There is certainly no doubt that electric power-system alternatives for what are typically glow engine powered models are becoming more popular and available each day. But even with the increase in popularity and availability of high-performance electric-power systems that many times provide performance equal to (and, in some cases, even better than) common glow engine set-ups, there are a lot of questions left to be answered when it comes to choosing and installing the right motor, prop, ESC and battery pack. The first step in finding the right electric-power system for your model is to determine the model's power requirements. 'Power' can be measured in 'watts,' as seen in this example:
Watts can be determined by multiplying 'volts' times 'amps.'
We can determine the power requirements for a given model based on the following Input Watts Per Pound Guidelines (using the RTF weight of the model, including battery pack).
• 50–70 watts per pound: Minimum level of power for decent performance, good for lightly loaded slow flyer and park flyer models
• 70–90 watts per pound: Trainer and slow flying scale models
• 90–110 watts per pound: Sport aerobatic and fast flying scale models
• 110–130 watts per pound: Advanced aerobatic and high-speed models
• 130–150 watts per pound; Lightly loaded 3D models and ducted fans
• 150–200+ watts per pound: Unlimited performance aerobatic and 3D models
As a note, these input watts per pound guidelines have been developed based on the typical performance of brushless motors. These guidelines may vary depending on the brand of motor as a direct result of actual motor efficiency and prop size/efficiency. With these guidelines in mind, we can determine the Input Watts Per Pound required in achieving the desired level of performance for a given model.
With this info in hand, we can now find a suitable motor based on the model’s power requirements. The following tips can help you determine the power capabilities of a particular motor and if it can provide the power your model requires for the desired level of performance:
• Most manufacturers will rate their motors for a range of cell counts, continuous current and maximum burst current.
• In most cases, the input power a motor is capable of handling can be determined by using the range of cell counts, continuous current and maximum burst current ratings per the manufacturer for reference:
Average Voltage (depending on cell count) x Continuous Current = Continuous Input Watts
Average Voltage (depending on cell count) x Max Burst Current = Burst Input Watts
And here are few quick tips to keep in mind when determining the average voltage of a given pack to use in your calculations:
The typical average voltage under load of a Ni-Cd/Ni-MH cell is 1.0 volt. The typical average voltage under load of a Li-Po cell is 3.3 volts. This means the typical average voltage under load of a 10-cell Ni-MH pack is approximately 10 volts and a 3-cell Li Po pack is approximately 9.9 volts. Due to variations in the performance of a given battery, the average voltage under load may indeed be higher or lower. These, however, are good starting points for initial calculations. Of course, you must also be sure that the battery chosen for power can adequately supply the current requirements of the power system in order to achieve the desired performance. In some cases, Ni-MH 1950–3800mAh high- discharge packs also make good alternatives at the expense of added weight and lower capacity, but also at a lower price than their Li-Po counterparts.
And there is the added benefit of no methanol required with this motor set-up and if you’ve ever nicked a finger on a sharp prop or modelling knife and then handled methanol, you will really appreciate at least this one of benefit.
