How powerful is it?

[Myron Beaumont]

With the availability of electric brushless motors now for models ,is it maybe a fact that the spec nos etc were originally meant to inform technicians in other (industrial ? ) fields to know just what to fit for a certain purpose & that is why so many of us find the nos undecypherable / meaningless /etc ? And if so ,now that so many modellers buy them from model shops /retailers  ,Why can't they simply put the KV & other relevant info on them or on the box they come in . Or maybe I'm wrong & some do already ? I've only bought two motors & they came with nothing I could understand until I read info on the forum .I certainly never needed to know the size of magnets etc.

Sorry if I'm slightly off thread!

G-umpy

[PatMc]

Posted by Peter Miller on 08/08/2009 08:33:06:

 

The point is this, an i.c. model will have the words "for .40 to.52 size engines"  or what ever size needed on the kit box or plan. Go into any model shop and buy an engine in that range and it will fly the model.

 

Sorry Peter that doesn't stand up to examination as these two recent threads show http://www.modelflying.co.uk/forums/postings.asp?th=30327&p=1

http://www.modelflying.co.uk/forums/postings.asp?th=30724&p=1

Add to this that engines of the same capacity come in several different versions with very different power levels & in some cases rpm ranges from the same manufacturer.

For instance a model that could be well powered by an OS61FX might struggle with an OS61LA but be adequate with another engine of smaller capacity.

What’s written on the box isn’t always (or very often?) satisfactory in practice.

 

If the same to could be said for electric powered models and electric motors  we wouldn't have threads like this one.

 

This thread isn’t about recommended power as stated or omitted on a kit or plan.

The OP asked about electric equivalents of ic power.

Off course if more realistic recommendations for ic power were included with kits & plans we wouldn’t have threads like the first link I gave.

 

In fact it after a quick browse of a couple of retailers most E kits either come with a motor (how many ic kits include an engine ?) and ESC and  sometimes battery or they recommend named motors, ESC & battery size.

Selecting the recommended power train is satisfactory with most E kits I’ve seen, albeit there might be cheaper alternative equivalents available.

 

On cc vs cu.in, these days ccs are only used on vintage engines. Back when ccs were a common size denominator ccs were normally used on diesels and cu.in. for glows and spark ignition engines, this was in the late 40s and early 50s, Before that ccs were used in this country for all engines, e.i. Nordec 10cc, and cu.in were used in the USA.

 

As I mentioned above even when capacity is given & understood it isn’t always a true indicator of the power or type of model that an engine would suit. I remember my disappointment in the performance of a c/l stunter after I modified it to take an Eta 29 when it had previously flown well with a much less powerful Frog 500 .

The Aeromodeller Plans Handbook recognised that it was unsatisfactory to grade engines only by capacity & IIRC from about 1955 had a system which at least attempted to grade engines of similar suitability into “power groups”. 

Whatever uniformity in ic engine capacity nomeclature we have today hasn’t been due to a concious effort by manufacturers. It’s been driven by the fact that glow engines overtook diesel in popularity as r/c became the dominant part of the hobby.

Affordable brushless motors have been around for less than a decade & although the reference numbers may mean different things with different makes at least there is some attempt to supply meaningful figures that give some indication of attainable power & rpm range with different batteries.

 

As electric power becomes dominant the nomenclature of electric motors & the potential of various power combinations will be recognised & exploited by the majority.

No doubt the modelling equivalent of Luddites will claim things were simpler in the IC AGE.  

[PatMc]

Posted by kc on 08/08/2009 12:06:27:

Thanks for the ref to the Wot4 site.  But why couldn't you tell us directly what motor and ESC is suitable?  Why not answer the original question simply?  Why make 'clever' comments that can only confuse the newcomer?     ( I posted my comments to clarify for beginners and cancel out the false info that Mills is equal to OS40 )

 

Peter Miller is right, just put the engine size or number on the box.   I wonder if the new Wot4 ARTF has the electric motor and ESC etc shown on the outside of the box like glow motors are shown.

 

When I made the post pointing up the irony in Ernie’s Mills75 & OS40 example of uniformity it was the last thing I had read before going for tea. The page was still open when I typed & posted my reply so as far as I noticed it was going to be the next post. For this reason I failed to notice your question about the Wot4 until you repeated it.

