MOTOR SPEED ERROR MAVIC MINI

[FlyBackOU812]

This is why I've never gotten any of those warnings and if everyone did the same ? this would be a non issue from here on out. Have DJI Start sending out with instructions and some velcro that would would litterally cost them less than a penny

 

[Zbip57]

I have the same error for some time. I replaced the Left Rear blade twice but still i get the error.
After reading this forum, I flight the MM for about 15 minutes in my room to test it. There was no error.
I looked at the aircraft dat file (using DatCon) and figure out that indeed this rotor speed is higher (about 25% from the other)
Motor:Speed:RFront 9949.0
Motor:Speed:LFront 9375.0
Motor:Speed:LBack 12521.0
Motor:Speed:RBack 10684.0
It looks like there is a problem with this rotor - what can i do ?

Your numbers are irrefutable evidence of a problem.

If the props are all in equally good shape, then the motors should all be turning at the same speed while in a hover. If your left-back motor is turning faster than the others, it's a clear sign that those blades are less efficient at producing lift, requiring that motor to spin faster to generate the required amount of lift.

Those left-rear blades need to be replaced with fresh straight blades.

If you've already replaced them twice before and are still getting the same error, then you need to examine how you are storing the blades between flights.

I've posted this image before, but I've added one new upward pointing red arrow. This is looking at the Mini placed upside-down onto its back. These are the rear motors and rear propeller blades. The blades labelled 1&2 are mounted on the left-rear motor.

This photo was taken with brand new blades installed on both rear motors of my Mini. You can see how high the tips of the new blades stand. Obviously, if you turn the Mini over and rest it onto a flat surface, the weight of the Mini will first fall onto the tips of these blades. Also obviously, in order to laterally mesh the blades over/under each other (as recommended by DJI), the blade tips from one side have to be forced down to fit folded under the opposite side's blades. (See blade tip#2)


Less obvious is the fact that this doesn't just bend the blade. It also twists the blade. Note the upward pointing red arrow. Here the trailing edge of the prop blade contacts the body of the Mini. As the tip of the blade is forced to bend down, the trailing edge is supported by the body of the Mini.

Where the trailing edge is touching, near the root of the blade, is much stronger than the tip of the blade. The root of the blade is thus better at resisting the deformation. However, the tip is not only being bent down, it is twisting.

This twist reduces the pitch angle of the blade, flattening the angle of attack of the blade. So while the blade root might still be producing lift, the faster moving tip end of the blade might just be slicing through the air producing no lift at all. In really severe cases of blade deformation, the blade tip may actually have a negative angle of attack at their tip producing downward thrust rather than upward lift.

This is a photo of the front blades viewed end-on. The front blades are less of an issue, as I showed in earlier photos, because when folded their motors are level with each other. I only used them for this photo as it's easier to get the camera under the blades to show this angle. But here you can see that applying an upward force at the trailing edge, while pushing down on the blade tip, causes a torsion in the blade that flattens the pitch angle.



Storing the blades (especially the rear motor blades) folded together crossed laterally under the body of the Mini is what's causing the blades to bend. Resting the weight of the Mini onto the tips of the blades obviously will make the situation worse. But merely forcing the blades to mesh together, with their tips bent under each other, is enough to deform the blades.

 

[Zbip57]

Where my finger is pushing would be sitting on the flat part of the fly more case in between the 2 motor cut outs which is exactly where the props are.

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I'm confused (but that's not new ? ).

Your photos show the un-deflected blades looking pristine and straight. Note the height of the blades tips. This is what new rear blades should look like. All good.



But then your video shows the blades crossed and held together by a velcro strap. Compare the height of those same blade tips now.

Only place that isn't exactly perfect is the tip of the one prop that rest under the other one.

Those blades sure look like they're bending, no? The tip of that one is pressed right down flat against the body of the Mini. How is that not causing permanent deflection???

 

[Zbip57]

In the next video I share you will see how easily those props intertwine themselves effortlessly with any bending or forcing into place. I literally only tap the back of my finger and barely touch them to get them into velcro position.

