I have seen similar behaviour with probably the Mavic mini but maybe the mini 2 as well.
great summary.
Drone did not descend and stopped recording
- Thread starter GiannisS
- Start date
If you flew in the same high wind speeds as before...I'm not surprised. In Cine mode the max tilt angle is shallower than for Normal mode, you should have tested Sport mode instead.
Yeah... the log would have shown signs of that if that was the case, & furthermore... interference doesn't just let all other stick signals through & just block the throttle.
This with thrust, gravity & vertical component is pretty basic... perhaps the attached picture below make it easier to grasp.
During you incident flight (that you earlier shared in post #1) your drone was nailed at max allowed tilt angle, the tilt was pretty much directed 45 degrees left of the heading direction (south east) when you passed those piers/docks in the incident flight & couldn't descend. There the thrust countered both the gravity & the wind coming from south and generated a heading speed. Go back to post #16 & re-read the 3 priority scenarios... these are programmed by DJI in the firmware. It certainly could be middle ground alternatives where for instance the heading speed is lowered some despite being commanded to max, this to prevent a kind of unresponsive descend command... but looking into your log seems to show that the heading speed was prio 1, then prevent drifting & last make it possible to descend.
The height is changed by your throttle stick... & the height is measured by a barometric sensor. If the throttle command doesn't reach the drone nothing is telling the drone to change height & the flight controller will see to that the barometric height value remains constant & unchanged.
DJI drones have height hold... meaning if the throttle stick is in neutral & doesn't send a value, the drone will keep it's present height. If the elevator stick is in neutral the drone will stop it's forward flight & start to hover... DJI drones doesn't have "Speed hold" per default.
So you need to ask yourself... if the signal is weak & according to your idea, the drone doesn't "hear" your throttle command... why did it hear your elevator command & kept the horizontal speed unaffected.
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Yes, a loss in gravitational potential energy can lead to a decrease in the rotational speed (RPM - rotations per minute) of a drone. When a drone descends from a higher altitude to a lower one, it loses gravitational potential energy. This energy is typically converted into kinetic energy as the drone accelerates downward due to the force of gravity which afterward is converted I believe to heat.
Now, if the drone's propellers are providing lift to counteract gravity during descent (as in controlled flight), the conversion of potential energy to kinetic energy doesn't directly affect the rotation speed of the propellers. This is primarily because while the drone is actively using its propellers to control its descent, the RPM of the propellers might not decrease significantly, or it might even increase if the flight controller compensates by adjusting motor speed to maintain stability. I believe that this is the point where we introduce the term signal response time but I might be wrong. So during controlled descent, I think the flight controller continuously receives feedback from sensors and adjusts the motor speeds to maintain stability and control the descent rate. The flight controller's response time plays a crucial role in this process, as it needs to react quickly to changes in flight conditions to keep the drone stable and responsive to pilot commands. So that leaves us with 4 different reasonable scenarios:
1) Signal Interference which potentially increased the controller's response time ( I was able to turn the drone into other directions so not our case)
2) Sent Commands to maintain stability (statistically because of weather conditions: in our case logically from the wind as @slup mentioned)
3) Sensor obstacle detection avoidance (not shown so possibly not our case)
4) issue with the optimization of firmware ( which is possible as well)
So from the aforementioned scenarios, only 2 are more reasonable for my case. However the drone today did it again which makes me fall a little bit more to the latest one. So I am thinking to go and ask for a replacement tomorrow. Hope someone changes my current thoughts cause I don't really want to go through this procedure.
I'll add this. Flying that far away you may eventually lose your Mini 4.
I think this is what happened. I was working on coming to the same conclusion over on
Drone did not descend and stopped recording
But hit a bit of a problem. At 645 secs the descent is arrested and the horizontal speed begins to slow. Normally this would be done by decreasing the tilt angle. But, it remains the same - about 29°.
Decreasing motor speeds would increase the descent rate. Increasing motor speeds would increase horizontal speed.