Using the principle of not re-inventing the wheel, there was no reason to answer your question directly when the info already existed online & it was quicker for me to direct you to it.

IMO I did answer your original question simply, just a little tardily.

 

Do lighten up, KC.

You seem to be the only one not to have noticed the smiley & Ernie appears to have taken the post in the lighthearted spirit it was made. I’m sure beginners up and down the land will be tearing their collective hair out at the confusion I’ve caused them over recognising the difference between a small diesel & medium glow engine - not.

[PatMc]

Posted by Myron Beaumont on 08/08/2009 12:32:27:

With the availability of electric brushless motors now for models ,is it maybe a fact that the spec nos etc were originally meant to inform technicians in other (industrial ? ) fields to know just what to fit for a certain purpose & that is why so many of us find the nos undecypherable / meaningless /etc ? And if so ,now that so many modellers buy them from model shops /retailers  ,Why can't they simply put the KV & other relevant info on them or on the box they come in . Or maybe I'm wrong & some do already ? I've only bought two motors & they came with nothing I could understand until I read info on the forum .I certainly never needed to know the size of magnets etc.

Sorry if I'm slightly off thread!

G-umpy

 I don't think the brushless motors we use have any industrial application.

 

The kV, max current & claimed power in watts is available with most motors these days.

In all honesty I ignore the power figure look at the motor's weight instead & estimate whether it will be suitable for what I have in mind with a given battery.

[kiwi g]

Well if they can put the rating on a IC motor why not do it for a brushless . at least it gives beginners an indication of size.

oh they cant because of what  ? number of turns, size of stator ,etc etc etc.

Funny thing though.

Eflite have managed to do a brushless and matched it to the Ic equivilant

I think its a resonable question for them to be asking. that can be answered without too much confusion or attitude.

 

[Peter Beeney]

   PatMc  Thanks for the link, I will try and give this some serious study. I’ve looked at similar in the past but unfortunately my head soon begins to hurt. Whilst this is indeed convincing, I’m still very reluctant to accept that the meters can so easily be fooled. As I said, if I can work up enough enthusiasm I will have another go at it. At the moment it seems to raise more questions than it answers. Looks like I’m back on the homework shift.
       What I do have a glimmer interest in and what also might be of interest to others, is the relationship between the power in to power out. Going back to what you said in your first post, about electric input power and i/c output, (shaft) power, let’s consider this from another angle.
    Can we make a comparison like this?
    I have a Seagull 40 with a JEN 0.56 as the power plant. This is a hack model, used sometimes for test purposes and I also use it on a buddy lead, for anyone wanting experience on fast low-wingers. It really does leg it round the patch. If it’s in a good mood, on a nice day, it will turn an 12 by 6 ACP at 11900 (static) rpm. This means that on take off, weighing 6lb, it will go vertical without seeming to slow. My calculations, if I’ve got this correct, show the output here to be 1.26 BHP, or 940 watts.
    Can we establish two facts here -

      Mechanical watts are exactly the same as electrical watts,

      And, - Electrical watts = volts times amps times power factor.

    So, our Seagull 40 has a very lively performance, on a power rating of 157W per lb. A ‘true’ power. If we then substitute an el motor, with an ‘apparent‘ input power of 940W, at the same 6lb, or 157W per lb, will it still go endlessly straight up?  I think unlikely. So now we double the apparent input rating to 1880W, or 314W per lb. Now it will go vertical but that’s a total assumption on my part, just for this exercise. In fact, let us assume the performance is now exactly the same as the JEN. Thus we instantly have a multiplier, 0.5, which we can use to bring our el input power into line with our i/c output. We could call it our ‘power factor’ but for the moment that might well not be strictly true. And, indeed, most of this is entire speculation; but a direct comparison with i/c would be nice.
    Therefore if I built an electric model with a power of 500W, say 50A by 10v, and then multiplied that figure by the slightly higher factor of 0.75, that’s then 375W, would this bring me closer to the true power at the propeller shaft?
    What we are missing here is the output power figures of the electric motor; I’ve have thought that someone somewhere would have done some experiments to get a result.
    To keep in step with Ernie’s original request, perhaps it might be an idea to dream up a system whereby we can perhaps get a fairly close comparison between electric and i/c power.
    Hope you understand some of this, because I’m not sure that I do        PB
    
 

[Martin McIntosh]

All that the manufactures need to do is state the RPM for a range of props for a given battery voltage. Simple.

