Propeller Guards

[TruGreen]

I don't get that, the rear motors are canted outwards slightly when the arms are folded - unless they've subtly modified the arms, this is why the motor speed error is almost always on the rear and on the side that folds underneath the other

well heres mine. not bent or deflected like you have pictured

 

[Zbip57]

well heres mine. not bent or deflected like you have pictured

Here's mine with new blades. Note the difference between yours and mine in where the tips of the blades touch against the folded front legs.

 

[Zbip57]

I think it's clear that there is a difference between your old props and my new props. And, even my new props are not all that new anymore. I installed them back in April and have flown the Mini often since then. The difference is that I've been storing mine on a shelf with the arms open, rather than packed away in the Fly More case with the blades bent to fold together.

This proves that the prop deformation is subtle and difficult to spot. It's only evident when directly comparing like this to a set of new prop blades.

Although there is an obvious difference here, the question remains -- is this a significant difference? Does it affect the performance? Should you change yours to new blades?

There is a very simple test by which the prop's efficiency can directly be measured. Do a hover test and plot the motor speeds.

Find a well-lit open space indoors (to eliminate influence from winds), press auto-takeoff, allow the Mini to hover hands-free for a couple of minutes, then press auto-land. Take the DAT file that logs all your flight data, and use CsvView to plot the Motor Speeds. At a steady hover the Mini's motors should all be turning at nearly the same speed around 9600 rpm.

See this post for details on how to find your DAT file, and how to plot the Motor Speeds in CsvView.
mavicpilots.com/threads/motor-speed-error-mavic-mini.86130/page-15#post-1121175

And see this post for plots of my Motor Speeds as I changed the prop blades one motor at a time. There was a very clear and definite improvement with new props as illustrated in the graphs, as evidenced by the motors spinning slower with better performing fresh blades.
mavicpilots.com/threads/motor-speed-error-mavic-mini.86130/page-13#post-1038918

If you can't figure out how to plot the Motor Speeds, send me your DAT file and I'll do it for you. It'd be really interesting to see whether that small amount of deformation on your props makes any noticeable difference.

 

[Pawpaw Woody]

Interesting observations indeed. After reading your initial headline I see a new variable that might be afoul
(after upgrading to the latest firmware (v01.00.0500) that came out in April 2020)
It is not out of the realm of possibility that a software glitch could be the culprit

 

[Zbip57]

It is not out of the realm of possibility that a software glitch could be the culprit.

There are people in this world who insist that Elvis is still alive, the Earth is flat, and the moon landings were staged in a Hollywood film studio. Similarly, there are people who insist they can see nothing wrong with their propeller blades, therefor the repeated error messages must be a software glitch.

It's not a software glitch.

The warning was there in previous firmware versions. The wording has changed. Previously it warned of Max Power Reached. Since v01.00.0500 it now says Motor Speed Error, or even more explicitly Propeller Speed Error. It even specifically identifies the affected motor by beeping the ESC on that motor and telling you to replace those blades. It is an improved feature of the new firmware, not a defect.

It's not as though I'm proclaiming (without proof) that my Mini can suddenly fly for an hour or more on a single battery after soaking the batteries for a day in olive oil [extra virgin]. p.s. Do not try that. Really, don't!

I'm not selling olive oil. I have nothing to gain from this. You can believe what I'm saying, or not. I won't be offended. I regularly fly my Mini over dense forest and deep lakes. The Mini would likely be irretrievable if it crashed anywhere here. Healthy propeller blades are essential.

The forums are full of reports from people claiming their Mini dropped out of the sky for no reason at all. Lots of people are wondering why they keep getting Motor Speed Error warnings, or Propeller Speed Error warnings. Well, here's a reasonable explanation, backed up with photos and data plots from flight logs. As illustrated above, the propeller deformation can be "barely noticeable". But it is there and it is noticeable if you know what to look for.

