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Watts are what we want!!!!!!!!

Old 10-22-2005, 09:02 PM
  #1  
GeraldRosebery
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Smile Watts are what we want!!!!!!!!

The major bit of information always missing in manufacturer information about new motors is the number of watts they can dissipate without melting. Nothing is perfect, but I think we all know that the rules of thumb are 80 watts per pound of model weight for general flying, 100 watts/lb for aerobatics and 120 watts/lb for 3D. Knowing that, I want to be able to select a suitable motor for my model by looking at its size and weight (common sense) and then the number of watts it can reasonably dissipate at full throttle. So then I know that if the motor can dissipate say 700 watts I can fly a 7 lb aerobatic model with it. Then I can select a battery - say a 5 cell LiPO - 18.5V and know that I had better be able to pull 700/18.5 = 38 amps out of it. In other words it had better be a 10C 4000mah at a minimum.

I look at all these new motors and see KV (fairly useless number) a number of cells count range (2-5 for example), number of winds - another useless number to most people. If the manufacturers took the time to actually test their motors to destruction before trying to sell them, we would all be way ahead.

What do you guys (and gals) think?
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Old 10-23-2005, 01:08 PM
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eflight-ray
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If your interest is just the sheer power of a motor, you would probably be better off going over to IC engines, that way you don't have to also look for maxium output from batteries and ESC's as well.
To be honest I would be more intersted in what power that propellers can put out and at what rpm's. That way I can then choose a motor capable of turning the prop I want at the speed I want to suit the plane I want to fly.
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Old 10-23-2005, 01:52 PM
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I don't think you'll see this any time soon. I remembered when I asked Razor for the max output of their motors. I was told that their were too many variables and he didn't want to be put on the spot for a hard number. Cooling has a big part in it. How long is continuous is a problem especially when dealing with a motor was designed in the pre-lipo days. With round cells, duration was 6 minutes, now who knows The ambient temp is a big factor also, especially for us in warmer climates.

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Old 10-23-2005, 03:03 PM
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The major problem with just saying that a motor can dissipate 700 Watts is HOW does the motor like its 700 Watts? You can get 700 Watts using 10 Volts at 70 Amps, 70 Volts at 10 Amps, 35 Volts at 20 Amps... There are literally hundreds of ways to get 700 Watts, and the motor can be designed to use any of these combinations. Many of these combinations aren't practical at all, and many aren't practical in a particular plane.

Gerard, you've got the process 90% down, but I think you're still clinging to the notion that "it's in the nose of the plane, therefore it's the source of the plane's power." Just switch two steps, and you'll find that you can power any airplane.

If you go back and read your description above, you'll notice that you paid absolutely no attention to the limits of the motor. You needed 700 Watts for that 7lb plane, and chose a 5S LiPoly at 38 Amps. At this point, all you have to do is go motor shopping, and find a motor rated to handle a 5S LiPoly at ~38 Amps. All motors are rated by their maximum cell count, and maxmium current handling capability, so you've got all the information you need to make the selection.
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Old 10-23-2005, 03:38 PM
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GeraldRosebery
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People are missing the point here. It really doesn't matter where the power comes from (within design limitations). Whether its 15 volts or 20 volts or 25 volts, 700 watts (in this case) is power and power = heat. Do you think the HP calculations of IC engines are calculated in the plane? No, it's on a bench dynamometer and 1 HP = 746 watts. A real number from the lab that says (for example) that DO NOT EXCEED a dissipation of 700 watts would be useful information. Sure common sense tells you to cool the motor or it will melt long before you reach 700 watts, but it gives us a starting point just as hp ratings of model engines or car engines for that matter give us a starting point to compare A and B. Now some manufacturers like MODEL MOTORS (AXI) are getting there with prop and amp recommendations, but DO NOT EXCEED 55 amps for 30 seconds does not mean a lot unless you also know the voltage and hence power.

