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Low KV X8 configuration, cant get it tuned :(

Posted by sturgessb on July 9, 2015 at 10:54am

Hi chaps

I've just finished a build of a largish X8 setup using the quanum 700 frame, its using multistar 268kv motors with 15x5 props running 6S using 30A escs ( http://rctimer.com/product-1323.html )

Now, ive tuned up countless 400-600 class quads and hexas with no worries but this rig is giving me a nightmware.

With default PIDS its pretty much unflyable unless holding a tight level with the sticks. If i whack the P up to around 0.3 and the D up to 0.2 it becomes a little more controllable but again only around the level. As soon as i try to pitch or roll with any enthusiasm it bobs and dives with a noticable overshoot. I have stab and pitch rate at around 5, moving this up to 8 or 9 only seems to help around the +-5deg rolls and still when rolling with any authority it bounces around.

So theres that, would be up for any ideas. Also Im having an issue with being underpowered. I know its not underpowered as my AUW is only 3.5kg and I have 8kg of potential thrust, i have even tested this by going into esc setup mode on the APM, and giving it the beans, it wasnt to rocket off into the sky. However when flying it barely wants to climb. Hover is around 70% stick, whereas theoretically it should be around 40%.

Whats going on then, any ideas welcome.

Cheers

Ben

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Replies

  • Russell Stutzman July 10, 2015 at 8:28am

    Your setup in ecalc predicts 69% throttle at hover. I don't think you have enough propeller surface for that low kv of a motor. Change to even a 330kv ups you to 47% throttle at hover by eCalc's results.

    Your current setup doesn't allow for a lot of adjustment ability and that might lead to some of the issues you are seeing. The motors don't have enough headroom to spin up when needed.

    • Anders Wikstrom > Russell Stutzman July 10, 2015 at 10:53am

      Just to compare...

      I have built an octo with

      Tarot Iron man 1000 carbon frame
      15 x 5,5 Carbon Prop
      Turnigy 4114-320 kV motors
      Turnigy MultiStar 45A 6S Esc
      2x5800mAh 6S lipo
      6.5 kG flying weight

      It flies very, very good! No need to tune as the frame and props are so stable.

      With this setup I have enormous power. Around 8 * 450 W equals around 4 HP.

      Maybe this helps?
      Anders

      • sturgessb > Anders Wikstrom July 13, 2015 at 1:51am

        I have changed to 15x5.5 props (the ones from a DJI s800) and this has made a HUGE difference, (Thanks for the tip).

        Power is well up now, and it hovers at around 45% throttle, and it flies super stable. Its still a little under powered because i need to lift 1.7kg payload (inc batteries). Im thinking 8s will solve this, but will probably need 14" props.

    • sturgessb > Russell Stutzman July 10, 2015 at 9:58am

      Ok, I'm going out for another test shortly this eve. I will take some video of the behaviour and the PID values.

      Do you think shifting to 8S would be a big help?

  • OG July 10, 2015 at 7:34am

    hello,

    sounds like you need to properly set throttle PIDS on your copter bottom left hand of advanced pid tuning in MP. Please post logs or screen shot of PIDS so people can help you find the issue.

  • Zwonimir July 10, 2015 at 7:00am

    Going through this too with a U8 170kV X8 setup. I got some good PID results with autotune, after working with the manufacturer of the esc board to get a flawless esc firmware which does not have any sync issues anymore.

    But I'm too struggling with power, although I should not. I calibrated the esc already, but I'm loosing thrust somewhere.

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