Efficiency of the original DJI Mini 2 propeller - Mavic Pilots

Test conditions: voltage for ESC power supply is 8.8 V, altitude 230 m, air temperature 23 °C
(Note: The efficiency measurement of each propeller affects the efficiency of the motor and ESC used. The measurement of the efficiency of very small propellers from DJI drones is affected by air temperature, air pressure and humidity, which are most noticeable when the motors start to spin up.)

The graph shows that if the propeller reaches rpm, it has an efficiency of about 8% (8.2% to be exact). However, as the speed increases further, the efficiency drops further, so further increases in speed are no longer relevant to the graph. 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.

But the propeller efficiency of 8.2% achieved is not dazzling, creating room for significant propeller innovation. The achieved efficiency of each propeller has a major impact on battery life. Instead of increasing battery life, reducing noise or increasing thrust may be chosen, but this requires additional modification of the propeller shape.

The achieved propeller efficiency should be of particular interest to propeller designers and manufacturers. For the average drone owner, efficiency may only be important when he or she starts to wonder how battery life could be extended, drone noise reduced, some record (maximum altitude, etc.) achieved. This can primarily only be achieved by replacing the propellers with propellers with significantly better characteristics.

Maybe some propeller manufacturer (from the EU, USA, Australia, etc.) will take notice of this chart (or all the charts I posted) and show interest in producing the first truly high performance 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.

If anyone has one unused original DJI drone propeller at home that I have not yet subjected to efficiency measurements, they can send me the propeller - the propeller may be used, slightly worn, etc. It will contribute to a good cause.

That would be awesome if they did, maybe a 5 blade race prop to get this thing to pick up some speed, I wanna go fast Ricky Bobby

I don't think it's possible to significantly accelerate a drone in level flight (for example in a tailwind). The maximum speed of the drone is probably limited by software. This has already been addressed somewhere on the forum. Increasing the speed would probably be possible with a DJI FPV drone etc.

It is possible to upgrade the propellers just to increase battery life, reduce noise or increase thrust (or a little of each). Increasing thrust will allow for increased speed when flying upwind, for example.

However, it's not clear (to me) at what RPMs the thrust is sufficient to break even with the weight.

I'm afraid I don't know exactly what you mean. I'll try to explain it in another way: If you start the engines, they reach about rpm. If you then start to increase the speed of the motors, and each motor reaches about rpm, the drone will start to climb and go into hover. However, at maximum speed in horizontal flight (upwind), the motors are at about rpm.

However, it is still true that when the drone is hovering (in no wind) the front motors have a slightly higher RPM than the rear motors. This is because the front of the drone is slightly heavier than the rear of the drone. I'm glad it helped you. I can talk about propellers all day if there's an audience. I bought the DJI Mini 2 just to test the upgraded propellers. I'm now thinking of a propeller shape that will allow for significantly longer battery life. I will then make a prototype propeller and measure the efficiency again. The goal is to achieve a battery life extension of 5 to 10 minutes. If I am successful, I will post a graph showing the efficiency comparison between the original propeller and the prototype propeller. This will set the stage for preparing to start mass production, that is if I can find a suitable company.

zenonn​

That's a good idea, but...

While I am able to model a propeller for the DJI Mini 2 that will have the same efficiency (or the same noise) as the Master Airscrew propeller, I don't have the time to look for the right propeller manufacturing technology using 3D printing (on home 3D printers). There would have to be someone here who would be willing to find the right technology to manufacture a propeller using 3D printing. If he finds such a technology, then he would post the exact instructions here on the forum. Anyone could then use such instructions to make a propeller at home. This would probably be welcomed by people who wish to use propellers in a distinctive colour.

scro​

I believe that an efficiency of 8% is not such a bad result. But it could probably be higher, maybe 10%. If I'm right, we'll see very soon, that is if I can model a propeller with 12% (or maybe 13%) efficiency. I'm already preparing for the modelling. It's taking me a little while because I have more work to do.

I chose the meximal speed (which the graph shows) for several reasons. One reason is that at higher speeds, the steady decline in efficiency would continue anyway. A similar drop in efficiency occurs with all propeller sizes. So I'm not interested in the lowest efficiency, I'm more interested in the golden mean - when the drone is hovering in place.

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I like the fact it’s not just more efficient, but quieter also.

Fascinating - but the cost is what is going to make this difficult to go mainstream - the video said a regular boat prop was $500, and one of these was $5,000 - a 10x increase in cost. That isn't going to pencil out for cost effectiveness unless they can get the cost down significantly.

