Does the APM really have to be in the very center of quad?

Posted by Edgar Scott on January 6, 2014 at 5:47am

Does the APM really have to be in the center of quad?

Reason I'm asking is TBS added a dampening system to the gimbal and I need to move the APM off to one side because of its size about a half an inch to the left, is that okay or will it mess things up?

Front and back it will still be centered, just off set to the left a little bit.

Big question of course is will I lose performance if I do this?

Thanks, Ed,

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Replies

  • Developer
    Randy January 6, 2014 at 5:40pm

    I think it's ok to be a bit off center.  I think the copter rotates around the COG so if the APM is far from that then when the copter does a hard rotation it will see not only a rotation through the gyros but also an acceleration because of the centrifugal force.  If that was sustained for a long-ish time (i.e. 10seconds?) then it could affect the attitude.

    So this might cause problems:

       - mount the controller on the arm

       - spin in yaw for 10 seconds

    It would probably end up having a bad roll and/or pitch angle after you stopped yawing...it would recover or so after a few seconds I think.

  • MR60
    Forrest Frantz January 6, 2014 at 8:51am

    The APM is never at the center in the center on any ship.  It's impossible.

    The center of the ship is a 3-D location (x/y and z) that shifts during flight.  Add a camera, the location shifts.  Change the battery capacity, the location shifts.  Torque the motors, the location shifts (the motor booms twist and bend).

    The APM has some pretty sophisticated mathematics on it to understand what is happening to it no matter where you place it.  I don't understand the mathematics, but I've had the APM several cm from the CG with not problem.  Just ensure that it's secured on something solid and stable.  You don't want it at the end of a diving board or too far from supports.

  • Edgar Scott January 6, 2014 at 6:46am

    Cool thanks guys, sounds like ill be okay.  Well I am only offsetting it at most 1 cm to the left for the new TBS bracket.

    Ed,

  • Euan Ramsay January 6, 2014 at 6:00am

    I run my normal disco with the APM forward of pitch axis, and very slightly right of roll axis (to accomodate a USB cable extension). Flies perfectly fine, and completed an Autotune no problem.

     

    "Terence" is a medium lift monster V quad (4.7kg AUW - APM 2.5 with 3.0.1), and has its APM 6cm forward of the pitch axis, and at one point 2cm right of the roll axis. It flew fine as well, thought I will admit I've not really been throwing it around. I have since moved the APM back to the centre of the roll axis (pitch is still 6cm forward), but not really noticed any difference in the handling. It even has a CoG 5cm forward of CoT, so is supposed to be unstable. It's not. It's rock solid and not pitchy in the slightest.

     

    I think you'll be fine. And there's always autotrim to help clear up any issue you might notice. Which I doubt.

     

    Semi-Related query: TBS are releasing a fix for the Disco Pro? Why? I only wish they'd do something about the CoG on the C-plane - so many horror stories (mine included!) about CoG being way off, even with a nearly total standard build.

  • Ben Cairns January 6, 2014 at 5:49am

    may affect the roll/yaw as the rotations will be slightly greater one way, so it may "bounce" on those axis, but pitch should be unaffected

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