BTW - I have to dispute that a small motor will get progressively hotter after 6 minutes (NiMH) and if it is still running safely at 6 minutes it will run safely for 20. (LiPO). I am confident that with the small mass of the these motors and the high currents they take, the ultimate run temperature is probably achieved in one minute or less after take-off
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Old 10-23-2005, 05:40 PM
  #6  
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I noticed in Axi ads that they also recommend a number of batteries/cells
if you figure out the voltage for the number of cells...multiply by max amps...you have watts, and since you are not to exceed a certain amp draw ( according to the ad)...you have the max watts....no?? also think that your 80 watts to the pound for sport flying is a little higher than needed
65 watts to the pound gives really good sport performance...I fly a 5 pound pattern model on just around 100 watts to the pound, and it does the whole routine easily..some larger scale models do well on less than 45 watts to the pound....it isn't just watts that makes the model go....it's prop thrust, combined with prop speed combined with watts...combined with the model design..one of my favourite models flies with a max prop rpm of 2400.....actual in air rpm is about 1100 to 1400...( it will fly on throttle trim only) no it's not fast, but thrust is more than model weight, and it does everything the full sized plane does and more ......when this model flies at an event...people stop to watch..you are going to find as you grow in the electric section of the hobby that you are going to learn how much rpm and horse power really don't mean anything...it's torque that turns the prop..
and if you can get into the habit of using the old noodle when you figure out the required motor battery prop combination, you will be coming up with some great performing models that the so called flight predicting programs say won't work...you have to remember that the factors used to build these programs are just drawn from information that the designer thinks is correct....if he is wrong, or off base at all on any of his assumptions, then the whole process is flawed....now suppliers are recommending ridiculously large power systems for an application, because they don't want to be wrong..so if a model needs 200 watts...they recommend 350 knowing that this much more power should " keep them safe" and for the most part it sorta works, but makes our hobby more expensive as we are buying power systems that are bigger/ more sophisticated than we really need.

Last edited by Ron; 10-23-2005 at 06:10 PM.
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Old 10-23-2005, 07:30 PM
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The point I was trying to make was, it isn't just the power from the motor, it's making sure you have the right prop on the plane, turning at the right rpm. And even then you will be using throttle to to reduce that power most of the time, unless you are in to hot-liners.
I can up the watts my motors take just by putting bigger props on them, i.e just loading the motor more, but those extra watts wont make them fly any better.
I still feel that there should be more useful info from prop manufacturers, such as their 14x8 turning 9000 rpm takes **watts, then I can choose a motor/volts/amps combination to suit.
Yes I can see the point of a 'do not exceed- rpm -watts' label, but if I was at or near that maximum, I would probably want to buy a better motor anyway.
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Old 10-23-2005, 07:33 PM
  #8  
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Gerry,

I don't think that's right. Higher amp draws will generate more heat due to the increased resistance, so you should be able to run cooler getting 700 watts at 14 volts than you would at 7 volts (half the amps).

Of course, the motor Kv won't necessarily let you use the prop you want. This year's vogue is "watts." Last year's was "thrust" (which is somewhat useful to 3D folks). Etc.

Unfortunately, matching a motor, battery and prop to a particular plane design just isn't as simple as knowing the thrust, max watts, Kv, Io or whatever as a single parameter. They all relate.