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Your next video shows only the front propellers (front arms folded back) meshing easily together. Because the front motors are mounted level with each other, their propeller blades will always slide easily together when folded together, not causing any bending stress on the blades. This is why the Motor Speed Error so rarely ever identifies a problem with the front propeller blades.









It's the rear blades that trigger the error messages most often. Because of the way the rear motors are mounted at an angle canted outwards, those blades cannot be folded meshed laterally across the body without forcing the blades from one side to bend under the blades of the opposite side.

 

[FlyBackOU812]

I'm confused (but that's not new ? ).

Your photos show the un-deflected blades looking pristine and straight. Note the height of the blades tips. This is what new rear blades should look like. All good.

View attachment 120423

But then your video shows the blades crossed and held together by a velcro strap. Compare the height of those same blade tips now.

View attachment 120424



Those blades sure look like they're bending, no? The tip of that one is pressed right down flat against the body of the Mini. How is that not causing permanent deflection???

There's a little play in each piece. 2 motors little play, 4 props, little play at screws thats atleast 6 little bit of play. Add those together theres plenty of play for that to be ok without warping

 

[FlyBackOU812]

There's a little play in each piece. 2 motors little play, 4 props, little play at screws thats atleast 6 little bit of play. Add those together theres plenty of play for that to be ok without warping

As far as the blade tips are concerned. Thats the thinnest part of the prop. Which would be the quickest to heal back to molded form. Now do that at the thick part of blade, not so easy to bounce back once its warped. Dont worry about the tips. The wobble, or out of balance issue that would cause problems comes from the thicker parts , thats part that needs to be protected the most. Which part of my props are protected the most?

 

[FlyBackOU812]

I'm confused (but that's not new ? ).

Your photos show the un-deflected blades looking pristine and straight. Note the height of the blades tips. This is what new rear blades should look like. All good.

View attachment 120423

But then your video shows the blades crossed and held together by a velcro strap. Compare the height of those same blade tips now.

View attachment 120424



Those blades sure look like they're bending, no? The tip of that one is pressed right down flat against the body of the Mini. How is that not causing permanent deflection???

Sorry bout the movie I apparently shared i just realized this. Trying to figure out what I didnt send in its place

 

[FlyBackOU812]

As far as the blade tips are concerned. Thats the thinnest part of the prop. Which would be the quickest to heal back to molded form. Now do that at the thick part of blade, not so easy to bounce back once its warped. Dont worry about the tips. The wobble, or out of balance issue that would cause problems comes from the thicker parts , thats part that needs to be protected the most. Which part of my props are protected the most?

I'm able to pay more attention at the moment. With that being said...this is really hard to navigate but im slowly figuring it out. Is there questions somewhere else ive missed

 

[Zbip57]

Don't worry about the tips. The wobble, or out of balance issue that would cause problems comes from the thicker parts,

It's not the wobble or out of balance that's causing the Motor Speed Error. It's a loss of lift from the propeller blades resulting from the blades being bent. And, it's the tips of the blades that are most susceptible to bending. I've made a short animation movie to demonstrate this.

The heavier inner half is stronger and is less prone to bending compared to the thinner outer half of the blade. But as the prop spins, the tip of the blade is moving much faster through the air, compared to the root, and consequently any distortion at the tip has a much larger influence on the overall lift of the blade.

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[FlyBackOU812]

It's not the wobble or out of balance that's causing the Motor Speed Error. It's a loss of lift from the propeller blades resulting from the blades being bent. And, it's the tips of the blades that are most susceptible to bending. I've made a short animation movie to demonstrate this.

The heavier inner half is stronger and is less prone to bending compared to the thinner outer half of the blade. But as the prop spins, the tip of the blade is moving much faster through the air, compared to the root, and consequently any distortion at the tip has a much larger influence on the overall lift of the blade.

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The part of tge propeller that is responsible for creating the lift is the inner portion of the prop where the blade is thicker and cupped, the tip of the prop being bent bent back a little wouldnt affect the lift the way you think it will. Thats why im not getting the warnings even if my tips are little flared up. Ive spent the last 2 days studying different propeller shapes and designs and what I say is facts. You are correct in many things you say throughout this thread but not all them pertain to the issue

 

[Zbip57]

The part of the propeller that is responsible for creating the lift is the inner portion of the prop where the blade is thicker and cupped [...]

