john stones 1 - Moderator Posted November 30, 2015 Share Posted November 30, 2015 Always a conundrum for myself choosing the motor prop combination, I have a Turnigy SK3 3548 1050kv it's a little on the powerful side but my thinking is prop it down on a 4c 3300mah lipo and 60 amp esc? then i'll get longer flight times and cooler running, prop will be figured out using the watt meter, I have some APC 14x7 and am guessing a little over 620w ? all guess work so far though. I have been using these SK3 for a while now and they've been very good. Feel free to chip in if my thinking's wrong. John Quote Link to comment Share on other sites More sharing options...
Glyn44 Posted November 30, 2015 Share Posted November 30, 2015 Hi John, I'm a fairly new builder, 2 models,to,date, and would gomdown the electric route but I would need a pointer or two, on how to best convert from ic, as I have never done it. So I shall watch this thread with interest! Cheers Glyn. Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted November 30, 2015 Author Share Posted November 30, 2015 There will be plenty of advice coming on set ups and mods for lipo installations Glyn, i'll be asking for reassurance myself re electrics. You'll be fine, looking forward to plenty pics though John Quote Link to comment Share on other sites More sharing options...
Steve McIntosh Posted November 30, 2015 Share Posted November 30, 2015 Following as well. First proper build and also going electric Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted November 30, 2015 Share Posted November 30, 2015 If, unlike John, you are not looking to use some existing kit and are therefore starting with a blank sheet as it were, then the best place to start is with the weight. Peter gives the weight of Ballerina as 71oz - so that is: 4.5lb as near as makes no difference. A plane will fly on about 50W/lb - but only just! 100W/lb is generally held to be a reasonable 'sports flying' level of power 130-150W/lb for good all round aerobatic performance More than that and you are in hooligan territory! Now I like to have a bit of power in reserve, and I do like my aerobatics - so I personally will aim for 150W/lb - plenty of power. You could redo these calcs at a lower level if you want something a bit more docile. So, going with 150W/lb and it weighs 4.5lbs - so our total full throttle power needs to be 150x4.5 = 675W. So John's estimate of 620W is pretty well in agreement with this. (What's 55W between friends ) Next up we need to work out the current at max power. Assuming we are using 4s - I doubt we'd get anything bigger far enough down the nose to balance. A 4s has a nominal voltage of: 14.8V. Power equals current times voltage so: 675=14.8 x I - where I is the maximum current. That gives us that I = 45.6A. So we need an ESC and a motor that can take that current. John's selected motor at 50A max looks OK, a bit close maybe? But OK. The real issue - as he says - is with the kV value. We want the motor to be knocking out around 10,000rpm at full chat. Given a supply voltage from a 4s of 14.8V we really need a kV of about 10,000/14.8 or 675. A bit lower than John's motor which is at 1050. John will tackle this to some degree by under proping. But I'll have a look around to see if I can find an alternative for myself - I don't have one of those SK3's on the shelf so I can afford the luxury of looking for a 50A 675kV (-ish) motor. For the ESC I would agree with John's suggestion of 60A, to give some headroom over the 45A max current. Regarding batteries - we've said a 4s. A 3000mAh capacity would give us 4mins at full throtlle - but given we have loads of power "in hand" in this set up full throttle is only going to be used for short bursts in verticals etc. So we'd probably get about 8mins comfortable flying time out of that. If we could go to around 3300 or 3500 then pushing 10mins should be possible. So, there's a basic spec - I'm off to browse round the shops to see what I can find BEB Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted November 30, 2015 Share Posted November 30, 2015 A Master-class in choosing an electric power train there from BEB..... I would just add to keep an eye on the prop size....I would think the Ballerina is probably fitted with a 10" or 11" prop when powered by an IC engine....fitting a 14" prop to an electric motor might mean you run out of ground clearance.... Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted November 30, 2015 Share Posted November 30, 2015 So, I've found this motor - max current 50A and kV720 that should do us. Cost £22 - (wait for the discount box!) For an ESC I'm looking at this one. At £24 not the cheapest - but a good ESC. I'll hold on batteries for now until I have examined which of my current stock will fit in the nose! But there we are - that's the set up I'll use. I'm sure others will be along with their suggestions to. With electric there is never just one "right answer", lots of different set ups will work fine - this is just one. BEB Edited By Biggles' Elder Brother - Moderator on 30/11/2015 19:10:16 Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted November 30, 2015 Author Share Posted November 30, 2015 Cheers BEB, I also have a NTM 3548 950kv max amp 55A, however both can be left in the box if better choices are there, just the Yorkie in me trying to save money I may as well be the first to say it, if flying lecky buy a wattmeter John Edited By john stones 1 on 30/11/2015 19:20:13 Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted November 30, 2015 Share Posted November 30, 2015 Absolutely agree with the Watt meter! We can do all these calcs but all they tell us really is how much power we want and what kit could deliver that power without the magic smoke coming out of the wires! But whether it actually will deliver that power depends very much on what prop you put on there - and you need to use a Watt meter to check! You can use these calculators that you can down load online. But to be honest I tend to find that the best they can do is give up a starting point. In previous electrical conversions I have done I've known them to be 10-15% out easily. So my advice is the only way to be sure is to use a Watt meter and test the setup. BEB Edited By Biggles' Elder Brother - Moderator on 30/11/2015 19:15:29 Quote Link to comment Share on other sites More sharing options...