First of all: I kinda admire your trust in the drone and surroundings to fly so far out of view - here it would be completely illegal to do so (VLOS requirement). Given the distance (and the messages) there can be signal interference but it can't influence downward stick only. I can follow all analysis given thrust positions, but I would imagine it to move a little, even in those conditions. I am wondering if this is not a situation where the drone assumes it is near 'a' surface and thus refuses to descend further. Did you see any markings about the distance below the drone (ie,. the distance with a arrow marker) and if so: what did it say? Did you make a screen recording of that part where it would not descend?
Something else you might want to try is to (physically) get near this location and repeat the flight. That would not only limit the risk of interference, but you would also be able to see where the drone is, relative to ground level.
Something else you might want to try is to (physically) get near this location and repeat the flight. That would not only limit the risk of interference, but you would also be able to see where the drone is, relative to ground level.
No need of a screen recording to find out, the log have that info... & it showed that the VPS sensor was outside (above) the sensor distance when the descend command was unresponsive. The VPS needs to report a distance smaller than 0,5m to halt a immediate descent, if the throttle is commanded there for a longer time (approx. 3sec), the auto landing will be initiated.
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From a PM discussion...
@GiannisS
Hello, I just saw your message and I really appreciate all the time you spent on the provided explanation. It really makes sense except for one part. You mentioned that the unresponsiveness to descend only occurred when I commanded max heading speed with a full elevator stick input, together with a negative throttle input for a descent. On the second flight that I had, the unresponsiveness to decent didn't only happen when I commanded max heading speed but also when the heading speed was commanded to not be maximum. So logically, I managed to reduce theoretically the tilt angle but the drone once more didn't respond. Is that normal as well?
Thank you fro your time and help!
My response:
The statement about "full" was mostly meant as a general description of your commands during the flight I analyzed, not meaning it can't happen with a lower command for horizontal speed than full (either by a elevator or a aileron command).
The elevator command in itself isn't the root cause... it's what those commands generates, a larger utilization of the max tilt angle.
If I paint up a theoretical situation to make you understand better...
Let say that your drone is hovering in a strong wind & the tilt angle is used to max (30 degrees for your drone in N-mode). You have both sticks in neutral position, and the drone manage barely to hold position & prevent a drift down wind.
In a case like this & as DJI photo drones automatically hold position when you have released the sticks... something else is telling the drone to tilt (pitch, roll or a combination) into the wind than you... it's here the firmware comes into the picture, & it's how they have coded it that decides how the drone will act without you touching the sticks.
The guys that program the firmware can't overrule the law of physics... if it is a coded tilt limit it's just a matter of time before you end up needing to prioritize between allowing drift, allowing commanded horizontal speed or allowing a decreased motor RPM. The coded priority is very likely not as black & white as I've painted it here, there is most probably middle ground alternatives where some drift can be allowed in order to make room for a decreased motor RPM for instance... but in general I'm pretty sure that's what going on in the firmware.
So... as you see, no elevator command was given at all in this theoretical situation... but the drone would be in a similar situation as in your real flight & by that, I bet that the drone had shown the same unresponsiveness to descend here. So the fact that you experienced the same descend problem even with a partial elevator command, doesn't make me suspect that the main cause for this is something else.
We will never know exactly how the firmware command the flight controller & we never know the exact extent of outside forces the drone needs to deal with (wind mostly)... we can only judge out from the data available (speeds, height values & attitude values).
That's why I wrote "The cause was likely wind related..."
@GiannisS
Hello, I just saw your message and I really appreciate all the time you spent on the provided explanation. It really makes sense except for one part. You mentioned that the unresponsiveness to descend only occurred when I commanded max heading speed with a full elevator stick input, together with a negative throttle input for a descent. On the second flight that I had, the unresponsiveness to decent didn't only happen when I commanded max heading speed but also when the heading speed was commanded to not be maximum. So logically, I managed to reduce theoretically the tilt angle but the drone once more didn't respond. Is that normal as well?
