Cumbernauld Model Flying Club

Electric Flight Basics

This page is written for "average" modellers so it skips over some of the detail of how electric flight systems work to keep it readable. The objective is to answer the most common queries that come up at the field and expand the knowledge of people coming to electric flight for the first time, either from scratch or converting from glow powered models. The information given here is applicable to relatively straight forward, normal sized models. Those contemplating very large models (petrol equivalent) or large multi-motor models need to educate themselves further.

Most modellers have many years experience of glow fuel powered engines. In general, these are designed to swing a particular size of propeller. So, if you fit a .40 glow motor, you'd put on a 10*6 propeller and forget about it. A .25 might have a 9*5, and so on. Pilots with quite a range of models might only have three sizes of propeller, being one each for a .25, a .40, a .61. If they have a four stroke, they will have a prop for that engine. There's no need to experiment by trying other sizes, these are the ones that work best.

Electric flight motors are not like this at all, and this is the first lesson that has to be learned. Any given motor, straight out of the box will run with a variety of propellers, and generate a different level of power depending on the propeller fitted (all other things being equal, of which more later). So the same motor could power a plane that would need a .25 glow, a plane that would need a .40 glow, and (in extreme) a plane that would need a .60 glow engine. The documentation supplied by most motor manufacturers and retailers is usually not good enough to give the exact capabilities of the motors so there most definitely is a need to experiment with different propellers (and maybe other variables) to get the performance needed for a particular plane. This takes us neatly to Rule 1.

A watt meter is connected between the battery (lipo) and the speed controller. You then (safely) run up the motor to full throttle for a short time (20 seconds?) while restraining the model on the starting bench and it will tell you the maximum current drawn (in amps), the maximum input power generated (in watts), and the minimum battery voltage during the test. The best kind have a memory function for these values so you can look at the meter once the prop has come to a halt, rather than trying to read off values while the motor is running at full throttle. You can get these now from China for less than £10, or from even the most expensive retailer for about £30. As we will see, the information from your watt meter will tell you how the plane will fly, whether any component will be under stress, and how you could improve the performance of the system as a whole.

You might be surprised how many people ask me for help and can't tell me what motor or speed controller is in their plane. Sometimes we can find out by opening it up and reading the labels but, equally, some motors and speed controllers just don't have labels or have lost them in use. It is almost impossible to help people under these circumstances. In an ideal world, you should have the data sheet that came with the kit, or a printout from the web of the specifications listed.

The major components of your electric flight system (motor, speed controller and lipo) act together as a single system. Each individual item has a specified maximum at which it can operate, usually defined by the number of lipo cells (voltage) and the maximum current (amps) that can be handled. It is really important to understand that your entire system can only operate at the capacity of your lowest specified component. For example, a motor capable of operating at 40 amps on 4 cells can only be used at that level if the speed control is rated for 4 cells and an absolute minimum of 40 amps and the lipo is capable of delivering 40 amps. If, say, the speed controller can only take 3 cells and 30 amps, that is the maximum at which the overall system can be operated, even although it is very substantially below the capability of the motor. In terms of input power, this would reduce your system from its potential of 600 watts (40 amps x say 15 volts) to around 345 watts (30 amps x say 11.5 volts). So you could be getting just over half the power available to you.......you would definitely notice this in flight performance! This leads us to rule 3.

Before we explore each component in detail, a word of warning.......you cannot always believe the specifications published by the manufacturers and retailers! There are two aspects to this, exaggeration and incompetence. Obviously, it is in the industry's interest that they present the very best capability for a given product. Otherwise you may buy somewhere else. To illustrate the incompetence issue, I have several motors in my possession, bought from a UK retailer where the specification on the web site, the specification on the motor box and the specification on the label on the motor were all different! This makes life just a little difficult when buying motors! The only way round this is to read any reviews you can find or talk to fellow modellers about what they are using. Once you've bought it, you need to experiment to find the best set up.

