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What size electric motor?


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I have inherited a package of electric motor A2212/10T 1400KV, with  8x6 props.  I wonder if this would be sufficient, with 3s battery,  to power my vintage Little Plank flying wing, designed as a glider, with, at the time, the option of.049-.10 ic motor power?  Other specs- 56" wingspan, with wing area 673sq in.  Weight approx 2lb.

 

Or would it require a larger combo?  Any thoughts appreciated!

Little Plank Photo.jpg

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You have missed out one important factor from the motor's specification - the wattage!

 

Phil G (of this parish) came up with a far simpler formula than those often prescribed for calculating the desired wattage of an electric conversion, namely 100 watts per cc. (Interestingly, this formula also works for helicopters, unlike most of the others!)

 

Your model was designed for 0.8 - 1cc motors, so around 100 watts would be ideal. Most motors in the .8-1cc range would run a 6x4 or 7x4 prop. Your motor seems to aim at an 8x6, which would normally be fitted to a 3.5cc (approx) motor. This indicates to me that your proposed motor is over the top for the model!

 

Having said all that, one of the beauties of electric motors is that they don't seem to mind being under-propped - unlike their i/c equivalents. The maximum rpm is governed by the kV rating, provided they are not over-loaded. 1400kV on a 3s pack gives a theoretical max rpm of around 15-16,000 rpm. If you run it on a smaller prop, then that should reduce the current draw (and hence wattage) to an acceptable level.

 

Provided the weight of the motor and battery is not excessive, and it physically fits, you should be OK. Just don't fit an 8x6 prop! Use something smaller! Maybe a 6x6 or 7x6?

 

--

Pete

 

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A quick check with eCalc indicates that an 8x6 prop (Cam Carbon folding) should be OK, pulling around 20 amps at WOT. However the pitch is a bit high and the blade will be stalled when the model is static, a drop to 8x5 should solve this. Would be useful if you could give more details of the motor or post a picture.

image.thumb.png.80d9ec73f56334ff1e6a0bde6489d5c8.png

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Thank you for your input guys- I used to be a know-all with ic engines (for 40 years), but I am now, with great difficulty, coming to terms with these new fangled electric motors- and escs- and battery combos!  (I think that 0.1 cu.in. would be nearer to 1.5cc, fwiw.  I would rather more, rather than less power- I can always throttle back)

Here's a photo of the motor/ prop combo, which I understand, is a Banggood package:

A2212 10t 1400kv motor- 8x6 prop.jpg

Edited by Tosh McCaber
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Ok, found it, discontinued Banggood motor, unfortunately it doesn't say how many amps it's rated to but it does say an 8x6 prop recommended for 3s. Might be worth rigging it up on a test stand with a receiver esc and battery and trying different size props to see how hot it gets or whether it burns out. Some of this type of motor are only rated to 10 amps continuous so start with a 7x4 prop and see what happens.

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Bottom line in my opinion, is it'll be marginal at best, and I'd go for something more powerful, however it depends on your aspirations. Here's my rationale:

 

I reckon it's probably one of these, as I have a couple but with a lower speed wind.

https://www.altitudehobbies.com/products/suppo-2212-10-1400kv-brushless-motor-park-400-equiv

That roughly agrees with the very rough "2 to 3 watts per gramme of motor" rule that you can look up for yourself.

I do recommend using a wattmeter for testing purposes when deciding what prop to use. Although another rule of thumb is that the motor will realise a speed of 70 to 80% of the Voltage x motor Kv . You can then look up the APC propeller performance data to give an idea of the likely power consumed.

 

The spec is 180W max, however 16A for only 60 seconds. For continuous operation you're probably talking 120 to 140 Watts or so.

 

If the weight of your model is 2 lb , and the motor / esc / battery will add another 5 ounces (if using a 1000 mAhr 3 cell LiPo) it's going to be marginal at best, taking in to account the watts per pound of model "rule" https://www.rc-airplane-world.com/watts-per-pound.html

 

For a sanity check, I'm using the 1000 Kv version of that motor with a 9x4.7 prop on a depron extra with an all up flying weight of about 1 pound. It'll do basic aerobatics but won't do unlimited vertical. Useful flight time is about 8 minutes.

 

Hope this helps.

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Many thanks for that analysis, Graham.  I'm getting the feeling that the motor may be marginal for the model- I'd rather use one a bit too powerful, where I can throttle back!

 

Now, I do (did) have a wattmeter, but it's not where I remember it in my workshop from a year ago, when I last saw it!  Looking just now for a replacement, GT Power seems to be a choice- (I'm sure that was the brand that I had) but there are various versions advertised- ranging from £12 on ebay- to £27 from model shops.  As well,  there are a lot of comments that wattmeters of all brands seem to be of dubious accuracy.

 

Can anyone out there make recomendations?

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Amazon have a good selection of wattmeters, I use the one they currently list for £9.99 cheap and cheerful.

I recommend soldering on bullet connectors to the leads when it arrives. Then make yourself some changeable ends so you can test XT60 Deans JST etc etc

 

LINK

 

Edited by Keith Billinge
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Thanks Keith.  As you say- cheap as chips!  Just ordered one- it'll be here vis Amazon Prime on Saturday.  I can afford to lose it if not successful.  (Having read some of the comments). 

As a matter of interest, although it's rated at 200A  (a few queries about that on the Reviews section!), what's the approximate maximum current that our largest rc aircraft model power systems would put out?

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2 hours ago, Tosh McCaber said:

........................

As a matter of interest, although it's rated at 200A  (a few queries about that on the Reviews section!), what's the approximate maximum current that our largest rc aircraft model power systems would put out?

Well over 200A, but you are moving into very specialist territory there.