Dave North
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Old 10-23-2005, 09:25 PM
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GeraldRosebery
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Originally Posted by eflight-ray View Post
The point I was trying to make was, it isn't just the power from the motor, it's making sure you have the right prop on the plane, turning at the right rpm. And even then you will be using throttle to to reduce that power most of the time, unless you are in to hot-liners.
I can up the watts my motors take just by putting bigger props on them, i.e just loading the motor more, but those extra watts wont make them fly any better.
I still feel that there should be more useful info from prop manufacturers, such as their 14x8 turning 9000 rpm takes **watts, then I can choose a motor/volts/amps combination to suit.
Yes I can see the point of a 'do not exceed- rpm -watts' label, but if I was at or near that maximum, I would probably want to buy a better motor anyway.
It's a good point, like with gas, you can always "detune" a motor by switching to a smaller or lower pitch prop. It is, in fact, one of the most useful and interesting characteristics of electric motors. I have an AXI 2826 on small 2 lb trainer type model that could easily fly a 4 lb model. By putting on a 9x3 instead of a 11x8 I keep the current (and power) down. The point I wanted to make is that it is often difficult to properly evaluate new motors as they come on the market. Not everyone goes to the trouble that Model Motors does to help you choose the right motor for the job. It is no small wonder that most of what I fly is AXI. The watt capability of a new motor would provide guidance to its applications.
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Old 10-23-2005, 10:23 PM
  #10  
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Gerald, I for one am not missing the point... It DOES matter where the 700 Watts come from. Motors have two limitations, temperature and RPM. Amps generate the heat, and Volts determine RPM. Pull too many Amps with a motor, and you melt the windings and/or kill the magnets. Run too many Volts on a motor, and you sling magnets or windings. That is why you can't just say, "This motor can handle 700 Watts."

The funny thing about this whole conversation is that you're doing things "the right way," but you don't realize it The motor doesn't even come into play in your calculations, if you step back and take a look.
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Old 10-24-2005, 06:24 AM
  #11  
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Hobby Lobby, for one, gives you a lot of useful information about the motors they sell. A range of plane weights for gliders and sport, for one. Also, recommended prop sizes, volts, amps and watts are given in a range setting, i.e., certain voltage with certain prop gives x amps and watts to prop. You would have to just check it out go see what I mean.
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Old 10-24-2005, 01:07 PM
  #12  
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One thing about Hobby Lobby's numbers is that they show "Watts to prop" while we use "Watts in" in our rules of thumb. That means you can actually get away with fewer Watts per pound, probably 15-25% less, because motor efficiency has already been factored in.

Why don't we use Watts to prop in our estimations? Mere mortals like us can't measure Watts at the prop easily, and it's different for every size and brand of prop in existence. It's not a linear relationship. In other words, the efficiency is not the same throughout a range of prop sizes. To borrow an engineering term, there is a safety factor built into the Watts-in rules of thumb that makes sure enough power gets to the prop as long as the motor is being used within its specified limitations.
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Old 10-24-2005, 04:35 PM
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Originally Posted by GeraldRosebery View Post
The major bit of information always missing in manufacturer information about new motors is the number of watts they can dissipate without melting. What do you guys (and gals) think?
Yes! That's what I want.
I just looked at a web site for a company that sells general purpose large DC motors. The first column in the selection list is horsepower (easily converted to watts). If I was designing an electric pizza tosser I would determine how much power I would need to toss the dough and select the motor based on that with perhaps some safety margin. That way I don't buy a motor that's too expensive or too heavy.

Power handling capability would be my first cut at selecting candidate motors for my airplane. Then I would use electricalc or something to fine tune it from there.

I have a Hacker motor that I will donate to the cause. Maybe I'll try to blow it up myself.
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Old 10-24-2005, 07:27 PM
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eflight-ray
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Here's a question then, you have a new plane, do you choose a motor based on watts, then decide what size prop and whether it's going to be geared or direct drive, or do you decide on the prop size first and choose a motor to suit. I do the latter.
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Old 10-24-2005, 08:16 PM
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Originally Posted by eflight-ray View Post
The point I was trying to make was, it isn't just the power from the motor, it's making sure you have the right prop on the plane, turning at the right rpm. And even then you will be using throttle to to reduce that power most of the time, unless you are in to hot-liners.
I can up the watts my motors take just by putting bigger props on them, i.e just loading the motor more, but those extra watts wont make them fly any better.
I still feel that there should be more useful info from prop manufacturers, such as their 14x8 turning 9000 rpm takes **watts, then I can choose a motor/volts/amps combination to suit.
Yes I can see the point of a 'do not exceed- rpm -watts' label, but if I was at or near that maximum, I would probably want to buy a better motor anyway.
I would think that there would be WAY more variables trying to specify watts to props even at a set RPM. A gear driven motor running a 14x8 would take less watts then a direct drive and then you have to figure gear ratio to watts for every possible combination. Then you'd have to also figure load as in AUW as well.

