Measuring a motor by Stator size is the most accurate method to use, since it is the stator that creates the power in an electric motor. This would be analogous to measuring an internal combustion engine by the piston displacement, since that is what produces the power in a gas or glow engine. You would never say my car has a 18 x 20 inch V-6 engine. It would be stated as 3.0 Liters or 183 cubic inches. In the same way, measuring an electric motor by the outer dimensions really makes no sense.
Due to the physical shape and construction of a motor, the outside dimensions are an inaccurate means of determining motor size. For example, a Cobra 2826 size motor has a stator that is 28mm in diameter by 26mm long. The external dimensions of this motor are 35mm in diameter by 46mm long, as measured from the mounting hole face to the top of the rotor end bell, not including the bump on the top that centers the bolt-on prop adapter. The E-Flite Power .25 motor, which has the same size 28x26mm stator lists its external dimensions at 35mm in diameter by 54mm long. This is because their motor design uses a taller back-plate and end bell, and measures all the way to the top of the centering bump on the top of the motor. If you went by their external dimensions, you would think that the E-Flite motor is larger, and therefore more powerful, when in fact they are the same size motor internally and produce similar power levels.
The companies that try and make it “Easy for Glow guys’ to size their motors, and call them a Power .46 or G-35 or something like that, to describe the motor as putting out similar power to a .46 or .35 glow engine, are actually doing a huge disservice to the modeling community. The problem with using this type of motor description is that the motor only puts out the suggested power with 1 or 2 props at a specific voltage. This is because unlike glow engines, that put out a constant amount of power, electric motors put out a constant RPM. As you change the prop size on an electric motor, the power output varies dramatically. Also, electric motors tend to use bigger props and turn at lower speeds than their glow counterparts, and this can cause huge problems for people that are new to electrics. For example, a .46 glow engine would typically use a prop like a 10x7. This engine will spin the prop somewhere around 13,000 RPM, depending on the power output of the engine. If you take a typical .46 equivalent electric motor and run it on a 4-cell Li-Po pack with a 10x7 prop, it will only spin at around 8500 RPM, and will make the power of a .15 size glow engine. This gives a very underpowered situation to an aircraft, and could lead to the model being un-flyable. You would need to run a 14x8 prop on this motor to get the power of a .46 Glow engine on 4 Li-Po cells with the motor spinning at 8000 RPM.
If you moved up to a 5-cell Li-Po pack, it would take a smaller prop to make the same amount of power. In this case, an 11x8 prop would make the power equal to a .46 glow engine, spinning at around 10,800 RPM. If you moved up to a 6-cell battery pack, then the same electric motor would product the power of a .46 glow engine with the original 10x7 prop, and would spin at about 13,000 RPM, just like the glow engine would. That is why prop selection is so critical for electric powered models. In a glow power model, the engine pushes its power into the prop, conversely, in an electric motor, it is the prop that pulls the power out of the motor.
All of the higher quality motor manufacturers such as Cobra, Scorpion and AXI use the stator size as the method for determining the size of their motors. All of the Cheap Chinese motor companies such as Turnigy, Leopard, Hobby King and others use the external dimensions of the motor. That alone should tell you something! In the end, the best way to keep electric motor numbering consistent is for all manufacturers to use the stator size as the method for determining motor model numbers. This gives you the true power output ability of the motor, and puts everyone on a level playing field.
Answer by: Innov8tive Designs (Admin) on Mar 20, 2015 10:56:00 AM