Fell from the sky

We’ve all seen this post before, and now it’s my turn. The first 3m 11 seconds were great. Second flight of the day (first was ~25 minutes, no issues) I was in a stable hover at about 21m. Position hold mode and doing just that, holding a position. Just turned to show a colleague the map on the Yaapu telemetry when it just fell. No warnings or messages until after it hit the ground.

DJI Flamewheel 450. (DJI esc’s and motors. This was an old resurrection project… oops)
Matek F405-CTR.
Arducopter 4.0.5
Jumper R8
Radiomaster TX16S

And of course the log file:

The only thing I noticed in the logs was the throttle input (RC3) dropped from 1528 to 1445. So I’m not ruling out that I bumped the throttle stick when I turned to chat. But this thing dropped like a stone so in Pos.Hold mode it should have been limited to a max descent rate (I hope?)

I’m also not ruling out mechanical failure. I kept everything pretty clean but maybe something got snow on it and caused a short? And they are older ESCs and motors so they don’t owe me anything.

If someone can check the log for me and rule out any setting issue or any failure there I’d sure appreciate it.


Looks pretty typical. Initial loss of thrust on Motor 3, Motor 2 drops (which is odd) , general lack of thrust to fly, Rapid Return To Earth.

Edit: It’s not odd actually with the motor order you have.

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I was just looking at the motor position as you edited your post. 2 and 3 are opposites on the F405-CTR.

So I understand you correctly: you see the climb in RCOUT3 as a sign that motor is loosing thrust, but since the flight controller is raising 3 it thinks it needs to lower 2 to balance.

When the altitude drops shouldn’t 1 and 4 also go up (more than they are?) to offset the descent? Or is having 3 at max limiting that?

No, this won’t happen. If it did it would spin like crazy. You have consider the attempt to stabilize in all axis.

That makes sense. The sum of 1&4 needs to roughly equal 3&2 or it will yaw from torque imbalance. I guess then since the FC sees 3 at max, 1&4 can only go so high.

I didn’t see anything physically wrong with motor 3, except that’s the arm that got broken in the crash. The wiring looks good and no obvious faults. Hopefully the FC didn’t get damaged in the snow and I’ll be able to power it up and check it out.

Thanks for your quick analysis!

Falling suddenly could be explained by an exhausted battery, but this seems not to be the case.

Just before altitude loss (vertical traces line above):

  • ATT.Yaw departs from ATT.DesYaw.
  • CTUN.ThO increases.
  • Vibration levels decrease.
  • Battery voltage increases.

Less power drawn from the battery; perhaps motor|ESC failure.

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