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Have I enough power?

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Ronos28/01/2020 16:45:49
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214 forum posts
8 photos

Sorry my mistake. It is a 580kv motor. DODO.

eflightray29/01/2020 10:17:34
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624 forum posts
132 photos

Some people love and trust calculations/calculators, personally I don't.

I tend to rely on experience, a wattmeter, and importantly, a tacho.

A wattmeter will protect the system from too high an amps draw, whether you are using a prop or a flywheel. Watts don't fly a model, likewise a flywheel don't tend to help the model fly wink .

But a tacho showing what rpm the prop is achieving is a very good starting point, add some experience, (keeping records), will give a much better idea of what will fly a model fly well.

Just from my own records, - 4Kg model, (63" Sparrow Hawk) - 14" x 7" APCe - 8300 rpm - flew the model really well, with plenty of aerobatics.

So to me I know what the prop rpm needs to be. The watts drawn, (amps), is almost unimportant, it's just a safety feature.

Sorry if I have stepped on a few toes, but the right prop turning the right rpm is what flies the model well.

Ray.

Edited By eflightray on 29/01/2020 10:18:46

Peter Beeney29/01/2020 11:36:14
1593 forum posts
59 photos

Exactly so. Ray, I’ve always thought along the same lines; although I’m probably even more basic. I start with the tacho and measure the unloaded kV first to make it is what it says on the tin, generally very close but definitely not always the case.

Then I can base everything else on that, relating the prop speed to the unloaded speed I can guess within reason what the current flow will be and also of course very importantly how the model will fly; that’s in conjunction to the already known wing loading, of course.

Then check for any hotspots with a thermometer and verify my current flow guess with a clip on power meter; and then if necessary some juggling with prop sizes when it’s in the air.

And as you say, perhaps not every one’s cup of tea, but then I’ve generally preferred coffee anyway…

#ilurvemytacho

PB

Ronos29/01/2020 12:43:14
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214 forum posts
8 photos

Thanks guy's, all my other warbirds are glow and if there are enough rpm there, then of you go to the field and fly. The tacho reads 8100rpm on the 14x7 APC electric prop, on my glow engines on average about the same, and they fly great, and I think they are all slightly above the weight that it says in the manual. I have run it at full throttle for the safe length of time for the lipo not to fully discharge, until it reads 3.8v on each cell and there are no hot spots, battery just warm. If it was a glow engine running 8100rpm i would be happy. Because this is my first electric on this scale just want to be sure.

Ronos30/01/2020 13:59:22
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214 forum posts
8 photos

HI all, here are my results with a wattmeter at full throttle with a 4s 5000mAh 40-50C discharge rate.14x7 apc prop

37.5A

16 VM

0.422Ah

27.55Ap

449.0Wp

6.7 Wh

could someone explain what the readings mean please.

Nigel R30/01/2020 14:31:23
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4081 forum posts
694 photos

449.0Wp is the peak power used in the system as a whole, i.e. 449 watts.

27.55Ap is the peak current draw, in amps. That tallies perfectly with the 450W peak power on a 4 cell pack.

However neither of those match up to a 14x7 doing 8100rpm, which is very odd. The 14x7 at 8100 needs almost double the power your meter is showing. I would say 450W is roughly equal to a .25 two stroke.

Just checking - it was connected between lipo and ESC?

And, did you hold it at full throttle for very long?

Piers Bowlan30/01/2020 14:33:57
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2168 forum posts
53 photos

Here is my 2p worth! You need a bigger prop;- 15x8 or bigger or alternatively 5s or 6s battery.

4s is nominally 3.7v X 4 = 14.8v, as I am sure you know, and allowing for the voltage drop under load, call it 14v. 14v X 37.5A= 525A and if the motor is (optimistically) 85% efficient it will give you 446.25 Watts at the propeller. aka not very much crying. Your Watt meter was giving you 449W at the propeller, which is pretty close to my 'back of the envelope' figures. It probably explains why the motor is getting barely warm.

Personally I wouldn't compare IC RPMs with electric flight RPMs as from my experience and to make a broad generalisation, I would expect a bigger prop and lower RPM in my electric models compared with IC although it does depend on the motor Kv and a few other things.

I will now get my head below the parapet!

Ronos30/01/2020 14:35:08
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214 forum posts
8 photos

Hi Nigel, yeah it was connected in-line and it was running at full throttle for the time it took me to write down the results.

Andrew Calcutt30/01/2020 14:40:38
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62 forum posts
1 photos

Still reckon you need six cells,1000 watts and 50 amps,personally I would want closer to 1500 watts.

Mike Blandford30/01/2020 14:57:43
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651 forum posts
25 photos

Something not quite right with those readings. You show 37.5A, but only 27.55A peak.
At 16V, and 37.5A you should have 600W.

Also with 16V in, I would expect the motor back emf to be between 13V and 14V. With a 580kv motor, 13V would be around 7500 RPM and 14V would be around 8100 RPM. Possibly the 16VM is a maximum value.

Mike

Ronos30/01/2020 15:51:12
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214 forum posts
8 photos

I appreciate all your input, and please don't take this the wrong way but, if i new what you were all taking about i might know what am looking for and know what all these numbers mean on this wattmeter. So going by the general census the top and bottom of it is, Go 6 Cell.

