Beginners Guide Part 4 - Electric Motors

Choosing the motor

We have come to one of the most important parts of every RC model, which is a main engine drive.
About this topic it can be written very much , because, indeed, there is a lot of details about the main drive, I will try to keep as much on the most basics that can satisfy the absolute beginner without being overwhelmed with the amount of data and options choosing a motor.

We divide the engine drives into two categories: internal combustion engines and electric engines.

Nowadays, electric motors with their efficiency and price are mostly the first choice for beginners, so I'll base this topic onto them.

Electric motors

Electric motors are divided into several categories:

The basic division is based on brushed and brushless motors. Brushed motors are of the older generation they wear and have the need to replace the brushes from time to time. Also they have less efficiency than brushless motors. By choosing the type of motor also depends choice of ESC (the brushed ESC has only two wire leads toward motor, opposite to brushless ESC which has 3 wires).
Brushless motors entered through the small door into modelling with CD drives, which included a motor and a ESC. For many, this has been the way to avoid the high cost of the modeling brushless motors at that time. Today, a solid motor can be bought for 10$, so brushed motors are not recommended anymore.
The main advantage of brushless motor is efficiency, which is typically higher by about 20% compared to the corresponding brushed motor. This is easy to understand because there is no friction on the brushes, sparking, and similar losses during operation. Brushless motors are generally simpler and have better construction. Hence their long life.

Another important advantage is that they can run large propellers without the use of gears, ie. they can have a large torque and a small number of revolutions, but also vice versa, a very high speed and a smaller torque required for the EDF (electric ducted fan) drive.

With this basic division, there is also a stator-based division ofelectric motors:

Outrunner motors have the magnets mounted on the outer casing and the outer casing is spun around the fixed coils in the center of the motor casing, hence the term "Outrunner"

Inrunner motors have the fixed coils mounted to the outer casing and the magnets are mounted to the armature shaft and this spins inside the casing, hence the term "Inrunner"

The inrunner motors are narrower and longer and generally have higher rotation speeds than outrunner designs which have a higher torque because of the chassis that acts as a flywheel and can spin to a larger propeller. The outrunner can be also recognized as a bell motor. The inrunners are most often used with additional gears to keep the tips of propeller blades remain at subsonic speed. The number of revolutions relative to the voltage is marked with KV and the inrunners can have up to 5000 KV, while for outrunners can be less than 200 KV. Thus, for example, the Inrunner with 4000 KV at 11 volts turns 4000 x 11 = 44 000 rpm. Most of the ESCs support inrunner and outrunner motor types. There are 10 times more manufacturers of electric motore than manufacturers who make combustion internal motors for modelers. Every other factory in China has its own brand. The most famous and the most expensive are Hacker, Kontronik, Neo, Scorpion.

A little about "the numbers" (we all hate them):

Dimensions of the electric motor, what do all of those numbers mean in name of outrunner motor?
let's say, you build an kit airplane, and manual says to use the 2826-6 2200 KV motor.

The first part (2826) is the dimensions of the motor. So 28 means that the diameter of the motor itself (bell) is 28 mm.
The second part (26) is the length of the motor itself without a fastening bracket.
That mark 6 sometimes indicates the number of motor poles, or something else, depends on manufacturer, I would not go into details about that now.

It means that 2200 KV (though the naming has no logic) indicates, as I have already mentioned, the speed of the motor revolution per voltage: Rpm/V

So, if such a motor is supplied with a 1V of voltage, it will turn in idle (meaning no propellers) 2200 times per minute.
Consequently, if powered by a 10V voltage, it will turn 22000 rpm, which is very fast.
This is one of the most important things about selecting a motor for your model, because the bad choice of motor can result in motor or ESC burn.
It is recommended that you always hold onto manual, what the manufacturer recommends, most often in the motors description stays ehat is the maximum current that motor can sustain (with this data you can choose how "big" the ESC you need) and which propeller at the specified battery voltage has the highest usability.
Engine power data is presented in Watts, and you can even get maximum power by simply calculating by yourself.
Let's say that motor pulls max 25A on a 3S battery with 6x4 propeller, then the calculation is simple:
Battery voltage x current = Power, or 11.1x30 = 277.5 Watts. For such "setup" you need a 30A ESC (It's always recommended tu use bigger ESC by 10%).
Quite a lot of power for a small, 50-gram motor. This is one of the reasons why more and more modelers use the electric motors. For those large models, internal combustion engines are still being used.