Thank you fro your time and help!
My response:
The statement about "full" was mostly meant as a general description of your commands during the flight I analyzed, not meaning it can't happen with a lower command for horizontal speed than full (either by a elevator or a aileron command).
The elevator command in itself isn't the root cause... it's what those commands generates, a larger utilization of the max tilt angle.
If I paint up a theoretical situation to make you understand better...
Let say that your drone is hovering in a strong wind & the tilt angle is used to max (30 degrees for your drone in N-mode). You have both sticks in neutral position, and the drone manage barely to hold position & prevent a drift down wind.
In a case like this & as DJI photo drones automatically hold position when you have released the sticks... something else is telling the drone to tilt (pitch, roll or a combination) into the wind than you... it's here the firmware comes into the picture, & it's how they have coded it that decides how the drone will act without you touching the sticks.
The guys that program the firmware can't overrule the law of physics... if it is a coded tilt limit it's just a matter of time before you end up needing to prioritize between allowing drift, allowing commanded horizontal speed or allowing a decreased motor RPM. The coded priority is very likely not as black & white as I've painted it here, there is most probably middle ground alternatives where some drift can be allowed in order to make room for a decreased motor RPM for instance... but in general I'm pretty sure that's what going on in the firmware.
So... as you see, no elevator command was given at all in this theoretical situation... but the drone would be in a similar situation as in your real flight & by that, I bet that the drone had shown the same unresponsiveness to descend here. So the fact that you experienced the same descend problem even with a partial elevator command, doesn't make me suspect that the main cause for this is something else.
We will never know exactly how the firmware command the flight controller & we never know the exact extent of outside forces the drone needs to deal with (wind mostly)... we can only judge out from the data available (speeds, height values & attitude values).
That's why I wrote "The cause was likely wind related..."
@GiannisS I noticed that your Mini 4 Pro has a max descent speed of 3 m/s. That's with full negative throttle. The spec is 5 m/s max descent speed which my Mini 4 Pro can easily achieve.
I gotta ask. Has your Mini 4 been modified, have extra equipment or otherwise not a stock unit?
I gotta ask. Has your Mini 4 been modified, have extra equipment or otherwise not a stock unit?
Good spot there, totally missed that.
Below is from the second flight the OP shared originally... Full 5m/s (11,2mph) is possible, then some time later the speed seems to be locked to max 3m/s (6,7mph) even though the throttle command is fluctuating just above full down.
The heading speed is constantly a tad below the max 12m/s (approx. 26,4mph), the tilt angle is fluctuating a bit but very near 25 degrees during both descent periods. The barometric height decreases as expected & the VPS height indicate a height above the sensor reach... & nothing odd exists in the DJI Fly Safe material either.
Strange...
The OP mentions switching to cine mode in that 2nd flight:
I don't know if this mode should be reflected in the logfile (I don't see it), but the descent speed would be limited to 3m/s in that mode.
The chart in post #30 only covers a period when the drone was in the ordinary GPS or Normal mode, (P-GPS), it's in the log & is symbolized by the pink background color in the chart (and written out in the chart legend under).
Droning on and on...
Well-Known Member
One thing that hasn't been mentioned is updraft.
I used to hang glide, and thermals were what we sought with earnestness. A 2 m/s thermal/updraft isn't extreme at all.
While I would expect the FC to compensate by decreasing RPM, there are operational limits.
Is the flying location a place subject to regular thermals? A cliff with an on-shore wind that creates a lift envelope at the cliff face?
Just another thought in light of this odd behavior.
I used to hang glide, and thermals were what we sought with earnestness. A 2 m/s thermal/updraft isn't extreme at all.
While I would expect the FC to compensate by decreasing RPM, there are operational limits.
Is the flying location a place subject to regular thermals? A cliff with an on-shore wind that creates a lift envelope at the cliff face?
Just another thought in light of this odd behavior.