The rest of the blade is there for a reason, otherwise DJI could save some weight by just cutting off the outer half of the prop blades.

The root of the blade is stronger and thicker because it needs to resist the leverage applied to it from the entire length of the blade. The blade becomes thinner and more flexible the further you move out to the tip because it's supporting less and less bending moment.

The blade is widest with the highest angle of attack at the blade root because, as the prop rotates, the root end is moving slowest through the air. Because the root end is moving at the slowest airspeed, it must be shaped more aggressively.

The tip of the blade is moving the fastest, so it needs a much shallower angle of attack to produce efficient lift. Because it is moving so much faster, it is MUCH more sensitive to any small change in that angle of attack.

If the blade is deformed, twisted in such a way as to increase the angle of attack, the tip might actually be stalled out (tip stall).

If the blade is twisted the other way, only slightly decreasing the angle of attack, the tip might actually be flat, merely slicing through the air while producing no lift at all. In this case the prop will need to rely more on generating increased lift using more of the root of the blade. The motor needs to spin much faster to provide the required lift. We've seen motor speeds of 16,000 rpm!

But if the blade is twisted even slightly more, further decreasing the angle of attack, the tip of the blade may actually be producing negative lift. Now the tip, travelling at much faster airspeed than the blade root, will fight against the positive lift generated by the blade root. At some point it no longer matters how fast the motors are spinning if the positive lift generated by the blade root is being cancelled out by the negative lift at the blade tip.

[...] the tip of the prop being bent bent back a little wouldn't affect the lift the way you think it will.

I suspect that merely bending the blade up or down doesn't have a huge effect in itself. The bigger problem comes from twisting the blade, thereby changing the angle of attack at the blade tips.

The blade tip, the fastest moving portion of the blade, is very sensitive to any change in angle of attack.

 

[FlyBackOU812]

The rest of the blade is there for a reason, otherwise DJI could save some weight by just cutting off the outer half of the prop blades.

The root of the blade is stronger and thicker because it needs to resist the leverage applied to it from the entire length of the blade. The blade becomes thinner and more flexible the further you move out to the tip because it's supporting less and less bending moment.

The blade is widest with the highest angle of attack at the blade root because, as the prop rotates, the root end is moving slowest through the air. Because the root end is moving at the slowest airspeed, it must be shaped more aggressively.

The tip of the blade is moving the fastest, so it needs a much shallower angle of attack to produce efficient lift. Because it is moving so much faster, it is MUCH more sensitive to any small change in that angle of attack.

If the blade is deformed, twisted in such a way as to increase the angle of attack, the tip might actually be stalled out (tip stall).

If the blade is twisted the other way, only slightly decreasing the angle of attack, the tip might actually be flat, merely slicing through the air while producing no lift at all. In this case the prop will need to rely more on generating increased lift using more of the root of the blade. The motor needs to spin much faster to provide the required lift. We've seen motor speeds of 16,000 rpm!

But if the blade is twisted even slightly more, further decreasing the angle of attack, the tip of the blade may actually be producing negative lift. Now the tip, travelling at much faster airspeed than the blade root, will fight against the positive lift generated by the blade root. At some point it no longer matters how fast the motors are spinning if the positive lift generated by the blade root is being cancelled out by the negative lift at the blade tip.



I suspect that merely bending the blade up or down doesn't have a huge effect in itself. The bigger problem comes from twisting the blade, thereby changing the angle of attack at the blade tips.

The blade tip, the fastest moving portion of the blade, is very sensitive to any change in angle of attack.

