More advice on Wot 4 motor?

[Clifford Stone]

OK, try to keep this short, but . . .

I have had some success with the Wot 4e balsa version as described by the instuctions; 2820/05, 3 cells, 11x5.5 . . . However, one had the naging feeling more power was needed as the 'old rough edges got knocked off'.

Time for the 4 cell 14.8v battery . . . wow!!!! Thats more lik it . . . two or three minuites in and I did my Red Arrow bit, turned on the smoke! Yep, fried the 50amp SEC. Postmortem, 11x5.5 draws 52amp, the cooling flow is not good IMHO on the woody W4 and I mounted the 50amp SEC high in the fuslarge, easy to get at?

OK, I got it wrong, on the positioning of the ESC, I chanced my arm with the 14.8v 4 cell with out checking with a watt meter. I might have got away with it if I had done a better job of throttle management . . . god it does move with the 14.8v battery . . . I mean fast and vertical, phew!!!!

Re group, lick wounds, two new ESC's on order, 50amp + 20amp burst and 85amp + 15amp burst, both with 'serious' heat sinks. I can induce more airflow and mount the SEC in a better position, waiting for delivery.

Now have a watt meter!! . . . All cleane up and rewired, I have a 40amp SEC, so I did some prop tests, very carfully!

3 Cell, KMS 2820/05

APC 12x6e 41amp - 452W

APC 11x5.5e 31amps - 346W

4 Cells:

APC 12x6e frightening!!! botted it!

APC 11x5.5e 52amps - 700W

APC 10x6e 49amps - 730W

. . . I have an 11x7e on order, might work on 3 cells but will the pich be to hight for any vertical performance?

. . . I favour the 4 cell rout, but with an appropiate prop, ESC match, however will the 'watts' be to much for the moter, spec., says rated at 400-500W.

Alternative, use a lower kv motor, I have a 2826/06 reated at around 730kv, but the watts are still 400-500w. there are higher watt ratings avaliable.

The alternative idea of gong to the lower kv is to take advantage of the bigger pro/torque they seem to offer? My Wat 4 has 8" of ground clearance so I can siwing up to 14".

Am I reading it wrong, will the big watts from the 4 cell, 14.8v cook the motor? . . . The spec., says the 2820/05 and the 2826/06 motors can take 14.8v's . . . confused?

Sorry, not as short as I would have liked . . . CJS

[Concorde Speedbird]

Errr, Saito 45s four stroke?

Sorry, no idea what any of this electric stuff means.

CS

[Clifford Stone]

Yep Speedbird, I'm the same, come from a background of many thosunds of hrs with ic. Long lay off, nearly 20 years, to return, no oil, no noise, not even any building if you dont want to. In the short time from posting to repy I have half answered my questions, only half mind.

Digging on the net helps, make a few mistakes, nothing changes, listen, learn . . . I will crack it!

CJS

[Turbycat]

one of our members has got a 80 size 4 stroke in the front of a Wot4 . I've got a Jen 57 2 stroke in mine running a 13 x 6. That goes well . No idea about the electrickery stuff. Tried it with a Ripmax Akrobat and a couple of ST foamies but the only electric plane I have left is my Mini Panic, and I'm not sure what set up I've got in that any more apart from the fact it uses 3 cell batteries!!

Andy

[David Davis]

Right Clifford, I'm far from being an expert on electric flight or even much of an enthusiast for the practice, but as a general rule of thumb, 100 watts per pound will fly the model in quite a spirited manner. Vintage models will fly on 75 watts per pound or even less and rates above 100 watts per pound will give you even more performance.

My "proper" kit-built WOT 4, foam wing, Irvine 46 up weighs 5.25 lbs, so I'd be looking at 525-750+ watts to fly a WOT 4 with an electric motor. I'd say you need a bigger motor to go with one of your new speed controllers but as I said above, I'm no expert.

Over to you.

[NigelH]

David, it looks like you're talking about the full-size model while the thread is about the 4 version.

[Clifford Stone]

A main concern using the motor I have, would it 'blow up' using the 4 cell, 14.8v battery? Digging into the net, I have found a few spec listings where 'Watts' are giver for 3 and 4 cell operation, 3 cell in the 400-500 area, and 4 cell 700'ish. These listing were not for my motors spacifically but sugest that there are two 'watt factors' depending on the cell count?

