glow engine to electric motor comparisons

Hi all,
Ive been modelling for years and used to work for a model engineering company. I say this to show that I can model, but what I would like to
know most of all is what size electric motors replace what glow engines?
I'm new to rc modelling and have decided to go down the electric route as it seems to be cheaper and cleaner, but I dont know what size motors to buy (all the buying will be done online) when the kits say the need such and such a size glow engine? I'm looking at a small C130 which needs 4 x .25 glow engines. A smaller version needs 4 x .051 glow engines.
Does anyone have a conversion chart or formula that explains how to come up with the right sized motor ?
Thanks
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go to www.flyhurricane.com and look in the Forums. They have some really good explanations of how to arrive at the correct size electric motor.

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Thanks Jim...
It makes a bit more sense now, esp with their motors, but Im still a little confused...
I hear the terms 280, 370 and 400 etc with regards inrunner motors. Is there any way of quickly converting between these brushed and brushless motors?
Karl
Jim Slaughter wrote:

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snipped-for-privacy@rhodesfamily.co.uk wrote:

Not exactly.
As I explained in the last post, its a can of worms that is a delight for those who enjoy technical detail, and a right royal pain for those who just want to bolt something on and fly it.
In *general* a motor of a given size and weight is *broadly* comparable in power input and output to another similar sized motor. BUT outrunners are multipole devices..they develop their power at lower RPM and have more torque. So the prop choices will be very different unless the inrunner is geared.
Also, efficiency makes a deal of difference. I think it was Matthew Orme who said that the actual MOTOR is more equivalent to an IC CRANKSHAFT. Its only PART of the solution. The pack is usually the heavier part, and really its almost better to think of the pack as being the starting point. Motors of high efficiency can develop enormous power for their size as well. Since the limitation is heat in nearly all cases, a motor that is 100% efficient can deliver any power you want without getting hot.
Now motors of up to 90% efficiency are definitely available. If you take a given size at 90% efficiency, and compare it with a motor at - say - 70% efficiency which is about the 'average' the 90% efficient motor can handle THREE TIMES the power of the 70% one..IF it has a big fat battery pack to supply the juice.
In most of my models the pack is the dominant weight, and cost. I start from there really.
The algorithm I would use with a model like the C130 is this
Find out the airframe weight and the wing area.
That enables a stab at power and stall speed to be obtained.
Now select a pack and 4 right power sort of motors and add that weight in and see if it still makes sense. Going for about 60W/lb for a model like that.
Being as how its big and slow, then find a gearbox and prop that draws the right amount of power, and has a static pitch sped at least twice stall speed..maybe 3 times. That will give a good flight pattern with enough thrust to get it off the ground, and into a decent climb, but still able to be throttled back for an efficient and scale like cruise.
I would use Motocalc (www.motocalc.com) to do these early guesstimations.
Once the power train is fairly well arrived at. I would get a range of props and a meter, and confirm the power levels, and a tacho to confirm RPM.
If all is within reasonable limits, I'd then fly the thing and try a few 'close' props in flight to find the best ones.
If it were me doing the conversion on a 4x .25 scale model, I would actually choose cheap old fashioned brushed motors - probably speed 600 or car race motors (buggy motors) as you only need one cheap speed controller - brushless motors need one per motor - and suitable deep ratio gearboxes (probably 3:1 or 4:1)to get RPM down to about 4-6k and then use a large diameter very coarse pitched propellor - probably a 4 blade Varioprop, somwhere in th 9-11" diameter and similar pitch area.. - to get somewhere near scale appearance.
Current is going to be massive..something like 25A per motor so the speed controller will be around 100A capability, and those sorts pf motors run well off about 12v, so I'd probably go for a 8000mAh 3s LIPO pack (around 11v). Not a cheap item.
That should fly a model up to around 20lb AUW, though I'd say it would be happier down around 15lb.
However it would NOT surprise me if the actual model couldn't be made a lot lighter and run on a lot less. I hate seeing scale models of large lumbering slow aircraft rushing around the sky like fighter planes and sounding like them too. You don't need massive structures to mount electric motors on, and neither do you need to build them like tanks. And they fly much more realistically and float in to gentle landings if you keep them light.. They can't of course fly in more than modest winds, but that is the price you pay for scale appearances.

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the inrunner referral to 280, 370, 400, etc. goes back to the original brushed motors that were rated that way. The brushless motors are a quantum leap over the old brushed. The problem with inrunner motors is they are very high RPM and very low torque so you hve to use a very small prop on them or use a gearbox. The Outrunner motors have tremendous torque and no need for a gearbox. If possible, stay with the outrunners.

