My Hexa tries to flip while taking off!!!

Hello,

it's been a while since my last discussion, especially because I have been very busy building up my hexa and during this period I haven't had any particular issue.

so far so good until three days ago when I tried my first take off, basically when I raised the throttle the copter leaned forward , it would end up in a crash if I hadn't released the throttle immediatelly.

There was obviously a problem on my hexa, I started surfing over the internet and I found this troubleshoting page which reports what can be the reason of tilting during take off.

I have doubled checked any tips in the link above but nothing solved my problem, as soon as possible I am going to recalibrate the IMU, but to be honest I have been following this procedure many times and I believe that IMU should be ok. 

Attached you can find log as well, I have checked vibration and mag_filed and they seems ok to me.

I would like to have some opinion from more experienced users. 

Best regards.

Marco

6_4s.BIN

6_4s.tlog

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Replies

  • Developer

    If there's an issue it's really most likely to be one or more motors/ESCs are out of order or are spinning in the wrong direction.  Looks like you're using an X Hexa copter and the motor output looks generally good but perhaps either 5 or 6 is off so I'd recommend double or triple checking those two at least.  Motor connection wiki is here but in any case:

    motor #5 = front right

    motor #6 = back left

    3702228662?profile=original

    Connect ESCs and Motors — Copter documentation
    • Hello Randy,

      thanks for your advice, I didn't know how to check for single motor output, looking at the graph ch6 gets the greatest output and ch5 gets the minimum output. 

      Can be this related to the fact uav leans forward? 

      Can I fix this issue recalibrating ESCs again and again? 

      what other kind of check would you do?

      If none solve the problem I will try tuning PID...after all the frame has been all designed and built by me and I suppose there could be some tuning to do. 

      best regards.

    • Developer

      the mission planner allows checking a single motor.  Checkout the wiki page above.  Best to check the propellers are spinning in the correct direction (as shown on the wiki page) and also make sure the propellers are all on the correct way (i.e. not up-side-down, are the correct props for that direction).

  • There is always a huge advantage to posting a video of the issue. Many can solve the problem just by seeing it. You might want to try that.
  • Marco:

    Assuming you have have confirmed that your esc's have been calibrated, the motor direction is correct, and props are correct,

    then,

    In the beginning (using stabilize mode) you will need to provide some correction with the stick to compensate for this initial tilt.

    but first you'll need to adjust stabilize P just a little.  Just enough so that when you start raising the throttle you will have adequate stabilization to take off without flipping.   This the tilting issue can be created from a stabilize P setting that is way to high or way to low.  If you have a big craft then likely you need a larger setting. Small craft, lower setting.

    Attempt to take off, stop when you see the tilting, adjust P in small increments, try to take off again.  Don't be aggressive.

    Your objective is to fly in alt hold mode.  Then do an autotune.  That should get you in the air.

    Read and understand the wiki sections on tuning the PID settings, and autotune.  check the blogs for settings that worked for others with similar size craft.

    • Hello Harry,

      what you are saying makes a lot of sense to me. 

      I have the same objective you suggested but I thought to skip the initial part of tuning P manually, but since the frame is all designed and built by me, tuning P looks more than reasonable to me.

      I will keep you posted on any breakthrough...

      regards.

      Marco

    • Marco,

      Once your mechanical issues have been resolved, tuning is absolutely required.  As a home built craft its more likely you need to guess at rate P, just to get in the air.  I believe the default PID settings are for the Solo, or the older "ready to fly" 3DR offering.  I'm sure you can find out what model the default PID settings are meant for, then you can compare your weight and power to the default settings.  Then you should be able to get an impression of how far off your current settings are.

      I have a heavy Y6.  7kg.  All home built, I spent weeks, and a bunch of broken plastic props, until I noticed a small increase in rate P made a small improvement in the tilting at take off.  A little more rate P and I was able to take off (with continuous corrections from the remote) and hover in alt hold mode.  After autotune my rate P doubled, now I'm steady as a rock, and no more broken props!

    • Hello Harry,

      making some pitch and roll correction during take off I have been able to fly in stabilize and althold mode, once it is in the air it flies relatively well but I see that sometimes it is a little twitchy, for this reason I would like to PID my exa. 

      I'm trying to fly my exa in autotune flight mode, unfortunately during my indoor test I can't invoke it.

      As you can see in the attached imagery once I switch on flight mode 3 pixhawk refuses to accept this mode and keeps the previous one. 

      3702815819?profile=original

      As you can see the PWM signal is between the treshold, but somehow it will be rejected by pixhawk.

      If I use a different flight mode 3 (anyone) it will be accepted, see image below.

      3691302633?profile=original

      As last thing I have thought that perhaps I couldn't set it as a flight mode so  i tried to use channel 7, which I normally use for RTL mode, but eventually even in this case Autotune flight mode will be rejected. 

      Regards.

      Marco

    • My advice first would be to stop using Flight Mode 3 for autotune and stick with ch 7.  little need for RTL during an autotune mission.

      So when using ch 7 for autotune you say " ...eventually even in this case Autotune flight mode will be rejected"  So what does happen?

      Prior to autotune existed (or with APM) we adjusted the PID settings on our own.  Plenty of good advice on the subject on this site.  Read First!  It sounds like you need more of a minor adjustment rather than a major change.

    • Hi Harry I think I have figured it out somehow how to make it work.

      I was basically trying to invoke autotune while the copter was still on the ground with its motors off, obviously it got rejected.

      So, still on the ground and no props, I have tried to arm the copter in stabilize mode switching to alhold mode and then invoke autotune, following this procedure the copter started tuning.

      Basically I have understood that if I want start tuning the copter needs to be in the air or armed.

      I have further proved that autotuning can be invoked either using chanel 3 or channel 7.

      Thanks

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