Why I do believe there's a HUGE problem with the Mavic Mini - Flew AWAY the 2nd day

[Deleted member 94047]

External forces such as wind and gravity can limit the achievable tilting angle if the center of pressure created by the air flow or the center of gravity do not conincide with the intersection between the diagonal lines joining the motors. I can't imagine basic design flaws of such kind occurring to DJI drones.

Ok, I am not sure I understand this. Isn't the tilt angle dependent upon the amount of force generated by the one or two motor involved? And if said force is not strong enough to overcome the pressure of the wind, then the tilt angle would be lower than what is otherwise expected? Or did I grossly misunderstand this?

 

[new2mavic]

Ok, I am not sure I understand this. Isn't the tilt angle dependent upon the amount of force generated by the one or two motor involved? And if said force is not strong enough to overcome the pressure of the wind, then the tilt angle would be lower than what is otherwise expected? Or did I grossly misunderstand this?

Perhaps a simple experiment can help. Try to drop a card onto the floor with the card initially put at an angle to the vertical. As the card drops, air will flow over it and exert a force on it. See if that force will increase or decrease the tilting of card relative to the "wind". Now imagine that the card is a drone flying into the wind. Will the wind help to increase or decrease the tilting angle of the drone ?

Now repeat the experiment but replace the card with a straw and see what will happen

Aerodynamically the drone is something between a straw and a card. You can draw your own conclusion on how the wind affects the tilting angle of the drone

 

[Deleted member 94047]

Sorry, but I am more confused because I can't visualize wind blowing vertically. This is how I visualize wind

 

[new2mavic]

Sorry, but I am more confused because I can't visualize wind blowing vertically. This is how I visualize wind

View attachment 90074

Dropping a card is something you can do inside any time. If you are not convinced that the effect of "vertical wind" is the same as that of "horizontal wind", you can always pick a windy day and let go a card in the wind as depicted in your picture and see what will happen.

 

[Deleted member 94047]

Ok, fair enough.

 

[slup]

Okay - this is another case of the Mini failing to achieve anything close to the specified tilt. The left back motor is maxed out, but the tilt is only around 10°:

View attachment 90063

Very similar to this event. This is starting to look like a real hardware or firmware problem.

@BudWalker

Now with access to the DAT file & the new eminent version of CsvView from @BudWalker I draw the same conclusion ... @amriceman , I think you should approach DJI with this.

I just choose to look at a small section of the flight (marked with the red line), close to the rails when the Mini were blown backwards even though full elevator were applied.



Rear engines were maxed to near maxed out (left rear more so, perhaps due to a slight aileron input?), but the tilt never came near to
the specified 20 degrees in P-mode. According to Airdata.com the wind at that moment were some where around 9,5m/s (with the Spark
profile which probably show a bit to high wind speed).

 

[BudWalker]

External forces such as wind and gravity can limit the achievable tilting angle if the center of pressure created by the air flow or the center of gravity do not conincide with the intersection between the diagonal lines joining the motors. I can't imagine basic design flaws of such kind occurring to DJI drones.

Tilt inclination angle is independent of wind. Tilt inclination angle determines air speed. Air speed and wind will then determine ground speed.

@Doppler

 

[Simmo]

and ergo???

 

[BudWalker]

2/2 in the logs that I've seen. Too few data points to conclude whether that's coincidence.

I tagged you over here

Mavic mini, unresponsive on forward command, logs for the pros.

The tilt calculations are now in CsvView 3.7.0 which is available for download at CsvView Downloads DatCon 3.7.0 also contains the tilt calculations. The tilt info is in the IMU record. The signal names were changed. IMU_ATTI(0):tiltInclination is the angle between the Z axis and vertical...

mavicpilots.com

But, after the initial post in an edit.

 

[sar104]

This is puzzling. The chart shows that the revolution speed of the two motors at the back were substantially higher than those at the front. The tilt angle should have continued to increase although the drone might still be unable to overcome the wind or maintain altitude. It seems that for some reasons spinning the motors faster did not end up in stronger thrust. These are just sensor data, may be the motor did not physically turn faster ? May be some of the prop screws has loosen resulting in the pitch of the props reduced ? May be the metal plate on which the prop screws go has deformed ? prop defects ?

No - the rear motors have to stay at higher speed than the front in order to maintain tilt.

 

[sar104]

@sar104 couldn't the cause of not being able to achieve max tilt, with full motor power, be that other external force, i.e., the wind?
Or would/should the drone have enough power to tilt to spec, in any strength wind??

As @BudWalker mentioned, tilt simply determines airspeed, and wind speed doesn't directly affect tilt. It affects it indirectly in P-mode flight because the FC is trying to hold position, and so it commands tilt values to produce airspeed equal and opposite to wind speed.

 

[amriceman]

slup and other kind people of the mavicpilots.com board, thank you for reviewing my case and for the active discussion!

