[Tutorial] Obtaining motor and propeller efficiency for UAVs, quads ...

Hello fellow DIYer. Last February, my collegue and I posted a survey here asking if you'd be interested in a dynamometer for drone motors and propellers. We are now releasing the production version of the dynamometer!
For the release, here is a quick introduction to motor and propeller testing, as summarized in the video above.

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?

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 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:


 

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?

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?

If you are interested by our dynamometer, have a look here. We offer 15% off until November 15 for DIYdrones readers using the code "DIY15".

It is an exciting time for my collegue and I, as this release is the results of almost a year of work! Please comment below, I will do my best to answer your questions!

Test conditions: ESC supply voltage 8.2 V, altitude 230 m, air temperature 23 °C and air pressure .2 hPa.
(Note: The measured propeller efficiency is always dependent on the efficiency of the motor and ESC used.)

The graph shows that when the propeller reaches rpm, it has an efficiency of about 8% (8.1% to be exact). However, as the rpm increases further, the efficiency drops further. These speeds produce a thrust of about 64 g and the four propellers thus produce a total thrust of 256 g. And since the weight of the drone is 249 g, at rpm it starts to climb upwards. So increasing the rpm further would not make any more sense in terms of the graph.

But the propeller efficiency of 8.1% achieved is not dazzling, creating room for significant propeller innovation. Every propeller designer should be interested in achieving the highest possible efficiency in a propeller, which will allow further modifications to significantly improve other parameters (noise, thrust, acceleration or battery life). Perhaps some propeller manufacturer (from the EU, USA, Australia, etc.) will take notice of this chart (or all the charts I have posted) and show interest in producing the first truly high efficiency propellers for the most popular drones. I can provide such a manufacturer with more of my aerodynamics knowledge to make the upgraded propellers the absolute aerodynamic cutting edge.

This looks to be a good read Propeller Performance: An introduction, by EPI Inc.

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?

The graph shows that when the propeller reaches rpm, it has an efficiency of about 8%.

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?