As the 2820/05 motor is sold clearly stating use up to and including 14.8v, I presume the 400-500w spec., is for 3 cell as a middle for diddle figure and that 700w seems usualy on motors running 14.8v, or am I reading this all wrong?

CJS

Edited By Clifford Stone on 28/07/2012 10:52:54

[Clifford Stone]

Titan speed controller arived this morning, 50amp continus, 70amp for 20 second burst! Its built like a tank, eclosed in a metal case with a large finned heatsink, not the smallest ESC, or the lightest, but I have plenty of room. I can now make a few cooling air flow mods to the fuselarge.

I thinking that I might get away with the APC 11x5.5 prop; 52amp, 700w draw? extra cooling + I'm told there is a 10% drop on amp draw in the air? This time I will be a bit more mindfull of throttle management . . . right thinking????

CJS

[PatMc]

Clifford, for any motor the current is the limiting factor, ESCs have a current & voltage limit. You need to balance prop size with voltage used to stay within the motor's current limit.

For instance if a motor has a 50A current limit & the prop size was changed at each voltage to keep to this limit, then using a 3s for 11.1v battery would give 555W & using 6s for 22.2v would give 1110W.

[Peter Beeney]

PatMc - I’m just musing here, and I’m not wishing to interfere in any way, this is just me rambling on about motors in general, but would it not also be fair to say that if provided we can arrange to keep the motor cool, or perhaps that should be cooler, with a good through draft of air, we may be able to increase the current flow. Albeit not a lot, perhaps; and it might need monitoring in the first instance, to ensure we haven’t overdone it. This would also have to apply to the ESC, too, this might also accept some increased current, but if we tried to give it too much welly in terms of voltage then I agree, it might still throw in the towel anyway.

Also in respect to your straight forward explanation of the watts consumed. Just to carry this a stage further, I would consider that if we wanted to increase the voltage but maintain the amps flowing at the same rate, we would have to increase the revolutions proportionally. To do this we then also have to reduce the torque proportionally, too, i.e., decrease the load. In our case it means reducing the size of the propeller, maybe to the point where, although it’s now spinning quite fast, it’s perhaps becoming too small to be fully efficient, and so the model performance will drop off anyway.

I do fully appreciate your quote was just an example to illustrate the current limit, but it also does seem to emphasise the fact that the amount of watts used is perhaps not always necessarily indicative of what sort of result you can expect from this.

And from what I’ve seen of some of the small electric models, arranging some appropriate air flow also might not be that easy. I’ve not taken that much notice, really, but I guess in some cases you might be able to spend some considerable thought on this. But it might be well worth it in the long run……

PB

[PatMc]

Peter, I was trying to keep it simple to make the point that for any motor the current is the limiting factor not the voltage. I did mention that the prop size would determine that current at varying voltages.

[Peter Beeney]

Pat, yes indeed, I’m quite happy with what you are saying. Certainly the prop size will be all when it comes to the correct load for the motor. However, a tiny problem I have is that I reckon that the power in watts consumed by the motor is just dissipated in heat from the motor. So if in your example with a 3S at 555W the motor is getting up to a certain temperature, the correct one, if we double the voltage which will increase it to 1110W it’s going to get much hotter. Even though we’ve reduced the prop size to increase the revs to maintain the current at 50A. So now if we want to maintain equilibrium and the lower temperature level and just retain the watts to the 555 level, we also have to reduce the current as well, so that means fitting an even smaller prop…… to increase the revs still further……. Which is where I came in, really, thinking that it might be beneficial if I can improve the cooling……

I know this is all hypothetical, and that you were trying to keep it all as simple as possible; and you would never get into this sort of situation in the first place! But it seems to me that in any practical cases I would need to cover all the aspects. So does it become necessary to get into it at a bit of a higher level? Certainly from reading about some exploits and also witnessing at the strip on one or two occasions some folks electrical ‘accomplishments’, it’s quite easy to get into a burn-up situation without realising it.

Think I’ll still go along with i/c for a while yet…

PB

[Clifford Stone]

OK, I,m getting the picture . . . I think? So, I thought look at the problem from the old fashioned point of view while I try to get the head around this 'current' thing that magicly turnes into 'amps and watts'? Oh boy . . .