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For anybody interested in looking at more data the attached is a log from a test flight with my Mini 4 Pro. Several runs were made by first applying full negative throttle and then full fwd elevator. Several combinations were tried 1) Sport, Normal and Cine mode 2) upwind, downwind and cross wind.
I didn't see any case of arrested descent. @slup pointed out that there is one case where vertical descent was maxed at 3 m/s; all other descents reached the max descent rate of 5 m/s. This occurred at time [609, 622] secs while the Mini 4 Pro was flying into the wind in Normal mode. However, there are other 2 intervals where the M4P was flying into the wind and achieved 5 m/s. One was in Sport mode, the other Normal mode.
DJIFlightRecord_2024-03-26_[13-11-29].txt
I didn't see any case of arrested descent. @slup pointed out that there is one case where vertical descent was maxed at 3 m/s; all other descents reached the max descent rate of 5 m/s. This occurred at time [609, 622] secs while the Mini 4 Pro was flying into the wind in Normal mode. However, there are other 2 intervals where the M4P was flying into the wind and achieved 5 m/s. One was in Sport mode, the other Normal mode.
DJIFlightRecord_2024-03-26_[13-11-29].txt
...but for the lazy ones regarding @BudWalker log above
Here's the 2 comparable stretches... both commanded with first a full throttle for a descent then a fill in of a full forward with the elevator also, both in N-mode.
Here the circumstances with approx wind speed & direction.
The red framed stretch is visualized in the first chart... head wind, it's here the vertical speed anomaly occurs.
The green framed stretch is visualized in the second chart... tail wind, here all behaves according to specifications.
(Click on all pictures/charts below to make them larger)
Hi,
There is much more to this 'not going down' on RC stick down command.
See my chart of my own mini4pro.
Flying at 20 meters baro height, not much wind this flight.
RC down stick and not descending! Drone pitch and roll angles not near the max allowed values.
Seeing this on other models as well.
DJI has changed how it handles RC down input when flying low with speed. It is now not possible to lower craft flying fast at low heights. This to prevent to enter an unwanted autolanding (0.5 IR height + stick down).
cheers
JJB
...but for the lazy ones regarding @BudWalker log above
Here's the 2 comparable stretches... both commanded with first a full throttle for a descent then a fill in of a full forward with the elevator also, both in N-mode.
Here the circumstances with approx wind speed & direction.
The red framed stretch is visualized in the first chart... head wind, it's here the vertical speed anomaly occurs.
The green framed stretch is visualized in the second chart... tail wind, here all behaves according to specifications.
(Click on all pictures/charts below to make them larger)
View attachment 173842
View attachment 173843
View attachment 173844
There is much more to this 'not going down' on RC stick down command.
See my chart of my own mini4pro.
Flying at 20 meters baro height, not much wind this flight.
RC down stick and not descending! Drone pitch and roll angles not near the max allowed values.
Seeing this on other models as well.
DJI has changed how it handles RC down input when flying low with speed. It is now not possible to lower craft flying fast at low heights. This to prevent to enter an unwanted autolanding (0.5 IR height + stick down).
cheers
JJB
Attachments
How high is "low heights"..?
In both charts in post #35 from @BudWalker test flight the heights were well above the VPS sensor reach...
-In the descent with a lowered vertical speed it started at 150ft height & continued down to 50ft.
-In the descent with vertical speeds according to specification, it started at 195ft & continued down to 65ft.
During both instances the VPS height was "flat-lining", indicating that the drone was above sensor reach or couldn't get a proper reading.
Hi,
Low heights starts above the IR range, max descend is slowed down from approx 40 meter baro height.
Fly full speed fwd +100% down stick, my mini starts with 5m/s rate of descend slowing it down to zero just above ground level. Problem is, my experience, that if the IR sensors and Vision isn`t working 100% oke, than false readings will slow down or stop a descend.
cheers
JJB
Hi BudWalker,
Found an better example where a (my) Mini4Pro does not descend with RC down input.
In the log from 9m19s , not descending with RC down input.
cheers
JJB
Attachments
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