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How to Measure Brushless Motor and Propeller Efficiency

plot trust and propeller efficiency

This tutorial is a quick introduction to motor and propeller testing, as summarized in this video.

Music Credit: Quantum by timbeek.com

Why testing your motors and propellers?

You must first ask yourself, what are your, or your end user’s needs? This question is important, as it will help you know what parameters to optimize for.

  • Do you want to fly longer to film uninterrupted for longer periods?
  • Do you want to carry a larger payload?
  • Do you need more thrust and power to go faster, or to improve handling in strong winds?
  • Do you have overheating problems, and your application requires you to minimize failure rate?

The final choice of power system depends not only on the airframe and payload, but also on your application.

What parameters should I measure?

The motor

NTM propdrive 35-30 brushless motor and Quantum 13-4 carbon fibre prop.

To fully characterize a motor, you need to measure the following parameters.

  • Voltage (V)
  • Current (A)
  • Throttle input (%)
  • Motor load or torque (Nm)
  • Speed (RPM)

The RCbenchmark software automatically calculates the following parameters for you:

  • Mechanical power (Watts) = Torque (Nm) * Speed (rad/s)
  • Electrical power (Watts) = Voltage (V) * Current (A)
  • Motor Efficiency = Mechanical power / Electrical power

The output speed is function of the throttle, in %, and of the load (torque in Nm). If you want to completely characterize a motor, you will need to test it with multiple input voltages and different loads. The throttle is changed with the software, and the load is changed with the type and size of propeller.

The propeller

For extracting useful propeller data, you need to measure the following parameters:

  • Speed (RPM)
  • Torque
  • Thrust

The RCbenchmark software calculates the following parameters for you:

  • Mechanical power (Watts) = Torque (Nm) * Speed (rad/s) ← same as the motor
  • Propeller efficiency (g/Watts) =  Thrust (g) / Mechanical power (Watts)

Notice that the mechanical power is the same for the motor and propeller. That is because all the motor’s mechanical power output goes into the propeller, since it is directly coupled to the motor’s shaft.

The overall system

The overall performance of the system depends on a well balanced combination of motor and propeller. Your system will be very inefficient if these two parts don’t match well together. Because these two parts have a common link (the shaft), the overall system efficiency is calculated as:

System efficiency (g/Watts) = Propeller efficiency (g/Watts) * Motor Efficiency

Where the system efficiency is in grams per watts of electrical power. Changing the motor, propeller, or even switching to another ESC will all contribute to changing this calculated system efficiency.

Moreover, the efficiency value will only be valid for a specific command input and mechanical load. In practice, this means that you will test you motor over a range of command inputs, and with multiple propellers to vary the mechanical load.

How to measure those parameters?

In summary, you need to simultaneously record voltage, current, torque, thrust, and motor speed, while at the same time control the motor’s throttle. By combining these readings you can extract the electrical and mechanical power, which in turn will allow you to get the efficiency values.

Dynamometer Thrust load cell

The RCbenchmark motor test tool was built to reduce the time and cost associated with building a custom test rig. The tool is capable of measuring all the necessary parameters while controlling the ESC, and recording the data in a CSV file for analysis.

Test procedure for static tests

Dynamometer test of a brushless motor and a propeller

For now, we will only cover static tests (we won’t talk about dynamic tests involving angular acceleration, estimating stall torque, etc…). Before starting your tests, we recommend:

  • Installing your propeller in pusher configuration, to reduce ground effects with the motor mounting plate
  • Have a reasonable distance between the propeller and other objects, again, to avoid ground effects
  • Having all safety measures in place to protect the people in the same room
  • Configuring your dynamometer to automatically cutoff the system should any parameter exceed its safe limit.

A simple but effective test consists of ramping up the throttle in small steps, and recording a sample after every step. Before taking the sample after each step, we allow the system to stabilize for few seconds.

In the video above, we manually varied the throttle from 0 to 100% in 10 steps. This procedure could also have been performed using the RCbenchmark’s automatic test or scripting feature, which we will cover in another tutorial.

The results obtained are shown in this CSV file.

How to use the efficiency results?

You can summarize a lot of data points using any plotting software. Here is an example obtained using the CSV file linked above:

plot trust and propeller efficiency

 

You can than compare this plot with other plots generated using the same method. Try comparing two plots, all with the same parameters identical expect one element changed, for example switching propeller.

