch and pottered/struggled around the circuit. A larger prop fouls the fuselage so not an option but I really have to get enough power to rotate off water so do i need more Kv ( i read somewhere that an outrunner can't do much more than 15,000 rpm so 1400 x 11.1v = 15540rpm seems about right. any and all help appreciated to a 'lekky newbie

 

Edited By richard cohen on 31/07/2016 17:55:29

MattyB31/07/2016 18:59:37
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Your Kv is too low; the stock one is supposedly around 1800 and some in the RCGroups thread are using 2200Kv motors. You either need to find a 6x4 3 blader or swap the motor. You could also try a 4 cell pack, but probably the motor change is the more economical solution.

Graham Bowers31/07/2016 19:22:33
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I'm afraid your replacement motor has too low Kv. Off the top of my head, I'd have expected 2200Kv would be needed.

The actual prop speed is going to be about 75% of the theoretical Kv * Volts value. So at 11.1V the prop speed would be 11.1 * 1400 * 0.75 = 12000 rpm as close as makes no difference.

You don't say what make of prop you have, but APC are good props and publish good data, so a 6x4 at 12000 rpm may be expected to produce 0.7 lb of thrust but the same prop with a 2200 Kv motor (11.1 * 2200 * 0.75 = 18000 rpm) may be expected to produce almost 1.6 lb of thrust. A big difference!

http://apcserve.w20.wh-2.com/v/PERFILES_WEB/PER3_6x4E.dat

It has to be said that assuming 11.1 V under load may be optimistic and the only way to really know what is happening is to use a wattmeter. These are instruments that will tell you the voltage, current and wattage under static conditions and really should be in the toolbox of every leccy flier.

I'm not sure where you read that an outrunner can't do much more than 15000 rpm but its simply not true.

Hope this helps

Graham

richard cohen31/07/2016 19:31:59
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cheers guys, clearly the guff i read about 15,000rpm for outrunners is tosh ! i have found a higher Kv motor - could you advise if you think this might suit - i dont want to spend a fortune on a motor for what is a fun fly model for me

this one seems a bit shorter than stock so i would need to space it out or shorten cowl

**LINK**

 

Rich

 

Edited By richard cohen on 31/07/2016 19:45:40

Graham Bowers31/07/2016 20:02:15
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That looks more like it.

Depending on what prop you use, you may end up pulling more current than stock, so careful you don't pop the esc. I know its more expense, but I really do recommend the use of a wattmeter. If you don't fancy splashing out right now, hopefully somebody in your club could let you use one. If overpropped a leccy motor will just pull more current and can cause damage to the esc.

Graham

Edited By Graham Bowers on 31/07/2016 20:04:23

Biggles' Elder Brother - Moderator31/07/2016 20:09:59
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Hi Richard,

well if the claimed figures are good you will have masses of power with that. 280W off a 3s with a 6x5.5 prop for a model that weighs in at just over a pound is huge. But OK rotating off water does take a bit more power so its probably good to have some in reserve.

One think I would check before committing to this motor is that your ESC can handle the current - they say 28A - you need a bit of head room over that and they recommend 40A. That's mainly because that is the next convenient size up! Its worth checking though.

BEB

Biggles' Elder Brother - Moderator31/07/2016 20:10:32
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Ah - snap Graham!

BEB

Andy4831/07/2016 20:38:10
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Posted by Biggles' Elder Brother - Moderator on 31/07/2016 20:09:59:

Hi Richard,

well if the claimed figures are good you will have masses of power with that. 280W off a 3s with a 6x5.5 prop for a model that weighs in at just over a pound is huge. But OK rotating off water does take a bit more power so its probably good to have some in reserve.

One think I would check before committing to this motor is that your ESC can handle the current - they say 28A - you need a bit of head room over that and they recommend 40A. That's mainly because that is the next convenient size up! Its worth checking though.

BEB

They changed over to 40amp ESCs some time ago. Bought mine last Christmas and it had a 40 amp ESC in it (supposedly!).

Graham Bowers31/07/2016 20:50:21
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Posted by Biggles' Elder Brother - Moderator on 31/07/2016 20:10:32:

Ah - snap Graham!

BEB

So, is it a case of "great minds think alike", or "fools seldom differ" ?-))

Oh, and I'd probably try the 6x4 first.

Graham

richard cohen31/07/2016 20:52:42
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289 forum posts
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cheers all for confirmation on the motor. Its currently fitted with a Turnigy 40 amp ESC. with regard to test current draw, i was going to make up a lead to connect my multimeter to the model - do i just need to connect in line on one of the 3 motor leads or does it go between battery and ESC ? once i know the amperage draw x by 11.1 v gives me the watts - what margin is safe on a 40amp ESC ?

Andy B 131/07/2016 21:03:26
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I've got a couple of models with them 2200kv motors in them, both weigh around 500grams and they have plenty of power. Just tested one and it is showing around 240w,24 amps on the wattmeter using a 6x4 prop and fully charged 3s battery
Graham Bowers31/07/2016 21:04:44
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In the DC wires between the battery and ESC. Measure both current through and voltage across, even if separately. I'm not convinced you will have 11.1 V at wide open throttle for long, but the numbers will tell. The 3 wires esc to motor have a chopped up waveform that a multimeter will make no sense of. Being conservative I'd not want to put much more than 30A through a budget esc.

Graham

Biggles' Elder Brother - Moderator31/07/2016 21:06:27
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Put it between the battery and the ESC. TBH current is more important than watts - well for the preservation of your ESC and motor! Watts are interesting of course - bear in mind using 11.1v will only give an approximate value - its only a nominal voltage. But it will be near enough.

40A is bags of headroom - if the values given on the website are right - you'd normally aim for 15-20% or so.

BEB

richard cohen31/07/2016 22:51:03
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289 forum posts
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thanks for everyones input. just placed the order for a 2200kv motor and made up a fly lead to connect multimeter in line. hopefully all will be fitted and tested in a couple of days and ready for some seaside fun when i go to the coast in a couple of weeks

Andy4831/07/2016 23:46:33
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Hope you've got a good multimeter, most won't handle that sort of current.

MattyB01/08/2016 07:54:03
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As per above recommendation most multimeters have a DC current limit of 10A, so forget it for this purpose - you need to purchase a wattmeter.

Graham Bowers01/08/2016 07:55:05
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Is it worth popping a new set of bearings in the original motor? I know you have a new one on the way, but you'll have a spare for next time, or another model.

Graham

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