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Electric Power Guide

Model Aeroplanes


Lots of electric powered planes about now, but if you don’t buy one ready to fly with all the bits, there is some mystery about where to start to put your own together. It’s not a precise science. With IC aircraft it was relatively easy, motor sizes were well established for all sizes and types of models, so you selected the right motor capacity and prop and off you went. With electric there are more variables to get right.

I haven’t gone into all the possible combinations of motors, props, ESCs, batteries etc., but below are some thoughts on how to go about selecting the bits you need for your electric project to be successful.

The following relates to sport planes in general up to 5 pounds or so (lb, equal to about 450 grams). Large aircraft or EDFs become more complex, although the general rules still apply.

First, think about the all up flying weight of the model. Thrust makes your model travel fast enough to fly, and thrust = power. In electric terms, power is Watts (W). A good rule of thumb for a trainer/scale aircraft is 75-100 W/lb. For something to be livelier or aerobatic you need more like 125-150 W/lb.

From this you can make a stab at what motor you want. The power rating in W needs to be the weight times the power level you have selected, i.e. a 3lb trainer needs around a 300W motor. Start with this figure. Motors are generally labelled or described with a diameter, a maximum power rating in Watts, and a kV figure (which we’ll come back to). The power figure is important – it’s a
never exceed figure, push the motor harder and you’ll burn it out. So the maximum power rating you have selected, plus a safety factor, is going to dictate the rest of the electrickery.

Don’t buy motors blind – manufacturers’ and suppliers’ websites indicate a range of prop size and battery combinations which keep you inside the max rating of the motor – use these figures! If they are not available, move on to a motor that quotes them. Use your common sense on the likely motor diameter and prop size for your project. For most sports models, you’re looking at prop sizes around the 9x5 to 11x7 range. If you use a bigger and/or coarser pitch prop than recommended, the power demand will increase. If the max wattage is exceeded in this way, the motor will be cooked.

The kV rating of the motor is simply the revs it produces per Volt (V) applied, i.e., a motor quoting 950kV will turn at about 10000 revs per minute (rpm) using a 3S LiPo battery. A 4S LiPo will turn it at about 14000rpm, a bit much for most models, which, with the right motor/prop/battery combination, are going to be turning around the 10000rpm, so you may need a lower kV.

In general, the bigger (heavier) the model, the higher the battery S rating (number of cells). Applying more Volts will make the motor go faster, so in general the bigger the model, the lower the motor kV will be to suit the prop. I’m only looking here at the typical 3S (11.1V) or 4S (14.8V) battery, common on every field.

So, we have a model weighing 3lb requiring 300W of power, turning the recommended prop (10x6?) at some10000rpm with a 950kV motor. Next we want the speed controller (ESC) to suit.

  • School Physics lesson:- WATTS = VOLTS times AMPS.
  • ESCs are rated in Amps (A).
  • Amps = Watts divided by Volts

We know the wattage (300) and the voltage (11.1), so the
minimum ESC rating we want is 27A. Although most ESCs will take a bit more than their quoted rating in bursts, nothing works well (or for long) at its operating limit. Given the cost differential, it pays to overrate the ESC, in this case 40A would be fine. You could use 80A, it would work the same, just cost more and be more bulletproof in terms of capacity.

Another thing about ESCs – they have an output which supplies the power to drive your receiver and servos. To do this they step down your battery power to about 5V. Most ESCs will state the number of (standard) servos that it will comfortably operate, generally 3 or 4. If you use more powerful and/or digital servos you stand a good chance of exceeding this, the result being overheating and/or a draw from the ESC which exceeds its capacity to provide. Loss of sufficient power to the receiver will mean signal and control loss, and a broken plane or worse.

To avoid this, if you think you are going to exceed the ESC limits, use a separate flight battery for receiver and servos. This can be a standard NiMh 4.8 or 6V pack, or a 2S LiPo with a voltage regulator. To intercept the power supply from the ESC, use a short extension lead and clip the RED (live) wire. This will stop the ESC supplying receiver and servo power and allow your separate flight battery to supply it. More complex and expensive ESCs have more sophisticated BECs for power management which I will not go in to here.

Overall, the best investment you can make is 1) a LiPo battery checker, and 2) a watt meter to test your setup. The watt meter plugs between the battery and the ESC, displaying V, W and A read-outs across the throttle range. Using this will give you precise test data to check and refine your power pack for power draw, prop size effects etc. With a bit of maths you will also be able to estimate and optimise your safe flight duration, always important with electric flight.

If you’re worried about duration, take your 100% charged battery, do a 3 minute normal flight, and check battery charge percentage remaining. If you’ve used say 30%, you’re using some 10% a minute, so with a safety factor of 20%, you’re OK to set your timer for an 8 minute flight with some reserve for a go around or other delay.

© Dave Burstow 2015

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