Stay tuned for more tutorials on how to analyze and interpret the results, allowing you to make smart design decisions for your designs.

Liked this tutorial/comments? Let us know below.

19 thoughts on “How to Measure Brushless Motor and Propeller Efficiency

  1. Hi, I have the Rcbenchmark device and after I pressed “Tare Load Cells” the trust are jumping between 0.000 to 4.000 in the highest
    also in the exel table I dont get the Motor and Propeller Efficiency.
    What should I do?
    Thanks, Daniel.

    1. Hi Daniel,

      What are the units of thrust? 1-2 g of noise is normal over the full 5 kg of measurement. You can use a script to average multiple readings for more precision.

      For the efficiency, are you reading a motor speed? If so, what is the “Main RPM” selected in the setup tab? Since the software supports and electrical and an optical RPM probe, you have to specify which RPM probe to use for calculation.

      Sincerely, Charles

  2. Hi,

    does RPM represent throttle stick position? I believe that (my motor) when I have 50% of throttle it doesn’t represent 50% of max power.
    So I think a curve is needed for linear power output, but that needs to be confirmed by measuring…

    1. Hi Jano,

      The relationship between RPM and throttle stick position is dependent on the firmware and on the motor loading. There can be non-linearity (exponents or quadratic relationship) due to the ESC firmware or the transmitter. Although this varies greatly, changing throttle value is often about equivalent to changing motor input voltage. If you want to understand how changing voltage affects the motor, we have two short videos about this on our Youtube channel. Cheers!

  3. Excellent tool, however, how about computing Ct, Cp and from this FoM (Hover) within your CSV file? I find this a much better measure than overall efficiency (g/W).

    1. Hi Dr Prior,

      This is a good point! The current software is design to acquire information that can be calculate in real time. Those constants (Ct, Cp ) require a fit of multiple data points. At the moment, the calculation can be accomplished in Excel, or automatically by modifying one of the included script (probably the Sweep – discreet script). If there is enough demand however, we would love to make a motor and propeller analysis software.

  4. What is the propeller coefficient formula? And what is the force that the propeller influences to the right and to the left, apart from the force that it affects back and forth?

    1. Hi Fatih,

      The thrust is proportional to the square of the rotation speed omega. In a spreadsheet, you can plot thrust vs RPM and create a fit going through 0 (0 thrust at zero RPM) and of second order. The result will be a coefficient C as in Thrust = C * omega ^ 2. C is your coefficient.

      What do you mean by forces to the right and to the left? If there is sidewind, the propeller could be pushed sideways due to drag.

      1. “Thrust = C * omega ^ 2” How do I calculate omega here?

        1. omega is rotational speed so called rpm 😀

        2. We replied by email earlier, but we will post my reply here for the benefit of other people: Omega is the rotation speed. It can be measured with a tachometer. Our tools include speed measurement too. The unit of omega is rotation per minutes or rad/s (more common in scientific publications. The unit will change the value and unit of the constant.

  5. Hi admin! I wonder for what purpose you used left and right loadcells?

    1. The two load cells are used to measure torque. The software is solving a system of equations in real time based on a simple calibration.

      1. Hi Admin
        Thank you for replying fastly and kindly. I wonder what kind of equations you use to calculate the torque from two load cells at the right side and the left side. And how did you measure and use voltage and amp.. by using electronic circuit or determining from other sources? I want to measure only torque.

        Thank you!

        1. You can check line 280 of the code for details.

          https://gitlab.com/TytoRobotics/RCbenchmarkGUI/blob/master/src/tabs/calib_wizard.js

          The calibrations is done with a calibration arm and a weight. Three measurements are made: without weight, with the weight in position A and with the weight in position B. Each time, the voltage of each force sensor is measured.

          The result is a very precise calibration, very good at rejecting parasitic forces.

          1. Woaw such a fast reply I appreciate your effort and I can’t tell how glad I am right now. keep up the good work.

            Thank You!

          2. It is a pleasure, good luck with your tests!

  6. Two separate systems will be formed, with the effect of the propeller in the first system,
    The applied force will be calculated, and the number of turns will be gradually increased and decreased
    For example 100 rpm in the interface or 130,123,540 different values
    By changing the speed of rotation from the force / rotation speed to the first system
    A graph based on the force and the propeller coefficient by means of the obtained coefficient
    To be obtained

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