Electric equals fuel engine size - RC Groups
Mar 31, 2012, 03:03 PM
Professor Doc Brown
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# Electric equals fuel engine size

Hi guys I am converting to electric etc from fuel engines etc but is there a table showing sizes of electric motors equals to .cu engines etc so I can know which motor is for what size planes etc. cos I bought a motor over lander brushless t4240 kv 890 with esc 60amps lipos 3-7 cells etc. chap said that is equal to .40cu to .52cu ish etc. If I were to buy a equalivent motor for plane with .cu 25 etc. what size the electric motor need to be etc
 Mar 31, 2012, 03:38 PM An itch?. Scratch build. There are some electric motors designated with engine size numbers 25, 40, 60 etc, theoretically as equivalents, but most motors have a 'size' designation, more like diameter/length. The main difference by going electric is you are not tied to a small range of props as used by a specific sized IC engine. There are so many combinations on electric motor, prop size and battery voltage, that a list is not really possible. You could search the - Glow to Electric Conversions forum - for the same model you have. Often you will then find people still use different motor, but it give a good base line of what works.
 Mar 31, 2012, 10:33 PM Registered User There is a useful "rule of thumb" which says "Watts = cubic inches x 2000"... so a .049 glow is ~100W.....a .25 glow is in the ballpark of 500W... a .40 glow is ~800W..... and so on. Another "rule of thumb" is that an electric motor can be expected to cope with 3W/g without over-heating... so if you are looking at 500W... you'll want a motor weighing about 165g. The correct Kv for that motor will be determined by the prop size suited to your aircraft and the voltage you want to use. Last edited by Dr Kiwi; Mar 31, 2012 at 10:38 PM.
 Mar 31, 2012, 10:47 PM Jack And once you have the Watts figured out as Dr Kiwi mentions you can use some other rules of thumb. Motor weight - Brushless outrunner motors will generate 2.5 to 3 Watts of power for each gram of motor weight. So if you need an 800 Watt motor it's weight would be about 800 / 2.75 = 290 grams. Watts per pound for various types of planes - 30 Watts per pound of ready to fly all up weight (AUW) is considered the minimum for flight. For various types of flying the following are consider acceptable power levels: 50-70 watts per pound; Minimum level of power for decent performance, park flyer/slow flyer models 70-90 watts per pound; Trainers and slow flying scale models 90-110 watts per pound; Sport aerobatic and fast flying scale models 110-130 watts per pound; Advanced aerobatic and high-speed models 130-150 watts per pound; Lightly loaded 3D models and ducted fans 150-200+ watts per pound; Unlimited performance 3D and aerobatic models So now you know the weight of the motor you need and the rating in Watts it should have. What else do you need to know? Well, the Kv of the motor is an important consideration. And to get that right you really need to know the prop you want to use, the RPM you want to turn it at, and the battery voltage you intend to use. All of those are needed to get the Kv right. The ESC needs to be sized so you are using it at no more than 75% of it's rated limit. The battery has to be sized and rated so that the demands of the motor do not make it get too hot and/or drop to too low a voltage under load. That should about cover it... Jack
 Apr 01, 2012, 01:59 AM WAA-08 THANK FRANK! Watts=amps * volts Increase volts means more efficiency... Increased amps means shorter flight times...
 Apr 01, 2012, 04:29 AM Registered User Well Overlander say your T4240 is good for 650W so if you take Dr Kiwi's rule of thumb, which many people seem to believe but which always comes out way too high for my taste, that makes it barely able to replace a .32. OTOH I still prefer the watts/lb idea so at 100W/lb that would make it good for a 6.5lb aerobatic model....which sounds a lot more like the sort of thing normally flown on a .46-.52. In my experience 300-350W will fly a model designed for a .25 2-stroke more than adequately. I think Ray's idea of looking for the model you want to convert or something very close and seeing what other people have used is by far the best idea . Steve
 Apr 01, 2012, 09:03 AM Registered User Then there's the Easymatch series on HK http://www.hobbyking.com/hobbyking/s...EasyMatch.html The motor # is eqeivalent to its glow counterpart. Most other motors on HK give the equivalent glow/gas in the specs. Gord.
 Apr 01, 2012, 09:30 PM Registered User Looking at EasyMatch seems to raise issues, and my "IC to Watts" conversion begins to look conservative! G10.....my IC to Watts = 200W... HK suggests 375W...3W/g for 140g = 420W G15.....my IC to Watts = 300W... HK suggests 575W...3W/g for 170g = 510W G25.....my IC to Watts = 500W... HK suggests 600W...3W/g for 190g = 570W G32.....my IC to Watts = 640W... HK suggests 800W...3W/g for 230g = 690W G46.....my IC to Watts = 920W... HK suggests 925W...3W/g for 303g = 909W G60.....my IC to Watts = 1200W... HK suggests 1425W...3W/g for 360g = 1080W So, Steve... do you think IC (cubic inches) x 1500 is better? That makes your .25 a 375W motor and .46 equivalent to 690W. Obviously electric can spin far larger props than IC...and so can make more useful work (~ thrust) from the same power in... my MVVS .15 is rated at 0.7HP (~500W) with a small prop at ridiculous RPM....but would you rather rate it at only 225W... my 300W seems a bit more useful. Last edited by Dr Kiwi; Apr 01, 2012 at 09:47 PM.
Apr 02, 2012, 12:01 AM
We want... Information!
Quote:
 Originally Posted by Dr Kiwi my MVVS .15 is rated at 0.7HP (~500W) with a small prop at ridiculous RPM
That's the problem with trying to work out a 'glow equivalent'. It all depends on which glow motor you are talking about. The Cox Baby Bee 0.49 produces 40W at 13000rpm, while the medallion 0.49 produces 60W at 17000rpm. An OS Max .40 LA with 10x6 prop produces 680W at 12100rpm, while the Thunder Tiger Pro .40 produces 1260W at 14900rpm. With such wide variations in power output, engine capacity is only a very rough guide.

