Mavic Mini uncommanded descent tests

[sar104]

@sar104 ——Would you be willing to do the test on a set of the new carbon fiber props for the MM? If so, I’ll have a set sent to you from China.

Sure - if I can get some I'll test them. I wasn't aware that any were yet available.

 

[Occams Razor]

Hi Sar,

First thank you for all of the experiments and great insight. This is the first time I've seen data based evidence to determine what is happening with the UD's. Which program are you using to produce those charts?

Randy

 

[sar104]

Hi Sar,

First thank you for all of the experiments and great insight. This is the first time I've seen data based evidence to determine what is happening with the UD's. Which program are you using to produce those charts?

Randy

Wavemetrics Igor Pro

 

[Occams Razor]

Wavemetrics Igor Pro

Thank you! I really appreciate the quick response.

 

[Huttcraft]

Hi SAR - thanks for your interesting work and patience with all of us.

So knowing what you know now, do you plan to make any changes to your own MM (eg hunt down aftermarket carbon fibre props)? Or will you just try to be careful not to distort the props when it’s stored?

Dave

 

[tribar]

dji WE have a problem, WE don't pay attention!
How to positions the propellers before putting the Mavic Mini in the Fly More Case....

 

[sar104]

Hi SAR - thanks for your interesting work and patience with all of us.

So knowing what you know now, do you plan to make any changes to your own MM (eg hunt down aftermarket carbon fibre props)? Or will you just try to be careful not to distort the props when it’s stored?

Dave

There is an ongoing hunt for aftermarket props, but not much available as yet. Personally I've been doing the opposite of everyone else - I've been trying to recreate the problem by deforming the props since the only reason, at least for now, that I have a Mini, is to investigate this issue.

 

[sar104]

Here is a view that shows quite clearly the deformation that is occurring:



The top propeller is from the rear left motor, and was on the aircraft for the flights depicted in the earlier graphs. The bottom propeller is brand new, unused. As you can see the profile starts of very similar, but towards the tip of the prop the old one is clearly twisted, CCW as viewed from the end. That is going to result in significantly less (if any) lift generated over the outer ½ to ⅓ of the length.

 

[Huttcraft]

The prop positioning infographic in the Flymore case is a bit misleading too. It shows the blades of the port side rear props overlapped in a way they don’t go (see pics).

 

[Herbie]

The prop positioning infographic in the Flymore case is a bit misleading too. It shows the blades of the port side rear props overlapped in a way they don’t go (see pics). View attachment 100294View attachment 100295

I always before each flight twist each prop about 45° to give it more lift.
Dont know if it works but never had any problems regarding decent or motors. On the prop pictures it looks as if the bottom one has negative lift at the tip.
Regarding the prop folding into the case mine works 100% according to the prop positioning infographic.

 

[sar104]

I always before each flight twist each prop about 45° to give it more lift.
Dont know if it works but never had any problems regarding decent or motors. On the prop pictures it looks as if the bottom one has negative lift at the tip.
Regarding the prop folding into the case mine works 100% according to the prop positioning infographic.

If you mean in the image that I posted, the top one is the one lacking lift at the tip - it almost has a negative angle of attack.

 

[Herbie]

If you mean in the image that I posted, the top one is the one lacking lift at the tip - it almost has a negative angle of attack.

Sorry yes the top one

 

[TDZHDTV]

I measured the combo case today with a vernier depth gauge, there is plenty of clearance (even with screw on motor caps), you just have to make sure you don’t trap them under the belly of the AC as identified by sar104.

 

[Nonprophet]

Here is a view that shows quite clearly the deformation that is occurring:

View attachment 100293

The top propeller is from the rear left motor, and was on the aircraft for the flights depicted in the earlier graphs. The bottom propeller is brand new, unused. As you can see the profile starts of very similar, but towards the tip of the prop the old one is clearly twisted, CCW as viewed from the end. That is going to result in significantly less (if any) lift generated over the outer ½ to ⅓ of the length.

Very interesting. So when people describe the prop problem is due to "flattening from improper storage" in reality the props are being twisted and not flattened, correct?

 

[sar104]

Very interesting. So when people describe the prop problem is due to "flattening from improper storage" in reality the props are being twisted and not flattened, correct?

Looks like it, although flattening is causing the twisting.

 

[Capt KO]

My original set deformed in the case using a rubber band. Was for-warned by getting repeated notices that it had switched to “Load” mode (with No load) causing decreased flight limits. Changing the rear props solved the problem. Since then I have them stored (securely in the case) using plastic caps that keep the blades folded, as shown in the diagram. No problem since and my only decent is from flying too low over water. But that’s total different.
Thanks sar104. We MM owners owe another debt of gratitude. ?

 

[rehkram]

Sorry if I missed it but given that this usually occurs after a period of normal flight (it would have to or we'd never have gotten off the ground in the first place), surely the sudden lack of lift, often after a fight with the wind in my own experience, indicates the blades' aerofoils are flattening-out out as the rpms increase. Lack of lift = no aerofoil.

So you could say that either a) the rpm needs a lower threshold limit to suit the blades, or b) the blades need to be sturdier to suit the rpms

I think personally that while improper storage may well be a factor, it's a bit of red herring in as much as the blades are under spec for the rpms. I believe this could happen straight out of the box.

 

[sar104]

Okay - here is the last piece of the puzzle resolved, I think. This test was flown with FW 0500 and brand new DJI props (A1) on the rear motors.



A few differences to note from the previous tests.

1. There is no pitch excursion on takeoff - it remained approximately level.
2. In sport mode the aircraft achieved the full specified pitch of 30° and the rated speed of 13 m/s.
3. In P-GPS mode the aircraft achieved the specified pitch of 15° and the rated speed of 8 m/s.
4. There is no sign of the front motors slowing down to try to maintain pitch.
5. Performance forwards and backwards was similar.
6. The rear motors still spin up to nearly 16 krpm, but are clearly generating more lift.
7. When flying backwards the front motors are now spinning significantly faster than the rear motors.
8. No motor speed errors were thrown.
9. No sign of uncommanded descent.
10. The last two runs in the test were sideways in sport mode -

The faster speeds required of the rear motors is interesting. The aircraft weighs 249 g, with 129 g on the front motors and 121 g on the rear motors when level, all other things being equal.

CONCLUSION

The Mini's problems appear to be due to a shockingly bad propeller design, presumably adopted to save mass. I find it really hard to conceive that DJI didn't discover this problem in pre-release testing, unless it really does only happen with a particular batch or batches of props. That seems unlikely though, given the number of reports. Or perhaps it only happens if the props are pre-deformed in storage, and DJI never let that happen. It certainly seems to be predominantly reported by users with the Fly More combo case. Very careless, either way.

 

[Xanadu]

Once more, thank you for doing all this work for us all MM owners.

 

[Nonprophet]

Seems like perhaps a DJI Mini prop recall is in order.....and it sure would be nice to have some stock carbon fiber and/or higher quality blades to choose from.