How To Size a Powertrain?

Correctly matching a power train combination for airborne applications can be daunting due to the many options. First, the desired flight characteristics or the intended mission attributes should be specified; for example, identifying whether a high speed or high-efficiency flight is the goal. Second, maximising voltage on the system is essential, as it reduces the thermal load on the power train as a whole. Higher voltage = lower current for the same power.

Propeller

Selection should start with the propeller, as the choice of application directly impacts it. For example, a lower pitch, longer propeller is more suited to long-range/high-efficiency flight when compared with a smaller/high RPM propeller better fit for higher speed efficiency.

• Large, slower spinning propellers are quieter and more efficient. Therefore, increase size, and lower RPM as much as possible to increase efficiency.

• Smaller propellers are quicker to change speed and, therefore, are better suited for higher dynamic performance and higher-speed applications.

The propeller tip speed must be kept below Mach 0.7 to keep aerodynamic losses and current levels within reasonable levels. Pushing above Mach 0.7 tip speed will result in excessive power consumption (and therefore thermal losses) for a minimal gain in thrust. Calculators such as these can be helpful for these calculations.

Motor

Upon selecting the propeller and the desired system voltage, the specifications of the motor can also be chosen. Generalising, the two main types of motors used available are below:

• Flatter, wider stators are designed for higher efficiencies. They are also easier to cool.

• Taller and narrower stators are generally better suited for higher RPMs.

Usually, a motor's kV specified by a manufacturer is the unloaded kV. Under load, the effective kV will drop by approximately 10-15%. However, this should be confirmed with the manufacturer for a given load.

When sizing a motor, the loaded kV must be selected such that the maximum system voltage x kV does not exceed the previous maximum RPM chosen for the propeller (remembering RPM = kV x voltage). This will ensure the greatest point of load occurs at 100% throttle.

ESC

Upon selecting the propellor, motor and operating voltage, the process of ESC selection is straightforward. Focusing only on the unit's power requirements, the use of a static thrust calculator can assist with finding the maximum current draw of the propeller at the maximum RPM. Then, combining this information with the operating voltage of the system, an appropriate ESC can be selected.

Example

9 Kg of thrust required (maximum):

• 15-inch propellor selected. 15x10x2
• 12500 RPM, as per manufacturer’s spec.
• 50 V, 12S system voltage
• Motor therefore selected to be: 280 kV

According to the thrust/power calculator, our maximum mechanical power at those RPMs is 4.9 kW. 4.9 kW / 50 V = 98 Amps maximum static current draw. Therefore, a 12S, 120 A ESC will be sufficient.

In summary, the steps are simple and as follows:

1. Size a propellor for the desired system thrust level
2. Size a motor for the desired RPM and power levels
3. Size the ESC, taking into consideration the system voltage and maximum current levels.