Ronos30/01/2020 16:11:27
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214 forum posts
8 photos

Fully charged battery at idle 16.74v, Ap 0.62. RPM at full throttle 8250. After 2mins at full throttle it reads.

Vm 15.09

Ah 1.555

Ap 37.61

Wp 609.2

Wh 24.2

worse,better,or get a 6 cell.

Phil Green30/01/2020 16:15:35
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1639 forum posts
344 photos

Ronos, please dont take this the wrong way but I would seriously suggest you put that 4.5kg scale model away for a while and get some electrics experience with a small sport model. 1000 to 1500 watts is an awful lot of power, its way, way above the average Sunday flyer's bag and if you are as inexperienced in this particular field as you say then personally I think you have a serious challenge of safety to yourself and to your clubmates. We appreciate that you're very experienced with large IC powered warbirds but electrics are different, and one-and-a-half kilowatts of electric is very very different. There are safety practises and throttle protocols that you've never had to even consider up to now - glows dont start themselves like electrics can and 1500 watts will take your your fingers off with ease, and it wont stop at that. I know you dont want to hear this but my suggestion is to gain some electrics experience with something more modest. Familiarise yourself not only with volts, amps and power, charging schedules, balancing, and more - but mostly the essential safety procedures of flying a large electric.

Thats my 2p anyway smiley

All the best

Phil

Nigel R30/01/2020 16:17:59
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4081 forum posts
694 photos

deleted, as you'd already re-run

Edited By Nigel R on 30/01/2020 16:20:41

Alan Gorham_30/01/2020 16:21:10
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1319 forum posts
146 photos

At the risk of sounding sarcastic I was going to suggest finding the Wattmeter instructions and reading those, but Phil has been a bit more diplomatic.

If you are not sure what you are trying to achieve here and can't understand the advice being given, then it won't do you any harm to do a bit of background reading.

Possibly work your way through these useful documents on the 4-max site to start with:

useful info

Ronos30/01/2020 16:31:47
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214 forum posts
8 photos
Posted by Phil Green on 30/01/2020 16:15:35:

Ronos, please dont take this the wrong way but I would seriously suggest you put that 4.5kg scale model away for a while and get some electrics experience with a small sport model. 1000 to 1500 watts is an awful lot of power, its way, way above the average Sunday flyer's bag and if you are as inexperienced in this particular field as you say then personally I think you have a serious challenge of safety to yourself and to your clubmates. We appreciate that you're very experienced with large IC powered warbirds but electrics are different, and one-and-a-half kilowatts of electric is very very different. There are safety practises and throttle protocols that you've never had to even consider up to now - glows dont start themselves like electrics can and 1500 watts will take your your fingers off with ease, and it wont stop at that. I know you dont want to hear this but my suggestion is to gain some electrics experience with something more modest. Familiarise yourself not only with volts, amps and power, charging schedules, balancing, and more - but mostly the essential safety procedures of flying a large electric.

Thats my 2p anyway smiley

All the best

Phil

Thanks Phil and no offence taken. I started of with electric foamies for example Hobbyking B17 on 5200Mah 3s, FMS BF109 on a 4s 3200mAh and a Eflite Spitfire 1400mm on a 3s 2200mAh, but these are plug and play no matching up to do. The motor and ESC for this VQ Hurricane is the one recommended by Hobby Plastics, so I assume its the correct set up, just figuring out what the best battery would be. I understand about the safety aspect between IC and Electric and have always used the throttle cut in both cases. I just fancied going electric on this size warbird and had a 4s lying about and just wanted to know if it was capable to do the job before forking out x number of ££ on the wrong size lipo.

cheers Ron

Nigel R30/01/2020 16:32:15
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4081 forum posts
694 photos

"Vm 15.09

Ah 1.555

Ap 37.61

Wp 609.2

Wh 24.2"

OK, now we're cooking on gas, well, nearly.

609 watts is still a ways under what I'd expect for that prop/rpm.

Nigel R30/01/2020 16:34:59
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4081 forum posts
694 photos

If you have a bigger prop I'd say test that.

15x8 would be my next port of call.

Or, 5s / 6s lipo, but start with a 13x6 and work up toward 1000W or so.

 

"5200Mah 3s"

If you have two or more of these, you can always run a pair in series to figure out a good prop for a single 6s lipo.

 

 

Edited By Nigel R on 30/01/2020 16:36:31

Alan Gorham_30/01/2020 16:44:34
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1319 forum posts
146 photos

I'd still love to know why you have to add 40 oz of weight to get it to balance...

Alan Gorham_30/01/2020 16:58:57
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1319 forum posts
146 photos

Having now visited Hobby Plastics website and looked at the page for this model it looks like the recommended power pack does not come with a propeller, nor does the website seem to recommend a prop. The specs for the motor recommend props from 14 x 6 to 17 x 10 so I am guessing that you have chosen a prop at the lower end of the range. I'd buy a 17 x 10, 17 x 8, 16 x 10 and try those props with the wattmeter fitted to make sure I wasn't exceeding the max current for the motor or ESC and see what that takes the RPM and wattage up to.

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