[PatMc]

Posted by Kiwi G on 08/08/2009 22:07:06:

Well if they can put the rating on a IC motor why not do it for a brushless . at least it gives beginners an indication of size.

oh they cant because of what  ? number of turns, size of stator ,etc etc etc.

Funny thing though.

Eflite have managed to do a brushless and matched it to the Ic equivilant

I think its a resonable question for them to be asking. that can be answered without too much confusion or attitude.

 

Who puts the rating on an ic motor & is it really meaningful ?

If you read my 1^st post in this thread I explained why.

Bringing beginners into this debate is a red herring. They don’t need an indication of equivalent ic/electric size. They are starting with no knowledge of ic power. Why would they want to learn about one power source then try to compare that knowledge with a different set of parameters when they can simply start where they intend to continue ? 

Do you mean the 1^st table here http://www.e-fliterc.com/Products/Motors.aspx  ?

Try taking the stated input Watts & dividing by the claimed equivalent motor capacity so that each stated power is per equal engine capacity. Then compare the results.

Someone’s having a laf.

 >>

No confusion or attitude just the facts.

[kiwi g]

If you read ernies first post he asked a simple question . and it was answered in the first four posts,.

Posted by PatMc  

Who puts the rating on an ic motor & is it really meaningful

 

Well lets see. umm umm   OS enya irvine asp sc jba revtech and almost all the other manufacturers oh except cox.

And it seems to have worked ok for the last 30 or more years

 

[kiwi g]

And thanks for the eflite link that will probably help more beginners than anything else on here

[Colin Bernard]

Further to the above, the eflite site mentioned includes a very useful document that gives some basic guidelines ...

 

http://www.e-fliterc.com/ProdInfo/Files/EFLDeterminingPowerRequirement.pdf

 

 

[PatMc]

Peter Beeney,

Glad you find the link useful but I won’t be studying it or anything like it. It reminds me too much of the time I wasted learning that sort of thing for work when it could have been playing with toy aeroplanes. However there is one point I may have missed. I mentioned I was surprised at the linear rise & fall off the currents shown on the 'scope. I assumed that the ESC was connected to a motor but if it wasn’t then the scan is pretty much as expected. 

 

Re the power input/output :

How did you calculate the BHP from the engine revs with the particular prop ? I have a Hyperion Emeter that has the facility to use prop constants in order to determine the efficiency & output power when measuring volt, amps, watts & rpm but I’ve not got around to trying it – too busy making & flying model aeroplanes.

Yes I’ve always reckoned mechanical & electrical power to be the same insofar as 1bhp = 746Watts.

My bible is Electrical Technology by Edward Hughes but here’s what Wiki says http://en.wikipedia.org/wiki/Brake_horsepower#Brake_horsepower

 

Remember that it’s the power driving the prop that measured on an ic engine not the power available from the prop. As I said previously our electric motors generaly are able to drive bigger props with thinner blades than the equivalent power ic motor so the conversion from different starting points to thrust is at least partialy offset.

I have two models that will climb more or less verticaly. A Veron Robot & Protech Unlimited. The Robot power is 115watts/lb using a fixed 8x4 prop @ 14500 rpm & the Unlimited 100watts/lb using a folding 8x4.5 @ 13200 rpm. Both motors are 1600kV inrunners. My Easyglider Pro does climb absolutely verticaly. Power is 137watts/lb, prop is a 10x6 folder blades on the MPX spinner drive unit which increases it to 11” dia & the pitch to 6.6” it turns @ 8600rpm. Motor is a 1000kV outrunner. The three models use 3s 2200 lipos.

Extrapolating that your 6lb model - 822watts input to an electric motor should be enough to climb verticaly.

