By Randy Butz

One of the fastest growing areas in the radio control airplane industry is electric flight. Brushless motors and lithium batteries provide enough power to make electric power a viable alternative to glow or gas engines. The purpose of this article is to discuss how to choose the proper power system and the proper charging of lithium polymer batteries.

An electric power system consists of three parts, battery, speed control and motor.  The first decision should be which motor is best for the plane. A good rule is that a sport plane will fly well with a motor that has about 100 watts per pound of airplane. This means that a 4 pound airplane should have a motor of about 400 watts. Extreme aerobatic or racing planes will need more watts per pound, anywhere in the area of 150 watts to 200 watts per pound. Lightly wing loaded planes such as trainers and Cub type planes can use fewer watts per pound.

Another consideration when choosing a motor is kV. This is a measurement of how many revolutions per minute a motor will turn per volt. A 1000 kV motor will turn 1000 rpm for every volt with no load or with no propeller. A fully charged three cell lithium polymer battery has 12.6 volts. This means that under no load a 1000 kV motor will turn 12,600 rpm on a three cell lypo battery. This is important in motor selection because different types of airplanes need motors that turn at different speeds. A light wing loaded trainer plane will need a motor that turns a larger propeller with a lower pitch at a slower rpm A racing plane needs a smaller propeller with a higher pitch turning at a higher rpm. A motor manufacturer will make several motors of the same size that have a different kV. When selecting a motor choose one that has the proper watts and a kV rating that matches the plane.

The next choice is the speed control. Selecting a speed controller requires some math so hang in there and it should become clear. Motors are rated by watts, speed controllers by amps and batteries by volts. To make all of these numbers come together Ohm’s law must be used, which is:

VOLTS TIMES AMPS EQUAL WATTS

V x A = W

This equation can be rearranged to determine which component fits with which. In this case a speed controller is being selected for a given motor. For example if a 500 watt motor is going to be used with a three cell lipo (12.6 v) to select a speed controller this calculation is made:

WATTS (500) DIVIDED BY VOLTS (12.6) EQUALS AMPS (39.7)

500/12.6 = 39.6

Using this calculation the minimum value for the speed controller is 40 amps.

Most speed controllers of 40 amps or less will come with a built in battery eliminator circuit (BEC). With a built in BEC the speed controller will supply the proper
voltage to the receiver to power the receiver and the servos. Without a built- in BEC the plane must be equipped with a separate BEC or a separate battery for the receiver and servos.

The third part of the power system is the battery. Battery manufacturers normally provide the following information with the batteries; number of cells, milliamps and a C rating. A typical battery that could be considered for the example being used is a 2200 milliamp hour, three cell battery with a C rating of 15. 2200 milliamps is the same as 2.2 amps. The C rating is how fast the battery will provide amps to the motor. To get the amp rating for a battery the C rating must be multiplied by the capacity of the battery. In this example it is:

C RATING (15) TIMES BATTERY CAPACITY (2.2 amps) EQUALS 33 AMPS

15C x 2.2 amps = 33 amps

As can be seen there would be a problem if this battery is used. The motor will draw about 40 amps at full power while the battery can only provide 33. If this setup is used the battery could overheat and catch on fire. One of two parameters of the battery must be changed. The milliamp hours or the C rating of the battery must be increased. If a battery with a C rating of 20 is used the calculation looks like this:

20C x 2.2 amps = 44 amps

If the capacity of the battery is increased to 3000 milliamps the calculation would look like this:

15C x 3 amps = 45 amps

The size of the propeller will be an important factor in the amount of amps the motor will draw. As the size and pitch of the propeller increase so does the amps the motor must draw to turn it. Using too small a propeller means that the motor is not providing all the power it is capable of. Using too large a propeller means the motor is trying to pull too many amps and the motor, ESC and battery could be damaged. The best way to determine if the power system is working efficiently is to use a watt meter. This meter attaches between the batter and the ESC and will provide the amp draw of the motor.

If at all possible use an ESC and a battery that have a larger capacity than needed by the motor. In the above example the motor will draw about 40 amps. If possible a 45 or 50 amp ESC should be used to ensure the ESC is not overworked. Also the battery should be able to provide 45 to 50 amps on a continuous basis. Using equipment that just barely has enough capacity is like driving a car full throttle just to maintain the speed
limit. This abuses the equipment and shortens its life expectancy.

After a flight the battery must be charged for the next flight. The battery should be allowed to sit for a period of time after use to allow it to cool before charging. Never charge a warm battery. A warm battery has not returned to its normal state after the flight and could be damaged if charged. A hot puffed up battery must be treated with extreme care and taken to a location to cool off where it will not cause any harm if it catches on fire. Batteries that are hot and puffed up have the potential to catch on fire, especially if the outside is punctured.

Lipo batteries that are charged at the wrong rate or are overcharged can catch on fire. The best way for someone new to charge a lipo battery is to get a charger that is dedicated to the lipo battery. With a dedicated charger the chance of incorrectly charging a battery is greatly reduced. Never charge a lipo battery using the car battery with the charger sitting on the car. It is better to lose just the battery and charger in a fire over damaging your car also. Lipo batteries should be charged in a fire resistant container away from people and structures. Do not charge or use a lipo battery that is puffed up or
damaged. The best way to dispose of a lipo battery is to place it in a container of salt water and allow it to discharge.

Is the IMAA website the proper forum for an article on electric power? The answer is yes. An ever increasing number of electric powered airplanes are showing up at IMAA sanctioned events. Just this weekend at Frank Tiano’s Giant Scale event in Lake Worth, Florida, an event where the minimum weight for the plane was 35 pounds, Rich Uravitch flew a twin engine OV-10 Bronco. Also manufacturers are producing more electric powered airplanes that are IMAA legal. Just to name a few two manufacturers make Piper Cubs, there is a Piper Pawnee, and there is a North American B-25 that are all easily converted to electric power and are IMAA legal. Electric equipment is becoming more powerful and more affordable so a larger percentage of planes at IMAA events are going to be electric powered.