While this is not intended to be a “full on“ treatise on model aircraft electrics it is hoped that it will help to explain some of the phraseology, meanings and formulae used to the newcomer to this branch of the hobby. It may also help and encourage some designers to expand into model electric flight by explaining the power set-ups required for the successful first flight of a new design.

Firstly there are inrunner and outrunner motors. Ignoring the technicalities, the inrunner is, generally speaking, a bit more expensive, can have a higher Kv ( thousand RPM per volt) and may be more efficient but may only be able to turn a small diameter prop in comparison with an outrunner of the same overall rating. It will also be able to turn a large diameter prop but may require a gearbox. An outrunner is able to deliver more torque and therefore able to turn a larger diameter prop without a gearbox and is less expensive. When looking at the specifications of motors you will find that most providers state a suitable range of prop sizes which is a great help.

The batteries in use today are mostly LiPolymer (Li-Po) so these are the ones I will mention here. However, even if you still used ni-cads, the formulae and information still applies.

Battery charging…….Some serious advice on charging li-po batteries.

Charge only with a dedicated charger and buy the best one you can afford, especially if you intend to do some serious electric modelling.

Charge the battery inside a charging bag or in a biscuit tin with an asbestos soldering mat in the bottom.

Charge outside and not in the house.

Follow the charger instructions to the letter.

Do not charge in the model or in the back of the car.

And, if we get some warm weather this year or not, do not leave the batteries in the car or they may overheat, and explode.

Discard any battery which has swollen or has been damaged in a crash. Do not be tempted to try it once more.

Only buy your batteries from an established dealer in the model trade.

You will need to remember these simple formulae……….

Volts x Amps = watts.

To find the approximate duration you will get from a battery, Divide the amps into the battery capacity which will give you the duration in percentage of an hour.

We will now discuss a circuit supplied by a 3S (3 cell battery = 11.1volts) li-po battery the circuit load of which is 44 amps which equates to 440 watts. ie The nominal battery voltage (11.1v) will drop to about 10 volts when the motor is running on load.

Volts, amps, watts! Watt the heck are they?

Volts is the pressure at which the amps are delivered around the circuit. Using a water analogy, the pressure required to pass one gallon of water past the end of a hose in one minute.

Amperes (Amps) is the measure of an electrical charge passing a point in a given time. Oh!! Really? Imagine an amp as the amount of water, say one gallon, coming out of a hosepipe in that one minute.

Watts is the amount of work that that amount of water will do. Say, the ability to turn a water wheel at 10 revs per minute.

Batteries are specified as an amperage, voltage and “C rating“. This is a most important piece to grasp if you want your batteries to give you a good life. Amperage is given as, for example, 2200Mah. If the C rating is given as 20C then to know what the maximum amps (amount of water) the battery can deliver you must multiply 2.2 amps (2200Ma) by 20 which gives you 44 amps. This is the maximum load that the prop must be allowed to place on the battery. Exceed this figure and the battery may overheat and explode whether or not it is a li-po or any other type.

To find what the load in a circuit is you must use a wattmeter. Connect this to your circuit, power up and run the prop at full revs. The reading you are aiming for is about 20% below the C rating (44 amps in our example) in order to preserve the longevity of the expensive li-po. Some may disagree with this figure and use a higher or lower one but it is the yardstick I use. If you get this figure ie. 35amps, the prop is the right one for the motor/battery combination. If it is higher then you must change the prop for one of a smaller diameter and/or pitch or fit a battery of a higher C rating.

There is also another unit in the circuit which must be considered in this equation and that is the Electronic Speed Controller (ESC). This will have a specification stating the number of cells it can handle and the maximum amperage. For our example it will have a maximum cell count of 3S (11.1 volts) and 50amps. It could be a 2S = 2 cells = 7.4 v, 1S = 1 cell = 3.7v, 5S = 18.5v = 5 cells etc. or any number. To explain then. 3S will be the maximum battery voltage it can handle ie. 11.1 volts . 50 amps is the maximum circuit load it can handle. In our example the circuit load (due to the prop load) is 35 amps but, as the battery maximum load may be as high as 44amps, one must have an ESC higher than this to manage any overload which may occur, hence the requirement for the 50 amp item. This will prevent the ESC from burning out; they can be expensive!

