Balancing the components in your electric power system is key.
Water Pipe Analogy
Do you have trouble keeping track of the difference between voltage and current? I like to think of a wire carrying electricity as a pipe carrying water. Voltage is like the speed of the water. Current is like the thickness of the pipe. To carry more current, the electrical wire has to be thicker.
High Voltage for Electrical Efficiency
The faster the water flows through the pipe, the more efficient it is at delivering water. Similarly, the higher the voltage in an electrical circuit, the lower the electrical losses. So to have an efficient electric power system, just make the voltage be as high as possible, right?
I wish it were that easy. The higher the voltage, the faster the motor will try and spin the propeller. The faster the propeller spins, the smaller it has to be to keep from overloading the rest of the power system components. A small fast turning propeller will not be very efficient.
High Current for Propeller Efficiency
So let’s say that you decide to keep the voltage low. Then you can turn a nice big and efficient propeller. The problem is then that the electrical wires need to be really thick and heavy. Even with thick wires, your electrical power losses will be much higher.
Balanced Speed Controls
Regardless of which way you decide to optimize your power system, it is going to cause problems with your speed control. They have both voltage and current limits that must be respected. If the voltage is high, then the BEC (battery elimination circuitry) just won’t work, either.
The power of an electrical system is measured in watts. It is voltage (volts) times current (amps). About 750 watts is equivalent to one horsepower.
There is a one to one trade-off between voltage and current. If you double the voltage, you get to cut the current in half and still keep the same power level.
Power System Rules of Thumb
We have all heard rules of thumb for sizing electric power systems. For example, one might say that you need 75 watts of input power per pound of airplane weight. The problem with these rules of thumb is that the efficiency of the motor and propeller can vary a lot. What matters is how much thrust you get out of the propeller, not how quickly you are draining your battery.
The key to having an efficient electric model airplane power system is balancing out the components. I have said this before, but it is such an important point that I wanted to repeat it.
If you are using my free calculator to optimize your power system, aim to make the motor and propeller efficiencies about the same. That will give you the best overall efficiency. Of course, you have to respect the limits of the individual components.