Brushless-glow equivalency

How can you determine if an electric for direct drive should be roughly equal to a .40 two stroke. Then how do you match-up an ESC and lipos.
Apparently money is the answer but how do you spend it well.
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agpete wrote:

One way that works well is to get Moto-Calc, assuming they've stayed current. It'll tell you motor, prop & battery.
You should plan on about 50 watts per pound for zippy performance, more (75?) for 3D and less (35 or so) for a lightly loaded trainer. Smaller airplanes can go less, and really light slow flyers can go even less than that. Few manufacturers will tell you watts out but you can get close by looking at current draw times supply voltage.
Once you know the supply voltage and expected current draw select an ESC that will handle both of those. It's probably a good idea to get one that will handle plenty of extra current for when you land in that tree and forget to cut power. Size your battery for two things: current draw and capacity. It used to be that LiPos would always give you long flights because you just couldn't draw to much current. Now they've got ones advertised for 15C, which would be about 4 minutes of flying at full throttle -- I'd go for a battery sized for 5C (i.e. amp-hours current * 5) to get reasonably long flights.
Or if your local hobby shop is good ask them.
Or call your favorite web store and ask them -- hobby-lobby is recommending e-flight sets for glow conversions.
--

Tim Wescott
Wescott Design Services
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Go to some of the retail motor sites and look at the performance specs of the motors. Most will tell you what props they will turn at what voltage. The AXI range is pretty extensive.
I bought an AZI 2820/10 for my first real foray into electrics. It is probably equivalent to a .25-.32 glow. This motor needs a 30A controller and 2200mAh Lipos at 15C for minimum performance. I will be using 1500s in a 3S2P configuration at first.
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Paul McIntosh
RC-Bearings.com
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That should have read that the battery was the minimum for full performance of the AXI motor.
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Paul McIntosh
RC-Bearings.com
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Goedendag ;-)
Lots of motor numbers/test: http://www.flyingmodels.org/motortest/index_e.htm
E-flight calculator: Usage and : http://www.badcock.net/motorcalc / Example: http://goldeneye.ethz.ch/motoren/electric/e_highlight/index_EN
Lots of E info: www.eflight101.com/
Vriendelijke groeten ;-) Ron van Sommeren http://home.hetnet.nl/~ronvans/ int E fly-in near Nijmegen, the Netherlands

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You can compare them on the basis of power output. A low-end .40 (e.g. OS 40LA) puts out about 0.7 bhp with a practical prop, a decent ball-bearing sport 40 about 0.9 - 1.0 bhp at practical rpm - not 16K or so where the ad writers like to rate them. The conversion is 1 bhpt6watts. A motor in the 500-600 watt range should give comparable performance, likely better as it gives you more flexibility to choose the most efficient prop/rpm for the job. A 12 cell (nom. 12V under peak load) pack at 50A should do nicely. I like batteries that are multiples of 6 cells, as you can use relatively cheap R/C car packs and put them in series. A 3000 mah pack is only good for about 3 1/2 minutes at that current, so throttle management is key to reasonable duration. Of course a 3 (or more) cell LiPo could be used instead, but for a 40-50 A load, it'll cost you a bundle and have you spending a lot more time on battery care and feeding, too.
Abel
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First, you completely forget the notion that glow engine = electric motor. That, and, "I'm not a newbie," are the two biggest hangups glow fliers have when converting to electric for the first time. Just throw away everything you think you know about electrics, and come at it as if you're a complete newbie. Tuck the information you have about glow engines away, because it's useless on an electric conversion.
You need to approach it from the plane, not the engine. A .40-size airplane weighs around 6 pounds typically. For glow-like performance with electric, the plane needs 100 Watts of power for every pound of final, ready-to-fly weight. Figuring a 6lb airplane, you need 600 Watts.
Watts come from the battery, not the motor, so next you size the battery. I can hear you saying, "But...??" But nothing. Just forget about the motor for now. Forget it.
"But..." Forget about the motor, okay? We'll get to it. Trust me.
Okay, you've got 600 Watts, so how to turn that into a battery... Well, a battery provides Volts and Amps, and Watts is Volts times Amps, so we simply break the 600 Watts into a reasonable combination of Volts and Amps. This is a bit of a leap of faith for most people, but from experience, 40 Amps is a good target current for a .40-size airplane. So, 600 Watts divided by 40 Amps is 15 Volts. That just happens to be the working voltage of a 4S LiPoly pack, or 14 NiMH cells. Can you tell me what size ESC you need now? Yep, one that does at least 40 Amps, and is rated to handle at least 14 NiMH cells or a 4S LiPoly. Ba-da-bing, ba-da-boom.
You've got enough information to pick a motor now too. Motors are rated by # of cells, and maximum Amps. As long as you pick a motor that can handle 14 NiMH cells (or a 4S LiPoly), at 40 Amps, you're golden. The motor can be geared as necessary to turn the right size prop, but if you're looking strictly at direct drive, look at AXi motors. They have extensive application tables that tell you what size prop, how many cells, how many Amps.
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Essentially true, but you really don't need 100W per pound with an efficient brushless. Those figures were more true for brushed motors. I converted my 4 Star .40 from an OS .52 4 stroke to AXI brushless. I'm only pulling 300W and it is almost as fast as it was with the OS. Probably 75W per pound is more realistic. Of course if you want unlimited vertical or 3D, more id better ;)
PCPhill

