Parachute critical sinkrate parameter "CHUTE_CRT_SINK"

Hello folks,

I am currently setting up several multirotors with parachutes and I am wondering how the logic behind the parameter CHUTE_CRT_SINK is working. Obviously I am trying to avoid a parachute being fired, if everything is still under control.

  1. if the critical sinkrate (let’s say 5m/s) is exceeded during a rapid descent under control (for example in STABILIZE mode with zero throttle → copter is practically free falling, but remains in attitude control), the parachute would NOT deploy? Can anyone confirm this? The only thing I can find about it is this discussion:
    Parachute: add documentation re CHUTE_CRT_SINK parameter · Issue #3066 · ArduPilot/ardupilot_wiki · GitHub

  2. the documentation says, the parachute will deploy if an angle discrepancy of more than 30° occurs.
    Parachute — Copter documentation

So one case I can think of, when CHUTE_CRT_SINK kicks in is, if there is a power loss, and the UAS remains in a level free fall and exceeding the critical sinkrate.
Is that the purpose or am I missing something?

Are you guys using this parameter, or do you set it to zero?
It’s kind of terrifying, because you can’t really test it. You “just” set it up, after checking your max sinkrate in the logs and put a safety factor on it and hope it won’t kill your setup.

Thanks a lot!

To be upfront, I haven’t tested a parachute in the air yet, but I’m setting one up so I’ll share my interpretation:

  1. I would have a look at some logs for this or similar quads you’re flying and see what is a max descent rate you may use for this application and set it higher than that. Note according to the wiki the parachute won’t auto deploy in Acro. The wiki doesn’t describe CHUTE_CRT_SINK as a crash detect parameter or criteria, so I think that if this is a drone you’re going to be doing some big dives with you could set this value high to prevent it from being an issue. Then test it to see if the crash detect still triggers.

  2. That would be 30 degrees between the target pitch or roll and the actual pitch or roll. If you’re more than 30 degrees different between the two you need to work on the tuning. :slight_smile:

In my experience quads rarely fall perfectly level. Usually only during radio failsafes in FPV and if that happens the radio failsafe should bring the quad back in arducopter. If there is a total power loss then the parachute won’t deploy any how. More likely is failure of one motor/esc/prop. In that case the machine is usually tumbling pretty quick and the 30 degree trigger should kick in.