There's no denying that the motors are mounted at an angle. With arms folded, clearly the prop blades on the rear motors cannot fold under each other unless one side or the other is bent to fit under the opposite side's blades.



Is the resulting prop deformation worth worrying about? That's totally up to you to decide. If you are already getting Propeller Speed warning messages with instructions to replace the blades, then I strongly suggest you take a closer look at your blades.

If your rear prop blades nest easily under each other when crossed with no apparent effort, then they must already be permanently deformed.

Do the hover test. If any of the motors is turning consistently faster than the other three, there's your proof. In hover they should all be turning equally at around 9600 rpm.

... see this post for plots of my Motor Speeds as I changed the prop blades one motor at a time. There was a very clear and definite improvement with new props as illustrated in the graphs, evidenced by the motors spinning slower with better performing fresh blades.
mavicpilots.com/threads/motor-speed-error-mavic-mini.86130/page-13#post-1038918

It's good to be skeptical. Don't buy olive oil from strangers.

But do check your own props. Test them in the comfort of your home. Do the hover test. But please, share your results, good or bad, as it may help convince others.

 

[Zbip57]

But please, share your results, good or bad, as it may help convince others.

I should also have said, please, if you can definitively prove me wrong in any of my observations, I will happily accept any better theories as long as they're demonstrated to be valid via a test that can be repeated and confirmed by everyone.

It is not acceptable to propose conjecture, such as that sunspots (or olive oil) might be causing Minis to crash, unless you can document a reasonable method for proving your theory.

 

[DanMan32]

I put my blades together and in the FMC case, no issues. They're usually loose enough that they can slip out of position when placing in the case. The biggest thing to look out for is that the bar in the case that crosses the body not press down on the props. If you angle the props, there's a good chance the bar will press down the tips against the body.

 

[Zbip57]

The biggest thing to look out for is that the bar in the case that crosses the body not press down on the props.

Yes. It's important to avoid anything that bends the blades. Bending the blades over the bar in the bottom of the case is certainly not good. Folding the arms with blades crossed, and then setting the Mini down onto a tabletop will also bend the bottom blades. That's all perfectly obvious.

But the whole point of my theory, as shown in the previous posts, is that even with the blades nested "properly" into the trough in the bottom of the Fly More case, the rear blades are still being bent simply by the fact that they are forced to cross over each other.

If, as you say, you are having "no issues", that merely means the blades have not yet been permanently deformed sufficient to trigger any warning messages. A hover test and plots of your Motor Speeds would demonstrate for certain whether there truly are "no issues" so far. Anything else is mere opinion.

If you've received no warning messages, your props may actually still be fine.

The threshold at which the Motor Speed Error warning message is triggered is set at a level at which the Mini will still continue to fly "normally". Mine did. People have even suggested it is set at far too sensitive a level, or maybe is even completely false (i.e. a defective firmware glitch).

Me, I'd much rather receive an informative warning message well in advance, giving me time to check and make any necessary repairs before something expensive happens.

My old 1990 BMW E30 has an extra float sensor that triggers a warning light if the engine oil level gets too low. It's alarming, but nice, to get that early warning reminding me that I'm an idiot for not checking the dipstick often enough. That early warning is much preferable compared to the 2nd stage next level oil pressure warning light that will eventually light up in the instrument panel. That one is much more serious as it means there's no longer any oil being pumped through the engine. If you ignore that oil pressure warning, the ultimate and inevitable expensive consequence comes when the engine seizes up tight and stops.

Even if you're having "no issues", it hurts nothing to do the hover test. It's simple, and you might learn something. It's fun just experimenting with the CsvView program to decipher all the other wealth of information recorded in your flight log DAT files.

 

[Pericles]

I always have the front leg propellers looking straight (at the camera side). Never have that issue. Only one time when i had guards + landing gear + 3 strobe lights on the mini.

Even after that only error, i put some soft material between all the blades for some hours, to ensure that the blades are running free. No problems after that.

