Glow vs. electric motor equivalency

Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge:

As a GENERAL RULE OF THUMB:

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

I can highly recommend RC Universe

Hope this is of some help.

Campy

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.

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

Goedendag ;-)

IC horsepower ratings are usually at unrealistically high rpm's, rpm's you will never use.

Vriendelijke groeten ;-) Ron van Sommeren int. electric fly-in, Nijmegen, Netherlands:

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

Barry ===== Home page

In article , Barry OGrady wrote: ...

| >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 --

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.

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

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