For some time it has been my practice to only buy one make of ESC, one which has available a programming card. ESCs may be programmed (set up) by the card or by the transmitter. The last is a real pain and not to be recommended unless you want a visit by the white coated men in ambulances! Instruction sheets come with the card and explain their use but, sometimes, do not advise which settings to install. My preferences are………

  1. Set battery type. In most cases “Li-Po“.

  2. Cut off. For all models, except gliders, set “soft“. Gliders usually have a prop brake to ensure that the prop folds.

  3. Cut-off volts. Usually has three settings, low, middle or high. These relate to the voltage at which the motor cuts. Low can result in a battery going to too low, voltage wise, for my taste and so I use a very conservative “high“ setting. This is good for battery long life but slightly reduces the duration of the motor run.

  4. Start mode. Normal is set for fixed wing models and any others are for helicopter settings.

  5. Timing. Low is set for most motors which a beginner will use. However, Low is meant for the very high Kv motors ie helicopter tail rotors and EDF motors and Medium for the middle range Kv items. For the high speed models such as pusher prop jet types use High as it will give a higher speed.

So, if your circuit is within all the parameters mentioned all will be well. But how do you know which motor/battery combination will provide enough power to fly your model in the manner and speed for which it was designed?

Some empirical formulae are given here and equate to the watts required for each pound of model weight when it is ready to launch and fly.

Old Timers. Sport gliders. 50 – 70 watts.

Sport aerobatic. Pusher jet type (Fast aerobatic.) 100 – 125 watts.

Scale types. Golden era. 70 – 100 watts.

Modern fighter. 100 – 125 watts.EDF 200 – 300 watts for hand launch. 300 – 400watts for ROG.

These are the figures which I find suit my designs and my type of flying and may be considered a little high.

Props. A reference point to keep in mind when choosing a prop is that, generally speaking, the slow models, old timers and the heavier types such as Scale models need a large diameter, low pitch prop coupled to a motor of low KV. (1000 RPM per volt with no motor no load) A fast pusher jet type needs a small diameter, coarse pitch prop coupled to a motor of high Kv. To get the best performance from your model a little experimenting with prop sizes may pay off here. Precision aerobatic types are a different fish and may need a large diameter, coarse pitch prop coupled to a motor of low KV.

Wattsawatt? You may ask. As in the water analogy circuit example given above, a watt is the power needed to turn that water wheel at 10 RPM. Also in the circuit you have volts and amps and if you multiply one with the other you get watts.

Example. A fast, pusher jet type weighing 2 lbs needs 100 – 125 watts per pound = 2lbs x 125watts = 250 watts.

You decide to use a 3S li-po battery = 11.1 volts but on load, ie with the motor running at full throttle the volts will drop to about 10 volts with a good battery. To find the motor you will need you must divide the 250 watts required by 10 volts = 25. This is amps. You now know the amperage of the motor to use so you look for a motor rated at 25 amps continuous and with a high Kv. As things stand today the highest Kv which will turn a prop is in the region of 2200 – 3200 Kv. Anything above this will be for EDF units or helis. Note. Low Kv = large diameter prop. High Kv = small diameter prop.

You now have the specifications for your motor which is one of 25 amp continuous rating from an 11.1 volt battery with a Kv of 2600.

So you may ask, “How can I get my plane to fly faster?“ There are two ways you can do this.

  1. Fit a coarser pitch prop. This will increase the electrical load, ie higher amps ,and you may have to reduce the prop diameter to keep it inside the maximum circuit load ie amps. The planes duration will be lower due to the higher amps.

  1. Fit a higher voltage battery. ie increase the cell count., and keep the same size prop..

To increase the duration of your motor run, increase the battery capacity from, say, 2200mah to 3000Mah. “Mah” means mille-amps per hour. 2200 m

Mah therefore equals 2.2 amps per hour. 2.2 amps per hour means that the battery is able to provide 2.2 amps for one hour or one amp for 2.2 hours.

As long as you keep the circuit load within the limits of the ESC, motor and battery, by reference to your wattmeter/multimeter, all will be well.

A point to remember when testing a motor on the bench indoors. Do not run at full power for more than about 20 seconds, or you may overheat it as there is insufficient airflow to do the cooling.

And lastly……keep everything cool man!! Plenty of `passing wind`!

And lastly again!! When you are changing a prop, or changing the settings on your tranny, or doing any work on the model DISCONNECT THE BATTERY.



Mike White