But we weren't referring to twisting we are talking at the moment, well at least I was, was just what tucking the tips or intertwining the blades together. That would causes a flare to the tip not a twist. And as we agree that wouldn't make enough difference to cause a problem, as I've been telling you I don't have the issue. The reason some might have had issues right out of the the box is because the velcro is much much thicker than the paper ones that come from factory. They aren't gonna fill the gap where it collapse under the weight of the drone especially so the hotter the plastic props get. I never said the paper ones were perfect I only got the idea from the paper ones. I got lucky that the velcro not only keeps them in place but they take up the air gap as well. By doing that keeps them from ever collapsing that gap. Not to mention further up the blade at the meat of it AND same place most of the lifting pitch is. Look again at my video. As long as intertwining them accross body as I store mine doesn't affect the lift or cause an issue then thats all that matters cuz they aren't gonn get deformed in any other way. Its impossible, there is no gap this way. Even if you put the weight of 5 Minies on top of them. So if you can agree to that then I can definitely 100% guarantee you that It never affected mine storing it that way the whole hot summer. I have no reason to lie I want to get to the bottom of this and if I was lieing I'd look the **** fool real fast. What I tell you is truth. This can be easily tested. As a matter of fact I'm going to do an experiment in time lapse. I can show you better than I can tell you. My only issue is I live in a no fly zone so I can't start the motors to find out if there's a warning after each attempt of proving it

 

[FlyBackOU812]

This the one video I want you to watch again. They all support each other making them 4 times stronger than a single blade

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[FlyBackOU812]

I took my props off and lined them up on a tooth pick to see if tucking the tips affects the shape. I cant see on. The last one the one I have circled is where all the lift happens

 

[FlyBackOU812]

I'm not just guessing at any of this

 

[Zbip57]

[...] the velcro is much much thicker than the paper ones that come from factory. They aren't gonna fill the gap where it collapse under the weight of the drone

The Standard Mini and the Fly More Combo (at least with the original case) both came in a box which raises and supports the belly of the Mini. The blades crossed under the Mini hang freely into a trough so the weight of the Mini does not rest on the blades. Nonetheless, simply storing the Mini with the blades meshed laterally across the body of the Mini is what bends the blades.

But we weren't referring to twisting we are talking at the moment, well at least I was, was just what tucking the tips or intertwining the blades together. That would causes a flare to the tip not a twist. And as we agree [?] that wouldn't make enough difference to cause a problem, as I've been telling you I don't have the issue.

When the blade tips are bent down, it causes the blade to twist and lose pitch angle. We are not agreed upon whether that would cause a problem. I'm convinced it does. But I still don't understand how you're getting away with it without seeing the same problem.

Back to your photo and video. Here are your undeflected blades appearing to be in perfect shape. If you push down sufficiently on the tips of the blades, it would normally close that air gap causing the root of the blade to contact the edge of the Mini's body. That happens either because the blades bend all the way from the root, or it's just pushing the folded motor arm down, or if the arm is already as far down as it will go then it's actually twisting the motor arm.

However, you've added a velcro strap around the midpoint of the folded blades and it acts as a cushion supporting the blades at their midpoint. Pushing down on the blade tips will therefor bend them only outboard of the velcro.



With the velcro strap around the blades, the blades are supported at their midpoint. With the blades overlapped and sandwiched in the velcro, the force applied by your fingertip is supported at that point. The blades are interleaved right over left, with blade-B at the very bottom of the stack and Blade-A pinched between the blades of the right-rear motor.



Here you can see that with the velcro cushioning the blades, the air gap at the blade root is preserved. However, compare this view to the first photo and you can plainly see that by forcing Blade-B to fit underneath the right-side blades, the tip of Blade-B is forced to bend all the way down to contact the right-rear motor bell. And all of that bending is now concentrated on only the outer portion of the blade beginning from the outboard edge of the velcro strap.

How is that not permanently deforming the propeller blade??? Does it just spring back straight each time to the original shape shown in the first photo above? I don't see how that's even possible.

 

[FlyBackOU812]

I put a velcro strip on top and bottom and I dont intertwine them a certain way, so all the tips get equally bent but just slightly. Apparently not enough im not squeezing them together im just slightly wrapping them just enough to keep them in place

 

[FlyBackOU812]

This is unassisted without the velcro. See how both the right and the left still sticks up depending on which side I tuck?

 

[FlyBackOU812]

This is unassisted without the velcro. See how both the right and the left still sticks up depending on which side I tuck?

 

[FlyBackOU812]

And I know what you're thinking is that even if im not getting the warnings my drones flights must not be what it should be. I have videos of me regularly going out 3 miles and back. Also have a recent one of me hovering in confirmed 35 mph gusts and my mini holds its own brother