Fast and uncontrolable descent in RTL / LOITER

Hi everyone !

I hope you could help me see through this issue I had recently.

I am using a TAROT 680 Pro hexacopter and after several nice flight exeprienced fast and uncontrollable descend rate (5m/s) while in RTL or LOITER.

It’s equiped with a pixhawk 2 running arducopter 3.5.2.

It’s equiped with T-MOTOR Antigravity 4006 KV380, T-MOTOR AIR 40A and 13x4.4 Propellers.

The total mass of the copter was around 5 kgs.

Regarding the logs it seems not related to vibration, nor the FCU or anything that could mess with sensors.

I would blame battery, but, along with power module which worked correctly, we add a lipo buzzer equiped and it never started to beep.

You will find some logs there :!AnS3nHy1xzlAlgz14N2OZ3HMA9hp

Thanks for you support guys !

Your copter is underpowered and using max throttle to just stay in the air with very little headroom for stability.

Any instability, such as trying to descend through your propwash (ring vortex) or wind gusts is going to get you into trouble.

Thank you for your analysis.
However that does not feel right because it was supposed to be dimensioned to have the right amount of power, given the TMOTOR specs:
1633 gs of thrust at full throttle per arm, that’s nearly 10 kg of thrust with 6 of them. And the copter was 6kg max.
So that leaves us with a bad ESC calibration?

EDIT: that would feel right as current drawn from battery is around 40A max (6.6A per rotor), when a rotor is supposed to draw 9.7A at 100%, and the other way around, as it needs full throttle to hoover without a current drawn that high.

Look at the power charts for your motor again.At 50% and 6S those motors and props are giving you 544g per or 3264g of lift.So that chart doesn’t surprise me.They’d be much happier with 15 or 16 inchers and then you’d have the lift.Just need extended arms is all.

Limited by the stock arms to 13 inch I run 4S and 610kv motors at 3.6kg.I may have to try 6S on it someday.

Way overloaded for that configuration. With those motors and props on 6S 2800g would be about right. You can’t use larger props so you need higher kv motors.

Thank you guys.
I’m ok to say that it was loaded, given the charts, that’s around 75% throttle to hover which is a lot.
Anyway, does my point on power consumption and ESC calibration seem legit?

I can see where you’re coming from but real world figures are always different to the paper ones.But the paper figures say that can’t fly any better than it is currently at that weight.Just not possible.Bigger props or smaller faster motors will improve things but 6kg take off weight is getting up there where the 960 class copters are.My Tarot X6 can run up to 8kg TOW using 18" props and 274kv motors on 6S.If I tried it on 15 inch props it would really struggle to fly.

I’m sorry but you’ve got your sums wrong somewhere.You need more power to lift that weight safely and that’s the bigger props/extended arms (you can stretch them to 960mm) method or new faster motors.Or lose a lot of weight.Don’t dwell on manufacturers claims or theoretical limits.I don’t think ESC calibration will help unless they are programmable and have been given the wrong program.

Personally I’d stretch the arms to use those lovely motors to the full with 16 inchers fitted.

I got your point.
I had one figure wrong, total takeoff weight was actually 4.5kg, and regarding the tmotor charts, it should about 60% throttle to hover.
Maybe at that point that makes more sense?