PixHawk Drone keeps descending and we don’t know why

So I made a drone since I’ve made other RC models before, and I’ve been learning about all the extra things that go along with drones. Such as the extra sensors, computer and software and firmware.

I really like the PixHawk but Mission Planner has bugs and it often doesn’t connect or freezes, or won’t allow the PIDs to be changed.

The Drone’s an X8 with 4 arms and two propellers on each arm, with one above the other.

In flight, or even just above the ground, it will often start descending, until the throttle stick is at the top and the drone even rocks back and forth a few times before meeting the ground.

The frame is rigid (carbon tubes and plates) and the flight controller is properly fitted with gel in the centre of the frame. No Bad AHRS or any other fail safes or warnings.

In the Datalogs show the motor PWM outputs going wild when the drone descends. Also in flight, some motors will go to 100% and others to idle!

I’m not sure what’s going on here.

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  • yes, some can have sync issues.  - you can test for sync issues using RX and radio only, one motor only.

    5.5kg .. how much thrust does each motor actually provide with the propeller you have ?  - you are also aware of not having actual redundancy with a setup that flies at so high level (% of available thrust)  , so you if you are looking for efficiency alone, you would have higher efficiency with just 4 motors - and still no real redundancy.

    ayway, seeing your logs, you are trying to fly 5.5kg on ~450W   that would require an efficency of  >12 g/W  ,   - if we assume your current sensor is not properl calibrated, you are maybe throwing 500W , it's still demanding rather great efficiency...

    All normal/newer/decent ESC's handle the default 490Hz.

    of course, having more stready PID's will increase lift a bit   - but mostly the thrust is not linear, so an ESC going between 70...100%  may in fact only provide only ~20% thrust increase.   - at *some* point there will be simply too little thrust change to have good attitude authority.

  • Are you sure about the props and motor rotation being correct for every motor? What size motor/prop are you using?


  • Hello and thanks for your replies!
    The drone weighs 5.5Kgs...
    Yes the power is intentionally marginal.
    The propellers are correctly mounted :)
    Power module is calibrated on current and voltage.

    The batteries are new and hugely over-spec for the current consumption.
    The motors and ESCs are rated at 3S and cannot go to 4.
    (That would also add more weight or decrease capacity for the same weight)

    It gets airborne and is basically controllable although NOT tuned.
    (I've done some basic increase in Rate P gain for pitch and roll) (It's almost time for an AutoTune)

    SO, I'm wondering why sometimes, for no obvious reason, it starts to descend and can keep going downwards even after increasing the throttle to full. The other scenario is it will start to do a graceful, large amplitude oscillation, and then descend, and if that oscillation diverges then it normally comes down on one skid and snaps the skid.

    I'm wondering if it's an ESC shutting down in flight, and being co-axial, one propeller turns the other so we never see a stopped propeller. The ESCs are not drone-specific ESCs. I know some have their own PIDs and adjustable options. 

    I don't know what the signal refresh rate for the ESCs is. I've read DJI ESCs cannot accept an "RC Speed parameter" of more than 400Hz. The PixHawk was at 490Hz so I've set it to 400Hz.
    Another setting I don't properly understand is the effect of changing the Outer P gains.
    So I'm wondering between poor PIDs and an ESC getting 'over-loaded'.

    If the PixHawk sends a signal to an ESC that goes low-to-high and back very quickly, will this produce a sync issue?

    Thanks fr all your help! :)

  • assuming your voltage/current sensor is calibrated:

    you start out at 12.4v  (assuming you have a 3S battery)   , which, each time you climb, voltage drop to  quickly drops as low as 11,2 v (3,7v)  so the battery pack is probably rather heavily loaded, or in bad shape. (high internal resistance)

    The second flight, It climbs for 5 seconds at ~73-78% throttle to gain less than 1meter.   - that's just hugely underpowered.

    again, IF your current/voltage sensor is ok, the power is nearly 450W at 70% throttle   if all propellers are mounted correctly, and motors working efficiently, that should be maybe as much as 3.5kg of thrust  (the g/W efficiency depends on many factors)

    so if if weights ~3kg - it's no wonder it struggles.

    if it weights  >2kg if should be flying well, and the reason may be reversed propellers, bad motors etc,etc. 

    you should recalculate the motors/propellers/thrust you expect at this voltage, vs weight of the thing.

    maybe, if ok within the max power of the motors, go for 4S og 5S battery packs.  

  • That was fast! Thanks for your reply! :)
    Re-attempting to upload a .bin

    2016-12-09 11-02-56.bin

  • For more professional/reliable GCS, try APMplanner2 , QGroundControl or mavproxy.  (depending on your level)

    you need to post the .bin, try uploading and sharing using google drive, or any other file sharing service, there are plenty, you are most likely to have at least one already.

  • The ESCs are not EMAX or SimonK.
    They are Pro's from RobotBirds.

    Here's a log screenshot since it wont let me upload a .bin file

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