To me, an old ic pylon racer, motor, battery and ESC cooling on the woody W4, IMHO, is not good enough . . . not if one wishes to put a real sparkel in its performance? . . . just my opinion.

So I have got the tame woodworm out

1 Original bulkhead:

2 After the woodworm had performed:

3 Giant woodworm created an extra hole in front of the UC, feed directly to the ESC and becaus of the fusalarge shape, faces forwards by a few degrees:

You will note, 6 extra holes in the cowling and the air scoop has been enlarged. The motor gets a good supply of air from the spinner ring, the clearance is the same size as the motor cooling holes and the distance between spinner and cowling is almost 10mm, the thickness of the prop drive boss.

Next job, do the watt?amp test again, and re look at the motors that quote watts for 3 and 4 cell, I dont like this compromise on power. We are not flying a Piper Cub here . . . I'm 65, going on 25 . . . CJS

 

 

Edited By Clifford Stone on 28/07/2012 19:50:00

[PatMc]

Peter, the heat generated = current^2 x resistance. The applied voltage is opposed by the back EMF. We are considering the case where the same current is maintained at 11.1v & 22.2v by varying the load therefore the different RPMs will have generated back EMFs that are equally different from the applied voltage in each case.

For example an Axi 4120/14 has an internal resistance of 41 mohms. If it was loaded to 50A the heat generated would be 50 x 50 x 0.041 W = 102.5W regardless of the applied voltage.

[PatMc]

Christopher, if this is your motor then according to Ripmax it's rated at 45A for 60 secs & has an internal resistance of 41mohms (same as the AXI) Therefore loaded to the max current it will generate about 83W in heat.

IMO a motor with lower KV to enable you to use a larger prop would have been more suitable for the performance that you want.

[Peter Beeney]

Pat, I think that’s exactly what I’m saying, too. If you maintain the current at a constant value the watts will remain at a constant level but in your example of a 6S you’ve increased the watts to 1110. This may be ok but if it’s not we have to reduce the current by increasing the back emf by fitting a smaller prop. Thus the voltage seen by the motor could then be at the 3S level.

In the case of the Axi if it’s loaded to 50A regardless of the applied voltage then to achieve this we have to adjust the back emf by adjusting the load, fitting a different prop, so the the same voltage is actually seen by the motor. I think the secret is actually in your statement, ‘if it was loaded to 50A’ . To to this, that’s maintain the 50A, if we applied different voltages, then we also have to change something else, too, the back emf, so that the motor always sees the same voltage.

Would it help if I did a description of how I think a motor functions and then we can kick that to death, but it might help establish a few facts on the way. If that might be acceptable we could perhaps start a thread in which this can be chewed over at all levels. I’m not sure if this is much of an idea or not, but it might be able to shed some light on the mysteries of motors. Or perhaps it will just add more confusion!

PB

[PatMc]

Peter, I don't see any mystery.

I've always specified maintaining the same current by adjusting the load to suit the supply voltage. So long as the current is the same, it doesn't matter what that the voltage is or what the power being consumed is, the heat dissipated in the motor will remain the same at current squared x motor resistance Watts.

When the voltage is doubled with a reduced load to maintain the same current level the motor "sees" that increased voltage but because of the higher RPM it generates an increased back EMF that results in the supply voltage minus back EMF being equal in both cases. In the example I gave this difference would be 2.05v regardless of the supply voltage

Edited By PatMc on 28/07/2012 22:34:45

[Peter Beeney]

Pat, again yes I agree, but if you keep the current always the same the voltage and the resistance will have to stay the same. Thus the power consumed will be just one value. You cannot alter the power without changing the voltage or the current, or both. Thus the power, in watts, will stay the same. You quoted - ‘When the voltage is doubled with a reduced load’ and this is exactly what I meant when I said previously about reducing the prop size to increase the revs, thus increasing the back emf and reducing the current. If you don’t reduced the load the power will rise and thus the heat dissipation, too.

