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       Electric Motors for Glow People,  by Greg Kamysz from MAXX products                                                                  www.maxxprod.com

Electric motors provide a clean and reliable power source for models.  Selecting a motor is not much different than selecting an engine when you look at the fundamental requirements. The basic principles that make airplanes fly should be used to select a power plant for the performance desired. Power to weight ratio and flight speed envelope make up the aircraft’s performance. This translates to thrust to weigh ratio wand pitch speed.  Considering power and propeller choosing and electric motor doesn’t have to be a challenge.

The power loading is the first parameter to consider.  Power to weight ratio for electric models is generally quoted in watts per pound (W/Lb.). This basically a performance gauges.  More power available per pound results in better aircraft performance.  Power loading holds truce for models all the way up to full scale.  Some examples listed below.
One horsepower = 746 Watts.
Piper Cub   65HP 1,220Lb  40W/Lb
B-174800HP               65,000Lb 55 W/Lb
Pitts Special           260HP         1,626 Lb120 W/Lb
Spitfire IV1440Hp         5,000Lb215 W/Lb

The chart shows that high performance requires a high power loading. The Cub flies sedately at 40 W/Lb and so will a model with this power loading.  Follow the chart below for Model aircraft power loading.

Mild ROG Performance    50 W/Lb
Mild Aerobatics          75 W/Lb
Aggressive Aerobatics   100W/Lb
3D or High Speed125 – 150 W/Lb
Competition           300 + W/Lb

There are many aircraft designed for internal combustion or glow engines.  We can also approximate the required power based on engine displacement. A survey of available engines resulted in the chart below in terms of watt per cubic inch displacement (W/ci).

Sport  2 or 4 cycle 1250W/ci
BB 2 cycle      1500W/ci
High Perf. 2C/4C   1800W/ci
Racing/ Ducted fan4000W/ci

Multiply the displacement of the recommended engine by the W/ci rating to find the amount of power required of the electric motor system. For example: a .40 plain bearing engines will make .40ci X 1250W/ci = 500 Watts of power. Consider that many.40 size trainer models weigh 6 Lbs, the power loading works out to 83 W/Lb.

Once the required power is known we can look at the motor and battery.  Look at the motor specifications for a power rating. Choosing a motor with a power rating equal to the requirement is safe. Erring on the safe side one would choose a motor capable of slightly more power than is required. The motor must be able to handle the required power and the torque to turn a propeller appropriate for the model.  A high speed model will work best with a small prop at high RPM, while a slower model will work better with a larger prop at lower RPM. Power is a product of RPM and Torque.  For a given amount of power one can have a lot of torque and low RPM or high RPM and low torque.  Getting a lot of both requires more power. RPM and Torque are related to the flight speed of the model.  Take a look at the Himax motors.  Find the ones that meet the power requirements.  Now look again at the prop size. Choose a motor that uses a prop size suitable for the model. Assume that the smaller props work best with models designed to fly fast.  On test flights it is best to try several props in the suggested range which draws an acceptable amount of current to see what works best. A difference of 1 inch in diameter or a couple inches in pitch can change the way a model flies drastically. If prop size doesn’t narrow the selection to one motor, consider a gearbox or the simplicity of an outrunner motor direct drive. Also consider the voltage required to see what kind of battery is needed. The battery must be able to support the current requirement of the motor with the chosen prop.  Check the current rating of the battery to make sure it will not be overworked.  The ESC must also be sized to handle the voltage and current requirements.
  
Comment by Gene Manno:  His description is good, but he stops a little too soon to make practical use of the information. This explanation tells us why we need the power, but leaves out the calculation of electric power. (Watts = Voltage times Current)  (Battery capacity current where Amps = MAh times C rating. Example: a 3S 2000Mah 20C battery will deliver a maximum of 40 Amps at 11.1 volts produces 444Watts). This tells you that the ESC the speed control unit has to handle at least 40 A and will turn the motor at 11 times the KV of the motor, and will deliver approximately 444 Watts.  Do the math for the model in question, and you will get an idea of the performance you can expect from it. 

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