I'll start with speed controllers as they are the easiest of the three components to understand. Generally, all you need to know is how many cells (therefore voltage) they can handle and the maximum current rating. A point to note is that this maximum current rating (even if it is not exaggerated) is specified for a controller mounted with an adequate airflow for cooling in flight (say, outside the fuselage). As we normally put them inside with limited or no cooling, we have to use them at less than their maximum current rating (say 20% to 25% less). So if you intend to run at 30 amps but the controller has limited cooling (say inside a foam model or seaplane), you need to use the next size up, in this case a 40 amp controller. This is less of a problem nowadays as there is little difference in price if you shop around. The most common way in which an overloaded system fails is that the speed controller literally burns out. A speed controller which is driven too hard will first of all get very hot (you might not be able to touch it) then, in extreme, will melt down and loose the magic smoke. At this point, if it was being used to supply the plane's receiver via a BEC circuit, your plane will crash with no control. (the melted controller is no longer supplying volts to the receiver!). Lastly, note that most controllers on sale are set up for outrunner motor use. If your motor is an inrunner (usually only in gliders or specialist applications) you will have to learn how to reprogram it. I'm not going to cover that here.

I find that the biggest problem people have with lipos is understanding the effects of C ratings. Now you do know what 1C is, don't you? (if not, you cannot charge your lipo as the safe charge rate is almost universally 1C). The easiest way to work out what 1C is is to think of your lipo capacity in Amp Hours, rather than Milliamp Hours (by dividing by 1000) and take that number as the 1C current rate in amps. So a 3200 Mah lipo is a 3.2AH lipo (3200/1000) with a safe 1C charge rate of 3.2 amps. Make sure you understand this. It is vital. You should be charging this lipo at 3.2 amps!

Your lipo will have a specified maximum discharge rate, expressed as a C rating. The most common rating on inexpensive lipos is 20C to 30C. You should ignore the higher number as this is only available for a few seconds, so count these as 20C. So our 3200 Mah lipo above (3.2AH), specified as 20C - 30C is good for 20*3.2 amps = 64 amps maximum. In practice, lipos that are run near the maximum C rating for the length of a flight will both get hot and puff up. This will shorten their life, perhaps considerably. So you should work on using no more than 80% of the maximum C rating in flight, in this case 80% of 64 amps = 51 amps.

For practical purposes and the target audience, lipo C rate maximums really only matter when using relatively small capacity packs, (say up to 2200 Mah) especially in high performance planes (for example, HK Funfighters and the like). A 20C 1800 Mah lipo will have an 80% limit of 1800/1000*20*0.8 = 28.8 amps. If you need 40 amps, that's no good. you would notice poor flight performance and you might well get pack heating and puffing. If you replaced this lipo with a 40C - 60C pack the numbers change to 1800/1000*40*0.8 = 57.6 amps, more than enough in our 40 amp example. At larger pack sizes in bigger models, there are usually few problems. So a 5000 Mah 20c pack will give you 5000/1000 *20*0.8 = 80 amps. For flight duration reasons, the most common electric models are set up to run at 30-40 amps. This is why I can run my 84 inch (20cc petrol class) Maule on Multistar 5200 Mah packs which are only rated at 10C (5200/1000*10*0.8 = 41 amps).

Sorry for the maths, but it's only one equation and you need to do it if you are near the limits.

Well done if you've stayed with us till now. There's good and bad news in this section. The good news is that in all the time I've been flying brushless systems, and helping others to solve problems with theirs, I've very rarely seen a motor destroy itself (obviously if you fly it into the ground hard, that's a different issue). When abused, they simply get hot (again sometimes too hot to touch). In extreme, the glue holding the magnets in place can melt and the magnets move causing problems but this can often be fixed with some 24 hour epoxy and a bit of care. Taking care that there is a cooling airflow over or through the motor really helps. Remember from above that the motor will draw more current with a larger propeller, and less with a smaller (all other components staying the same), so it is relatively easy to adjust the current the motor is taking by simply changing the propeller. (note that the diameter is much more important than pitch in this situation - motors are relatively insensitive to small pitch differences.)

A few words on motor specifications. These can be extremely confusing.......

Motor manufacturers use different labels to describe their motors.

Some have adopted numbers trying to emulate the previous generation of brushed motors (380, 400, 480, etc).

