Efficiency of the original DJI Mavic Mini propeller

[Dynamicus]

Test conditions: ESC supply voltage 8.2 V, altitude 230 m, air temperature 23 °C and air pressure 1000.2 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 8588 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 8588 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.

 

[Yorkshire_Pud]

How are you defining efficiency?

Just for your reference I am attaching two Mavic Mini motor RPM DAT traces, one is for an indoor flight that was primarily a hover, the other is from an outdoor flight where the drone covered significant distance but then lost connection.
A very high RTH height had been set, truthfully accidentally, and the pilot did not, at that time, know how to stop the climb, the climb was significant and sustained.
Once the drone started back towards home the throttle was closed for most of the journey back. Once the drone was close and low there was some messing abount to relieve the tension.

 

[Dynamicus]

I use a wind tunnel to determine the efficiency of the propellers. But propeller efficiency is mainly useful for propeller designers. For the average drone owner, efficiency may only be important when he or she starts wondering how to extend battery life, reduce drone noise, achieve some kind of record (maximum altitude, etc.). This can primarily only be achieved by replacing the propellers with propellers with significantly better characteristics.

 

[Zbip57]

I think the question is more general, as in what is what is meant by "efficiency"?

I think we all understand the concept of light bulb efficiency, as a ratio of how much actual light is produced compared to how many watts of electrical energy is consumed, where much of the energy is wasted as heat.

What are you actually measuring to determine propeller efficiency? Speed and Mass of air moved vs energy input? 8% efficiency seems extremely low.

 

[Yorkshire_Pud]

This looks to be a good read Propeller Performance: An introduction, by EPI Inc.
What does anyone, @sar104 maybe, think?

 

[Zbip57]

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 8588 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?

 

[Yorkshire_Pud]

I am in the process of reading through your post but intial thoughts (which may well be wrong - I am very, very rusty on this stuff ) are, that the 'velocity' would be the change in speed, relative to the surrounding unaffected air, imparted to the air 'ejected' by the propeller.
I suspect it will come down to the engineering foundation of a "Free body diagram" drawn around the propeller and considering property differences of the air entering and leaving the diagram but as I say I am very, very rusty on this.

This is why I was/am hoping @sar104 will chip in.

 

[Zbip57]

For sure it's going to be a complex explanation. Even just the "change in speed" can't be a simple measurement. On a rotating propeller, the tip of the blade is moving much faster through the air than the root of the blade. So that blade is twisted with a much fatter section at the root with a much higher angle of attack versus the smaller flatter tip. Are both the root and the tip thus moving the same "amount" of air and are both accelerating the air by the same amount? The total "change in speed" of the airflow across the length of the blade must be really difficult to measure, whereas measuring the total generated "thrust" is relatively simple.

Obviously there are a bunch of different losses along the path from the battery to the end result of how much actual lift is generated, losses in the battery itself, the wiring, the ESCs, the motors, resulting in heat losses, then the props themselves generate lots of vortex drag and noise, etc.

But only 8% efficiency sounds ridiculously low. That can't possibly be right, can it? Or maybe that was just a typo and it's actually supposed to be 80%?

It should be easy enough to measure how much power is driving the propeller, and how much thrust is generated by the prop. I'm just curious where the 8% comes from.

 

[Yorkshire_Pud]

It's why "computational fluid mechanics" are such a big thing.
You have to take your hat off to the old timers who used slide rules to design stuff before computers did the number crunching..