In other words it would be like saying it takes X number of horsepower to run a given size set of tires on a vehicle.
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Old 10-24-2005, 08:40 PM
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ForestCam
I was thinking along the lines of the prop manufactures using some standardised testing equipment. There must be some sort of lab equipment that would give a useable measurement for the power that the prop requires. It would then be up to the uses to determine what sort of drive they are going to use, to achieve the rpm they want.

Wouldn't you find it useful to have a graph included with each prop you buy showing what power is required to turn it at specific rpm's?.

Perhaps the graph would have to be for 100% efficiency to standardise the lab test equipment. But at least you would then know how efficient you system is by comparing your rpm and watts with the graph.

It's just an idea, it may be just too expensive to do, but the first manufacturer to do it would certainly get my attention.
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Old 10-24-2005, 08:51 PM
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Originally Posted by eflight-ray View Post
Here's a question then, you have a new plane, do you choose a motor based on watts, then decide what size prop and whether it's going to be geared or direct drive, or do you decide on the prop size first and choose a motor to suit. I do the latter.
Either way you somehow know how to choose a motor that's not way to heavy and beefy for your application or so wimpy that it's gonna get fried.

It's not a huge problem. I manage to pick out motors by looking at the available specs and seeing what other people use (thank goodness for internet forums like this). But, having a maximum power rating listed as a spec would help me get in the ballpark easier.
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Old 10-24-2005, 09:29 PM
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Wait a minute... You're going WAY overboard here. The point of this discussion is to make it EASIER to select a motor, not more convoluted and difficult.

Every motor on the market now is rated with its maximum power, in terms of maximum cell count (Volts) and maximum current (Amps). Having it in this form is more useful, because it's in "battery" terms, not some ambiguous scalar (directionless) value. These values give you an idea if the motor's a "screamer" (low voltage, high current) or a "torquer" (high voltage, low current). If you need the motor's capacity in Watts for the "warm fuzzies," it's a simple calculation.
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Old 10-25-2005, 12:57 AM
  #19  
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Make it easy. 100 watts per pound. Figure 40 amp draw on the larger stuff, 20 on the smaller stuff. Find a battery that can support the amp draw and voltage. Go pick a motor that can run at that voltage and current. Fly the plane and try a couple of different props to optimize your plane.

Will this work? Yes. Are there other thoughts out there? Yes. I think that you will fnd this method will work 90% of the time.

The only time I overpowered based on this was with a Cub. I have found that I seem to like a little more power, so I normaly shoot for 125 watts per pound.

Hope this helps.

John
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Old 10-25-2005, 03:40 AM
  #20  
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Originally Posted by Matt Kirsch View Post
One thing about Hobby Lobby's numbers is that they show "Watts to prop" while we use "Watts in" in our rules of thumb. That means you can actually get away with fewer Watts per pound, probably 15-25% less, because motor efficiency has already been factored in.