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snipped-for-privacy@rhodesfamily.co.uk wrote:

Basixally it goes roughly like this
Take the engine capacity in cu inches, and multiply by 2000
That's about what WATTS you need.
Its crude, because a geared electrc motor running a big prop slow will need less than that on s slow flying model, whereas to replicate a high revving tuned pipe you will need more. but as a starring point thats a fair rule of thumb.
So being that your C130 is fairly slow and probably WOULD use either gears, or a slow revving outrunner, you wouldn't need 4x500 watt motors on it, but more like 4x300W.
A better way to go is to use wttas per lb.
A tanky old C130 needs only about 50W/lb to lumber round the sky successfully, so if its is say 15lb it would need a total of 750W, or about 4x200W motors.
THEN you need a tool like Motocalc to work out what gears and props and pack to use. Or ask someone else who knows.
The problem - or the advantage - of electric is that there are engines and gearboxes that will operate efficiently over a MUCH wider range of RPM than a Glo engine. This means that prop selection is non trivial.
For example. perhaps the motor I have used the most is the $5 speed 400.
That will run a range of props at around 90W in/65W out from - say - a 4x4 right up to a 11x11. Depending on how its geared. And what voltage its used on.
With electric you cannnot think
get right motor stick one of 4 props on it go fly.
You have to think in terms of the whole powertrain..prop,box, motor and pack.
An interesting example for you..60" span WWI biplane. Scale prop is around 20"
Weight (estimated 4-6lb) suggests it should fly on about 300watts - about 1/3bhp shaft after losses, so something like a .15 glo equivalent.
Calculations suggest that the prop needs to turn a shade over 2000 RPM with a pitch of about 22 inches.
Show me a .15 or .25 engine that can do this!
You would PROBABLY put a 60 or 90 4 stroke in that to swing that size of prop (20x22). I am putting in a thwacking gearbox and a much lighter electric motor.
Contrast a tuned pipe 36 or so, able to swing an 8x8 at 18K RPM and propel a small model at 140mph. THAT actually NEEDS the RPM and the power, and you are looking at over 1500W of electric to replicate it.
So, we have two extremes..in the first case our slow biplane with a huge prop, we can use a 300W motor to simulate a 90 4 stroke, in the second we need a 1.5KW motor to replace a tuned pipe 36 2 stroke.
There is about a 15:1 difference in 'what electric power equals what size of IC engine'.
If you are truly interested in the detail of all this, I suggest you take your question over to e.g. rcgroups. Where we can go into it with pictures, diagrams and calculations that simply cannot be done here.
Either in the power http://www.rcgroups.com/power-systems-13/ or scale http://www.rcgroups.com/scale-electric-planes-10/ fora.

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Wow, Can open, worms everywhere!!! :)
Thanks for taking the time to go into so much detail. It is far from the simple conversion I thought it might be. I'm not too afraid of the math as I like to work stuff like this out to be honest, but I really do need to know what is available to me in terms of equipment - youve mentioned gearboxes etc which I had no idea existed!! Once I know the formula and other details such as the watts, pitches and rpm etc I should be able to work this out 'fairly' simply.
I'll have a look at the forums youve suggested.
I suppose I ought to ask about props while Im here...
Eventually I'd quite like to build a B-17 (quite a large scale one) and would like to know if triple bladed props are available? Also what do the numbers on the props mean? My park flier has the numbers 8 x 3.5 on them. What on earth does this mean?
Can you tell how green I am at this yet?? :)
The Natural Philosopher wrote:

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snipped-for-privacy@rhodesfamily.co.uk wrote:

No greener than me when I re-started the hobby..only about 7 years ago..
8x3.5 is the diameter and pitch of the propellor. The puitch is how far it would advance in a sort of ideal situation for every revolution.
The 'pitch speed' is the pitch times the RPM. It sets an upper limit on the model speed. Experience tells us that it needs to be at least twice stall speed, and 2.5-3 is better, or you end up with a model that no matter how hard the engines rev, will barely stay in the air.
If you really want to get to the bottom of all this I have some serious recommendations.
First, take out a free subscription to RCgroups and ask the silly questions there. Ignore smart alecs who give you a hard time.
Secondly go to www.motocalc.com and download the 30 day evaluation copy and play with it. I'd advise buying it outright - its not very expensive $39. Its better than any book and most of the algebra is done for you.
Thirdly, the other two items you really need if you want to stray off the beaten track of 'use this, it worked for me' is a decent tacho, and a 'whattmeter' The original Astro Whattmeter is a device that goes between the battery and everything else., and measures voltage and current and watts, and indeed how many mAh you have drawn..and tells you what you are pulling. Or at the least some form of decent clamp on DC - I stress the DC - current meter is almost mandatory. In an ideal world none of this is necessary, but one mistake and you can blow and expensive motor, pack and controller. It pays to KNOW you are safe. Other even better whattmeters are out there. Some download data to a PC which is useful for keeping a record and drawing pretty graphs and teh like.
Finally, I'd say that doing all the above taught me a lot, taught me how little I knew when I started, and how little the average glo pilot knows, or cares about how his stuff actually flies the plane. Not fast enough on a 25? Stick in a 40. You have three props: a 9x9, a 10x6 and an 11x4. That's it, Try all three and keep the one you like.