Thanks for posting the video - very interesting.

Hi, when flying smaller (5" to 10") copters in fast forward flight, three-blade propellers are almost standard, because they produce less vibration. The most commonly accepted reason seems to be that with three blades, the oscillation of angle of attack of the blades in forward flight is more balanced than with two blades (where always one blade has a high angle of attack, and the other has a small angle of attack, producing vibration).

Link to Wing Flying

But on larger copters, I still mainly see two-bladed props.

I recently built a custom 3-blade propeller hub for the T-motor MF propellers that we are using on our 5 kg, 18" quadrotor. I noticed some vibrations while flying at higher speeds in a large wind tunnel (video), and I thought that the 2-bladed props might be the reason.

And today I tested the new 3-blade setup, and I like it much better. I was flying the same auto mission with both setups. The mission is to oscillate several times between two waypoints at 30 meters distance with 5 m/s and with 7 m/s.

Vibrations decrease by a factor of 3, the power consumption is almost identical, the mean throttle drops. What is the reason that 2-blade props are so popular on larger drones…?

Edit: I accidentally flipped the labelling in the vibration table… Now it is correct. 3-blade has of course a much lower vibration level in forward flight.

You have raised an interesting topic because our props on multi rotors are operating as props were never intended to operate.

From a fixed wing perspective it is well tested that more blades is less efficient, where the prop directly faces the oncoming air flow.
The most efficient in this case is actually a single blade prop, and yes, they were a thing, although you don’t see them much anymore.

But considering the direction of airflow on multi rotors its a whole different story.

It would be interesting to hear if people have actually tested the characteristics of props in horizontal for ward flight.

Im just going down the same path. My copter has max power and hover time with 11"-2.
But i noticed with 10"-2 it flies much more sporty and can do a flip with half the height loss (in spite of the lower max power).

Now if a 10"-3 gives me the max power of the 11"-2 and the handling of 10"-2 that would be great.

Want more information on drone propeller efficiency test? Feel free to contact us.

For efficiency ecalc predicts a loss of 3 minutes out of my 34 minutes hower time, which is not too bad if the performance gains are what i expect.

It seems the bigger the prop, the smaller the efficiency losses because the blades are further apart and the lower rpms mean more time until one blade reaches the turbulence of the previous one. Also the hover RPM of 3 blades is lower helping here.

So it is clear that efficiency suffers more on small, high kv builds, but even there the handling gains seem worth it. So why dont we see 3-blades on larger general purpose copters? While hover time may decrease a bit, in dynamic flight the drag decreases and max flight distance improves again. I’m excited to try soon, I could be wrong.

If anybody has experience with 3-blade influence on filter parameters and thrust expo i would be very interested.

I think the “efficiency benefit” of 1- or 2-blade props vs. 3-blade is a misunderstanding. I had the same opinion for a long time, but now I think differently.

2 or 3-blade props with the same diameter do not have a noticeable difference in efficiency. But, if your aim is to generate a specified maximum static thrust, then you will use a smaller 3-blade prop and a larger 2-blade prop (as these will generate the same amount of static thrust at a certain input power).

Naturally, the larger 2-blade prop will have a higher efficiency (due to the decreased disc loading related to Froude efficiency).

To conclude:
At same diameters, there is hardly a difference in efficiency, but using 2-blade props instead of 3-blade allows to use larger diameters (at the same power input), and that gives you more efficiency.

Dr. Martin Hepperle, a pretty renowned aerodynamicist states the following:

[…] The Number of Blades has a small effect on the efficiency only. Usually a propeller with more blades will perform slightly better, as it distributes its power and thrust more evenly in its wake. But for a given power or thrust, more blades also mean more narrow blades with reduced chord length, so practical limits have to be considered here. The chord length can be increased while decreasing the diameter to keep the power consumption constant, but a diameter reduction is usually a bad idea in terms of efficiency, as long as the tip mach number or tip cavitation is not an issue. […]
from: JavaProp

Keep in mind that the propellers need to be as light as possible for motors with a small diameter, otherwise the motors will not have enough torque to accelerate them. Some propellers from MasterAirscrew are awfully heavy. I had the best experience with very cheap 3-blade props from ABS, but they are not available anymore. And you can convert every folding prop to 3-blade when you mill your own hub from carbon.

You’ll also need to increase the D-term quite a lot On my .
Here are more props:

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