SIMON CRAGG Posted June 17, 2020 Share Posted June 17, 2020 Getting myself in a tangle! For my latest project (Lb 5 low wing balsa model), I have got a Turnigy SK3 3542 - 800, Rated at 42a 625w. According to e.calc: 4s 12x6 = 582w / 41.95a 11x7= 515w / 36.8a All ok so far. So why are my figures so wildly out? 12 x 6 Dynam = 839w / 53a 11 x 7 APC = 757w / 48a The only prop that is reasonably close is 11 x 6 APC @ 666W / 42A. I am trying to stay within spec, especially the amps.Does it matter if I go over the Watts? Can anybody shed any light on why my figures are so different to e.calc please?. Quote Link to comment Share on other sites More sharing options...
extra slim Posted June 17, 2020 Share Posted June 17, 2020 Numbers look wrong to me.. Given W=V*A.. then the top set of figures have the voltage at around 13.8 or 13.9V.. which divided by 4 puts the lipo at 3.4V!!!!.. Your figures gives 15.8V which under the load is much more like it... There is probably a wrong setting in the eCalc Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted June 17, 2020 Share Posted June 17, 2020 I've found e-calc to be fairly close to what I see static, but you do need to check all the boxes - cell count, capacity and C rating, cooling level, battery state, prop make and type - the lot! It's still only a guide as battery age (eg IR), charge state, ESC timing and even the air density will affect results. Then there's the motor manufacturers tolerance and final quality control, or lack of. Bit like IC in that there are so many factors that affect performance except that with leccy you can easily measure just how good or bad your setup is and the effect of changing a prop/timing/battery etc. Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted June 17, 2020 Author Share Posted June 17, 2020 Ok thx. I have tweaked the figures in e.calc. but they are still low compared to the readings I am getting. If I stick with the 11x6 @ 42a 690w will that be ok, or do I need to get the watts below the spec. of 625w as well?. I believe its amps that kills motors, not watts? Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted June 17, 2020 Share Posted June 17, 2020 yep you should be good at 42A, it's all about I^2R, how much heat the motor is dissipating. Besides which once airborne the current will drop slightly as the prop unloads and I doubt you will need to be flying at full throttle for long periods. Quote Link to comment Share on other sites More sharing options...
PeterF Posted June 17, 2020 Share Posted June 17, 2020 When checking motors you should also check the rpm figure compared to ecalc. There are many reports where someone gets a motor of the wrong KV. A motor that has a KV 10% higher than expected will have a power demand around 33% higher (power required by a prop is proportional to rpm cubed). Normally the rpm measured is around 90% of the measured voltage x the KV as the prop load slows the motor somewhat. I have one plane where the measured rpm is higher than the measured voltage x the KV and drew a lot higher current, clearly either a mislabelled motor or some other out of tolerance build. Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted June 17, 2020 Author Share Posted June 17, 2020 Thanks Bob, understood. Does it matter if the Watts are higher? Still cannot understand why e.calc is so different. Anyway, as long as I dont damage to motor I'm happy! Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted June 17, 2020 Share Posted June 17, 2020 Posted by SIMON CRAGG on 17/06/2020 18:53:16: Thanks Bob, understood. Does it matter if the Watts are higher? I would think not within reasonable limits, in fact the specs say 4-5S so 42A on 5S would be more like 740W or so. 600W will give you 120W/lb, plenty for an average srorts model. Edited By Bob Cotsford on 17/06/2020 21:38:31 Quote Link to comment Share on other sites More sharing options...
PatMc Posted June 18, 2020 Share Posted June 18, 2020 Posted by SIMON CRAGG on 17/06/2020 16:58:23: I am trying to stay within spec, especially the amps.Does it matter if I go over the Watts? No, the winding resistance for this motor is given as .037ohms, therefore at the max current of 42A the power dissipated as heat = 42 squared x .037 Watts = 65.27W This is independent of the applied voltage. Quote Link to comment Share on other sites More sharing options...