Some suggest an equivalent glow motor size (Power 25, G40, G110, etc) but re-read my comments at the very beginning.

Some use the outside dimensions of the motors in mm (2820, 3536, 4380, etc).

Some use the inside dimensions of the motors in mm (2820, 3536, 4380, etc).

Some provide you with the number of wire turns in the motor (3537-07T, etc).

As you can imagine, this is all very helpful (not!).

Two features that are very slightly more useful are motor weight and "Kv" rating.

In general, the heavier the motor, the more power in watts can be handled (so it should be proportional to the plane you are putting it in).

The higher the Kv number, the smaller the propeller the motor is designed for (so match to the propeller size needed by the plane).

So a 3536 1000 Kv motor might be designed to swing a 10*6, a 3536 1500 Kv motor might be designed to swing a 7*6 or 8*6.

This is by no means an exact science and breaks down quite a bit a bigger motor sizes (16*8 and above) where low Kvs are the norm.

By the way the term Kv was an early mistake that stuck in the industry. It doesn't stand for kilovolts or anything startin "K", "v".

It's the RPM the motor will reach with no load (i.e no propeller) at 1 volt - a lot of help (not!)

As with other components, read the reviews and any guidance available. Ideally you need to know what current your motor will draw from your intended flight pack (number of cells), using a propeller appropriate for the model. This will also give you the watts that will be generated. 

You will need several propellers covering the range of sizes quoted for the motor. Note that different brands and blade shapes will give varying results so try to stick to one brand throughout. I use APC E series which are readily available in most sizes. Remember that diameter is more important than pitch for increasing power. As with glow, new propellers can be razor sharp so take the worst of the sharp edge off by your favourite method before use.

Let's assume you have an aircraft and your flight system and you want to ensure that it will fly well. Make sure that you understand the maximum current that the overall system can handle (see above). It doesn't matter if the flight system is installed or whether you are going to use a test stand. The process is the same.

The first thing to do is to weigh everything (the aircraft with servos, etc inside, and the flight system including the battery you are going to use. Get the overall weight in pounds, not grams or kilograms. Now consider what power you need for it to fly well. For sports planes, aim for 150 watts per pound. This will allow normal aerobatics to be flown. Lightweight built up gliders might get away with 75 watts per pound, where as 3D hovering might need 250+ watts per pound. These are all rough estimates and will vary from plane to plane depending on drag, total wing area etc.

Set up your plane or test stand so that you can work on it safely (without getting near the propeller), put on the smallest propeller from the range suggested for the motor, and connect the watt meter between the lipo and the speed controller. Now, watching the current reading on the meter, progressively run the motor up to full throttle. If at any time the current reading goes over the maximum your system can handle, stop immediately and rethink. If all is OK, note down the maximum amps and watts achieved. Do not run the motor for more than about 20-30 seconds at a time or it will overheat. If you want more power and are under the system maximum rating, you simply change the propeller to the next size up and run the test again. You can continue doing this till you reach the system maximum or the power level that you wanted to achieve. That' it.

If you still can't get to the required power level, first check your theoretical lipo maximum current using the sums given above. next look at the minimum voltage figure given by the watt meter. Cheap and old lipos may not be able to hold their voltage up under load, resulting in a poor power figure (watts = volts * amps). Try a larger capacity, different brand, or newer lipo for comparison. This will often sort the problem.

Assuming your motor and speed controller are rated for it, the next step is to up the number of cells in your lipo (say from 3S to 4S). This will give a dramatic lift in power for a relatively minor increase in weight. Before buying, make sure your new lipo will fit in the battery bay of the aircraft! If you use the same propeller, the power will increase a lot because both the voltage and the current will be higher and watts = volts * amps. This brings us to rule 4.

This is quite a dramatic effect, and in some cases, might nearly double the power of the system. The current (amps) may now be well over the maximum ratings of at least one of the system components. So you need to approach testing carefully, starting with a propeller at least a couple of sizes down from the one you used with the lower cell count. Simply complete your tests as before.

If you still can't get the required power output, you may have to look at changing out one or more of your flight system components. This system can't give you more as it stands. Try to find which component is restricting you most and change that first.