Why don't we use Watts to prop in our estimations? Mere mortals like us can't measure Watts at the prop easily, and it's different for every size and brand of prop in existence. It's not a linear relationship. In other words, the efficiency is not the same throughout a range of prop sizes. To borrow an engineering term, there is a safety factor built into the Watts-in rules of thumb that makes sure enough power gets to the prop as long as the motor is being used within its specified limitations.
They give you volts and amps. If you want those watts just multiply volts by amps. The point was they have a lot of data to help, if people would just use it. Like I said, they give a range of planes weights they would be good for and give you some numbers. Better than most.
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Old 10-25-2005, 01:08 PM
  #21  
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Even better, you don't even have to do any calculations at all. I just got Hobby Lobby's latest catalog in the mail, and they now feature an entire section of larger glow planes converted to electric. Every common size and flying style is covered. All you need to do is find a plane of similar size, weight, and flying style to the one you want to convert, and copy the power system. The information is also on their website if you don't get their annual catalog.
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Old 10-25-2005, 03:23 PM
  #22  
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Why does everyone always get so wrapped up in watts per pound? Yes, it's an indicator, but it's not the end-all solution. I've hovered a plane on 66 watts per pound. I've also had a setup at way over 100 watts per pound that would hardly fly. Watts per pound is not the only indicator of performance. I'll get off that soapbox now <G>.
Gerry,.. if you need 700 watts of power, a motor doesn't have to DISSIPATE 700 watts in heat. It only has to dissipate the number of lost watts. For instance, say a motor is setup (propped/geared/batteried) so that it's consuming 700 watts from the pack, and that setup is say 75 to 80 percent efficient(about the range your axi's are going to be in. Then there are 140 to 175 watts of waste heat that the motor has to dissipate,.. not 700. Thats going to be something probably in the 41XX range,.. an 11.5 to 14.5 oz motor. Now go to a geared in-runner that's more efficient,.. say a B50 S with gearbox,.. and say it's 85% efficient with a particular setup,.. then it only has to dissipate 105 watts of waste heat. (side note**,.. see the geared inrunner advantage,.. 3 to 5 oz weight savings, and more actual power to the prop ).Now you get into the windings you mentioned earlier. Specific winds can go either high current and low voltage or low current and high voltage,.. so depending on how much battery you're using to get those 700 watts,.. you could have a low voltage and high current setup , or a high voltage and low current setup. the same physical motor, but with different windings, may be able to handle 700 watts in one wind at 100 amps and 7 volts, or 1000 watts in another winding, with 30 volts and 33 amps, so you can't really say a particular motor can handle "X" watts, as it depends on windings,.. and voltage/amperage combination .Another factor to consider is duty cycle.You can shove an enormous amount of power through a small motor, if only for a few seconds. For example, the B50 S motors are stated on the hackerbrushless site to be 900 watt PEAK motors, good for 35 continuous amps and 50 peak amps for 15 seconds. I know of one particular wind B50 S that was used at one time in F5B on 27 cells and VERY VERY HIGH current,.. basically triple the stated specs,.. but duty cycle was 3 to 5 seconds at a time. Most of the manufacturers are going to be conservative enough (900 watt B50 example above) so that people don't burn things up constantly (above example, 900 watt peak specs, however has been run at over 2.5 KW in very short bursts) Another example the A20 L motor is rated by the manuf as 6 to 15 amps with 19 amps peak on 3S (roughly 150 watts, with 190 watts peak). I know personally that it's fine at close to 300 watts with 3D flying, or 200 continuous,.. so would you have the manuf state it's a 300 watt motor, 200 watt motor, or 150 watt motor? They take the conservative route and people are happy. I guess my point is that they can't really do what you're asking because there are too many variables involved. The ones that do state those numbers have to be on the very conservative side for the above reasons.
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Old 10-25-2005, 04:38 PM
  #23  
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I'll give you a real life example that validates some of the points that have been made in this discussion. I have a Modeltech Simply Magic that I originally converted with an AXI 4120/14. 4s lipoly and an APC 13x6.5e. Pulled 40-45 amps and flew well. When the 2826/10 came out I replaced the 4120 but left the 4s pack. Changed to an APC 11x7e. Still pulled 40-45 amps but the plane was now about 1/2 pound lighter due to difference in motor weight plus I removed two oz of lead from the tail. Plane flys better with this combo. Probably due to weight reduction, but the 11x7 is probably closer in rpm and power to the .40-.46 glow that the plane was designed for.