Better to just forget about glow engine equivalents, and calculate electric motor size directly from the weight of the model. Use your desired W/lb to get the required power input, and 3W/g to choose a motor size. Battery voltage must be matched with Kv to get the required power from a suitably sized prop.

Quote:
 Obviously electric can spin far larger props than IC...and so can make more useful work (~ thrust) from the same power in
An outrunner electric motor is generally closer to a 4 stroke glow engine with similar weight and prop size. Unfortunately this doesn't help much, because few glow models specify a 4 stroke. Both the 4 stroke glow and outrunner electric tend to work better with a larger prop in slow flying and sport models, while a 2 stroke or inrunner electric is better for sleek high speed models.

Quote:
 Originally Posted by welshwizardred If I were to buy a equalivent motor for plane with .cu 25 etc. what size the electric motor need to be
The cu x 2000 rule says you need 500W, and the 3W/g rule says 167g. The HK G25 870Kv weighs 193g and so should be good for 580W, plenty enough for a typical '.25 size' model. In some cases you may be able to get away with less power and a smaller motor.

I have the Hyperion P-51 25e, which is designed for glow or electric. I am using an Apache APL30-14T 850Kv outrunner, which weighs 145g and runs on 3S with a 12x8 prop. It draws 330W and 94W/lb. Two friends who used a glow engine on the same model found that a .25 was too weak, and they needed a .35 to get satisfactory performance.
Apr 02, 2012, 04:13 AM
Registered User
Quote:
 Originally Posted by Dr Kiwi So, Steve... do you think IC (cubic inches) x 1500 is better? That makes your .25 a 375W motor and .46 equivalent to 690W. Obviously electric can spin far larger props than IC...and so can make more useful work (~ thrust) from the same power in... my MVVS .15 is rated at 0.7HP (~500W) with a small prop at ridiculous RPM....but would you rather rate it at only 225W... my 300W seems a bit more useful.
Yes, 1200-1500 would be more like the value I'd go for....but then I'm basing my views on a limited range of actual conversions I've done. Years ago I replaced a decent .46 in a model with about 550W of electric power and it flew almost identically. I've replaced a .32 with 400W and that flew very similarly. Both of those are around 1200W/cu in but they are actual running wattages not just multiplying battery nominal volts by ESC power rating. Even so I'd say 1500 is plenty. But then having too much power available to play with never does any real harm either, except perhaps paying more than you need to .

Steve
 Apr 02, 2012, 08:36 AM Jack I agree with the 1500 or so number too. I think maybe the 2000 dates back to the days of brushed motors and NiCad and NiMH batteries. When I converted a SIG Rascal 40 to electric I had the 800 Watt number in mind and bought a motor that could get up to that power level (Torque 4015-570, 10 oz./280 grams). But I flew the plane at a little over 7 pounds and 625 Watts and it had more than enough power. Very short takeoff rolls and people who had flown that plane on .40 fuelies said that it had better power than those. The Rascal was a wonderful plane, the thin semi-elliptical wing was a big factor in that I think. I could throttle back and cruise the pattern at 25 or 30 Watts per pound and full throttle was seldom needed or used. Jack
 Apr 02, 2012, 01:13 PM Registered User I think we've now got ourselves into a similar situation to those with glow motors who read max. power 1.5hp and think that's what they're getting even though that was probably on about an 8" prop at 19,000rpm and they're running a 10" prop at about 13,000. I suspect people are buying electric motors that claim to be good for say 1000W but actually using them at 60-70-80% of that even though it's a lot easier to measure the power (input power at least) with electrics. So perhaps the "2000W" means buy a motor that the manufacturer claims is good for say 500W for a .25 and then when you're actually running it at nearer 350W but it's not getting stinking hot you'll still have enough power to fly well. At least that's my theory . Steve
 Apr 02, 2012, 02:39 PM An itch?. Scratch build. We keep coming back to the prop though, just quoting watts isn't the total answer for someone wanting to do a conversion. Do IC fliers expect to use the same prop as was on their IC engine and just fit a motor that will turn it the same rpm ?. As electric power can be a pretty mystifying thing. Or do they realize that with electric you could change to a much more suitable prop for their plane ?. It would be interesting if a current IC only flyer commented on the posts to see if they really helped to understand what a conversion needs.
 Apr 02, 2012, 05:53 PM Registered User Years ago I replaced a decent .46 in a model with about 550W of electric power and it flew almost identically. I've replaced a .32 with 400W and that flew very similarly. Steve, Ray's post above got me to wondering...... what prop would those two planes you mention have used with an IC .46/.32 respectively......and what did you use on their "IC equivalent" 550W/400W electric motors?
 Apr 02, 2012, 06:58 PM Registered User Can't remember what the .32 used, probably 9 or 10" but the replacement electric motor ran an 11x6 prop. The Irvine .46 ran an 11x6 or 11x7 but the electric worked better on 13x6. I don't really start from the prop but where possible I'll generally choose a motor Kv that allows me to run a slightly larger prop than is typical for the IC version. I agree with Ray that although the simplest possible choice is just to use the IC prop size and choose a motor to get it running at the same speed it's usually possible to get a better setup. Since I prefer pull over speed I normally end up with a more thrust-efficient large diameter relatively low pitch prop (and sometimes bigger wheels to give me ground clearance ). Steve