A lot of people are confused when they first get involved in electric flight as to which motor, speed controller, battery combination to use to power their model.  This is the first in a series of posts that will look at how to determine and choose the best power train for your electric model…
The first, and most useful, development in the world of electric flight are the manufacturers who actually label their motors with equivalent 2-stroke i.c. motor sizes.  The best know of these are the E-Flight Outrunner series, which stretch all the way from .10 size motors right up to a whopping 1.80 size unit!  They aren’t the cheapest on the market but are very high quality and certainly worth the bucks if you can afford it.  We’ll look at i.c. to electric conversions in another post.
However, if you are looking at building a custom power system, maybe from E-Bay or another online seller, then you will need to work it out for yourself.
The first thing we need to look at is what sort of model you are building and how much power you need to fly it…
With electric models, power is measured in ‘Watts’.  Effectively the more watts a motor can produce, the more thrust is generated at the propeller.  Different types of model have different power requirements, a slow flying trainer needs far less power than a balls-out 3D model.  To give a basic idea we use the following:
Trainer/Sports Model:     90W/lb
Powered Glider:    120W/lb
3D Model:    175-200W/lb
This is just a basic guide, a lot of my models are of the 3D type and so I aim for 200W/lb.  I have designed a couple of smaller sports models that have used 90W/lb and have flown very nicely.
You will notice that the figures given state watts per pound (lb).  You will need to look at the finished flying weight of your model and then work out the power from there.  For example, if your ARTF spitfire weights 3lb 8oz flying weight then you will require (90 x 3.5 =) 315 watts minimum from your motor to get a decent flying performance.
So how do you work out how much power a motor will give?
All motors will give you basic information in their advertisement (if they don’t then don’t go there).  This will hopefully include a ‘Continuous Current’ rating and a ‘Recommended Input Voltage’ – these are the two figures you want to look at.
Quite simply multiply one by the other, for example:
Motor Continous Current = 30A
Recommended Input = 11.1v
Therefore 30 x 11.1 = 333 watts
The 333 watts is what the motor will generate at full throttle and assumes you are using the recommended prop size and your lipo isn’t losing all of its charge when under load.  Therefore, always select your motor to give slightly more power than you need.  You don’t have to fly at full throttle the whole time and flying at lower throttle settings will give you a longer flight.  The only way to know for sure is to use a wattmeter when you have your set-up in the workshop – but that does’t help when you’re buying.
Have fun out there and drop me a line on the contact form if you want to ask an electric flight question…

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