Reproduced by kind permission of Nigel Hawes and RCM&E
SO YOU’VE DECIDED TO TAKE THE PLUNGE INTO ELECTRIC FLIGHT BUT ARE CONFUSED BY ALL THE NUMBERS AND UNSURE HOW TO MATCH UP THE POWERTRAIN COMPONENTS? NEVER FEAR, ELECTRIC FLIGHT GURU NIGEL HAWES IS HERE TO EXPLAIN IT ALL IN LAYMAN’S TERMS!
One of the main problems in taking up Electric Flight, especially if you are currently an i.c. flyer, is the sheer baffling array of motors, batteries, speed controllers and propellers on the market, each having a crucial effect on the other.
With an i.c. engine, the parameters are virtually set in stone; sure, you could use a different fuel, different glowplug, different silencer and a small deviation in prop size, but the performance or more specifically the power output will be lucky to show a 10% deviation.
In short, an i.c. motor specified as being capable of producing one brake horsepower will, under ideal conditions, produce one brake horsepower - and that’s it. They are pretty foolproof, and the data that goes with each different size or make of i.c. engine is pretty much established. As far back as when I was eight years old (and that’s longer ago than I’d care to remember!) I knew that a Merco .61 needed a 12 x 6 prop, straight 80/20 fuel, and a Taylor glowplug. For a slightly more agile aircraft, an 11 x 7¾ prop, 5% Nitro fuel and a Hot-Spot plug gave it the edge - and that was all we needed to know!
Little wonder then, that a dyed in the wood i.c. flyer could pick up three electric motors with 3630-4/780, 22/30/3E, Z3025/12 and on the labels and wonder what on earth it all means!
Well, an article in last year’s special attempted to explain what all the various numbers mean but even when you have mastered the art of motor nomenclature, there are still many other considerations regarding ESC and battery choice, as well as which prop to use!
Conversion from i.c. to Electric, or even just starting in radio controlled aircraft taking the more sensible electric route, can still be extremely confusing so hopefully by the end of this article it will all be a little clearer, and might hopefully save you either from a significant wasted expense or at worst a serious injury: electricity can be dangerous.
THE KEY TO IT ALL?
Sadly, many of the manufacturers and distributors are largely to blame for much of the confusion, each numbering or labelling their product in a different way in the futile hope of avoiding direct comparison with a rival’s products. Some even put charging plugs on their batteries which will only fit their own specified charger, and it has to stop.
Fortunately, the tide is slowly turning and the way many motors are marketed is beginning to become much clearer. In the same way as i.c. engines are rated by their brake horsepower output, electric motors (as they always have been but in most cases, somewhere in the background) are rated by their MAXIMUM WATTAGE.
This being the case, it allows us to immediately visualise what the motor might be suitable for, i.e. whereas 35 years ago we would have deemed a Merco .61 as being suitable for a Gangster .63, we can pick up a 300-watt electric motor and deem it as suitable for a Ripmax Spitfire.
As I touched on in the last RCM&E Special, wattage (or rather the power-to-weight ratio of watts per pound) is the key to success in electric flight.
And the reason that the 300-watt motor is suitable for the Ripmax Spitfire is that this will be a 3lb model, and 100 watts per pound is by today’s standards an almost foolproof power to weight ratio. Naturally, a fast jet or extreme 3D type model will need over 200 watts per pound, and a Piper Cub or powered glider will happily buzz around on 50 or 60 watts per pound; but aim for in the region of 100 and you will never be far out.
Armed with this equation, we can start to figure out our model’s basic power requirement simply on the basis of its all-up weight. For example if we predict our model will come out in the 5lb region, then we are looking at needing a 500-watt set-up for it. Of course we need to know what proportion of that 5lbs will be made up of the batteries and motor itself, but this information is readily available in the product specification.
