User sar104 on the Mavics Pilot forum has done extensive testing with his Mini to better understand the relationship between deformed props and uncommanded descents.   The full discussion is here:  https://mavicpilots.com/threads/mavic-mini-uncommanded-descent-tests.86237/    Ice_2k originally referenced this information on another thread but it seemed to get little attention.  Sar104's information and conclusions are based on relatively controlled testing and data versus the anecdotal information that is running rampant in this forum.  Out of everything I’ve read, I lean towards sar104’s explanations because it is data based and there is logic to his analysis.  For those of you that want a summary, here is my understanding.  I apologize if I misstate his information.

  

  

• He was able to flatten the rear props by improper storage in the Fly More case in just 12 hours.  With the flattened props, he was able to produce an uncommanded descent (UD) while flying sports mode and pushing the drone to max RPMs.  Previous to flattening the props, he was not able to produce a UD.
  

• He believes that at or near max RPM’s the props will experience additional deformation and will not be able to sustain the necessary thrust to achieve the required front pitch.  The drone will reduce the front motor RPMs to try to maintain pitch because it cannot increase the rear RPMs at max. This reduction in the front RPM’s will cause the drone to descend.  It looks like the FC prioritizes forward pitch over altitude and causes the drone to descend.
  

• He repeated the same test the next morning and felt that the rear props reduced some of the deformation but he was still able to reproduce the UD.
  

• He then runs the same test under firmware 1.00.05.  He observes that the flight characteristics have slightly change but with the flattened rear props, he is still able to produce a UD.  He concludes that 1.00.05 gives additional warnings but does not eliminate UD’s.
  

• A common misconception on this forum is that you can achieve forward pitch with all 4 motors running at roughly the same RPMs.  This shows that many users do not understand the basic mechanics of quadcopter flight.  The rear props always need to spin at a higher RPM than the front props to sustain forward pitch for horizontal velocity even when you have all good propellers.  All of his data demonstrates this if you know how to interpret the data.  In addition, he specially states this in his description of what is happening during the drone flight.   If you do not believe this, then please show your RPM data with forward horizontal velocity to prove otherwise.
  

                  

  

Finally, at the end of his discussion, I asked him what program he is using the chart his data.  It is Wavemetrics Igor Pro and seems to be a great way to graph RPM’s vs Pitch vs Horizontal Velocity vs Vertical Veolcity.

Randy

Hi Randy,

Thank you very much !

This is exactly what I am looking for.

I just create a thread about The way the motor RPM are running.   I received  informations  about that   but,   I'm not convinced.  
The link  ???

Well. I just did a simple test and my results do not align with that opinion. There was a very brief imbalance in front-to-rear motor speeds when the drone begins a forward direction, but I saw that the motors quickly returned to a balance speed even while the drone moved ahead. The same happened in reverse, a brief imbalance followed by a balanced speed.

However, the tester may have seen very small imbalances due to more exact measurements. Can he quantify the differences?

Update:

I will at least sleep easier to night

See  Here

Cheers !

Hi again !

Just one more thing,

Randy, You wrote: ''...The rear propellers must always rotate at a  higher speed than the front propellers to maintain the forward pitch for a horizontal speed even when you have all good propellers.''

I wouldn’t say that. (with respect),

Here's the exact behavior:  Quote: ''.....The FC knows the exact combination of front and rear RPMs to keep the AC at the right pitch to achieve a forward speed and maintain a certain altitude. ''

See:   Here  Occams Razor

Cheers !

JGFly Posted at 5-2 11:42
Hi again !

Just one more thing,

OH my God,  I just realize that Randy and Occams Razor are the same member   

I really hate growing old  

GaryDoug Posted at 5-1 19:47
Well. I just did a simple test and my results do not align with that opinion. There was a very brief imbalance in front-to-rear motor speeds when the drone begins a forward direction, but I saw that the motors quickly returned to a balance speed even while the drone moved ahead. The same happened in reverse, a brief imbalance followed by a balanced speed.

