I am a recent graduate of M.Sc. Mechanical Engineering from Ottawa, Canada. My colleague and I designed a dynamometer and motor tester specifically from UAVs and quad designers. We actually started this project after spending weeks to build our custom solution.
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For the release official product release, here is a quick introduction to motor and propeller testing, as summarized in the video above. The tutorial is done with
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
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?
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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.
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
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
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:
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.
What next?
For the launch week, we are offering 15% off to multirotorforums readers. Use the code "
We want to publish more tutorials, with more details about certain aspects, such as automatic tests, installation, automatic kV testing and pole counting, motor theory, dynamic tests, scripting, etc. Anything in particular you would like to learn about?
Hi guys,I am a recent graduate of M.Sc. Mechanical Engineering from Ottawa, Canada. My colleague and I designed a dynamometer and motor tester specifically from UAVs and quad designers. We actually started this project after spending weeks to build our custom solution.For the release official product release, here is a quick introduction to motor and propeller testing, as summarized in the video above. The tutorial is done with our RCbenchmark dynamometer , but is should be applicable to DIY solutions too.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.The final choice of power system depends not only on the airframe and payload, but also on your application.To fully characterize a motor, you need to measure the following parameters.The RCbenchmark software automatically calculates the following parameters for you: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.For extracting useful propeller data, you need to measure the following parameters:The RCbenchmark software calculates the following parameters for you: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 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: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.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.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.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: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 You can summarize a lot of data points using any plotting software. Here is an example obtained using the CSV file linked above: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.For the launch week, we are offering 15% off to multirotorforums readers. Use the code " MRF15 " until November 15th.We want to publish more tutorials, with more details about certain aspects, such as automatic tests, installation, automatic kV testing and pole counting, motor theory, dynamic tests, scripting, etc. Anything in particular you would like to learn about?
A very good read indeed! Thanks for that.
The article describes propeller efficiency as the ratio of generated propeller power divided by the engine power required to drive the propeller. But how do you measure the propeller power?
It's relatively easy to measure the thrust (force) generated by the propeller, but power is defined as
work per unit of time, which is
force x distance / time, or force x velocity. So how are you measuring the velocity?
Propeller (propulsive) hp = thrust {lbs} * velocity {ft/sec} / 550
The article refers to propellers driving an airplane, where velocity is the speed of the airplane, i.e. the velocity of the air being fed into the propeller. It's not a measure of the velocity of the air driven back and leaving the propeller.
Looking at our drones in hover, that velocity is essentially zero, isn't it? Or is it actually the velocity of the air being pulled down into the propeller at the point where the air first touches the prop blades?
I'm still confused. The article shows an aircraft propeller operating at about 85% efficiency when the aircraft is travelling at high speed, but very low efficiency 0% when the aircraft isn't moving 0-mph. That makes sense, only because all that engine power being consumed to spin the prop to generate thrust is all totally wasted if the aircraft isn't moving.
It's like idling your car's engine while waiting for a traffic light to turn green. The engine is actually still doing useful work running your air conditioner, powering your headlights and radio, etc. But if your engine efficiency is only being measured in mpg, you're seeing zero efficiency whenever the engine is consuming fuel but the car isn't moving.
Is that why the Mini's propeller is shown as only 8% efficient? Because in hover it is pulling in stagnant air rather than being fed a high velocity air stream?
Dynamicus said:
The graph shows that when the propeller reaches 8588 rpm, it has an efficiency of about 8%.
Click to expand...
8% just seems like a ridiculously low number, hardly worth the effort. Or is that just a reality of hovering flight? You're burning energy, but going nowhere?
Clearly the props are generating significant thrust, enough to sustain hover for ~20 mins, and propel the Mini up/down, forward/back, etc. That's why I'm having a hard time wrapping my head around that 8% efficiency number. What is that actually measuring? 92% of the available battery power is wasted?
I mean really, if you think about it, all of the energy is totally wasted if the drone takes off, flies around, and returns to land at the original spot. You've burnt all that energy and ended up exactly back where you started from. That's pretty much the definition of zero efficiency, isn't it?
A very good read indeed! Thanks for that.The article describes propeller efficiency as the ratio of generated propeller power divided by the engine power required to drive the propeller. But how do you measure the propeller power?It's relatively easy to measure the thrust (force) generated by the propeller, but power is defined as, which is, or force x velocity. So how are you measuring the velocity?The article refers to propellers driving an airplane, where velocity is the speed of the airplane, i.e. the velocity of the air being fed into the propeller. It's not a measure of the velocity of the air driven back and leaving the propeller.Looking at our drones in hover, that velocity is essentially zero, isn't it? Or is it actually the velocity of the air being pulled down into the propeller at the point where the air first touches the prop blades?I'm still confused. The article shows an aircraft propeller operating at about 85% efficiency when the aircraft is travelling at high speed, but very low efficiency 0% when the aircraft isn't moving 0-mph. That makes sense, only because all that engine power being consumed to spin the prop to generate thrust is all totally wasted if the aircraft isn't moving.It's like idling your car's engine while waiting for a traffic light to turn green. The engine is actually still doing useful work running your air conditioner, powering your headlights and radio, etc. But if your engine efficiency is only being measured in mpg, you're seeing zero efficiency whenever the engine is consuming fuel but the car isn't moving.Is that why the Mini's propeller is shown as only 8% efficient? Because in hover it is pulling in stagnant air rather than being fed a high velocity air stream?8% just seems like a ridiculously low number, hardly worth the effort. Or is that just a reality of hovering flight? You're burning energy, but going nowhere?Clearly the props are generating significant thrust, enough to sustain hover for ~20 mins, and propel the Mini up/down, forward/back, etc. That's why I'm having a hard time wrapping my head around that 8% efficiency number. What is that actually measuring? 92% of the available battery power is wasted?I mean really, if you think about it, all of the energy is totally wasted if the drone takes off, flies around, and returns to land at the original spot. You've burnt all that energy and ended up exactly back where you started from. That's pretty much the definition of zero efficiency, isn't it?
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