And I’m convinced this is exactly what is perhaps happening in some cases. If the voltage is increased, by using a higher voltage battery pack, without changing the prop, it’s most likely the I squared R value will also increase, in fact because of this mathematical formula, if the voltage is doubled, the power goes up by a factor of four, a situation that might not have escaped the notice of some pilots, as the smoke starts to flow steadily from the front of the model when they’ve tried to get a touch more power from the standard arrangement. Just by fitting a prop size that is too big, the motor is slowed so that the back emf becomes less and then the applied voltage effectively become greater and so the power increases can probably do the same thing.

So in Clifford’s case, I would consider that he’s certainly going in the right direction and ensuring a good flow of air. Don’t overdo it, though, and weaken the nose area too much, a motor ripped itself out of a model at the strip the other day, just after take off… … and so far we’ve not found it…

PB

[Clifford Stone]

Posted by Peter Beeney on 28/07/2012 23:50:41:

So in Clifford’s case, I would consider that he’s certainly going in the right direction and ensuring a good flow of air. Don’t overdo it, though, and weaken the nose area too much, a motor ripped itself out of a model at the strip the other day, just after take off… … and so far we’ve not found it…

PB

I have been mindfull of the possible weakening of the front bulk head, there is one point where I did think the wood worm had been a little close to over enthusiastic, fortunatly I was able to epoxy a triangle brace on the inside of the motor mount to more than compensate. It is all fairly sound, a full flat bulkhead is generaly over enginered and will take what is in effect lightening holes . . .

It amazes me, with lazer cutting technology, the only air flow hole on the W4 woody is no bigger than a special issue stamp, most of which is blanked off by the battery! Poor thinking . . .

Lots more cooling, propper throttle management, we are close to the top limit, but not over . . . ???

CJS

[Peter Beeney]

Clifford, that sounds like a very good idea. However, when you talk about proper throttle management there might also be another little situation you might like to consider.

One very important characteristic of the electric motor is the fact that it has maximum torque at start up. The reasoning goes a bit like this, at least in my opinion.

When the motor is at rest, and the battery voltage is then applied, because there is no back emf this voltage is at it’s maximum; and therefore the maximum current will flow. If your motor has a resistance of 41 milliohms then a 3 cell pack will result in 11.1V / 0.041 = 270 amps. Then the I squared R figure become 270 by 270 by 0.041 = approx. 3000 watts. Or we might just transpose the formula to V squared divided by R to get exactly the same figure, but slightly faster. Dividing by 746, as that equals one horsepower, we get a figure of about 4. So there could be a twisting motion momentarily applied to the woodwork by the motor trying to turn against the prop of up to 4 HP. If we do the same trick with a 4 cell pack we will have 7 hosses briefly stampeding around on the loose!

This situation does not last long of course, as soon as the motor stars to turn the current starts to decline. But there is no doubt the the electric motor develops a lot of torque on start up, so they can accelerate very quickly.

Regarding the poor thinking about the design and the air cooling holes etc., I’m afraid I’m rather cynical and so I might slightly construe it differently; to some extent, anyway. I’ve said many times that I think the model trade in general is perhaps not entirely adverse to the odd disaster or two, or maybe even a slightly more frequent mishap or more, so this might not be the casual approach that it seems. But, as I said, that’s only me looking at the downside, it’s probably just simply that they don’t consider the power train very much.

Anyway, the very best of luck with it, I must admit, I’m sometimes not adverse to zipping models around at full chat myself! I do like a bit of pokey occasionally……

PB

[Clifford Stone]

Peter, you old cynic you . . . you are sounding like I often feel

Regarding the maths, if only I understood them . . . However, understanding the maths or not, I do apreciate the instant torque of electric motors, its amazing the amount of power there is there in that instant.

Its this 'torque power' that I wish to use, rather than out and out speed. hence I will be looking at the equivilent motors (physical size) that have a lower kv figure, something like kv 900-1000? Swing a larger prop with low'ish pitch = a much more efficient prop disc. I'm thinking 13" or even 14", the problem is finding the right pitch in 'e' type props. An 11"x4" or 5" pitch would suit my existing set up, no such animal in an 'electric thin blade', the 11x5.5" just about fits? Unfortunatly its all about mathamatic formuli, no one seems to look at things in the old fashoned way or my way of 'gutt feeling' . . . I have always believed 'there is more than one way to skin a cat' . . . ? Figures are slightly off perhaps, but the end result is what I want . . . Oh for a 12x4"e prop?

