Crash Inspection

Hi everyone. One of my drone just crashed with similar pattern in three flights. Can someone help me on the steps to debug?

Log 1 : here
Log 2 : here
Log 3 : here

The drone was perfectly stable but all of a sudden, it toppled and crashed to the ground. Pixhawk doesn’t seem to loose power during the flights. But I can’t understand the cause. Please guide me to debug.

I looked at log 3.
It was going Ok until Motor2 loses thrust, maybe ESC desync, and is commanded to maximum output - Motor1 is commanded to minimum output to try and keep the copter balanced, but attitude control is lost.

On the way down Motor2 recovers and starts providing thrust again - you can see Motor2 and Motor1 end up with similar outputs.
But Motor4 then Motor3 are also battling with desyncs and attitude control is never recovered.

So most likely 3 of 4 motors lost sync with their ESCs at different stages. I dont think it was slipping props because they wouldnt all do it and recover - slipping props would probably stay slipping.
What motors, props and ESCs do you have?

Hey. Thank u so much for responding.

We are using T-motors MN5008 motors and T-Motors 18 inch propeller with Air 40 T motors ESCs.

Link to motor : TMOTOR

We are using the recommended set of propellers and ESCs from T motors only. Notably we did not face this issue on previous drone at all.

Question : Why would desync happen if we are using recommended configuration?

I find that just because items are list on the same page doesnt mean they will work perfectly together in your build.

Is it the 5008 340kv motor you have?
What is the take off weight?

Where are the ESCs in relation to the motors? Are the ESCs right out on the arms near the motors?
If so this is a very undesirable configuration.
Maybe provide a couple of photos of your copter.

The ESCs should be inboard with shorter power wires and possibly extra capacitors. It’s OK to have long motor wires.
Check this information about long wires and ground loops

  • Yes, the motors we are using is MN5008 340 KV. Escs are Air 40A 6S. Here are the propellers

  • Drones take off weight was 4.8 Kgs

  • ESCs are enclosed in a CF frame very close to the battery. They are placed one on top of the other in pairs of two as shown in the image. They are not under the motors.

  • Loops are avoided as described in the link you shared.

This model has been flying very reliabily for our usage, but this issue is repeated thrice now. Please recommend changes if there are any.

Below is an image for reference.

I think those ESCs will be borderline with those motors. You might have to change to a good known-brand 4in1 BLHELI32 ESC, or individual BLHELI32 ESCs, that can handle 40 amps with ease.

Even though ecalc says you combination is quite good, those Navigator series motors seem to be built for long flight time with minimum takeoff weight. I say that because they seem prone to desyncs.
We have no idea about the firmware in the T-Motor Air ESCs and how it really operates - that’s we I recommend a BLHELI32 ECS where we know the firmware and settings, and they seem reliable - plenty of people are using them with a wide variety of motors.
The added advantage will be getting the BLHELI telemetry data such as voltage and RPM, and that can feed the harmonic notch filter more directly. If you go down that path call out if you need help, the BLHELI data is worth it.