 

As stated earlier to get a comparison of power it’s the battery that needs to be considered first not the motor. Using cells of 2200mAH capacity & drawing 12C for the reasons given, it would be possible to use Ernie’s motor on 2s to produce 167 watts, 3s 250 watts etc up to a probable practical limit of 6s 500watts. The practical limits would be the ESC’s max voltage capability & the diminishing size of the prop.

[PatMc]

Kiwi G,

I have several OS, Irvines & other motors. I can only remember seeing max bhp @ specific rpm given with OS & a suggestion of prop sizes by most if not all engine makers but never the size of prop that the rpm was achieved at. So not exactly a meaningful rating.

 

The table in the Eflite link I gave is as useful as a chocolate fireguard but of course a beginer wouldn’t know what the comparisons meant anyway as he/she would have no experience of ic

[PatMc]

Posted by Martin McIntosh on 08/08/2009 22:13:20:

All that the manufactures need to do is state the RPM for a range of props for a given battery voltage. Simple.

 You're quite right - and Scorpion Motors do just that for some of their range. Here's an example from Micron's site you can follow the link back to see the others.

Model Motors (Axi) do a less comprehensive set of figure with all of their motors plus if you use the "Fast Axi Set Up" on their home page it guides you to a suitable selection of motor, esc & battery combos.  Himax & other manufacturers also make available data & graphs online for various props & voltages with their motors.

 

 

Can't say I've ever heard or seen anything similar from an ic engine manufacturer.

Edited By PatMc on 10/08/2009 00:16:08

Edited By PatMc on 10/08/2009 00:18:06

[kiwi g]

Pat mc . theres one thing you are missing. no one said that a beginner had no experience of IC,

 he may be flying IC but looking at converting to electric ,. so he possibly knows what a 25 size 40 size 60 size Ic will do.

You are talking power related  rpm /hp . fair enough there can be huge difference in two IC motors of same capacity, but ernie was asking a question of what his motor will compare to. even gave the motor /ecs / battery sizes I do beleive. and the average answer was approx a 20 size motor.

Even timbo was game enough to a comparison.

If e flite can put a compared size on electrics that gives a motor size /prop range  ecs recomendation then theres no reason why someone else cant.

So a chart is required for those who dont understand stating IC size /electric motor / prop/ ecs/ battery.

Yes we all know that if the batteries bigger the motor will pump more power and the disscusion will go on. and on

ALL a electric beginner wants is a comparision so whenm he looks at motors he can say ,oh thats close to what I need,.

Im sure theres clever people reading this that could do that for beginners ,

On other forums ernies question would have been answered ,not disscussed.

[PatMc]

Kiwi G,

I answered Ernie's question in my first post giving justifications why no direct comparison was possible. I summed this up with a suggested equivalent ic motor size.

 

Eflite's chart is demonstrably meaningless. There's no uniform relationship between the size of each ic engine & electric motor wattage they're supposed to equal. So your beginner who knows ic inside out would be completely misled.

 

The discussion took place after the question was answered, do pay attention or I'll have your crayons & colour in book confiscated.

[Ernie]

Good morning all

 

I got the information that I asked for at the beginning of this little exercise, and thanks all for that.  (a 20?)

 

I think all this discussion is just great, because it throws up all sorts of related issues.

It also is interesting which topics really get people going.

 

Now where did I put that downwind turn

 

ernie 

[kiwi g]

so patmc is saying that eflite engine size and recomendations for there planes are wrong.

Just saw a guy in the goodbuy IC thread call a motor a 60 size electric.

Strange how so many people still compare electrics against  IC

Edited By David Ashby - RCME moderator on 10/08/2009 11:01:44

[kiwi g]

I do agree with one thing patmc. and thats the salto glider you have in your photo album.

 

[PatMc]

Kiwi G,

I'm sorry if my comment offended, it was just bit of off the cuff  banter made at 00:40 before I went to bed. I would remove it but I appear unable to edit posts after a cut off period.

However in my defence I was feeling more than a little frustrated that you were criticising my advice either without having  read fully or understood it & you seemed to have missed the point regarding the Eflite chart.

I think continuing the discussion over why the conversion comparisons don't work is becoming like flying a control line so don't feel any reason to recycle them in this thread at least.

 

Glad you like the Salto it's one of my favourite designs. Sadly the one in the pic needs major surgery to the wings. The wing joining tubes have have come loose from the spars due to age & too much aerobatic flying.  