 

[Zbip57]

I always have the front leg propellers looking straight (at the camera side).

I'm sorry. I don't understand what you mean. Can you post a photo?

 

[Zbip57]

well here's mine. not bent or deflected like you have pictured...

@TruGreen Have you had a chance yet to do a hover test? I'm very curious to see your results. ?

My Mini had always been carefully stored in the Fly More case, and my props looked just like yours. They slid together effortlessly when folded to nest into the case. It didn't seem to make any difference which ones went over/under the opposites side because the blades sat pretty much level and parallel to each other when crossed. And, as far as I could tell, the Mini flew perfectly fine and normal like that.

Yes, I received the occasional Max Power warnings, but I ignored them because the Mini always kept flying anyway. It was only when DJI changed the warning in firmware v01.00.0500, specifically telling me that I should replace the propeller blades, that I considered taking the warnings more seriously.

If you purchased the Mini as a Fly More Combo, you will have received spare sets of props. I did. I've never crashed my Mini, never run it into branches or anything, always hand-catch when landing to avoid sand and stones, etc. I believed my original props were still perfect as-is. But I had spare props to play with, so why not change them. If I ever did crash the thing and needed more replacement blades, I could always just pop the original blades back on, as I could see nothing at all wrong with them.

But, as I explained in my previous posts above, once installed the new props were obviously different and the Motor Speed data plots conclusively demonstrated that they work better.

If you're thinking of doing your own hover tests (please do!), here's my two-minute test for comparison with all new prop blades installed, all except for the right-front motor which still has the original pair of blades on it. The motor speeds average out to around 9600 rpm.



For comparison, here's a hover test posted in another forum by someone else. Note, at hover, the left-rear motor (blue line) is spinning consistently much faster than the others. That's because the flattened left-rear prop blades are not producing sufficient lift. Consequently the right-front (red) other counter-clockwise rotating motor is working much harder as well. All of the motors are running faster to make up for the loss of lift from the left-rear.



Here's a quick one-minute test with mine. Note that it takes about 30 seconds for the Mini to stabilize itself into a steady hover. So be patient and let yours sit in a hands-free hover for at least 2 to 5 minutes. I loaded the CSV file into an Excel spreadsheet to calculate the average motor speeds, discarding any values under 800 rpm to eliminate the takeoff and landing portions of the graph. I should have cut off the entire first 30 seconds, which account for the LBack distorting the average. You can see that in the second half of the plot the motors are all running at near equal speed.



For one more comparison, here is the same hover test but this time with the DJI prop guards (cages) installed. In case you are nervous letting your Mini hover hands-free indoors without the prop guards fitted, here you can see the effect the extra weight of the cages has on motor speeds. Again it took about 30 seconds for the Mini to find a stable hover, after which the motor speeds equalized at around 1000rpm faster than without the guards installed.



As you can see, the hover test is an unbiased and accurate way to conclusively measure your propeller blades' effectiveness. Unless your blades are obviously cracked or broken, there's really no other way to tell if they're working okay. The propeller deformation can be quite subtle and difficult to detect visually. The hover test is quick and simple.

 

[Zbip57]

If we are going to prove, or disprove, this theory we need lots of people to post their own hover test Motor Speed plots for comparison.

There's no denying that there are many reports of Motor Speed or Propeller Speed errors. People are getting tired of being told to replace their propeller blades. I'm convinced this explanation of how the props are being damaged makes sense.

If you are still skeptical, or see nothing at all wrong with your props, and you are convinced your blades are fine, the Hover Test is the simplest way to accurately measure the performance of your propeller blades. Knowing whether or not they are functioning at their best will give you confidence in making the decision to continue flying, or not.

If enough evidence can be collected to prove the cause of this problem, maybe DJI will step up with a proper solution.

If you don't know how to get or post your Motor Speed plots, I'm happy to help.