Going back to i.c. engines for a moment, there are different designs of motor for different types of model; a specific ducted fan motor such as an OS .91VRDF will be tuned to produce its best power output at maximum revs, and has a short stroke to achieve this, whereas the longer stroke OS .91FX is tuned for the more general type of club model it will be used for.
In the same way, electric motors are “wound” for specific applications and this is expressed as a motor’s Kv, which means its “RPM per volt”. If you take a 1500 Kv motor and run it at 10 volts, the shaft will spin at 1500 RPM for every volt applied making 15,000 RPM. This is however a “no load” shaft speed so when you fit a prop, you won’t expect to get the same 15,000 RPM!
Motors with high Kvs of between 3000 and 4000 are generally used either for fast pylon or EDF models where the prop or impeller is of very small diameter. They can also be very useful for use with a gearbox; if you gear a 4000 Kv motor through a 4:1 reduction gearbox, the gearbox output shaft will have an equivalent of 1000 RPM per volt and so on.
Motors of much lower Kv, typically of around 1000, are wound for more general applications such as everyday sport models and are capable of turning fairly large props on the basis that it is far more efficient to move a large amount of air more slowly than it is to move a small amount of air very quickly.
Unless a model is of a specialised type, when I’m looking for an everyday motor to put on one of my prop-driven models I tend to look for a motor with a Kv of around 1000 as a starting point. As your understanding of electric flight progresses, motor Kv figures do become a lot easier to understand and work with.
Lithium Polymer flight batteries are currently enjoying their heyday, which is understandable as the more recent generation of Li-Pos (as we refer to them) give excellent power output, good voltage stability and are much lighter than their equivalent power density NiCad (Nickel Cadmium) and NiMh (Nickel Metal Hydride) predecessors.
But they do have their disadvantages too! For a start, as with any new-ish technology, they initially had a crippling price tag which has gradually decreased to a sensible level but only fairly recently. Secondly, as they are wrapped in a plastic sac rather than a metal canister, in a crash situation they are particularly susceptible to damage, and even in careful use their cycle life (i.e. the number of times you can recharge and use them before the capacity reduces significantly) can often be as few as 50 flights.
Finally, their Lithium Cobalt Dioxide chemistry is inherently unstable, and unless used within very strict voltage parameters they can either be extremely dangerous or rendered completely useless. Charge any cell above 5.5v and they will reach “thermal runaway” and explode, discharge them much below 2.4v and they will effectively die.
At the same time as Li-Pos have improved in performance, not only have NiMhs done the same but have become significantly cheaper too. The Li-Po versus NiMh argument will smoulder on for a good while yet, and it is not the purpose of this article to pick a winner! All I am saying is that BOTH are suitable for electric flight applications, and it is the pack VOLTAGE that is one of the more significant figures that we need in electric flight calculations.
Generally, I look for 10 volts as an “ideal” pack voltage under load (i.e. running in a model with a prop fitted to the motor) as not only does it make the calculations easier, it is also high enough to achieve decent wattage figures without requiring silly currents.
A motor drawing 30 Amps from a 10-cell NiMh or 3s Li-Po that is comfortably showing 10 volts under load will consume 300 watts (30A x 10v = 300w).
A motor drawing 30 Amps from a 7-cell NiMh or 2s Li-Po that is comfortably showing 7 volts will only show 210 watts (30A x 7v = 210w).
To get this wattage figure up to 300 watts using a 7-cell NiMh or 2s Li-Po, you will have to prop the motor to draw a whopping 43 AMPS! (43A x 7v = 301w).
As you can see from this, the higher the input voltage, the less current the powertrain will need to run at in order to produce useful wattage. This is why in highly efficient set-ups people used to use 30+ NiCads or NiMhs and more recently up to 10s Li-Pos; at 40 volts input, you only need to draw 25 Amps to consume 1000 watts which is 1 Kilowatt!
