This is a two-part beginner’s guide to electric flight basics – hopefully in a simple to understand form. It is not a complete A-Z of everything you need to know, but should help to de-mystify some of it, so put the kettle on, get the sticky buns out and come over to the dark side….
First off I should say that as in all things aeromodelling, there is no better way to learn than actually taking the plunge and trying it – keeping in with your local club’s electric flight wizard will also be invaluable. However it is all too common to dive in and end up making a poor choice which could end up both costly and with disappointing performance too.
There are now several electric flight specialists retailing everything you need, and the better ones will advise you about which components to buy after they have discussed your requirements – after all, they want you back for the next model!
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With this in mind, let’s try and arm you with some useful knowledge which should at least give you the confidence to have that discussion and actually be able to understand what the retailer is saying. Also don’t forget that there have been several articles in RCM&E recently which are really helpful to beginners – indeed a series was recently featured by the excellent writer Tony Jones. Remember also, that many models now come with recommendations as to the most suitable power-train, and that’s obviously the easiest and usually the best route to success.
I’m going to approach things from a slightly different angle to the norm, and expand in a little more detail on the four main components of a modern electrically powered model in the context of what you may already be familiar with if you fly internal combustion (i.c.) powered models.
Rather than lecture you about the right type of model to choose, I’ll describe the various factors to consider for different model types – after all you may have long since retired the big high-wing trainer and now hanker after a nice sexy jet job so who am I to dissuade you!
One thing to consider here is cost. Even more so than with i.c. power – the bigger and faster you go, the pricier things get. Sure there are plenty of relatively inexpensive foamy type rocket-ships around that can be successfully flown on similarly inexpensive power-trains, but if your dream is for a big 1/4 scale Lancaster bomber, then get ready for a chat with the bank!
The Graupner Elektro Kadett is a fine electric trainer/sports model and an ideal introduction to the electric genre
Some of you may want to try your hand at converting an existing model that perhaps was i.c. powered, or maybe a glider from yesteryear that still has an old buggy motor still stuck up the sharp end. Almost anything is possible these days and getting more practical every day – thanks in no small part to the volumes of equipment coming out of the Far East.
One major factor to successful electric flight is getting the power-to-weight ratio right and this can be linked to the type of model. Any shortfall in an electric powered model will be more noticeable than in an i.c. powered model. In trying to keep things simple lets break these down into just three types for now.
There is a generally accepted rule of thumb when it comes to calculating power requirements for our needs, and before I reveal what that is, I had better explain just how we measure electric power – look out here comes the first techie bit!
Unlike our old friend the i.c. engine which is commonly measured in cc or horsepower or even just “it takes a .50 size two-stroke mate” – electric power is different, and is measured in watts. Watts is derived from the formula V x I – where V is voltage, and I is current, or amps.
The rule of thumb I mentioned earlier is that we need to aim for around 100 watts for every pound (WPP) of weight – that’s the final all-up weight (AUW). This does depend though on several factors, not least of which is the model type as outlined above. Our modest everyday high wing model will probably be more than happy on the 100 watts per lb rule; the lightweight foamy park flyer will probably manage on a bit less, say… 80 WPP, but for a decent performance our jet will be much happier at nearer 200 WPP.
A good charger is one of the first things you’ll need
Factor in the effect of drag which will differ across model types, wind speed and flying style and you can see why this is a rule of thumb only! Slower flying scale models will also probably be happy at around 75 – 80 WPP. Having said that, if during your planning stage you aim for 100 WPP then you’ll not be far wrong and even if it proves more than you need, you do have a throttle.
If any one part of an electric flight set-up causes severe head scratching and frustration, then it is motors. As someone once said, the great thing about standards is that there are so many to choose from! I cannot hope to unravel all the mystery in this article, especially when it comes to motor numbering and lettering, not least because every manufacturer seems to want to use different designations for their particular products – and even some of these don’t follow a standard.
Most of them however do provide within that strange looking number, some important information such as the diameter and length of the motor, however there are more important things which you need to establish before choosing from the thousands of different motors available, such as Kv (revs per volt), typical prop sizes, the battery required and our old friend Mr Max’ Wattage.
Some motors have’equivalent’ i.c. designations like this .46 outrunner motor from E-flite
Kv is the number of revs per minute that the motor will provide for every volt supplied to it. So, looking at a well known and good quality range such as the AXi – lets try and simplify the data. Each AXi motor has a 6-digit (metric) designation eg: 22/10/38. The first two numbers (22) represent the diameter of the stator – the fixed part in the middle of an outrunner motor. The next two numbers (10) represent the length of magnets (attached to the rotating case) and the third set of numbers (38) represents the number of wire winds, sometimes called turns. Those with more winds have a lower Kv, but they generally have more torque like a four-stroke i.c. engine, can spin a nice big propeller at slower speeds and use less current than lower turn motors.
