Putting It All Together

Matching a brushless motor, lipo battery and ESC may be confusing at first, but understanding how each relates to the other will demystify this issue.

The first step is to determine your power requirements. Since the objective is to drive a plane, you have to determine what your plane needs to fly the way you want. For example, is the plane a trainer, scale model, aerobatic or full 3D? Each will need a different power plant. One criteria to determine motor sizing is based on watts/pound (total flying weight):

• 50-70 watts/pound: Minimum level of power for decent performance, good for lightly loaded slow flyer and park flyer models
• 70-90 watts/pound; Trainer and slow flying scale models
• 90-110 watts/pound: Sport aerobatic and fast flying scale models
• 110-130 watts/pound: Advanced aerobatic and high-speed models
• 130-150 watts/pound: Lightly loaded 3D models and ducted fans
• 150-200+ watts/pound: Unlimited performance 3D models

Another method is to determine the required thrust/weight ratio. For this you start with the total flying weight of the plane:

• 50% thrust to weight ratio: Minimum required for "sedate" flying
• 80-100% thrust to weight ratio: Recommended for trainer type planes – enough power to get out of trouble easily
• 120% thrust to weight ratio: For unlimited vertical performance and 3D flying
• 200-300% thrust to weight ratio: For speed and wild aerobatics with light weight foam planes

Almost all motors sold will spec their power requirements such as seen below:

In this instance, you can estimate total watts by multiplying the volts by amps – in this case 12 volts x 22 amps (continuous) = 264 watts.

Note that there is no thrust data – not all will give thrust data. Scorpion is one that does gives excellent data on each motor:

Note that the propeller used will have a large impact on power requirements. For more on Propeller Selection, go HERE.

In general, slow flying props are designed to deliver high thrust at relatively low rpms and "E" props are designed to enhance speed over thrust. For example, if you have a scale biplane, a slow fly prop might be more appropriate; a war bird, eg a P51 Mustang, typically flies faster and will use an "E" prop.

Once the motor is selected, you need to match a battery to the motor's power specs. Let's assume your motor/prop combination will draw 15 amps. The lipo battery that will meet this requirement must be capable of delivering at least 15 amps on a continuous basis. This is where the battery's "C" rating comes in.

Lipos are rated by voltage and capacity; capacity is stated in mah – milli amp hours – a measure of how much the battery holds. As an example, if you look on this label you'll see six key numbers (circled in red):

The "25C cont" means 25C continuous; the "40C burst" refers to how fast the Lipo can be rapidly discharged for a SHORT time period, something like 15-30 seconds; look at this as the "supercharger" rating – to be used rarely. The second set of numbers – 55A cont/88 burst – are what this battery can deliver to the system considering its capacity – 2200 mah.

The two ratings – mah and "C", combine to tell you how intensively this battery can be used with the following formula:

In our example the battery at a minimum MUST deliver 15 Amps. If you over-discharge a lipo battery, it will get VERY hot and probably catch fire; at a minimum it will ruin the battery. To be safe, it's a good practice to use a battery that will deliver MORE THAN what's required – the motor will draw only what it needs, you can not "force feed" it with a larger battery.

Once you've matched your model's power requirements to a motor/prop/battery combination, you need an ESC that will handle the power required. ESCs are rated in Amps, so you need an ESC that will handle the maximum required Amps. In our example of a motor drawing 15 Amps, you need a 15 Amp brushless ESC. These ratings are typically written on the ESC:

This one is rated at 35 Amps at a maximum of 15 volts. It's a good practice to use an ESC that's rated for at least 30% more Amps than you need – it will run cooler and you will not stress it as much as one rated closer to your requirement. In our example where the motor will draw 15 Amps, an ESC rated for 20 amps will give you a nice safety margin.

A little homework up front will go a long way to keeping your flying trouble free. If your motor does not have any data, you need a wattmeter to help in your setup. A rule of thumb that can get you started is that in general a motor will deliver 100 watts/ounce of motor weight. However, it's only a rule of thumb and only real world testing will give confirm a trouble free setup.