Sunday, April 29th, 2012 10:34 am GMT -7 Sunday, April 29th, 2012 10:34 am GMT -7Sunday, April 29th, 2012 10:34 am GMT -7
Inrunner electric motor rotor

I happen to know a little about putting together efficient RC airplane power systems.





I sometimes envy my friends that fly gas powered model airplanes. They have it so easy when it comes to putting together a power system. Any engine that is the right size will work fine. Even the choice between two and four stroke engines is not as critical as it used to be. Buy the propeller recommended by the manufacturer. Not a lot of choices there, either. Take a guess at the size of the gas tank depending on how long you want to fly. If the gas tank is too big, it’s not a big deal. Done.

Then I remember about the noise from gas airplanes (specially the two cycle engines), the oily messes, the long drive to the gas flying field, and the hard starts. Hmm. I love my electrics!


The flexibility of electric power systems can work against them. There are just too many choices, and it is not obvious at all to a beginner what are the right components to use in their airplane.

You won’t go far off by following the recommendation of the airplane manufacturer or retailer. Problem is, these recommendations are usually biased one way or another. Not common, but sometimes I see an undersized power system recommended, apparently to make it seem like the total cost of the airplane is less than what it really is. Much more common is for the recommended components to be limited to what the manufacturer or retailer sells. If what they sell is not a great fit, don’t expect to get a helpful warning.

Too many times you are on your own. Sometimes the retailer makes no recommendations, and neither does the kit manufacturer. If you acquired an old kit secondhand or you cobbled together your own Frankenplane, you are pretty much out of luck.

Regardless of your situation, here are some tips to help you have efficient electric power system in your model airplanes.

The Key

Normally the efficiency of the battery pack and speed control are not the main factors that you need to worry about. The key to having an efficient electric power system is to balance the efficiency trade-offs between the motor and the propeller. For high efficiency, the motor wants to turn fast and the propeller wants to turn slow.

I suppose that is a strong argument in favor of an inrunner with a gearbox. The problem is that nice planetary gearboxes are expensive, heavy, and prone to crash damage. When inexpensive outrunners with easily replaceable shafts hit the market, pilots switched in droves. Not being able to turn as large a prop seemed like a small price to pay. Nowadays, inrunners with gearboxes for model airplanes are a rare sight at a flying field.

Battery Pack

The higher the voltage of the battery pack, the more efficient will be the entire power system. This is because electrical power losses have a linear relationship to the voltage but a quadratic relationship to the current. Double the voltage and you double the losses, but double the current and you quadruple the losses. Since power is voltage times current, it is to your advantage to make the voltage as high as possible.

Electronic Speed Control

Hold on a second before you go off and double the number of cells in your battery pack. You see, your electronic speed control (ESC) is probably not going to like it. Most are not designed to handle more than four cells in series. That is less than 17 volts.

Part of the problem is the BEC (battery elimination circuitry), which has to step down the voltage for the receiver and servos. Linear BECs, by far the most common, are best used with two or three cell battery packs. Any more cells in series, and you should be using a switched BEC. These cost more and are bulkier, an issue with very small airplanes. Few ESCs come with switched BECs.

So-called high voltage speed controls, designed to handle up to twelve cells in series or 50 volts, have become more common over time. But they may not help you have a more efficient power system. Read on to learn why.

Electric Motors

An outrunner motor is one where the magnets spin around the stationary core containing the wire windings. Because of their nature, all outrunner motors are brushless. The speed control must use electronic commutation to quickly switch the direction of the voltage across the windings.

The superiority of outrunners has nothing to do with the efficiency of the motor itself. Inrunners and outrunners have about the same efficiency. The difference is that at a given motor weight, an outrunner has more torque and therefore can turn a bigger propeller.

The speed of a given motor is directly proportional to the voltage. The higher the battery voltage, the faster the motor wants to spin. But the electric current is what gives you the power to actually turn a propeller. Remember: speed is proportional to input voltage and torque is proportional to input current.

The stationary wires in the core of an outrunner motor produce a strong magnetic field when a battery is connected. An unwanted side effect of this electromagnet is the generation of heat. What limits the amount of power an electric motor can produce is its ability to get rid of this heat. The heat comes mostly from the current flowing through the electrical wire windings.

When choosing a motor, I always favor the ones that are short and have a large diameter. They will have better cooling, and therefore can handle more current. More current means more torque for turning larger propellers. With better cooling, they will also last longer.

One more tip. Grab one of your motors. Make sure its wires are not connected to anything. Now turn the shaft. What does it feel like? Does it turn smoothly, or does it only turn in discrete steps? A motor that turns smoothly is said to be low cogging. The amount of cogging in a motor is mostly related to how it was designed. For example, Axi motors are generally low cogging. Other brands are known for making the strength of their magnets very well known. Bottom line: the amount of cogging in a motor is not an indication of quality or efficiency. I know it is very tempting to be more impressed with high cogging motors, but do not be fooled.


The amount of energy required to push the air back goes up with the square of the velocity that you push it at. But the power that you get out of a propeller is linear with the acceleration of the air. In other words, the faster the air coming out the back of the propeller, the less efficient the propeller is. Ideally what you want to do is accelerate a lot of air a small amount. Think of a helicopter.

You will soon run into practical problems if you try and do this. The bigger the propeller, the more torque from the motor you need to turn it. The more torque you want from the motor, the more current it needs. More current, less efficiency in the motor. Also, if you start increasing the current, pretty soon the motor will burn up.

There is another huge problem. As the propeller starts turning slower, its Reynolds number goes down. If you get close to 100,000 then its efficiency will drop dramatically.

Best Efficiency

So what is the answer? You need to balance the motor efficiency with the propeller efficiency. Focusing just on one or the other won’t work. The power system as a whole has to work together. This is exactly when my electric power system calculator comes in handy. Without it, you really cannot tell where the loss of efficiency is at.

Normally the best that you can hope for is about 90% motor efficiency and 90% propeller efficiency. That works out to an overall power system efficiency of about 80%.

Because of other real world constraints, many times you cannot get anywhere near there. That is just the way it is. Do not beat yourself over the head because the best you can do is 70% overall efficiency. After all, the whole point is to have fun.

Optimize for cruise conditions. That means a given airspeed and given throttle setting. I try and make sure my power systems have good performance over a wide range of flying conditions, but that is even harder to do.

I would say that anything below 50% is poor, 50-60% okay, 60-80% good, and over 80% excellent.

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