Motors with fewer winds have a higher Kv and are good for turning small props (and ducted fans) at high speeds, but do tend to use more current than the lower Kv motors. Going back to the designation ‘outrunner’ for a minute, this simplistically means the moving part is outside, and with an inrunner, the moving part is… you guessed it – inside!
Generally speaking inrunners are high Kv motors, and outrunners are low Kv, although many outrunners these days are being used in situations where, not long ago an inrunner would have been the usual choice, furthermore, outrunners are normally easier and cheaper to produce.
Sticking with these two main types, let’s make a selection. For a high speed model, a low-turn high-revving inrunner motor with a small prop would be right, but for a trainer, or draggy, slower flying biplane, choose the high-turn, higher torque outrunner motor and a nice big slower turning prop to produce the thrust at lower speeds.
Other brands of motor will actually state the Kv and maximum amps or watts that the motor can handle, some also specify the maximum voltage possible, although without the maximum wattage available this is of little help, but will give a guide as to the battery size recommended
PROPS AND POWER
Electric motors will burn-out if the current flowing through exceeds the stated maximum, although they are not particularly worried about the voltage applied in its own right. However, our old friend Ohm’s law dictates that if the volts go up, then so does the current and this must be allowed for when choosing your battery pack. The other thing that happens with electric motors which does not apply so much to i.c. is that they demand more current when you stick a bigger prop’ on. They won’t actually slow down very much, but just keep drinking more and more current in an effort to maintain their designed Kv.
Prop size matters and the best way to see what’s going on is by using a watt meter
For this reason, propeller selection is very important, and a prop’ which is even just slightly too big, on a given battery size (volts) could easily cause a burn-out. Kv also tells you roughly what sort of voltage the motor is intended for because RPM is proportional to Kv. A ten-turn electric motor could spin a 12 x 8″ prop on three Li-Po cells, but needs to come down to a 10×6 on four cells – the latter develops much more power, but needs to be allowed to rev harder at the higher voltage to avoid consuming too many amps and possibly burning out.
So generally speaking a high Kv means a low cell-count and viccky verkky. Now, how can this knowledge help us choose the right motor for the job? Well, there are several different ways to go about this decision, including one that I tend to favour which is to choose a set-up that will provide the power needed (watts) in the correct ‘style’ for the type of model.
So to revert back to the first section about model selection, let’s say we have a largish slow-flying high-drag scale type model. Presumably we know the expected or recommended AUW (all-up-weight) so we should be able to start narrowing things down a little already. If the AUW is say 8lb and the model requires a nice big prop to fly at scale-like speeds, then we would choose an outrunner motor, which will be happy working at around 600 watts (8 x 75 watts per pound). Now how we get those watts from our motor is determined by the three things discussed already – volts, amps and of course, the prop size.
A watt meter takes all the guessing out of the equation – amps, volts and watts can be measured across various prop and motor combinations
Most motors will state the recommended prop size, so choose one which is happy swinging the size you intend to fit, but be prepared to experiment with a few alternative sizes. I usually decide what size battery I’m going to fit (in terms of volts) and this depends to some extent on how much weight the pack will be, as this affects the AUW. Now, our power requirement is 600 watts, and we achieve this by a combination of volts x amps. Let’s say that due to reasons of weight and space, we have chosen a battery of four Li-Po cells in series (forget the capacity for now – its volts we are interested in at the moment), this equals around 14V.
To get our required 600 watts, from a 14-volt battery, will require around 43 amps (14 x 43 = 602). We also want it to swing our prop at an optimum RPM which should ideally match the motors peak efficiency, and again for simplicity, we will aim for around 9000 RPM. We will therefore need a motor which spins at around 650 Kv, (9000 / 14 volts = 642)
Now we could also achieve these 600 watts by running a 10amp motor from a 60v pack, a 20amp motor from a 30v pack or a 60amp motor from a 10v pack and so on. However, each of these different motors would have a different Kv and so would develop the required power on a different prop size – so we should choose a combination that develops the power we want on the prop size we need. It follows that motor around the 650 – 700 Kv mark, and is happy at 43 amps should fit the bill.
Control of the motor is achieved by the use of an electronic speed controller – better known as an ESC. The important two basic rules to remember here are:
Next month I’ll look at batteries and radio equipment along with charging and balancing.
Click here to read Electrickery Pt.2.
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