 

[Pawpaw Woody]

If we are going to prove, or disprove, this theory we need lots of people to post their own hover test Motor Speed plots for comparison.

There's no denying that there are many reports of Motor Speed or Propeller Speed errors. People are getting tired of being told to replace their propeller blades. I'm convinced this explanation of how the props are being damaged makes sense.

If you are still skeptical, or see nothing at all wrong with your props, and you are convinced your blades are fine, the Hover Test is the simplest way to accurately measure the performance of your propeller blades. Knowing whether or not they are functioning at their best will give you confidence in making the decision to continue flying, or not.

If enough evidence can be collected to prove the cause of this problem, maybe DJI will step up with a proper solution.

If you don't know how to get or post your Motor Speed plots, I'm happy to help.

I do not know how to get Motor Speed plots.... Please help! and thanks

 

[Zbip57]

I do not know how to get Motor Speed plots.... Please help! and thanks

Find a well-lit open space indoors (to eliminate influence from winds), press auto-takeoff, allow the Mini to hover hands-free for a couple of minutes, then press auto-land.

Take the DAT file that logs all your flight data, and use CsvView to plot the Motor Speeds. At a steady hover the Mini's motors should all be turning at nearly the same speed around 9600 rpm.

See this post for details on how to find your DAT file, and how to plot the Motor Speeds in CsvView.
mavicpilots.com/threads/motor-speed-error-mavic-mini.86130/page-15#post-1121175

 

[WooZie]

Hi, this is my test, my drone is 11 months old, with original props, what do you think? The drone was kept in its original bag, with the props fixed with velcro. Thanks

 

[Zbip57]

Hi, this is my test, my drone is 11 months old, with original props, what do you think?

That's a really odd plot. Both the right-side motors are turning significantly faster than the left-side motors. It looks as though the Mini is leaning to the left struggling to hold position against a strong crosswind coming from the left side. ?

 

[WooZie]

it is possibly to return its own wind returns from the wall...who knows

 

[Zbip57]

it is possibly to return its own wind returns from the wall...who knows

To see if that's actually the case, repeat the test in the same location but with the Mini turned 180° to face in the opposite direction. If the difference is being caused by the wind direction, you should then see both left-side motors turning faster than the right-side, i.e. the exact opposite of what's shown in your previous plot.

Wind will interfere with the results. I did a couple of tests to plot motor speeds comparing hover to full forward or full reverse. I though the wind was dead calm at ground level, but apparently there was a steady breeze blowing above treetop height, coming from the rear-left corner of the Mini.

You can see here the left-rear motor (blue) is turning consistently much slower than the others while hovering. The Mini was leaning back and to the left to hold position against the breeze coming from that side. With full forward stick input, the rear motors spin faster than the fronts in order to to pitch the Mini forward. At full rearward stick input, the front motors spin faster than the rears to pitch the Mini rearward.

 

[Zbip57]

With the Mini still facing the same direction, and the breeze still coming from the left-rear, you can see how the motor speeds vary when giving full left or full right stick input. At hover the Mini was still leaning slightly back and to the left to hold position against the wind. But with full stick input to either side, it's always the motors on the high side that are spinning faster than the low side.

 

[Zbip57]

So, yes, for sure the motor speeds will vary depending on what the Mini needs to do to counter wind interference while trying to hold a steady hover. That's why it's important to do the hover test in a location without wind, preferably indoors, and preferably in a large room where the Mini's own propwash won't interfere with the results.

If your motor speed plots show a consistently large difference in speeds (like yours did), I would recommend installing a new set of prop blades and repeating the test under the same conditions. If that makes no difference in the results you'll know there's nothing wrong with either set of props. But I suspect you're going to see a very noticeable improvement with new blades.

For comparison, here's my short one-minute hover test with brand new blades. Note how it took about 30 seconds for the Mini to initially stabilize itself into a steady hover, and how for the last 30 seconds the motor speeds are all nearly equal around 9600 rpm.