As 3s Li-Pos and 10-cell NiMh flight batteries are extremely popular and available these days, this makes an input voltage of 10v very easy to achieve and for the purposes of an everyday electric model flyer is a good starting point.
CELL PERFORMANCE CAPABILITIES.
Without putting too fine a point on it, with early generations of Li-Pos and certainly a few years ago when NiMhs first hit the scene, their voltage stability under load was rubbish. Of course their constant development for the portable powertool and mobile communications market saw this improve dramatically but Li-Pos are still sadly oversold and overrated by some distributors and the result can easily be a ruined battery.
In order to avoid this disappointment, we can observe a very simple set of rules: buy the highest “C” rating battery we can afford, and run it at around HALF of its claimed performance rating. This means that if you buy a 20C rated Li-Po and run it at 10C, it should be working well within its capabilities and give a decent service life.
But what IS this “C” rating? It is simply the multiple of the pack’s capacity.
If you have a 3000 mA/h Li-Po that is rated at 20C, then its theoretical maximum current handling capacity is 60 Amps. Although some vendors would love to tell you that it can sustain this constantly, your flight will last exactly 3 minutes (i.e. 1/20th of an hour), the pack will get extremely hot, and with regular such use its cycle life will suffer terribly.
The same applies to an older generation 3000 pack rated at 10C; run it constantly at its theoretical maximum of 30 Amps and whilst the flight should last twice as long, i.e. SIX minutes, your pack will be working at its limit the whole time.
It is far better to run the 3000 20C pack at 10C, as this will both allow occasional bursts of more than 30 Amps (even 50+ Amps if you must!) and give prolonged life.
Some of you reading this may have already noticed a relationship between motors and batteries at this stage: a 300 watt motor that draws 30 Amps at 10 volts, and a 3000 mA/h 20C Li-Po that can supply 30 Amps at 10 volts until it is exhausted! Put this set-up in a model which will weigh 3lbs or just over, and you also have your ideal power-to-weight ratio
It is at this point that the penny begins to drop, and you have taken the first step towards conquering basic electric flight calculations. Not only is the wattage of the motor significant, but also calculating the wattage a flight battery can sensibly deliver constantly without being pushed too far, is also a vital step in the right direction.
Keeping the figures simple, a 5000 mA/h Li-Po working at 10C will give 50 Amps at 10 volts; put this in a model that will weigh 5lbs and use a motor rated at 500 watts and once again you have your ideal set-up. It really is this easy!
If only more newcomers to electric flight would buy a Li-Po from a shop or trade stand at a show with this equation in mind, then perhaps there would be less frustration, disappointment and potential hazard in the hobby.
THE SPEED CONTROLLER.
Sadly, the Electronic Speed Controller (or ESC as we all refer to it these days) is one of the most misunderstood and maligned items in Electric Flight; this explains why so many of them get burned out, overheated, melted and generally abused!
Unfortunately only the genius minds of people like Mike Merrick of Mtroniks in Yorkshire (01943 461482) really know what’s going on inside that little box of tricks but over the years Mike has babbled on in technical language at me until some of the basics have actually penetrated my extremely dense grey matter.
The way the industry has gone is to produce these units ever smaller, to the point at which they have almost become a fashion accessory. Sadly, as Scotty continuously ranted to Captain Kirk in Star Trek, “ye cannae change the laws of physics!” - heat needs to be dissipated and the larger the heatsink, the better the dissipation.
A modern sensorless brushless speed controller has a very complicated task in that it has to start a 3-phase motor from a standstill, relying only on electronic feedback from the motor windings following a “kick” pulse, then continue the motor turning in the correct direction under extreme current loads and particularly in the case of high pole count motors, unimaginable commutation speed (i.e. the higher the RPM, the faster the ESC has to analyse the winding feedback and the FETs have to switch the phases).
Add to this the complications of programming various degrees of timing advance, switching frequency, input voltage cut-off point, soft or hard start-up strategy and brake options, and you have an extremely sophisticated piece of technology Velcroed to your model!