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Remember, static readings of pitch speed don't mean much. When the airplane gets in the air, the prop/motor will 'unload' and you will get far better performance than the static numbers.
wrote:

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Jim Slaughter wrote:

More absolute bollox., Static readings of THRUST don't mean much because when the airplane gets in the air the prop/motor will unload and you will get far *worse* performance than the static numbers. Electric motors of high efficient do not show much RPM increase at all with plane speed. A *100%* efficient motor would stay at the same RPM regardless of airspeed..its speed is dictated solely by the applied voltage.
I repeat, rules of thumb learnt and grandad's knee flicking gassers DO NOT APPLY to electric.
What happens with an electric is that the thrust decays, (and the current drawn) more or less linearly as the pitch speed is approached becoming zero at (or around) the pitch speed - depending on how this is defined.
If the pitch speed is no greater than the stall speed the model will *never* fly on its wing. At best it can be persuaded to flutter around in a nose up attitude like a hummingbird on methedrine.
No matter how much *static* thrust it has.
This is an unlikely condition with an IC engine, because the power and RPM regime dictates that only a really fine pitched prop would show this sort of behaviour. Its an extremely likely scenario with electric models, because they are designed to swing far bigger props slower.
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whatever you say. Where do you get your information? Mine comes from a very sophisticated company called E-Power Test Labs. That use all sorts of dyno equipment to produce static results and then telemetry to measure 'in the air' data.

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Jim Slaughter wrote:

Oh, a degree in engineering and electrical sciences..the experiences of a thousand people or so on the e-zone who fly electric models and pass their experience on..plus test data done of 30 motors and props or so over a few months..
I could continue..
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The easiest way to determine the motor is not by using watts, but by using thrust. You know wht the airplane weighs, say 4 lbs. That is ready to fly by the way. 4 lbs is 64 ounces. So if you can find a motor manufacturer that gives you thrust numbers or gives you the weight of an airplane the motor will fly and with which prop, that is the way to go. Forget watts! It varies so much by what TYPE of airplane it is. a 4 lb trainer will require far less power to fly it properly, say 60 to 70%. For aerobatics you'll want 80-90%. For 3D you will want 100-110%.
wrote:

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Jim Slaughter wrote:

but you also need speed no good having 100% thrust to weight if it only pulls the aircraft at 20mph and the stall speed is 30mph or you could end up with a plane that will prop hang forever but will not fly forwards

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funfly3 wrote:

Precisely. AND I've seen those planes do just that. They call them '3D' shockies' with > 1:1 thrust to weight on stupidly fine pitched props..they don't actually FLY at all. Often the CG is so far back that they are unstable in level flight ANYWAY. They are really just helicopters.
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Are you implying that those constantly hovering 3D flyers aren't really good airplane pilots because they're not really "flying" on the wings anyway? <G>
Ed Cregger
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Ed Cregger wrote:

how the hell do you get to that question from his answer? the 3d pilots skill was never in question
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Yeah, my thought exactly. It takes some skill to just hover!

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Oh, I can disprove that!
A long time back, before I became the "consummate" skilled RC pilot that I am now, (harrumph! <g>) and I was trying to takeoff in grass that was too long for the short wheels and low belly of the little .15 powered Lazy Bee. I finally broke ground almost at the runway's end, and had to pull up, or crack up in some very tallll weeds.
The plane was not ready to fly yet, but I made it, anyway. Did I say that it wasn't ready to fly yet? <g>
It went into mushing flight, and I had but a few inches between the wheels and the weeds. I pulled back more. And more. I was keeping my altitude, and it was by this time pointed almost straight up, hanging on the prop.
I stirred the sticks madly, keeping it from settling into the weeds. A few of the guys standing around were starting to turn their heads, pointing, and nudging to the next, saying, "Hey, look at this! I didn't know he could do a torque roll! Why didn't you do it a little closer to you, so you could see what it was doing?"
One of them asked, "Hey, when did you learn to do that?"
"Right now!" I replied. "It sorta' just happened!"
Well, finally, luck caught up, just as skill ran out. It finally flopped over into the weeds, after hovering for nearly a minute, I guess, but I was really sweating it. No damage, since it had no speed, and the grass was tall.
So really, not much skill is needed to hover. Lots of luck can take skill's place, at least for a minute, or so! <g>
Skill is preferable though, I think! ;-)
--
Jim in NC



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Lighten up some. I'm just teasin' ya'll. <G>
Ed Cregger
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