*SOLVED* Agricultural Quad Crash Analysis

Greetings fellow enthusiasts,

I’m reaching out with a situation concerning my recently assembled Quadrotor. The build was completed using:

  • Cube Black
  • Here2 GPS
  • Hobbywing X8 Power System
  • 2x 6S 22000 mAh batteries in series configuration
  • An Agricultural Sprayer frame.

Having carefully executed the setup and the preliminary adjustments, the UAV performed well on its inaugural autonomous missions.

Regrettably, an incident occurred today. After tweaking the WPNAV_SPEED parameter upwards, my quadrotor experienced an abrupt spin and consequently crashed, resulting in total loss of the craft. Thanks to the safety precautions in place – multiple observant spotters with radios ensuring a clear operational area – no personal injuries occurred.

In order to unravel the mystery surrounding this mishap, I could use some help with the analysis of the craft’s log. My primary suspicion is an electronics failure. However, one of my spotters mentioned an unexpected approach of birds towards the craft prior to the crash. While it remains unconfirmed if the craft was hit by a bird, this could also be a plausible cause of the crash.

I’d be incredibly grateful for any assistance you could offer in making sense of the log file to determine the likely cause of the crash.

The crash log can be found here: 00000047.BIN - Google Drive

Now I am off to disassemble the wreck to see if anything of value can be saved :melting_face:

In a way this is more about what it wasn’t, than what it was that caused this crash. I think vibrations are the key.

For about 10 seconds before the pitch and roll departs, you can see vibrations are steadily climbing. This does not occur elsewhere in this flight or the previous flight, so this is atypical and likely a good indicator of the root cause.
Something happens right around 22:24:13.8 resulting in motor 4 commanded to minimum and pitch and roll are changing - things get out of control from here.

  • It could be like a weight shift → battery or payload came loose?
  • Or broken arm or folding mechanism?

Vibrations do not go absolutely crazy, so it’s unlikely to be a prop or motor coming loose or breaking.
It does not appear to be thrust loss or motor/ESC desync, since motors are only commanded to maximum briefly in turn to try and recover stability.
There is no special voltage sags that would indicate an electrical issue, and unfortunately your current monitoring is not working correctly.

Before the incident attitude control and tuning looked particularly good for a quad with such big props. Because of that I haven’t mentioned wind gusts, since I feel it would have been possible to recover from a strong wind gust, even if there was some period of instability.
It could be handy if you observers have anything to add, even if just to rule out the wind. Although wind at altitude is usually not apparent from the ground, there’s no signs of wind prior to the incident.

Once a copter is resurrected I would set:

BATT_FS_LOW_ACT,2  // or 3

You do not want to wait until critical battery voltage before starting RTL, and have taken no action at low voltage. Also get current monitoring working if possible.

And configure the Harmonic Notch Filter, this is just a starting point but should be close

INS_HNTCH_ENABLE,1  // set this then refresh params to see the rest
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I cannot express enough how thankful I am for all your help and support in solving this mystery. It’s incredible how this community bands together in times like these.

With your suggestions, I revisited the remains of my quad and examined the fourth motor. I noticed that its mount had shifted around 20 degrees to the right. Initially, I thought the impact from the crash caused this shift, but after finding the absence of the screw that runs through the arm and the motor (which is supposed to prevent this kind of tilt), I’ve come to believe that the motor got loose while still in the air.

So, it seems like the loose motor shifted during the flight and triggered a pretty violent yaw spin.

Sadly, there isn’t much left to salvage, apart from the flight controller and a few motors. But, every cloud has a silver lining, right? I am very much looking forward to my next build as the cause of this crash is now solved!

Big thank you and greetings from Austria!

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You are correct - I was looking at the log again and yaw is first to go!

The amazing thing is: this is a quad, it was yawing almost out of control, pitching and rolling probably ±100 degress and when you switched it to RTL it still managed to turn towards the launch point and did it’s best to go in that direction before crashing relatively close to the launch point!

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one time i accidentally configured an x-quad as an h-quad (i was switching builds over from an h-quad). h-quad has opposite yaw direction from x-quad. I took off, and although the copter was spinning wildly, it still maintained altitude and I was able to steer it towards myself and catch it out of the air (the frame had a long vertical handle that made this easy). the behavior was impressive even though I horribly misconfigured it.