As a result of what we learned when testing for the record attempt we made last year, by flying the first electric powered model aircraft across the English Channel, my attitude towards ESCs has changed dramatically.
Although we were only drawing a constant 15 Amps, we managed to melt several 40 Amp ESCs during testing and even got a 60 Amp unit to thermally shut down! The reason for this is that at 15 Amps the FETs are actually doing more switching, and therefore working far harder, than they would be at approaching the stated 40 Amp maximum.
This explains why I now buy the most substantial ESCs fitted with the most efficient FETs I can afford, as some of the cheap and nasty units around really are a false economy. The ESC pictured is the one Mike and the Mtroniks design & development team made especially for the attempt, and you can see the size of the heatsink it uses! This thing never even got warm during the crossing, and that means that no energy was wasted in creating heat - something few people even consider when purchasing an ESC.
Mtroniks apply the same principle to their commercial range of model aircraft ESCs, and you can probably imagine why I use them almost exclusively these days.
HORSES FOR COURSES.
When you buy an ESC, as well as considering the quality and efficiency issues, you must also make sure that it is suitable for the intended application, and that doesn’t just mean its current rating. If you are following our pet example of the 300-watt set-up, this will require an ESC that can sensibly handle a constant current of 30 Amps. However, a 30 Amp ESC is the last thing you should be looking for!
When you consider the price differential to a 40 Amp unit, which will usually be less than £20, is it really worth risking your model on a piece of electronics that will be working at its limit? I tend to think not, and insist on using an ESC with at least 25% in hand, as in this example. For a 40 Amp application, I won’t use anything less than a 60 Amp ESC. This may seem a little paranoid or untrusting of the label - but in the same way as a Li-Po, an ESC working well within itself will pay dividends not only in service life but also in terms of reliability during that service life.
When buying an ESC you must consider the input voltage you are using, and indeed may choose to use in the future. A model flying on a 3s Li-Po will receive an amazing shot in the arm if the input voltage is raised to 4s; but thanks to our good friend Ohm’s Law, this also increases the current! So you need to be certain that the motor and Li-Pos can deal with this extra demand, whilst the ESC needs to be capable of the extra current AND the higher cell count.
Some ESCs are only suitable for 6-10 NiMh cells or 2s-3s Li-Pos, so if there is any chance that the power bug might sting you somewhere down the line, it may be well worth investing in a good quality ESC capable of high currents and higher voltages than you might intend to use at first, as things might change when you taste the potential!
With the price and availability of a 60 Amp programmable ESC capable of 7-16 NiMhs or 2s-5s Li-Pos these days, there is no excuse for losing a good model through the silly use of a sub-standard speed controller. You have been warned!
If you are using a Li-Po, you must ensure that the Power Cut-Off (PCO) function of the ESC is capable of protecting the Li-Po from being over-discharged. ESCs can do this in two different ways: some can be programmed to shut motor power off at different voltages (e.g. 9v for a 3s battery, 6v for a 2s etc.) and some simply shut down at around 70% of the voltage detected when you connect the Li-Po up.
The latter are often described as having an “auto-detect” PCO and are popular with those easily baffled by the complicated programming sequence employed by some ESCs - but they must ONLY be used when the Li-Po you are connecting is fully charged. I would personally always err towards a programmable unit (after learning how to programme it correctly) because if your Li-Po isn’t fully charged, then 70% of its connection voltage will definitely fall below the safe voltage-per-cell level when the PCO shuts the motor power off.
For example, a freshly charged 3s Li-Po showing 12.6 volts will be shut down at 70% of this, which is a fraction under 9 volts, and 3 volts per cell is a nice safe level. However if you have already had half a flight with your model and landed to make an adjustment, you may be connecting the battery back up at 11 volts; in this case the PCO would shut down at just 7.7 volts which could render the weakest cell unusable, and thus destroy the pack.
