Crash due to unexpecrted, comanded full yaw command

Hi there.

I’m flying a 700-class with latest heli fw (4.5.4). Everything went fine (like 6 flights, still to be fine tuned, but largely flyable) until i raised rotor speed through the “heli” interface on QgroundControl (almost the same of MP there). I had the helicopter spinning like crazy like 5 seconds after take off, then crashed. I was 100% sure it was a failure on tail servo, but with much surpise, tail servo is still 100% ok, and it was the flight controller to issue the yaw, putting it to full stroke right yaw. I had a “vibration compensation ON” message in the logs, so probably it was the root reason.

BUT the question is: why it put yaw at full end position? I could not find a failsafe position for that into parameters

logs attached
2024-07-09 16-26-05.bin (612 KB)

That is very wired, it looks like the copter is trying to command yaw left to compensate observed yaw right.


I would suspect reversed yaw servo or possibly loss of tail rotor effectiveness (not necessarily aerodynamic).

Hi @Steve79 can you post a log previous to the crash event.

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So the data indicates that the aircraft was yawing to the right. Does that match reality? if so then the flight controller had the proper input but there must have been something mechanical or aerodynamic. very curious.

hi @Ferrosan here you go: Dropbox

@bnsgeyer as far as i can remember (things happened VERY fast) it begun spinning to the right. After crash i actually had very minor damage, mostly cosmetic, apart broken blades, while all mechanical parts were good, including tail and its servo, so i could check everything mechanically, and all was/is still functional and not broken/burnt. Only thing i can add, tail servo was VERY hot, like 70°C. Which is strange, because i tried it afterwards, even putting it into stall, and barely became warm.

@LupusTheCanine well, seeing this (i missed it) can lead to a mechanical failure of some sort. Seeing the FC doing opposite yaw is quite assuring it’s doing its job correctly, at least.

If the grub screws on the tail drive pulleys are all good and there is no “slipping” in the transmission it can only be the actuator at fault or too low headspeed (what’s your current main rotor rpm?).
Pre:


Crash: