RC Electric Basics


1. What motor, battery and speed control do I need for my ____ model?

This is the number one question everybody asks when building their first electric R/C model or converting an existing aircraft to electric power. Of course there are the three most important rules of electric R/C, and they are:

1. Keep it light.
2. Don't let it get too heavy.
3. It shouldn't weigh very much.

The next solid rule is Watts Per Pound. A good number to strive for is 100 watts per pound. That will give you good performance for most models. A lightweight glider type can get away with less and a 3-D model will need more, but it's a good starting point. For example, a small model weighing 16 oz will fly just fine pulling 100 watts out of the battery. A 6 pound airplane will need at least 600 watts for good performance.

OK, how do we figure watts? If you have a wattmeter, it's simple, just plug it in between your battery and speed control, run the motor up and it reads out in watts. Some of them measure volts, amps and watts all at once, but the only one you really need to measure is amps, you can calculate watts easily. Watts is the total amount of electric energy consumed from the battery, and is the product of volts times amps. You already know the battery voltage. If you figure 4 volts per cell, that's what you will have from a freshly charged battery. It will run down pretty fast but it's always good to start with the maximum voltage. Now we go to the Guppy model that you flew. It weighs 15 oz and uses a 2 cell 800m battery, pulling about 10 amps from the battery pack. 2 cells at 4 volts per cell equals 8 volts times 10 amps which works out to 80 watts. 15 oz is .9375 lb, 100 watts per pound is 93 watts, pretty close to the measured 80 watts used by the Guppy. That amount of power won't pull it straight up but it's still plenty for the type of model, able to loop from level flight.

Now, how do we figure what battery we will need? Let's go back to the Guppy, where our small motor needed a 2 cell battery. (more on number of cells later) Lipo batteries all use the "C" rating for the amount of amps they will deliver. Most of them over-rate them somewhat, so it's a good idea to be a little on the conservative side. If a battery has a 1000mah rating, that means it will deliver 1000 milliamps for one hour. 1000 milliamps is 1 amp, nowhere near enough to fly our plane, so we must pull a lot more power out of the battery. A battery's "C" rating pertains to how many times it's rated capacity can be drained from the battery without damage. Therefore, if our 1000 mah battery is rated at 10 C, then it can deliver 10 amps without damage. If you exceed this value, the battery's life will be short. My first set of lipo batteries were rated at 3 C, this was over 10 years ago and I was pulling at least double that amount on each flight. After a couple months flying, they were toast. Most batteries are now rated for at least 20 C. In the case of our 800mah battery in the Guppy, we can pull 16 amps from the battery without damage, but it will have a pretty short flight and I always try to use no more than about 12 C to give a good safety factor. 10 amps divided by 800mah equals 12.5 C, pretty close to the 12 C max for good battery life, and it will give us good flight time. A higher C rating will not give you a longer flight unless you are pulling too many amps from the battery. If you want more flight time, use a larger battery instead.

Now for the speed control, what size do we need? They are always rated in amps, so if you know how many amps your motor will draw, get the next size up speed control. If your motor pulls 10 amps, the next size up will usually be 15 or 20 amps. In the case of the Thunderbird control I used in the Guppy, you have a choice of 9 amps or 18 amps. I used the 18 amp model. It doesn't hurt to go larger, just don't use a smaller one. There are a couple of other factors to look at when choosing a speed control. Most all of them will work with two or three cells, but many will not accept four cell batteries. Check on this first. Another thing to look at is the Battery Eliminator Circuit, or BEC, built into most, but not all speed controls. What this amounts to is a 5 volt regulator to supply voltage to your receiver and servos, saving you the weight and trouble of a receiver battery. Some of them are only good for 1 amp, plenty for a receiver and two small servos but not enough for more or larger servo installations. Some larger speed controls have a 5 amp BEC which will handle pretty much anything within reason. If this isn't enough, like when you have 5 or 6 digital servos, an external BEC is available. Here's an example of an airplane that I converted from a 60 glow engine to electric power. A Hangar 9 Ultra Stick, fairly light for its size, made a good conversion. I used an E-flite 60 motor, 400kv (more on this later) Castle 75 ESC and an APC 14x8.5 prop, plus a 6-cell 4000mah battery. It pulls 45 amps from the battery (24 volts x 45 amps equals 1080 watts) giving 166 watts per pound. This level of performance is enough to pull it straight up, not quite enough for 3D but plenty for normal flight. The Castle ESC has a 5 amp BEC, allowing me to replace the receiver battery and remove the throttle servo as well. With the lighter weight of the E-flite motor, I was able to make up for the weight of the battery pack and equal the total weight with the previous glow engine and fuel tank.

The only other change I had to make was to lengthen the landing gear for better prop clearance with the larger prop. The original prop on the glow engine was a 12x6 and the electric version needed a 14x8.5 for equivalent performance. The landing gear that came with the Ultra Stick was a little flimsy anyway, so it needed some extra help. I cut the axles off the ends of the gear and added a 3/16in music wire gear on the bottom of the aluminum original, adding two inches to the ground clearance while making the whole installation stronger. Four little clips held the music wire extension to the bottom of the gear, up one side and down the other. The entire conversion was very successful, having more power than the glow engine and no more mess to wipe off at the end of the day. A new hatch on top of the nose gave access to the battery and speed control, and a square hole on the bottom let the speed control heat sink hang out in the breeze for cooling.

2. What do those numbers on the motor mean?

We have another can of worms here, because different brands use different systems. Let's go back to the small motor on the Guppy. It's a 2208/12, as listed on the sticker on the motor. 1800kv is on there also. The first number has to do with the physical size of the iron motor core, in this case it's 22mm in diameter and 8mm long. You can only measure it if you take the motor apart. The last number after the "/" means there are 12 turns of wire around each of the poles in the laminated iron core. The number of turns determines the speed of the motor, the fewer turns the faster the motor will run. I know it sounds backwards but that's how it is, which brings us to kv.

3. What does KV mean on my motor?

KV stands for RPMs per volt, or more accurately, thousand RPMs per volt. If we have a motor rated at 1000kv and we apply one volt to it, the motor will turn at 1000 RPM with no prop. A more realistic example would be a 2-cell battery pack at 8 volts for 8000 RPM. The higher the KV rating, the higher the RPM. That doesn't mean a motor with a higher KV will have more power, but it does have an effect on how many cells it needs and what size prop to use. Most of your smaller motors have a high KV because they usually use fewer cells. Larger motors tend to have a lower KV so they will need more cells. You can have a 1000 KV motor that uses 3 cells and the same size motor rated at 400 KV for 6 cells. Both motors will have the same power (rated in watts) but the 400 KV motor will be more efficient because it draws fewer amps and will have less loss in the wiring and speed control. Higher voltage just works better when you get into the larger, more powerful motors. I told you it was complicated.

4. OK, that's all interesting information, but I still don't know what motor to use in my Sky Tiger 40.

Here's your ace in the hole. The E-flite series of motors use sizes that pretty much match an equivalent size glo engine. We know that most Sky Tiger models end up with a .40 glo engine or maybe a 50 4-stroke. An E-Flite 46 would be a good option to use, they will tell you which battery, speed control and prop to use and you'll be home free. E-Flite tends to be a little pricey but it's a safe bet. The prop sizes they list tend to be a little inaccurate because different brands of props will be different. This is the route I went with when I needed a replacement for a .61 glo engine. I used an E-Flite 60 and an APC 14x7 like the sheet said, turned out to be perfect. I was showing the prop to someone when he told me it wasn't a 14x7 but a 14x8.5. It had been marked wrong in the package but had turned out to be the right prop anyway. The bottom line: You will have to experiment a little to find the right prop, just like you would with a glo engine.

5. What kind of speed control do I need?

I'm partial to the Castle Creations line because I have one of the Castle Link USB interfaces to do any programming changes on my computer. It's sooooo much easier than trying to use the transmitter sticks and listen for the beeps. About the only thing I usually change from the default setting is the brake and maybe motor rotation. Some models also have a logging function where it saves recent data in the speed control's memory from your last flight. It is useful to go back and review the amps used and battery voltage, gives you a more accurate picture of in flight data. As mentioned before, you need to read the specs on each speed control before you buy it, quite a few models are only good for 3 cells. As for the AMP rating, bigger is always better. If my motor draws 40 amps, I like to go with a 60 amp model or better for a safety factor. Again, check the BEC, or Battery Eliminator Circuit to see if can handle your installation. For the small models, a 1 amp BEC is fine for the two or three micro servos, but it's not enough for a larger model. A 5 amp BEC will be OK for any but the really big models, then you should be using a receiver battery anyway. If you have 4 or 5 larger servos and they draw too much current, the BEC will just shut down if it gets too hot, then you will lose all control. You know what happens next. When in doubt, you can disable the BEC and add a receiver battery, just cut the red wire from the ESC to the receiver throttle channel. Don't try to use both a receiver battery and a BEC, they will fight each other. I like the Castle warranty policy also. I let the smoke out of a 75 amp ESC and they sent me a new one at no charge, no questions asked.

6. How About Batteries?

The one thing you have to remember about batteries is: They all go bad no matter what you do. Consider them expendable. Some batteries are cheap and some are expensive but they're ALL delicate. I've had good and bad luck with both cheap and expensive, although the higher priced ones seem to have more power, they still give trouble. If you treat your batteries gently, they will last longer, but if you always run them down until the motor cuts off, or you pull too many amps on a regular basis, you're gonna buy more batteries. I try not to pull more than 12c from a battery, and run it down only about halfway before recharging.

7. Connectors.

These aren't nearly the problem they once were when meltdowns were common, the large red Deans connectors will handle plenty of power. But I prefer the XT60 connectors, once available only from Hong Kong, now all over EBAY. They're much easier to solder, especially with the larger wire sizes. For smaller models, the black 3 pin Deans connectors are what I use, good for up to 20 amps and much more convenient than the larger Deans models or XT60's. I use the outside two pins, with the black wire nearest the notch. Some batteries, small ones that is, come with JST connectors already installed. I consider these inadequate for electric power and I replace then with the small Deans plugs. The JST connectors are a small, red version of servo connectors, and in my opinion are just too wimpy for electric power use.

8. About That Charger.

I prefer the "4 button chargers" available just about everywhere. They're known by this name because they all have four buttons in a row below a digital readout. Somebody must make a chip with the battery charger program in it because no matter who makes it, the operation is almost identical. But there is a difference in the amount of power they will handle. Some will charge only 2 or 3 cells and others will go up to 8 or more. If all you have are small 3 cell packs, any of them will do, but if you need to charge large 6 cell batteries, you need to pay close attention to the specs. I was given a charger not long ago and it worked fine with the smaller packs but it would only charge my 6 cell battery at 2 amps. Since I normally charge it at 4 amps, it took twice as long to charge. I ended up giving it away. To be safe, get a charger rated at 150 watts and 6 cells so you can charge anything within reason. Be careful, a LOT of them are only rated at 50 watts and they try to hide the fact from you the customer.

9. What The Hell is a 400 Motor?

We have another history lesson here. Several years ago, Graupner in Germany came out with what they called a Speed 400 motor. It was a small, cheap, brushed can motor actually made by Mabuchi, but it became very popular with lots of small airplanes designed especially for this motor. As delivered it would only accept a small prop, maybe 5 inches, but adding a gearbox allowed it to pull more efficient props. It would develop about the same amount of power as an average .049 glo engine, so small models were all it could handle. When brushless outrunners came along, the 400 motor made the transition and lots of "400" motors were on the market. They weighed half of what the old geared, brushed motors did, being about 80% efficient compared with the previous 50%. A motor that's only 50% efficient wastes half of its battery power as heat, so the new 400's were not only lighter but more powerful also. I have a "400" motor to be used as a more powerful replacement in the Guppy. It's labeled as a 2212/13, which means its core measures 22x12mm with 13 turns around each pole. It's rated at 1000kv, that means it will need a 3 cell battery instead of 2. With the same 8x4 prop as used on the Guppy's 370 motor, it pulls 10 amps from a 3 cell battery, same as the 370 motor. Since it has more cells it will have more power even though the amps are the same, 120 watts instead of 80. The motor can handle more power with a larger prop but I'm gonna leave it the same so I will have the same flight time. Motors smaller than the 400 size will still use the same numbering scheme, such as 370, 300, 250, 180, etc. You can get an idea of what kind of power they will produce by comparing them with a 400 motor. They come in different KV ratings and will use different prop sizes, you'll have to experiment to find the best one.

10. Building a Model For Electric.

Your choice of model will go a long way toward your success, the lighter it is, the better it will fly. If you're converting a model intended for a glo engine, some changes will need to be made. Most electric motors are much lighter than the equivalent engine, so weight and balance need attention. If you add a new firewall to replace the old one, it can be moved forward to get more weight out front. You'll have to make room for the battery in the fuel tank area to get it to balance, plus a hatch for battery access. If there's any way to make the hatch on top, that's where it needs to go, if not then it's got to go on the bottom where it's not convenient to change the battery. Don't worry too much about cooling the battery, just remove it after flight and it will be OK. Do make provisions to cool the ESC, if it gets too hot it will shut down to protect itself. If it has a heat sink, cut a hole and let it hang out in the breeze, it will never get hot.

11. Flying Electric is Different.

You have a limited amount of power to deal with. The old practice of taking off wide open and never throttling down until ready to land just won't cut it. I never use full throttle unless I need it for a maneuver like a loop or a roll. Even takeoff is usually easier at less than wide open, especially with the larger props used on electric models. If you try to fly it like your engine powered models, you'll have a short flight and a hot battery. Hot batteries have a limited life span. If you find you need full throttle just for takeoff, then you don't have enough power. Every model should maintain level flight at half throttle or less. If not, it's time for a bigger battery, motor, prop, etc. The brake can be used in more than one way, number one is to stop the folding prop on a glider for less drag. But if you don't have a glider you may not want a flat, floating glide path. If you leave the brake inactive (the usual default) then the prop will continue to windmill when coming in for a landing, adding a lot of drag to the approach. Landing is easier if you can make a steeper approach without gaining a lot of speed. Try the brake both ways to see which one you like. I always land before my battery gets nearly dead, makes for longer life cycles and you don't have to worry about the ESC shutting down your motor when you least expect it. Short flights are easier on the battery and take less time to recharge. And the best part of the whole electric experience is:

You don't have to clean it up after a day's flying.

User comments

Great stuff, but as a "newbie", I have a rather fundamental question: I would like to build a replica "flying" model, ideally a biplane (Fokker D.VII?), at 1:72 scale. This equates to a wingspan of no more than 4 inches. With an electric motor and battery weighing no more than 10 grams altogether, all-up weight (i.e. wing loading) doesn't seem to be the limiting factor, but ... It seems to be the "stickiness" of the air at this scale that is the problem. There seem to be no RTF models at this scale. Has anyone tried to tackle this? How would the speed and pitch of a "scale" airscrew bear on this? I am not looking for "FROG", and RTP would be perfectly acceptable. Ho hum, I may end up just having to experiment ....
RogerV - 26/11/2018
There are people doing true "micro RC". Your best bet is an image search on one of the major search engines for __micro RC aircraft-- and sort out the small from the really itty-bitty. Martin Newell has some 1/96 warbirds flying and is sharing his experiences at
Davidterrell80 - 28/11/2018
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