200A will cover probably 99.9% of normal RC use 😀

 

Dick

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I asked for one of these for my last birthday: https://cpc.farnell.com/tenma/72-2985/clamp-meter-mini-200a-ac-dc/dp/IN07620 . It is not as cheap, but it does DC current without needing to plug it in to the power train. Most clamp meters are AC only, but this one is DC as well. I have found it really useful for modelling and for other electronic projects, and it seems good even at low current. I used it this week for checking 20mA for LEDs.

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All I would say Tosh Is: Don't leave that watt meter out in full bright sun, its got the sort of screen on it that will be damaged by too much direct sunlight. (Ask me how I know)

 

My biggest ESC is 80A so I've never tested anything near the 200A limit of this particular meter. But for your average club flyer this is just the ticket!

 

Keep it in your flight box and it'll be fine.

KB

Edited by Keith Billinge
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Hi Tosh, I suspect your motor’s suitability will become clear when you run it with your new wattmeter attached.

I have a similar (?) Hobbyking Turnigy 2826/10 1400kv motor which indicates 150W, pulling 15A, on an APC 7x5 prop. This was after running it for 60s. On the fully charged (and well used) 1350mAh LiPo it was initially indicating 170W incidentally. At the end of the 60s full power run it was barely warm and I never normally fly at full throttle for 60s!

There was once an oft quoted ‘rule of thumb’ that 100w per lb would fly most sport models, 150W an aerobatic job and 200W+, DF. 
Your plank is a lightly loaded glider so I would try your motor and see, but then maybe it depends on how you want to fly it. 👿 

good luck.

 

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My wattmeter has arrived!  I have firstly checked it out on my little Eyelash model. (See my separate entry:  https://forums.modelflying.co.uk/index.php?/topic/24961-forum-members-new-models-lets-see-them/page/169/&tab=comments#replyForm) 

I'm reading a peak 18.75amps, and 214 peak watts on the wattmeter.  Now, I'm running on a 20a esc on the Eyelash  Am I in danger of burning the esc out, although the peak amps would be only when the plane is at full throttle, which, of course, is only a percentage of the flight (8 minutes on my timer)?  Maybe 30 amps to be safe??

One thing that I'm still not sure about is the dangers of motor burn-out.  When does that happen??

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If it was me, I'd go up a size to 30A and I do routinely under-run my ESC's.

 

I imagine the ESC has a BEC circuit that is powering the receiver and servo's, so if you lose the ESC you don't just lose propulsion, you lose control - I've seen it happen, but not to me.

 

The instructions for the ESC may well say it's rated at 20A continuous with, for example 25A for a 10s burst. Any you may well be OK.  But for a few more £ and grammes I wouldn't risk it, considering the potential consequences.

 

I've never seen a motor burnout however I suppose they happen. My personal view is that checking the motor temperature after the flight, and smelling it, would give sufficient indication of impending problems.

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I concur Graham.  30a will be a sensible solution, since, along with the cheap and cheerful motor on the Eyelash, Bangood provided in the package, the present ESC (unmarked, other 20A on the cover)!

 

Now I have the Wattmeter, I shall use it to test out the motor/ESC/battery package for my Little Plank- I'll update the thread as and when.

 

Anone have a motor burnout?

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I find all this stuff fascinating. The Frsky kit I have has the capability to measure and record the lipo  voltage and currrent from sensors on the model, I just need to get it done. Then I'd have real data on static full throttle and in flight, including throttle stick position. A hark back to a past life, in which I was a test engineer on off road machinery, which only flew for fractions of a second 😉

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19 hours ago, Tosh McCaber said:

My wattmeter has arrived!  I have firstly checked it out on my little Eyelash model. (See my separate entry:  https://forums.modelflying.co.uk/index.php?/topic/24961-forum-members-new-models-lets-see-them/page/169/&tab=comments#replyForm) 

 

I'm reading a peak 18.75amps, and 214 peak watts on the wattmeter.  Now, I'm running on a 20a esc on the Eyelash  Am I in danger of burning the esc out, although the peak amps would be only when the plane is at full throttle, which, of course, is only a percentage of the flight (8 minutes on my timer)?  Maybe 30 amps to be safe??

One thing that I'm still not sure about is the dangers of motor burn-out.  When does that happen??

 

Unless you are regularly flying at full throttle for long periods I think you will be ok - remember that 18.75A is with a fresh pack, it will only get less as the battery is discharged. Also remember that the prop "unloads" in flight anyway, meaning current draw will almost certainly be lower (pretty much always true unless the prop is stalled during static tests which can give you some confusing results).

 

Re: the motor burnout point, obviously you can look up the max burst and continuous currents for the motor if you can find them, but those figures are often unreliable. Instead for the cheaper brushless motors most of us use, then (assuming installed in a standard "tractor" model with average cooling) go with the 3W/g rule of thumb. For your Eyelash that means that if you motor is anything less than ~70g it is probably likely to be under stress at the wattages you are flying at, though obviously it is influenced hugely by cooling, flying style and throttle use. 

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Firstly, as MattyB says, the motor unloads inflight so the max amps will be less, maybe 15A not 18.5 perhaps? If you don’t have adequate cooling the heat tends to build up throughout the flight. At the end of a flight the motor should be warm to the touch. If it is too hot to touch it is too hot! Cooling is essential so make sure there are not only air entry holes around the motor but exit holes too, further down the fuselage. Likewise, if the motor smells hot 🥵 that is the smell of the insulation around the windings beginning to melt ☠️.

Not all ESCs are born equal. A good branded one may be more reliable than an unbranded no frills one, but how do you know? I tend to be rather conservative and like plenty of headroom (30a ESC with a motor pulling 20A ) probably a bit OTT, heavier and  more expensive 😳.

So your 20A ESC is likely to be fine Tosh!

 

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