PROPPING IT UP!
The final link in the chain (and its significance is often overlooked) is the humble propeller. It still amazes people coming from an i.c. background that this simple piece of nylon or GRP can make such a difference in electric flight but from the outset you must treat the propeller with the utmost respect. Not only is it the one item that can rid you of a finger with one swipe, it can also conspire to destroy your flight battery, motor or ESC in a puff of smoke - or in the worst case scenario ALL THREE.
You may think this is a little dramatic, but it is completely true. Anyone who has inadvertently put their fingers into the spinning prop of a .15 size i.c. glow engine will have got a smart whack that may even have drawn blood; but nine times out of ten the motor will stop instantly. Do this with a .40 and you can sustain a nasty gash that may require hospital attention as the increased power of the motor can mean it has two or three swipes at you before it stops.
Electric motors are DIFFERENT. No matter what size the motor is, as long as there is power being applied, it WILL attempt to turn, and keep turning - even if it becomes so overloaded it melts in the process. What this means is that an encounter with a spinning prop can result in your fingers being slashed relentlessly again and again, until the power is cut.
If you are entering electric flight for the first time, PLEASE remember this and never become complacent when it comes to safety around the prop of an electric model - they can be FAR more damaging than you would ever imagine.
MEASURE FOR MEASURE.
Apart from the obvious safety aspects, incorrect prop choice probably causes more problems in electric flight than any other mismatched component, and it is SO easy to get right, as long as you have the correct equipment.
The equipment I’m referring to is some sort of measuring device such as a Wattsmeter or Power Meter, and these are available from most model shops now at very low cost.
There was a time when even I used to poke about in the dark, trying to establish whether or not I had chosen the correct prop by how hot the motor or NiCad pack got after a couple of minutes! Looking back it was crazy!
When you consider the cost of a melted motor, blown ESC or overloaded Li-Po, paying around £25 for a meter suddenly fades into insignificance and your enjoyment and success in electric flight will suddenly take an upturn, as you will finally be able to see at a glance exactly what is going on.
These meters simply plug in between the flight battery (whether it be a NiMh or a Li-Po) and as you open the throttle of your (well secured) model to full, the various readings instantly indicate the important information you need: current in Amps, power consumption in watts, battery state under load in volts, and in most cases the number of milliamp-hours of battery capacity given up (although this will only be of any use if you are measuring a complete ground run from fully charged to fully exhausted).
As you analyse this information, you can see whether the wattage exceeds the motor’s capability, whether the current exceeds the ESC or battery’s constant handling capacity, and whether (in the case of a Li-Po) the battery voltage is being pulled down too low by the current being drawn. It is therefore also very useful in establishing the quality of some of these unknown brands of Li-Po from the Far East - if at 10C it is showing less than 10 volts, then your Li-Po is a lemon.
You simply CAN NOT participate effectively in electric flight without one of these, so please make it one of the first items you buy!
You may prefer to stick to an established or tried and tested set-up recommended by a colleague or article, but before flying ANY new set-up you need to put the meter on it just to make sure everything is as it should be. What you find is that the more you do this, the more inquisitive you become and before long you’re testing every set-up in sight - it’s addictive!
It’s a good idea to have a few different size and different makes of prop to hand, and starting with one which is a good bit smaller than you imagine you will need, run it up to full throttle and read off the results. If they are a long way below what you need, you simply increase the diameter and pitch of the prop until you find the one nearest your requirements.
What may surprise you is that on our 300-watt example, if you try five different makes of 9 x 6 prop, you will get five different readings and there may be anything up to 50 watts of difference between the highest and lowest reading! This is why in all the charts I have done for my FLY ELECTRIC column in RCM&E, I ALWAYS specify what make of prop was used as well as the size, as they really can vary greatly.