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I must disagree. I built a Great Planes Tracer kit as an electric. I had two Tracers with OS 46FX's powering them. I ran a 6s3p arrangement on the old 1500 mAH packs, rated at 7.5C and an AXI 4120 (I think) that pulled about 30A. It weighed almost exactly 6 lb, and ran at very close to 610 W into the motor.
I got close to 100 flights on it. Although it was as fast as the .40 size versions, it lacked just a little of making unlimited vertical, which the glow powered planes had. It was a dandy plane.
Unfortunately, the pilot made a dumb mistake.
I really think you need 100W/lb to get glow performance. 120 would be better.
-- Mike Norton

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Mike Norton wrote:

That is true, if that is what you want...in fact a racing 36 in a 'mad' airframe you need getting on for 300W/lb..almost impossible even with latest electric technology..
Remember that airframes designed for glo have been optimised to glo sort of power delivery: you can build a fra lighter airframe that will exceed teh glo performance on less power with electric...but its a bigger deal than 'conversion'
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| I really think you need 100W/lb to get glow performance.
Ultimately, `glow performance' is a mighty broad brush. `Glow performance' varies widely, and so will the watts/lb needed to approximate it. A pylon racer or 3D plane will need a lot more watts/lb than a slow flying glider, even though both may have previously had `glow performance'.
As for brushless motors, at this stage in the game, if your plane uses more than say 200 watts or so, you really should have a brushless motor, especially if you're using LiPos. Sure, a brushed motor may be cheaper, but the added efficiency of a good brushless motor means you can save money on your batteries, more money than the additional cost of the better motor.
--
Doug McLaren, snipped-for-privacy@frenzy.com
In a real emergency, we would have all fled in terror, and you would
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Doug McLaren wrote: Sure, a brushed motor may be