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Old 10-25-2005, 05:26 PM
  #24  
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Originally Posted by Duster52 View Post
Hobby Lobby, for one, gives you a lot of useful information about the motors they sell. A range of plane weights for gliders and sport, for one. Also, recommended prop sizes, volts, amps and watts are given in a range setting, i.e., certain voltage with certain prop gives x amps and watts to prop. You would have to just check it out go see what I mean.
Yes, Hobby Lobby is one outfit that understands what information modelers need to make a choice of combinations. They really are trying very hard to help. I buy a lot of stuff from them as a consequence - Go figure!
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Old 10-25-2005, 05:36 PM
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Originally Posted by gwright View Post
Why does everyone always get so wrapped up in watts per pound? Yes, it's an indicator, but it's not the end-all solution. I've hovered a plane on 66 watts per pound. I've also had a setup at way over 100 watts per pound that would hardly fly. Watts per pound is not the only indicator of performance. I'll get off that soapbox now <G>.
Gerry,.. if you need 700 watts of power, a motor doesn't have to DISSIPATE 700 watts in heat. It only has to dissipate the number of lost watts. For instance, say a motor is setup (propped/geared/batteried) so that it's consuming 700 watts from the pack, and that setup is say 75 to 80 percent efficient(about the range your axi's are going to be in. Then there are 140 to 175 watts of waste heat that the motor has to dissipate,.. not 700. Thats going to be something probably in the 41XX range,.. an 11.5 to 14.5 oz motor. Now go to a geared in-runner that's more efficient,.. say a B50 S with gearbox,.. and say it's 85% efficient with a particular setup,.. then it only has to dissipate 105 watts of waste heat. (side note**,.. see the geared inrunner advantage,.. 3 to 5 oz weight savings, and more actual power to the prop ).Now you get into the windings you mentioned earlier. Specific winds can go either high current and low voltage or low current and high voltage,.. so depending on how much battery you're using to get those 700 watts,.. you could have a low voltage and high current setup , or a high voltage and low current setup. the same physical motor, but with different windings, may be able to handle 700 watts in one wind at 100 amps and 7 volts, or 1000 watts in another winding, with 30 volts and 33 amps, so you can't really say a particular motor can handle "X" watts, as it depends on windings,.. and voltage/amperage combination .Another factor to consider is duty cycle.You can shove an enormous amount of power through a small motor, if only for a few seconds. For example, the B50 S motors are stated on the hackerbrushless site to be 900 watt PEAK motors, good for 35 continuous amps and 50 peak amps for 15 seconds. I know of one particular wind B50 S that was used at one time in F5B on 27 cells and VERY VERY HIGH current,.. basically triple the stated specs,.. but duty cycle was 3 to 5 seconds at a time. Most of the manufacturers are going to be conservative enough (900 watt B50 example above) so that people don't burn things up constantly (above example, 900 watt peak specs, however has been run at over 2.5 KW in very short bursts) Another example the A20 L motor is rated by the manuf as 6 to 15 amps with 19 amps peak on 3S (roughly 150 watts, with 190 watts peak). I know personally that it's fine at close to 300 watts with 3D flying, or 200 continuous,.. so would you have the manuf state it's a 300 watt motor, 200 watt motor, or 150 watt motor? They take the conservative route and people are happy. I guess my point is that they can't really do what you're asking because there are too many variables involved. The ones that do state those numbers have to be on the very conservative side for the above reasons.
Gary,

My point was to raise the issue that many manufacturers try and sell products with little or no useful information. Consumers should not have to guess whether a product offered may be suitable for a particular application. The "dissipate 700 watts" does not mean into heat, partly (mostly one hopes) it goes into into work energy creating thrust. Most motors are rated in hp (e.g. 1/2 for an washing machine, 1-2 for pumps etc.) That hp rating can be transferred into watts. 746 watts = 1 hp. It is by no means is the ultimate piece of infomation. But if manufacturers could say that this "typically" with 3s or 4s LiPO's the motor can produce X or Y watts to the prop, it would provide useful information. I was looking at an ad yesterday that offered motors and gave the length, diameter, weight and kv - totally useless information for making a power choice. That is what I mean. I wonder just how many motors they sell?
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Quick Reply: Watts are what we want!!!!!!!!


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