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For any coaxial or dual-motor propulsion systems, we can use the overall efficiency of the whole propulsion system to determine the global performance:
The thrust stand measures the two thrusts generated by the propeller A & B on each load measurement unit and the voltage and current A & B of the power chain on the power and control console A & B. The overall efficiency has the unit of measurement kgf / W (kilogram-force per Watt).
The equation above assumes that the dual-motor system uses a sole power source which generates the same voltage for both motors and ESCs. If you are using different power sources for different motors, you may have two electrical power combined.
If you are using the continuous recording or the automatic script control from the software, the output CSV file contains the Thrust A + B (kgf) and Electrical Power A + B (W) and you may simply add a column to derive the combined overall efficiency.
The combined overall efficiency helps to determine the whole dual-motor propulsion system if it is sufficient and efficient to generate the required combined thrust. Please take note that this efficiency may also related to:
- Axial distance between two propellers
- Radial offset distance between two propellers
- Throttle input of two motors
As the upstream propeller is usually more efficient than the downstream propeller, the combined overall efficiency may differ when the controlled speed of each motor is different. Due to the aerodynamic turbulence and drag effect, you may sometimes observe negative thrust for the downstream when the speed difference is too large.