IanR Posted November 30, 2015 Share Posted November 30, 2015 I follow your sums, BEB, but can I ask - where does 10,000 rpm for the motor come from, and, will this alter if we use Steve's 10" or 11" prop or stick with John's 14x7 prop? Thank You. Ian Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted November 30, 2015 Author Share Posted November 30, 2015 My motor, props etc are just what I have in stock at the moment, please don't take them as recommendations, I posted for input from others more experienced and you learn from the replies given. John Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted November 30, 2015 Share Posted November 30, 2015 Can someone give me a link to the plan please, I cant seem to find it...... Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted November 30, 2015 Share Posted November 30, 2015 It's the free plan in this months RCM&E Dave.... Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted November 30, 2015 Share Posted November 30, 2015 Ahhh!!! thats why I cant find it on myhobbystore!!!! Have to go to Smiths tomorrow then Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted November 30, 2015 Share Posted November 30, 2015 As for electric set up, I will be doing it in Depron so aiming for a AUW of about 3lb - so something around the 500W I guess on a 3S set up.... Might well see what I have in the shed with a KV of around the 1000 ish mark so a smaller prop can be used Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted November 30, 2015 Share Posted November 30, 2015 Posted by IanR on 30/11/2015 19:27:40: I follow your sums, BEB, but can I ask - where does 10,000 rpm for the motor come from, and, will this alter if we use Steve's 10" or 11" prop or stick with John's 14x7 prop? Thank You. Ian 10,000rpm is just a typical max revs for a medium size model aeroplane. If you look at 40-60 size engines you'll see that around 10,000rpm is representative of max revs - its the same for electric. If we spin the prop slower than this there is no real problem but we are not getting all we could out of the prop. If we go much higher than this we start to run into regimes the prop wasn't really designed to operate in. If we go very much higher we might even run into safety problems because centripedal forces in the prop may be higher than it was designed to withstand. So 10,000rpm is a good target. In practice if we get anywhere between 8 and 12,000 that will be fine. Looked at from the other angle though things are a lot more complicated. Put simply (this is isn't absolutely true but its a good explanation of the basic concept) the motor will try to reach its kV value at full input throtlle. So, if we have a 675kV motor supplied by 14.8v it will try to reach 10,000rpm. It won't get there because of loading issues - that's the complication I'm over looking - but lets just say that it does try to do that. Now it will try to do that whatever prop is mounted on the front and how much power it will need to spin that prop at 10,000rpm. Suppose its a tiddler of a prop - say 8x4 - well no problem, spinning that up to 10,000rpm won't take much power - let's say 300W and that will only need 300/14.8 amps - about 20A. The motor is rated up to 50A so as I say no problem. We are underworking the motor somewhat - and so lifting a heavier motor than we need for 300W - but that's a different issue. The plain fact is there would be no electrical problems with that arrangement. BUT,...if I were to go daft and say put a 17x10 on the same motor what will happen? Well it will still try to reach 10,000rpm at full throttle and that will need a lot of power - maybe 1200W say. That would require a current of over 80A! and that is exactly what the motor will try to draw. But its only able to withstand 50A - something has to give - the motor will burn out. So the moral of the story is: you can't do any harm by putting too small a prop on an electric powered model - you just don't get as much power is the system could give you. But you can do very great harm by putting too big a prop on an electric powered model - becuase it will still try to spin it at the same speed and draw too much current. So what we do is design the nominal system, as John and I have done above, to be able to deliver the power we want. Then basically we "guess" at the prop size - based on previous experience - and run it all up very carefully on the ground with a Watt meter in the circuit. If at full throttle I'm drawing around 650W (which is basically my target) I know that prop is spot on. If I'm drawing a lot less than that, I know I can go up a bit in prop size - maybe an inch on the pitch or on the diameter - and try again. On the other hand if I reach 650W at 3/4 throttle, I immediately back off and I know the prop I've fitted is too big and I must come down in pitch or diameter otherwise I am going to burn the motor or the ESC out. I wish I could give you a magic formula for prop-size on electric models - but the truth is its a lot of "suck it and see"! BEB Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted November 30, 2015 Share Posted November 30, 2015 As BEB observes there is a lot of "guestimation" in getting a good electric set up.....you will quickly find that a bag of different props to try are an essential requirement. Make a few assumptions...do a bit of maths & try it out on the wattmeter. Then tweak the prop size to get the figures required. Unless your initial "guestimate" was a mile off the set up can usually be "zero-ed in" with a different prop or three... Quote Link to comment Share on other sites More sharing options...
