Strange crash in Alt-Hold APM 2.6 Arducopter 3.1.2

Hi

I had a small crash this morning while testing some video shoots with my Y6.

APM2.6 ublox GPS with external mag, Rctimer HP4108-380kv top props 13x5,5 bottom 15x6,5 + 40A rctimer ESC.

frame type 10.

4.5 kg with 5 S 5800 and Nex7 with gimbal hover current is around 36 A

the problem started in the last few min after descent from 50 m to 3-4 m i was flying forward and the copter didn't hold altitute and landed a bit harded (nothing broken) i put full throttle but no response.

the battery use was 3800 out of 5800 good SLS LIPO ! all cells well balanced

could someone please have a look at the attached log and tell me what went wrong ?

Thanks

Thomas

 

2014-02-26 09-48-45.log

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Replies

  • Thanks for the answers

    it is probably due to the smaller props on the upper Motors that was the only change i've done recently.

    all started at around 50m i dropped the throttle a bit do descent but my gps (futaba) reported not falling, then i dropped the throttle to 0 ! result was fast descent and i did a quick stop at around 4 m.  

    i will go back to the same props on upper motors and let you know the results.

    Thanks

    Thomas

    • ok here another log with same props on top and bottom (needs again PID tuning :-))

      does it look ok now with throttle out around 600 ? or underpowered ? copter lifts fast if not in alt-hold !

      thanks

      Thomas

      2014-02-26 14-34-55.log

  • MR60

    Hi,

    Looked at your log.

    We can first see that your throttle out stays very high (between 70% and 100%) which means you are underpowered versus the weight. It means also you do not have any power reserve to lift your copter.

    Then there is something strange that happened at about 4.5 minutes into the flight where for a strange reacon the throttle Input goes to zero (you have cut the throttle ?) and vibrations get very high. this happens in 20 seconds before the seconds before the end of your log (flight).

    It is too bad you do not have voltage nor current measurements active so we can't see what happened with your battery level.

    Intuitively I would say you are flying with too much weight, underpowered and that toward the end of your flight you quickly descended, creating lots of vibrations and your battery was low enough so as to not be able to sustain the weight and you crashed because even with throttle out at 100% the lift was too low (that explains why you had the impression that the throttle did not respond anymore).

    Just a thought

  • Can you give an indication of how much reserve power you have at liftoff and what the voltage drop is across the battery (difference in voltage idle at ground and in stable hover?).

    What may have happened is that as your voltage decreases, it loses power on the motors. It's not just a question of voltage and current though, it also involves how the motors are rated and where their efficiencies are. On a 5S the voltage drop is a bit higher than 3S, so you get larger fluctuations at the motors. Eventually it means that the amount of electrical power you need to stay in the air (mechanical power) is higher than usual, resulting in even higher current requirements.

    If the battery has a discharge rate close to this 36A, then you may get closer to those limits. Every battery has some internal resistance, so when current increases, it also increases  the voltage drop, requiring even more current and so forth. Eventually it just doesn't cope with the current requirements and doesn't deliver enough power to the motors.

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