Selecting the propeller

Let's go further on the propeller selection, which is also a very important item in every "setup".
If you do not know which propeller to select, look at the motor datasheet.
For this specific example data says that a 2S-3S battery is recommended for the motor, 2S is recommended for propeller 7x4 and 3S for propeller 6x4.

The propeller number tags refer to propeller diameter and propulsion (pitch). To shorten and not confuse beginners, the diameter indicates a thrust or how strong the propeller is, or how much weight it can pull.
The pitch indicates how much air flow velocity can be achieved, therefore, higher airflow rate means higher air velocity of the model in the air.

Now many beginners, as I have been, think, this is great. I'm going to take motor as fast as possible and a bigger propeller, with a big pitch, the plane will take off like a joke because it can pull well, and it will be a fast in the air like a rocket.
It does not go that way.
If I put a bigger propeller on the mentioned 2200 KV motor, say 9x6, yes, I would get a thrust of at least 2 kg and speed over 100 km/h. Theoretically.
But the engine with such a propeller, due to its size and components that create motor load, would pull up to 80A of electric current with full throttle. And what would happen?
After one second or two, motor or ESC would burn, but in my experience ... probably both.

In short, it is very important to select a good "setup", ie. a ESC-motor-propeller combination for certain types of aircraft.

Please note again:

USE SETUP AS MANUFACTURER OR DESIGNER OF THE MODEL RECOMMENDS IF YOU ARE BUILDING YOUR OWN MODEL.

For your own calculations you have a lot of On-Line calculators where you enter or choose the motor you have, add the propeller to him, and it calculates the load, thrust, max speed, flight length and everything you might be interested in.

One of these is at:

http://www.ecalc.ch/motorcalc.php?ecalc&lang=en

There are also off-line programs that are regularly updated with new models of motors, propellers and ESCs, and which I personally use and can be downloaded at:

www.drivecalc.de


Please note that all these programs give you approximate values, but again good enough to avoid burning of your equipment and model.



Why  are there motors with lower KVs and higher KVs anyway?

This depends on the type of aircraft, for example, a modern military aircraft model or what's called JETs, due to its small wingspan and narrow profile, will require higher flight speeds. In such models, high speed motors with small propellers and large pitch or EDFs are used.




Models of classic airplanes, such as Cessna, Piper,WWI Warbirds ... use slower motors with larger propellers because they are not fast flying models, but they need power, good towing, keeping them in the air at low speeds.




Propellers are also classified as electric propellers and gasoline/nitro propellers.

When choosing an electric propeller,again, you have more sub types:

SF propellers, or so called Slow Flyer. These are propellers that run at small revolutions and, as the name suggests, serve for slow flying models. Mostly often they are made of noticeably soft and elastic plastic and have a limited maximum rotation speed (up to about 9000 rpm).They do not tolerate larger RPMs (revolutions per minute) and can easily break, with result in severe injuries.




Standard and sports propellers. These are propellers that can withstand high rotation speeds, they are made of solid plastic or reinforced with fiberglass. They are used on high speed motors ( with bigger KVs).






Wooden propellers. If you are building scale models from the First World War, one of the cool detail is a wooden propeller that "beautifies" the whole model because at that time, all propellers were made of wood.






Carbon fiber propellers. The most expensive and lightest of all, exceptionally strong and lightweight, there are also SF versions that is used with drones (multirotors). If you can afford this for yourself that's great. They have excellent performance and can withstand great rotation speeds.







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