My general feeling about the Wot 4 is that its simple lines seems to lull people into the thinking of a poodling 'trainer' . . . nothing could be further from the truth, with a bit of 'thought out umph' up front, it can be a real performer, not too precise but great fun to fly.

As far as the thinking of poor cooling air flow . . . Wot 4 obviously has the 'ic stamp' all over it, and so it should, thats where most of us first encountered the joy of flying a Wot 4. Sadly, IMHO, that ic thinking has not been well adapted . . . time over again or if I built??? assembled another, I would do things a lot different . . . hindsight is a wonderfull thing

CJS

[NigelH]

I know what you mean. The new ARTF Acro Wot doesn't come with an electric power option in mind. Very short-sighted, in my opinion. A large proportion of the new 40-size models from other manufacturers come designed for electric power (Seagull even redesigned the Harmon Rocket 3 with this in mind) as well as glow.

[NigelH]

BTW, knowing the model / flying style and estimated AUW generally leads to a fairly easy way to choose an electric power system, with some associated research. Although not as easy as just picking, say, a 46 glow engine or a .60 glow engine, it's not a big deal to find something reasonable.

E-Flite brought out their 'Power' series motors to fit in with the glow motor ratings and other, generally budget makers, have followed on. For example, an E-Flite Power 46 or a Turnigy G46 would be good choices for a model also designed foir a .46 glow.

[PatMc]

Posted by Peter Beeney on 29/07/2012 09:25:01:One very important characteristic of the electric motor is the fact that it has maximum torque at start up. The reasoning goes a bit like this, at least in my opinion.

When the motor is at rest, and the battery voltage is then applied, because there is no back emf this voltage is at it’s maximum; and therefore the maximum current will flow. If your motor has a resistance of 41 milliohms then a 3 cell pack will result in 11.1V / 0.041 = 270 amps. Then the I squared R figure become 270 by 270 by 0.041 = approx. 3000 watts. Or we might just transpose the formula to V squared divided by R to get exactly the same figure, but slightly faster. Dividing by 746, as that equals one horsepower, we get a figure of about 4. So there could be a twisting motion momentarily applied to the woodwork by the motor trying to turn against the prop of up to 4 HP. If we do the same trick with a 4 cell pack we will have 7 hosses briefly stampeding around on the loose!

Peter, happily this is not the case or there would be more electric model fires than electric model fliers. And an electric motor doesn't produce max torque at start up.

Due to the inductive reactance of the motor windings & frequency of the pulses from the ESC the current could never reach anything like the figures you are quoting even if the motor was completey stalled.

Looking at the results from an Eagle Tree logger there is no current spike at start up.

Re the overheating - IMO keeping the current to within the specified limit for 60 secs then allowing a reasonable period before applying full power would be more effective than increasing the ventilation.

Christopher, if you want to use a larger dia prop & have more power with the same weight motor then you need to increase the cell count & find a motor with lower KV.

[Clifford Stone]

Posted by Clifford Stone on 29/07/2012 11:48:05:

Peter, you old cynic you . . . you are sounding like I often feel

. . . . An 11"x4" or 5" pitch would suit my existing set up, no such animal in an 'electric thin blade', the 11x5.5" just about fits? Unfortunatly its all about mathamatic formuli, no one seems to look at things in the old fashoned way or my way of 'gutt feeling' . . . I have always believed 'there is more than one way to skin a cat' . . . ? Figures are slightly off perhaps, but the end result is what I want . . . Oh for a 12x4"e prop?

CJS

I have tracked down a sourse of 13x4"e and 13x6.5"e APC props, plus a Turigy D3548/04 quoting 50amp, 910w,1100kv and 2.15kg of thrust, thats the sort of power I measured in some of my pylon racers. Not so keen on the kv figure, but it is obviousy built to take a bit of abuse??? There is also a new Turigy 70amp ESC. The 13" props can be trimmed as required . . . a bit of my old fashoned modeling coming to the surface . . . .

I'll have a word with the supplier in the morning about the motor.

CJS

Edited By Clifford Stone on 30/07/2012 01:51:19

Edited By Clifford Stone on 30/07/2012 01:52:05