However, the tester may have seen very small imbalances due to more exact measurements. Can he quantify the differences?

The tester had flattened his rear prop so he artificially created a greater differential between front and rear RPM's than normal.  Try this test - operate your drone at max horizontal speed in sports mode for 10 to 20 seconds and then see what the data shows.  

Occams Razor Posted at 5-2 14:46
The tester had flattened his rear prop so he artificially created a greater differential between front and rear RPM's than normal.  Try this test - operate your drone at max horizontal speed in sports mode for 10 to 20 seconds and then see what the data shows.

My response had nothing to do with anomalies. It was only about the motor speeds in normal use.

I think part of the problem is his whole theory depends on completely flattened props, but answers no questions for those who straight out of the box almost received uncontrolled descents and further more many of those it only happened once and after that without changing props their craft flew ok, one prominent member here jjb had an uncontrolled descent and his findings were that it was not his props and also he continued to fly with same props.

I think purposely damaging props to achieve his results only shows one thing if you’re props are completely damaged you have a problem, now he maintains that this was caused through storage so he stored props the one way he knew would damage them and so his test was not done with a sample few props from a few people it was done in a way he knew would guarantee failure, so it’s not really relative to everything that’s going on, my opinion.

If we have an idea we can purposely set up a test that will show what our idea is proving .

There's nothing inherently wrong with setting up an experiment to prove an idea. Some of Einstein's theories were proven in just that manner.

There seem to be 2 causes of uncontrolled descent (UD) here.

1) Deformed rear props
2) Height sensor error (as per JJB's case)

The 2 may or may not be related but it would be good if DJI look at this as 2 seperate issues. Making people change rear props will eliminate (1) but will not address (2). Question is, how should the quad react if the IMU received 2 diff data from height sensor and baro? Which should it follow?

In JJB's case, in previous firmware, the craft slows down all props causing UD. This points to programming logic issue as craft decides to follow the erroneous height sensor rather than baro (although the baro may fail too). Perhaps if both sensors show discrepancies, IMU should cross ref GPS data to determine height (if this us even possible)?

If only the mini have redundacy systems installed.

Throwing this out there to try to help with understanding of possible bad blades.

Bad blades don't have to be deformed or flattened to result in low thrust, once blades are put under a load.   
A bad blade can be weak and as more thrust demand is put on blade, the blade starts to deform (bending) or flattening (less pitch).
Point being, when blades are stationary, blades appear fine.  When blades are at high rpm, under a high load to produce thrust, blades deform or flatten, but you can tell with your eyes.

Have seen something similar with carbon fiber arrows.  Arrow meets all normal specs, length, diameter, weight, straightness, and balance.  Arrow looks absolutely perfect to eye.  But arrow has a dominate weak side from manufacturing process.  Resulting in to much flex in arrow in one direction, which results in arrow being off from a group of arrows.  You can't see arrow flex with your eye as it's fired from bow, but it does.


Do hope problem gets resolved, no mater what cause is and final solution is.

GaryDoug Posted at 5-2 18:19
There's nothing inherently wrong with setting up an experiment to prove an idea. Some of Einstein's theories were proven in just that manner.

Yes you’re right but the result is known before you start the test, you can’t fly a drone using Flattened props we already know that, there is no need to test that, but we also know that brand new craft with new props had uncontrolled descent , so in other words props that appeared to have no problems, and that’s what needed to be tested because this was the part that was unknown , if you take all the brake fluid out of the car, wouldn’t you know exactly what was going to happen ?
These props were purposely flattened out knowing that they couldn’t and wouldn’t fly correctly and they didn’t .

I think we have gone a little while since new FW which is noticeably better and have not seen any drop outs or uncontrolled descents, so just a coincidence or was something fixed, even tester above admitted to a change optimizations on the drone, but he continued to try testing with something he knew wouldn’t work.