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Mar 12, 2002, 07:44 AM
Registered User

Motor power ratings

I thought I'd start this following a question in the thread "Who has extensive brushless knowledge"

Martinl asked - "Is there a easier way to show the equivalent brush to a brushless motor?? when we compare a brushed motor to in simplicity 05...25IC 25-40.....40-46IC"

It would be really great if manufacturers / distributers could indicate an equivalency rating for motors or ESC motor combos;- for instance "Equivalent to a .25 IC engine when used with 10 x 2000mah cells and a 9 x6 prop or to a .40 when used with 14 cells and 10 x 6" etc.

This would make it much easier for Efly newbies (and some not so new) to select a power train for their model

Any of the manufacturers / distributers out there want to comment?

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Mar 12, 2002, 08:09 AM
Balsa Flies Better!

The more I play with electrics, the less I like this approach- it always seems to come up with less than optimal electric power systems. 9 times out of 10 you will do better by discarding any preconceived notions from glow, and examining the airframe at square one. Simply put- glow engines match propellors to the engine requirements, electrics allow matching propellors to airframe requirements- a better way to design IMHO.

Mar 12, 2002, 08:27 AM
ChrisP's Avatar

Not a manufacturer, not a distributor just an engineer :

The model is pulled through the air by a propeller. The air doesn't know (or even care) if the propeller is driven by an internal combustion engine or an electric motor. All that matters to the air is that a prop of a certain size (diameter & pitch) is rotating at a certain rpm.

The only thing that complicates life slightly is that the power of internal combustion engines is (usually) given in hosepower and electric motors (usually) in watts. There are some exceptions - like buying an 0.25 hp electric motor for a lathe or German cars (my neck of the woods) who's power is now given in kW.

A direct comparison, however, is dead easy as 100 kW = 136 horsepower (apologies for my use of the conversion factor for German hosepower in this case, but I've been here too long).

So a 0.15 hp 2.5cc glowplug engine would be directly equivalent to 0.15 x 735 = an 110W electric motor. Say something like a fairly standard Speed 600 pulling 19A on 8 cells (8.8v x 19A x 65% effficiency).
Mar 12, 2002, 08:36 AM
Registered User
Chris, agree with what you say, I've been playing with this electrickery stuff long enough to have a feel for it but when I started the lack of a ready comparison to what I knew worked IC wise really slowed me up. If comparisons were easier I think more people would slide (on exhaust residue?) over to electrics.

Mar 12, 2002, 08:55 AM
Balsa Flies Better!


If you do these conversions you will rapidly come to the conclusion that glow motors have far superior power to weight than electric motors when everything is included in (even the weight of the fuel- often omitted.) However, the mfg of electric motors are not stupid- there's a good reason not to try and come up with a direct comparison. Glow motors use very inefficient props- they are often way too small for the airframe.

Example- take a cheap 40 which is often rated at 1.5 hp or about 1100 watts. Most of these hp ratings are nonsense since they are given at rpms far from where the engines are operated. How many times have glow mfg stated 1.5 hp at 16,000 rpm while the recommended operating limits for the motor are 3,000-12,000? In practical terms- somewhere between 600-800 watts will swing a 10 x 6 prop at 11k-12k rpm- effectively similar to a glow 40. Hence the 40s true output is closer to 1 hp, not 1.5.

Now look at propellor sizes- A typical 40 size airplane has a wingspan of around 4 ft. A prop should be 1/4 to 1/3rd the size of the wingspan (1/4 works well in lots of cases) hence a 12 inch prop. You can swing this size prop with an e- motor on 500 watts, and the additional efficiency will more than make up the power difference between the glow engine and the electric motor.

I've been down this road before. I had a model, and I was convinced that to get it to fly properly, all I needed to do was spin the same prop as was called for on the glow motor version at the same rpm, at the same weight, and I'd have similar performance to glow. Typically what happens is the electric version comes out heavier, and then people whine about the lack of performance of the electric. However, if you look at the airplane unbiased by glow engines- you will have a much better shot at coming up with a suitable power plant. My best conversions have not involved matching watts to horsepower, but matching propellors and motors to airframes. This flexibility is where electrics shine.


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