How to Select Electric Power Systems:

It's actually not that tough to determine what size motor to use for a particular application:

Let’s first take a look at motor nomenclature.  The primary motors in use today are the Brushless Outrunner Motors.  These motors can be identified by:
•  Weight – The overall weight of the motor, e.g., 24g (aka Blue Wonder)  This is the motor that is typically used on the LightFlite Bug.
•  Dimensions & kV – e.g., 28-20-1380, where diameter is 28mm, length is 20mm and the kV is 1380 rpm/volt.  kV is useful for determining how “hot” the motor wind is.  This the no load rpm the motor will turn per volt input.  Low kV is good for slow flyers, high kV is good for fast movers.
•  Brushed motor equivalents – e.g., E-Flite 400 (roughly equivalent to a brushed Speed 400 motor, which was called a 400 because it was 40mm long.
•  Glow engine equivalents – e.g., E-Flite Power 25  (roughly equivalent to a .25 glow engine)

The main problem with all of these different schemes is that it’s hard to compare motors from different manufacturers.  However if we know the Wattage rating of the motors, then we can easily compare them.

If the motor specs don’t list Wattage, you can compute it by multiplying the max continuous Amps by the Volts supplied by the battery pack.  Assume 3-1/3 volts per cell for LiPo packs, so a 3-cell pack would be good for 10V (under load).  So, if we had a motor with a 10A max, then the wattage would be 100W on 3 cells.

Okay, so now you understand how to compare motors.  Let’s take a look at how we can determine how much motor will be needed via the following steps:

1)  Determine what the all up weight will be for your plane, in pounds.

2)  Determine what kind of flying you want to do with it:
     •  50-70 watts per pound; Minimum level of power for decent performance, good for lightly loaded slow flyer and park flyer models
     •  70-90 watts per pound; Trainer and slow flying scale models
     •  90-110 watts per pound; Sport aerobatic and fast flying scale models
     •  110-130 watts per pound; Advanced aerobatic and high-speed models
     •  130-150 watts per pound; Lightly loaded 3D models and ducted fans
     •  150-200+ watts per pound; Unlimited performance 3D and aerobatic models

3)  Multiply by the number of pounds (from step 1) by the average watts per pound value (from step 2) to see how many total watts are needed.  For example, if you have a 24 oz (1.5 lb) sport aerobatic plane, you will need 1.5lb x 100W/lb or about 150W.

4)  Now you can start looking for motors rated for at least as many watts as were computed in step 3.

5)  Once you've selected a motor, and know the Amp draw, it's easy to pick a speed control that will provide that many amps.  Just make sure it's rated for the number of cells you're planning to use.

6)  Now you need to select an appropriate battery pack.  Battery packs have a “C” rating that indicates how much current can be safely pulled out.  To compute the max current for a 20C 2100mAH pack, multiply 2.1A x 20C which yields 42A continuous.  Higher C packs will run cooler and allow you to run smaller/lighter packs.

Hope this helps with your new electric setup!

(see attached .pdf for printable version)