IanR Posted November 30, 2015 Share Posted November 30, 2015 Thanks BEB. I've copied the above and put it into my RC folder for future cogitation. Ian Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted November 30, 2015 Share Posted November 30, 2015 IanR your biggest ally is a Watt Meter..... essential kit Quote Link to comment Share on other sites More sharing options...
PatMc Posted November 30, 2015 Share Posted November 30, 2015 Sorry BEB but I think that the explanation of the relationship between RPM, supplied voltage, Kv & load in your last post is either ambiguous to the point of misinformation or just plain wrong. A 675Kv motor connected to a 14.8v supply will turn at 10000rpm with no load & take a couple of amps. Fit an 8x4 - the rpm will drop & as you say the current will rise to a moderate figure, possibly around 20A. Fit a 17x10 - the rpm will drop drastically with a correspondingly drastic increase in current beyond the motor's limit. The point being that with a given KV & fixed voltage the max rpm will always reduce according to the magnitude of the load. It is this reduction in rpm that actually causes the current rise. Getting back to the OP, I've been flying a Ripmax Druine Akrobat for a couple of years until I managed to rip the u/c out of it earlier this year (It's in the winter repair & modify schedule) For those unfamiliar with the model, it's a semi-scale aerobat along the same lines as the Ballerina. Mine weighs 68 oz & flies very well with 4s 2650 lipo, a 60A ESC, an E-max 2826/06 950 Kv motor fitted with an APC 10x7 prop. Figures with this combo are 14.83v, 31.73A, 470W, 10800rpm. I also tried the model with a couple of other props before deciding that the 10x7 gave the best all round performance : APC 11x5.5 prop - 14.56v, 38.78A, 564W, 10200rpm. APC 11x7 prop - 14.47v, 39.79A, 575W, 9900rpm. It's also worth mentioning that mine isn't the current version of the Emax 2826/06 motor & that I measured it at an actual 850Kv as against the stated 950Kv. Sweet point to get max power without overstressing most motors seems to be when they are loaded to produce around 80% of supplied volts x Kv at full throttle. If the motor can't reach near to this figure it will tend to overheat, if it revs much higher then it will run cooler but is being under-used. Quote Link to comment Share on other sites More sharing options...
Geoff S Posted November 30, 2015 Share Posted November 30, 2015 I agree a so-called watt meter is an essential piece of kit for electric flight set ups but really all we're interested in is current. Knowing current it's easy to work out the power (watts). The meter just takes over that small chore. We have a few F3a competitors in our club and they all fly with expensive set ups ... and none of them have a clue as to how much current their motors draw! I'm sorry I won't be taking part in the mass build because the Ballerina looks a lovely model. I have a lot on my plate right now. I have a high performance 2 metre electric soarer to build - one designed by a fellow club member called Thermal Magic and a Dennis Bryant sixth scale SE5a which will also be electric - probably 4S but could be more. Even in retirement, time is still short Geoff Quote Link to comment Share on other sites More sharing options...
Peter Miller Posted December 1, 2015 Share Posted December 1, 2015 As point of interest for comparison purposes, My OS FS40 Surpass turns an 11X6 prop at around 9 to 10,000 rpm Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted December 1, 2015 Share Posted December 1, 2015 Posted by Geoff Sleath on 30/11/2015 23:45:07: I agree a so-called watt meter is an essential piece of kit for electric flight set ups but really all we're interested in is current. Knowing current it's easy to work out the power (watts). The meter just takes over that small chore. True up to a point Geoff but without a wattmeter or an extra voltmeter in situ you don't know the watts because you don't know what voltage your battery is supplying. All batteries experience a voltage drop under load....granted this likely to be small & hence the difference in watts between the actual voltage & the nominal voltage will be small but, more importantly, you don't know how well your battery is coping. If the current drawn is pushing the voltage much below 3.5 volts per cell I'd suggest that your battery isn't coping & will soon puff up & possibly die. Without a wattmeter or voltmeter you simply wouldn't know this.... Wattmeters are so cheap these days it just doesn't make sense NOT to have one...... Quote Link to comment Share on other sites More sharing options...
Robin Etherton Posted December 1, 2015 Share Posted December 1, 2015 Another thumbs up for the explanation BEB. Suddenly this electricary starts to make sense!👍👍👍👍👍👍👍 Robin Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted December 1, 2015 Share Posted December 1, 2015 PatMc - yes you are correct. Which is why I went to some lengths to point out that the explanation I was giving was a simplification that didn't take loading into account - you clearly missed that bit! I was trying, I think sucessfully, to get the concept across to people for whom it is unfamilier territory. It is a well established practice in teaching to simplify in order to communicate a concept. - I know I have been teaching for 30 years! Refinement of understanding comes later. BEB Quote Link to comment Share on other sites More sharing options...
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