Glow vs. electric motor equivalency

I' m sort of starting over in modeling. My last experiences were in the late 1950s when control line models were finished with dope and silkspan, Ambroid was the best glue,
and control line flying was king (only a few "older" folks with real $ could afford RC). Needless to say, CA glues, monocoat, foam, and electric power are all new to me. BTW, this group has been great about answering my "newbie" questions!
I am still confused about electric motor sizes. A brushed 400 motor is nothing like a 400 in a brushless design. I thought that motor dimensions could serve as a rough way to compare one brushless motor to another, but when you throw in "turns", voltage, etc. this falls apart. No one speaks about glow engine equivalency, yet this is something that would make sense to someone who grew up with glow engines as I did.
Here is my question...... 746 watts is equal to 1 hp. Many brushless motors give watts as a rating (if not, it can be calculated by multiplying volts times amps), so I should be able to calculate the hp for a given brushless motor, shouldn't I? If I then looked up with the hp ratings for glow engines of various sizes (I could use non-ringed OS motors as generics) I could make a table comparing the wattage of brushless motors to their glow equivalent.
Will this be valid? Am I missing something? Is there a better way to judge the power of brushless motors?
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Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge:

As a GENERAL RULE OF THUMB:
75 - 100 watts per pound for a trainer 100 - 150 watts per pound for sport flying 150+++ watts per pound for 3D
You will be in the right ballpark. The plane wll fly with these wattages. The weight is based on the plane ready to fly with the batteries installed.
I am just as confused as you as to what happens when you change the number of turns, put a gearbox on, change diameter or pitch on the prop.
I have been using the above formula for converting my 40 size planes to electric, and so far, I have not been disappointed in the performance.
I have been using a Towerpro 3520-6 or -7 motor, a 60 amp ESC with a 4s 4,000 - 4,400 mah lipo, a 12x10 or 13x8 or 10 APC e-prop and a separate battery for the receiver for my 40 - 46 powered planes. I get equal (or better performance ) than the glow engine provides along with USUALLY 2 10 - 12 minute flights of sport flying. If I am doing a lot of aerobatics and/or wide open throttle I usually get 1 flight of about 12 minutes per pack.
I can highly recommend RC Universe http://www.rcuniverse.com/index.cfm It is an excellent site with a lot of real great people.
Hope this is of some help.
Campy
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Goedendag ;-)
IC horsepower ratings are usually at unrealistically high rpm's, rpm's you will never use.

The e-flight subfora at http://www.rcgroups.com are more active
Vriendelijke groeten ;-) Ron van Sommeren int. electric fly-in, Nijmegen, Netherlands: http://home.hetnet.nl/~ronvans/
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BCRandy wrote:

Its a pretty good way wih some caveats.
All motors do not have the same efficeincy, so a 50% 200W motor only produces 100W at the shaft etc.
Glo engines produce heir power at quite high RPM. Typically well above the optimum for prop efficiency, therefore most glo planes ( 200mph stuff excepted) runs at sub optimal RPM and at consideranly less power than the motor is capable of. Also, since electric motors tend - in the case of outrunners or geared motors - to deliver power at a more sensible RPM into a bigger prop, the electric power to match a '1 bhp glo' may in fact be less than 300W..
With those caveats, your rule of thumb has considerable merit. I tend to feel that around 2KW per cubic inch input is a decent enough guide, which places .40 glo at around 800W input.
The other way is to simply use the watts per lb formula..and pick a prop whose pitch speed loads the motor up to its rated current draw, and whose pitch speed is 2.5 - 3 times the stall speed. That is actually a very precise way to go for subtle reasons.
If you want a sporty performance, 75W-100W/lb is a decent target.
You will note that a typical 5 lb glo plane running at 800W is very sporty indeed.
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Are you sure there are no motors with the same efficiency?

The specified power is the shaft output, not the input. I have an electric scooter motor rated at 100W which requires 140W input.
Barry ====Home page http://members.iinet.net.au/~barry.og I do not represent atheists or atheism
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...
| >so a 50% 200W motor only produces 100W at the shaft etc. | | The specified power is the shaft output, not the input. | I have an electric scooter motor rated at 100W which requires | 140W input.
In the case of R/C motors, the specified power limit is usually the input, not the output -- probably because 1) it's easier to measure, and 2) it makes your motor sound better. And you can put even more power into the motor if you want, and it'll probably work -- for a while, anyways. And under many conditions, it won't even last at this power level.
Also, the efficiency varies, depending on the current, voltage, RPMs and load (which are all intertwined, of course.)
The problems with comparing the peak horsepower of a IC engine to the power used by a electric motor are several --
1) the glow engines will generate that much power at a certain RPM. If your plane is not propped to be able to take advantage of that certain RPM, you'll see lower performance. This is usually the case, by a significant degree.
2) usually that certain RPM is quite high, and gearboxes aren't normally done with IC powered R/C planes, so you're usually turning a smaller prop than would be ideal (unless it's a pylon racer, of course.) So you lose efficiency there.
3) taking the max voltage (above this, the motor spins too fast and it may break) and multiplying it by the max current (above this, the motor heats up too much) of an electric motor to get a max wattage is pretty bogus.
In general, if you take a properly chosen (good quality, appropriate Kv rating, perhaps a gear box) motor rated at 750 watts, and 1 horsepower glow engine for the same plane, with properly chosen props for both, the electric motor will perform significantly better. 4 strokes are a little better, as they can swing a bigger prop more slowly, making them more efficient.
Still, there's little need to guess. Go download Motocalc at http://www.motocalc.com/ and let it help you pick an appropriate motor. Electric power systems are well suited to accurate modelling on a computer like this.
--
Doug McLaren, snipped-for-privacy@frenzied.us
"People who do stupid things with hazardous materials often die."
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Just so you know and just in case you are interested there is still a large subgroup of control line flyers out there. Big enough that Tower and Topflite brought out almost ready to fly versions of the Nobler and Flite Streak and have developed two new models in addition. Another company importing CL ARF planes and still kitting many that you may be familiar with is Brodak. Even SIG makes an ARF CL along with a number of kits. Electric has also arrived in CL with timers that weigh a few grams and control the power. Bob Furr
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