 

 

 

 

[kc]

A lot of this is like listening to Professor Stanley Unwin trying to explain things. 

We want the bottom line ......what motors etc will fly what planes?   Where do we get this info?

 

 

 

 

[PatMc]

For the hard of thinking just follow the Axi link I posted at 00:15 this morning. 

Or if it's an e power kit model read the recommendation that probably came with it. 

[kiwi g]

Hey patmc . those will help people and so will some of the other links. people dont seem to bother to look at the tutorials because its easier to ask on the thread / hence threads like this , like I suggested a couple of days ago maybe RCME could drop some of the common trouble shooting tutorial links on the side bar as some links are possibly becoming difficult for people to find if new to the site.

It may save the same problem going around and around.

Just a suggestion team.

Perhaps just above latest posts.

[Peter Beeney]

         Colin Bernard.
            Colin,   Thanks for your link. As it so happens, this is the item I was reading some time ago, and then I suddenly felt like I should go and lie down in a darkened room! Like PatMc, I’ve given up on this stuff for the time being.
 
    PatMc, The BHP for a JEN 57 is given on the notice board as 1.8 at 17000 rpm. I transposed this into a little formula for motor calculations, but as I said, it might not be correct. However, I don’t think it can be far off the button, just judging from the sheer pace of the model. Plus the fact that rpm against bhp curves usually seem to give the best reading before the peak rpm reading is reached. If we allow for a 10% unwinding factor, that gives us a flying figure of 13000 rpm, then my guess is that this model hares around at up to 65 - 70 mph.
    I mentally tried to substitute an electric motor for the JEN. Yes, I know, I know, but I did say tried. All highly suspect.
    For a direct comparison, I reckon I need a motor that will spin a 12 by 6 standard ACP, not an E prop, at 11900 rpm static. This has to be indefinite, not in 15 sec bursts as some motors advertise. I think that will require a battery output of maybe up to 1500W but I do admit that at the moment the evidence suggests less. To be realistic I would also need a duration of at least 5 minutes flat out. The battery would have to be of such a capacity as to maintain the voltage for the full five minutes. 1500W could be generated by 25v by 60A. So this could be a 7s, 5Ah pack. Chunky!  This all may be all well wide of the mark but until or/if/perhaps I do a few more investigations I shall remain on the fence. There are a number of electric aerobatic models at the patch, Extra 300 types etc. And they fly really well, brilliant performance. But when you pick them up they are only,  like a pole dancer’s uniform, (I’m told!!), nearly there!  Hardly any weight at all. I’m all for that, always add lightness, but I think they can also be a bit fragile, even a perfect landing can wipe the u/c off. It also implies, to me anyway, that the watts to weight ratio is higher. And, at the moment, nobody seems to know too much about the weights of the aeroplanes anyway. I shall have to take my scales along.
    What we seem to need, but I’m not sure that we’ll ever get, is a shaft output figure, as measured by a dynamometer. This, I suspect, is what is used to measure the i/c engine crankshaft output. Then, to some extent, we can forget about what we are putting in.
    Perhaps I’m too cynical, but I’m not always convinced that the consumer always gets the fairest of treatments. I like electric. It’s a great way to go; and I’m sure will continue to evolve, very quickly; but maybe until we get some proper measuring sticks and datum points to make true comparisons, there might be a modicum of confusion.  
    Love the Veron Robot, that evoked a few memories of a bygone era. I made a Veron Skylane, must have been back in the sixties, Veco 19, single channel radio, can’t remember the name, and somewhere along the line I made a couple of TerryTone? rx’s to boot...      PB  
 

[kiwi g]

I am agreeing with what petes saying.

 just a broad comparision would be good

Arf electric planes are fragile. My funtana rips the under gear out on the slightest wif of touch down and Ive damaged it twice just by moving it around. . it runs a het 600/32? and 4 cell lipo 2500 If Iremember correctly. I also have the mini funtana which i dont even bother with undercarridge . every flite it comes home to be repaired. it uses a dynam motor and 3 cell .usually broken engine block and battery  mount damage.

Foamies are definatley worth investing in.....

Cheers kiwi g