That statement is, doing the maths, false, for most smaller sizes of motor.
To get - say - 100W out of a 60% efficient can, takes 160W To get the same out of a 85% efficient brushless, takes 117W.
The difference in cost between the (say) speed 480, at $10 with a $20 speed controller is and a $50 Mega 16/15 brushless and $50 speed controller is about $70.
The TOTAL cost of a 3s2200 LIPO pack to power teh brushed motor is not $70...the difference between a 1600 and 2200 pack to 'make up for the brushed inefficiency' is almost negligible.
The only downside on the brushed setup is increased motor weight. That is an issue ONLY with ultra high performance models. Otherwise given the battery weights with LIPOS, its almost a non issue.
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| Doug McLaren wrote: | Sure, a brushed motor may be | > cheaper, but the added efficiency of a good brushless motor means you | > can save money on your batteries, more money than the additional cost | > of the better motor. | > | That statement is, doing the maths, false, for most smaller sizes of motor.
You didn't read the post you're responding to very carefully. I qualified my statement before the part that you quoted --
As for brushless motors, at this stage in the game, if your plane uses more than say 200 watts or so, you really should have a brushless motor, especially if you're using LiPos.
If you're going to correct me, at least try and read my post carefully enough to make sure I'm wrong first.
| To get - say - 100W out of a 60% efficient can, takes 160W | To get the same out of a 85% efficient brushless, takes 117W.
Which part of 160 watts is higher than 200 watts?
In any event, perhaps 300 watts would be a more definitive cutoff, but I still stand by my statement -- the larger your setup, the more you want brushless, even if it's just for economic reasons.
I was really aiming at setups where a speed 400 or speed 480 motor wasn't adequate anymore. Your basic speed 480 can motor might be able to handle 200 watts, but it won't last long at that power level. And once you start getting into the larger brushed motors (larger than a 480) the price differences between them and the equivilent brushless become smaller and smaller.
| The difference in cost between the (say) speed 480, at $10 with a $20 | speed controller is and a $50 Mega 16/15 brushless and $50 speed | controller is about $70.
My main point was economics, so don't forget that if you are pumping 170 watts into it, that speed 480 motor won't last long, so let's count a few of them (because the brushless motor will last much longer.)
Now, you could get a higher quality speed 480 motor, with rare earth magnets and such, and you may even be able to replace the brushes seperately, and the efficiency will improve, but it'll cost almost as much as a brushless motor ...
| The TOTAL cost of a 3s2200 LIPO pack to power teh brushed motor is | not $70 ...
... except that you're forgetting at least two factors :
1) if you put in a bigger pack, it'll weigh more. So you'll need even more power, which will require an even bigger pack for the same peformance. I'm talking about performance in a plane, not on the test bench.
2) most people have more than one pack for a plane. Three sounds more reasonable. So triple the difference ...
(Though to be fair, the brushless cost benefits will go down if you have several planes that all use the same battery packs.)
| the difference between a 1600 and 2200 pack to 'make up for the | brushed inefficiency' is almost negligible.
3 cell 1500 mAh LiPo pack at CBP = $52. 3 cell 2100 mAh LiPo pack at CBP = $79
Triple that difference and you get $81, which is *more* than the added money that you spent on the brushless setup. And this is a case where we're looking at power levels *lower* than the break-even point I'd proposed, and I'm ignoring any additional capacity needed due to the additional weight (perhaps I should be looking at the three cell 2500 mAh pack for $100 instead?).
| The only downside on the brushed setup is increased motor | weight. That is an issue ONLY with ultra high performance | models.
Well, it's a small issue, but it's not a non-issue. All that weight adds up. Also, comparing that Mega 16/15 brushless to a speed 480 is hardly fair -- the Mega can put out a lot more power. Comparing it to a speed 500 or maybe 600 motor would be more reasonable -- and that bigger motor will weigh a whole lot more.
--
Doug McLaren, snipped-for-privacy@frenzy.com
That which does not kill me can still really hurt.
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Actually, from what I have seen, the price difference grows quite a bit. A speed geared 600 can motor is about $35.00. Equivalent brushless is $80 or more. Go any larger and the difference grows (except you won't find many can motors larger than 700 size).
--
Paul McIntosh
RC-Bearings.com
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Paul McIntosh wrote:

De Walts...sort of 'speed 800'. $50 for a '40 sized' can motor. Or use two in a belt drive and get a '90 sized' electric motor for $200 :-)
http://www.modelairtech.com/belt.html
If I ever go 'quarter scale' electric, that's what I'd use...
Mind you a 6s 5000 mA/h pack to drive it is going to be nigh on $400..
Other than that motor combo, its astro Cobalts that cost as much as, if not more than, a brushless..but they are as good as, or better than..a brushless!
I have no doubt that a suitable motorcycle starter motor capable of delivering a couple of brake horsepower would also be not THAT expensive....in the same way that weed whacker motos are turned into cheap IC power plants for larger models.
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Doug McLaren wrote:

That is only true when cheap brushed motors cease to become available.
You can get 200W out od a $15 soeed 600, 300W out of a buggy motor, and there are varous dewalt type motors that will do 600W or more.
Beyond that point you run into the problem that any motor you buy is custome built for the RC market and brushed or brushless makes almost no difference to cost or performance. The speed controllers for brushed arer cheaper though, and efficiencies on a decent neodymium brushed or brushless of say 600W capability are very similar, as is the price.

