Facing a new problem. Need help.

My config on the problem quad is:
frame: OpenRacer with 5mm fatty arms
FC: SkyStarsRC F722 Mini HD Pro 20x20
ESC: REDUX32 PRO F4 70A 128KHZ 20X20
motors: iFlight XING2 2207 1855kv
GPS: Matek m10q 5883
receiver: Radiomaster RP1

At the end of a recent race, I crashed my quad and it fried a motor. So I replaced the motor and it ran fine again. But a few days after that, the motors have started running fast out of the blue for 3-5 sec periods at random times, while not moving the throttle, causing the quad to fly up and shake. These short 3-5 sec episodes are getting more and more frequent.

I started to troubleshoot.
- I checked all the motors and even upgraded to BLHeli32 v32.10. Problem persists.
- I replaced the ESC. Problem persists.
- I replaced the FC. Problem persists.
- I reflashed Betaflight to the new 4.4.3 version. Problem persists.
- I disabled the GPS. Problem persists.
- I checked all cables. Problem persists.
- I disabled AirMode, which help a bit but I hate flying without it and it's not addressing the root cause of the problem.
- I put the OpenRacer0 O3 canopy (+ VTX/cam/ant) on my Vista OpenRacer2 frame (+ ESC/FC/motors/GPS) and everything ran flawlessly. This means the O3 unit and the Boxer controller are not the source of the problem.
- I checked the frame and it is OK.
- I recorded the phenomenon on the BlackBox 3 times during a 50sec flight. I am attaching the tracings.

Josh Bardwell thinks the gyro is damaged or something else is damaged causing bad gyro readings, which causes the PID loop to freak out. But the problem persists after replacing the FC.

Any idea where to go next?
I have just bought 4 new motors, a new FC and a new ESC and a new RP1, that I will install when received next week. Rebuilding the quad from scratch should solve the problem. But I'd like to know where the problem is exactly.

Benoit

Hmm, that's hard to tell. If you actually replaced everything except the frame, it leads to the frame. Take another look at the frame, if there aren't microcracks somewhere, I would focus on the screw holes. Even so, I doubt that a crack that you don't notice at first glance could be causing these instabilities. And it's definitely instability if it behaves better without Airmode.

(25-Nov-2023, 10:49 AM)MomoBrut Wrote: Hmm, that's hard to tell. If you actually replaced everything except the frame, it leads to the frame. Take another look at the frame, if there aren't microcracks somewhere, I would focus on the screw holes. Even so, I doubt that a crack that you don't notice at first glance could be causing these instabilities. And it's definitely instability if it behaves better without Airmode.

I felt it was extremely unlikely to be the frame, since there was no obvious crack or instability. But I had two unbuilt spare OpenRacer frames, so I said "what the heck!". I built a new frame, and transferred the motors and all the electronics to the new frame. I tested the drone with the new frame this morning, knowing very well that there was no chance in hell this would ever solve the problem.

Well I was wrong. It solved the problem!!! Thanks MomoBrut!!!

I'm glad I wasn't completely wrong. If you have the frame completely disassembled, inspect it, something must be visible, during the inspection, try to bend the parts, if there is no interlayer delamination, and really check the screw holes, especially the ones for the motors ...

(25-Nov-2023, 02:12 PM)MomoBrut Wrote: I'm glad I wasn't completely wrong. If you have the frame completely disassembled, inspect it, something must be visible, during the inspection, try to bend the parts, if there is no interlayer delamination, and really check the screw holes, especially the ones for the motors ...

upon closer inspection, I saw an arm cracked horizontally. See attached picture.  Is this really enough to create that problem?

Hmm, interlayer delamination ... as I assumed ... YES, this definitely causes vibration and instability in the motor control.

It's easy to fix if you have epoxy for lamination. Then it can be glued and the arm will be like new. If you can't do such things, just replace it with a new arm and that's it, the quad will behave well again. unless there is another crack...

(25-Nov-2023, 02:52 PM)MomoBrut Wrote: Hmm, interlayer delamination ... as I assumed ... YES, this definitely causes vibration and instability in the motor control.

It's easy to fix if you have epoxy for lamination. Then it can be glued and the arm will be like new. If you can't do such things, just replace it with a new arm and that's it, the quad will behave well again. unless there is another crack...

I just rebuilt another OpenRacer frame, as I had a few backup ones, which enabled me to solve the problem, as stated above. The advantage of the OpenRacer frame is that it is inexpensive.

I find it fascinating that such a tiny crack causes so much problem, given that both split arms parts were firmly held together between the top and bottom plates by two screws, which hid the problem.

Below is a handy link to a tutorial thread for repairing a broken / delaminated arm with epoxy resin...

https://intofpv.com/t-how-to-repair-a-br...copter-arm

(25-Nov-2023, 03:06 PM)SnowLeopardFPV Wrote: Below is a handy link to a tutorial thread for repairing a broken / delaminated arm with epoxy resin...

https://intofpv.com/t-how-to-repair-a-br...copter-arm

I threw the whole problem frame in the garbage. 
There might have been more than one issue, besides the one I identified. 
Not worth my time to deal with further problems with that specific frame.
I still have a few back up full frames, as well as many spare parts.