It’s quite an overwhelming volume of the information to process for me now! So from what I understand, there could be a fault in the Mavic Mini controllers algorithm which calculate RPM ‘s for each motor in order to make the AC to assume a proper tilt for the wind conditions it faces at the moment? I don’t think it actually measures the wind strength and direction, but it could sense it from the extra air resistance when the drone is supposed to go to a full speed in respond to the stick command, but could not actually achieve it. So it should re-distribute the power between four motors in order to increase the tilt, but only one left-back motor actually gets that extra-power? Could it be a correct assumption?

 

[BudWalker]

slup and other kind people of the mavicpilots.com board, thank you for reviewing my case and for the active discussion!

It’s quite an overwhelming volume of the information to process for me now! So from what I understand, there could be a fault in the Mavic Mini controllers algorithm which calculate RPM ‘s for each motor in order to make the AC to assume a proper tilt for the wind conditions it faces at the moment? I don’t think it actually measures the wind strength and direction, but it could sense it from the extra air resistance when the drone is supposed to go to a full speed in respond to the stick command, but could not actually achieve it. So it should re-distribute the power between four motors in order to increase the tilt, but only one left-back motor actually gets that extra-power? Could it be a correct assumption?

Motor speed isn't calculated and there isn't any reason to suspect that the motor speeds are incorrect. In fact, motor speeds aren't relevant here. You're correct that the wind vector isn't determined.

For a given stick input the power to each motor is adjusted up and down until the desired ground speed and direction are obtained. The FC isn't adjusting motor speeds, it's adjusting the amount of power to each motor. If ground speed needs to be higher then the power to the back motors are increased from whatever it currently is to something higher.

But, there are limits. If one of the motors is presently at 100% power it can't go any higher and increasing the power to the other motors will cause the MM to tumble. So the total thrust is limited if any one of the motors is at 100%. In your case the left back isn't getting any more power because it's already at 100%

Like motor speed the FC isn't trying to adjust the tilt inclination. But, the FC probably wants to limit the inclination for flight stability reasons. Theoretically, as long as there is enough downward thrust to hold altitude the inclination could be increased beyond what the FC is comfortable with.

Saying that tilt inclination is limited or doesn't meet specs is just another way of saying that one or more of the motors is maxed out at 100%

If you still have your MM can you do the experiment described here
Mavic mini, unresponsive on forward command, logs for the pros.

 

[amriceman]

BudWalker,

Thanks for a detailed explanation! From what I understand, the max power (thrust) output for motors can be achieved only in the sport mode. But my drone was staying in the P-mode all the time. At least before it went into RTH mode, which
I believe
also uses P-mode. So how come only the left-back motor was peaking out to the max, while the other motors were working in moderate RPM’s?

 

[sirbadger]

I'm no expert (should probably stop typing at this point..) but given what's been said the left motor was giving all it could to keep the direction/speed/stability it needed (due to wind), if the other motors increased but the left couldn't give anymore then it couldn't go the direction it needed. I guess it could also tumble out of the sky?

ETA, I've replied to try and answer but also see if I've understood what is being said.

 

[sar104]

I'm no expert (should probably stop typing at this point..) but given what's been said the left motor was giving all it could to keep the direction/speed/stability it needed (due to wind), if the other motors increased but the left couldn't give anymore then it couldn't go the direction it needed. I guess it could also tumble out of the sky?

ETA, I've replied to try and answer but also see if I've understood what is being said.

That doesn't fully explain the event though, because the aircraft should have been able to tilt to 20° to hold position or move. It didn't achieve anywhere near that, and yet one motor was maxed out.

 

[sirbadger]

That doesn't fully explain the event though, because the aircraft should have been able to tilt to 20° to hold position or move. It didn't achieve anywhere near that, and yet one motor was maxed out.

Ah ok, see, no expert me! Should it be able to give 20° no matter the wind speed/direction and aircraft direction?

 

[sar104]

Ah ok, see, no expert me! Should it be able to give 20° no matter the wind speed/direction and aircraft direction?

Exactly - if needed for the commanded speed or to hold position/RTH. Now 20° may not always be enough to hold position (wind speed over 8 m/s), but it should dial up the tilt to the maximum in trying.

 

[BudWalker]

BudWalker,

Thanks for a detailed explanation! From what I understand, the max power (thrust) output for motors can be achieved only in the sport mode. But my drone was staying in the P-mode all the time. At least before it went into RTH mode, which
I believe
also uses P-mode. So how come only the left-back motor was peaking out to the max, while the other motors were working in moderate RPM’s?

So how come only the left-back motor was peaking out to the max, while the other motors were working in moderate RPM’s?

Because increasing the power to the other motors will cause the MM to tumble. The left back is being commanded to 100% but it's not delivering 100% power.

 

[amriceman]

Okay, so it was a technical fault after all? I hope DJI folks monitor these threads (they better do), and take it into considerations for further improvments of their products...