Richard Clark 2 Posted June 19, 2020 Share Posted June 19, 2020 Posted by SIMON CRAGG on 17/06/2020 18:53:16: Thanks Bob, understood. Does it matter if the Watts are higher? Still cannot understand why e.calc is so different. Anyway, as long as I don't damage to motor I'm happy! The Watts are merely the RESULT of the voltage, current, and prop size. And the current of any given motor is dependent on the number of cells (voltage) you apply and the prop size. So given your chosen motor and your 11x6 prop all you have to do is run it at full power for a minute or so, stop it for brief intervals during that test, and put your hand on the motor to check it is 'warm' NOT 'hot'. and the thrust is 'OK'. In practice how hot it gets is far more important than the actual Watts. If you happen to have a wattmeter you can use it to measure the Watts but it's not essential. If you don't think you have enough thrust go down a cell and gradually increase the prop size until the motor gets 'warm' again. But of course you may find that you still don't have enough thrust. Tough, buy a bigger motor, you can use your present one in a smaller plane.. It's that simple. No calculations necessary. (Though I am a mathematician by trade ) Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted June 19, 2020 Author Share Posted June 19, 2020 Thanks Richard, not a method I was familiar with. I have used the Wattmeter at every stage, but not felt the motor, I have found it very interesting playing with various set ups on e.calc, and usually its not far off. From what I have learned, its the amps that are important, so in this case I am trying to get as close to 42a as possible, but never over?. Possibly sold the model to a club mate now, but after 40 years of ic, I find electric very rewarding when I get it right!. Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted June 19, 2020 Share Posted June 19, 2020 Posted by Richard Clark 2 on 19/06/2020 05:53:42: Posted by SIMON CRAGG on 17/06/2020 18:53:16: If you don't think you have enough thrust go down a cell and gradually increase the prop size until the motor gets 'warm' again. But of course you may find that you still don't have enough thrust. Tough, buy a bigger motor, you can use your present one in a smaller plane.. It's that simple. No calculations necessary. (Though I am a mathematician by trade ) I would increase the prop diameter and reduce the pitch, removing a cell reduces the max power a motor can deliver. Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 20, 2020 Share Posted June 20, 2020 Hi Simon, Don't get too despondent. I too get the same with eCalc when I compare it's results with my own static tests. Usually the current in eCalc is about 25-30% less that what I record. And that's with all the boxes filled in as accurately as possible. It's a great tool to play with though when looking at prop affects etc. Quote Link to comment Share on other sites More sharing options...
Dickw Posted June 20, 2020 Share Posted June 20, 2020 Posted by Roger Dyke on 20/06/2020 15:26:34:...........I too get the same with eCalc when I compare it's results with my own static tests. Usually the current in eCalc is about 25-30% less that what I record. And that's with all the boxes filled in as accurately as possible. It's a great tool to play with though when looking at prop affects etc. You can get the same thing in MotoCalc unless you turn OFF the option "Account for heating in motor performance calculations". With that option turned off you get remarkably accurate results. Is there a similar option in eCalc? It's probably intended to allow you to see how things perform once they have warmed up later in the flight, but I don't like or use it. Dick Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 20, 2020 Share Posted June 20, 2020 Don't see any facility in eCalc to be able to do that. It just says "all data without guarantee - Accuracy +/- 10%". Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted June 20, 2020 Share Posted June 20, 2020 Underneath the motor manufacturer selection box there is a selection which I believe is motor cooling, this defaults to medium. If you change this to excellent I would think that would equate to a fully exposed can. Using calculators such as E-calc and Motocalc can save a small fortune as you don't need to buy a large range of props to find out what sort of effect different sizes will make, or different cell counts, or different C ratings etc. Edited By Bob Cotsford on 20/06/2020 16:23:05 Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted June 20, 2020 Share Posted June 20, 2020 Posted by Roger Dyke on 20/06/2020 15:26:34: Hi Simon, Don't get too despondent. I too get the same with eCalc when I compare it's results with my own static tests. Usually the current in eCalc is about 25-30% less that what I record. And that's with all the boxes filled in as accurately as possible. It's a great tool to play with though when looking at prop affects etc. What are you selecting for the battery Full, Normal or Low, if you are comparing results on a watt meter with a fully charged battery then you should select full on Ecalc, otherwise it is calculating the results with a 50% full battery. I do find Ecalc matches up pretty close to my Multiplex and Unisens sensors, but a lot depends on the quality of the Lipo too. Edited By Frank Skilbeck on 20/06/2020 16:51:38 Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 20, 2020 Share Posted June 20, 2020 Thanks for that Frank. I'll give it a go. Quote Link to comment Share on other sites More sharing options...
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