Abovce 400 W yes..but so do the advantages of the brushless get smaller and smaller..

Oh , it will last OK if you are careful with it...the secret is to get te voltage right..:-)

Indeed. Slightly more, but the ESC is cheaper..

10% more on a LIPO pack weight is almost irrelevant. Unless you are going for utter contest power, the pack weight will be around 25% or less of the model weight..10% of 25% is hardly earth shattering in rterms of performance decrease.

Only when they are running Nickel technology that flies for 3 minutes and takes an houir to charge. I have ione pack for three planes. It lastys most of te afternoon, and then I charge it while flying another pack in another three planes :-)
I fly at about 2:1 charge to fly time ratios. Charging 20 minutes for ten minutes of flight is not onerous..although it is really an hours charge the evening before for 3 x 10 minute flights before I even get OUT the charger..

Indeed, but so what? That is my point...a permax 600 - a decent 200W plus motor, weighs 6.7oz...a mega 16/15 weighs 2.70z..also capable of 200W or better (but a buggy motor will also fior similar weight to the can 600 do what a mega 16/15 will) anyway both are about '.19' power.
They are both goiung to draw similar currents - the mega a little less. I have a pack that weighs 4.8oz (2200mAh) that will drive either well.
So the advantages of 'going brushless' are 4 oz weight saving in a power train that weighs 11.5 oz and is likley to be powering 2-4lb of plane.
so its an 8% weight increase to use the can motor, and save about $80.
If you take the brushless and nickel cells, to get similar run times you woopuld beneed 10x3300 cells - weight 20oz, so AUW is 22.7oz for NiMh cells against 11.5oz...a whoppping 11 oz heavierr for an exopesnive brushless motor that WILL need tio be that heavy, or else you WILL need 3 opacks of lighter weight simply because one pack WILL go flat on you in under ten minutes..
In short, what motor you use is nowhere NEAR as crucial as what battery you use..and in ANY size a LIPO cell is half the price to 9 SUB C's - that 9 because you WILL need three packs and each LIPO cell is about three times the voltage of a Nickel cell..
My contention is therefore, that its cheaper to get one decent LIPO pack than a trunkful of crap Nickel cells, and that alone will make your model hugely better performing.
Whether you choose to run a hacker B50 at $300 up the front, or a Modelair belt drive with a 600W De Walt can costing $100, is hardly an issue after that..except the controller on the de walt will also cost you half the price...
Sure, it will weigh in a bit heavier..but so will your wallet...
Its enough to match a 40 power plant...For $105 with belt drive...http://www.modelairtech.com/belt.html.and it will top 70% efficiency too.
You won't get an AXI 4130 for that money..and they are not much lighter either..and the ESC will set you back more...
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That was a VERY good primer on how to do it! Thanks for making the process so clear.
--
Paul McIntosh
RC-Bearings.com
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On 28 Oct 2005 14:01:13 -0700, snipped-for-privacy@rochester.rr.com wrote:

Very Well said. I also tell people interested in Electric flying to start with the battery. After a litte explanation they usually agree that it's much simpler than they thought.
Ken Day
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agpete wrote:

A 40 will produce about a horsepower, or 750 watts, at around 10-12KRPM. On a 10x6 sort of prop.
If you want to directly replace that, you need a 750 watt motor that will turn a 10x6 at similar RPM.
However, most glo aircraft are overweight and overpowered, and a 10" prop is usually too small and 10K RPM too high for efficent use of the power.
So many more sedate '40' models can fly on half a horsepower with a larger prop revving slower. Achieved by a 300-400 watt motor with either a gearbox, or multipole (outrunner) design.
Aim for about 70-100W/lb for a typical sport model, use LIPO batteries, and buy a halfway decent motor and you will not go wrong...
..and ask here http://www.ezonemag.com/ because no one uses Usenet anymore...much..
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