Quite commonly, the specific electric-only props such as Master or APC “E” will give the highest wattage, with cheap plastic i.c. props at the bottom of the pile. You may not be as aware of it on an i.c. model, unless you have an expensive test-rig and dynamometer all you can practically do is fit a few different ones and attempt to gauge which one “feels” the best when you fly the model, when testing different props with a Wattsmeter in the circuit, you can instantly see the difference in the way props perform and this does help immensely when fine-tuning a set-up.
SPOT THE DIFFERENCE!
People constantly ask me why it is so difficult to compare electric motors to an equivalent size of i.c. motor, and I have to explain why it is virtually impossible.
The major difference between i.c. and electric is that whereas with an i.c. engine you have to stick to the recommended prop or thereabouts, whereas with electric motors you can diversify further than you would ever imagine. For example, a 300 watt motor of 1000 Kv could quite happily swing a 12 x 6 prop on a suitable 3s Li-Po at around 7,500 RPM on a powered glider, but the same motor fitted to a fast pylon model could be equally effective with an 8 x 7 prop on a 4s Li-Po spinning at over 12,000 RPM. An i.c. engine can simply NEVER be this flexible as its port timing is tuned to a very narrow RPM band; above or below this “powerband” the engine simply isn’t efficient.
So due to this flexibility, one single electric motor can in fact be comparable with SIX different i.c. engines, depending upon what prop is fitted and how many cells are being used.
There are some motor manufacturers such as MVVS who are now daring to market their motors as direct i.c. counterparts, but this in my opinion can only be done in terms of MAXIMUM output, i.e. that a certain electric motor “running at or near its maximum wattage rating” will power a model “normally powered by a .40 i.c. engine”.
PUTTING IT ALL TOGETHER.
So hopefully using the information above, the whole process of matching up the components of your electric model will now be a lot more straightforward.
To summarise, power to weight ratio is the key, so for every pound of model weight (ready to fly with the chosen motor and flight batteries installed) aim for 100 watts. Once you know the wattage you need, buy a motor that will easily handle this wattage, and choose a Kv that will swing an appropriate prop size on the number of cells you will be using. Most motors these days come with some data or a chart which give you a rough guide anyway; for example EVERY motor bought from BRC HOBBIES (0191 430 1834) is rated by its wattage and contains suitable prop information on various input voltages.
Choose an ESC that is at least 25% higher rated than the current you will be drawing, establish that it will take the input voltage you plan to use, and if using Li-Po flight batteries make sure it can be programmed to protect them from being over-discharged.
You will need a battery that is not only capable of running constantly at the current you will be drawing, but also has sufficient capacity to give a worthwhile flight duration, and if it is a Li-Po, use an individual cell charger/balancer to ensure longevity of the pack.
Finally, and in my opinion most importantly, get your hands on a measuring device such as a Wattsmeter so when you have all the components of your powertrain installed, you can confirm the volts, Amps and watts at full power to make sure that these fall within the parameters you know to be observed; you can always increase or decrease the prop size if the figures are above or below expectations.
If you follow these simple rules, and get little bits of help and advice from those who have a proven successful track record in electric flight (avoiding “theory” experts like the plague!), you will be very unlucky if your first venture into the previously “unknown” isn’t completely successful.
As some may tell you, the 100 watts per pound rule is perhaps a little extravagant; some models will fly really well on 80, others will manage on 60! But for take-off from the ground, power to get out of trouble, and maintaining authority in adverse weather conditions, it is far better to have plenty in hand than find yourself struggling with marginal power.
And of course if you don’t need all the power available, remember that the throttle stick on the transmitter is PROPORTIONAL and not an on-off switch! Electric models seem to be remarkably efficient at cruise settings; this was one of the keys to our cross-channel flight success - so good throttle management with your electric model will result in flight duration extended further than you might think.
There is no mystery to electric flight, it is in many ways simple mathematics and observing a few simple rules that brings good results - so bite the bullet, join the fray and discover what you have been missing!