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Thread: Unstable flight

  1. #1
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    Angry Unstable flight

    I have a new build and have faced a problem that I cannot resolve, I am hoping to get some help.

    My setup is:
    APM2.5
    TBS discovery frame, 30A ESCs and 900kv motors.
    uBlox GPS
    Turnigy 9x tx/rx

    At the moment, I have a completed build, props and motors are balanced and turning in the correct directions. The frame has been leveled, center of gravity just about spot on.

    The problem I am having is that as soon as i lift off, the quad seems to drift off to one direction every time and the direction of the drift seems random each take off. This is particularly true when i take off from a less than level starting point. I have also noticed that within the Mission Planner, when I move the frame around, the artificial horizon does not move, whereas the information below this (altitute, yaw etc) updates normally.

    Any ideas will e greatly appreciated.

  2. #2
    Navigator cardboard_boks's Avatar
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    Sounds purely like a tuning issue. Are you trying to run any auto levelling or are you just in gyro/rate mode?
    I'm not sure how the APM tuning is done (I have multiwii) but if you do have PID settings I would raise P a bit to start with.
    A general rule for tuning the P term-raise until oscillations start then back off a bit. To lower P and things will fell sloppy.

  3. #3
    Sky King j5coat's Avatar
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    if you are trying to tune in manual or acro mode it still flies like crap. I tried the same setup as you and thought the apm would be a good controller, well it is if you like to fly in stabilize or do the mission planner, but in acro mode it is not like any other controller, its very unstable. i posted a question on the apm forum and the basic response is no one flies it in acro so they dont really work on it. here is the thread i started over there, has some good info for setting a better acro mode. http://diydrones.com/forum/topics/tu...o-mode-for-fpv

  4. #4
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    It might have to do with the fact that the Disco isnt a normal "X" quad, the motors are angled weird. Check this out...

    http://diydrones.com/forum/topics/of...nually-setting

  5. #5
    Navigator cardboard_boks's Avatar
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    I don't think it is a geometry issue, my disco flies great in quadX mode. My pitch pids are slightly higher to compensate for the large masses of battery and gopro at the front and rear of the frame. Multiwii seems pretty robust to odd copter cg's and motor positions though, I can remove the gopro (in its protective case) and the copter doesn't care at all. On another other occasion I had a tri turn into a T copter mid flight due to folding arms and it was still very flyable.

    @Elto did you ever resolve your issue in the end/what flight controller are you using??

  6. #6
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    Quote Originally Posted by cardboard_boks View Post
    I don't think it is a geometry issue, my disco flies great in quadX mode. My pitch pids are slightly higher to compensate for the large masses of battery and gopro at the front and rear of the frame. Multiwii seems pretty robust to odd copter cg's and motor positions though, I can remove the gopro (in its protective case) and the copter doesn't care at all. On another other occasion I had a tri turn into a T copter mid flight due to folding arms and it was still very flyable.

    @Elto did you ever resolve your issue in the end/what flight controller are you using??
    Forcing the rate controller to compensate for a weird frame config is like not balancing a Multicopters props/motors and just damping vibrations on the camera platform, sure you might get decent results but its better to eliminate the source of the problem.



    Anyway, I dont think thats your issue either but you should look into doing the change those people have done. I think your problem is how you're tuning your copter. You should start it in acro mode, take the accelerometers out of the equation and just tune the gyro's. Check out one of the many arducopter tuning guides at DIYdrones.
    Last edited by gvickers; 8th January 2013 at 01:20 AM.

  7. #7
    Moderator Derek_S's Avatar
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    I dunno if you are new to APM, but I would think you need to tune it 1st (in STABILIZE mode) by adjusting the RATE KP value, probably up some. This is easiest to do by linking that parameter to a knob/potentiometer on your Tx and just slowly upping it until it wants to oscillate, then back a little. You just set the min and max values to say 30% +/- deviation from the default value, and center the knob. You can actually tune all the major parameters such as Loiter and WP navigation this way (one at a time) and I think that is my fav feature of the APM).

    It does seem that the APM really is intended to fly in a STABILIZED mode, but I did fly mine in ACRO a little and found that it wasn't impossible, but certainly felt a little weird. Granted, my new Multiwii FC (Flip FC from Witespy) also feels that way, almost like a toilet bowl effect. Anyways, I fly them both in STAB/LEVEL modes mostly for FPV anyways, and only switch to Arco at higher altitudes to do flips or whatnot if I feel like it.

    If you are trying to fl in acro mostly, I cant really be of much more help (but RATE KP should improve that flight mode, too). I hate to say it, but if you want to fly mostly acro, maybe pick up a CC3d/opencopter FC as they seem to be perfect for that.
    Last edited by Derek_S; 9th January 2013 at 01:59 PM.

  8. #8
    Moderator Derek_S's Avatar
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    The next thing to do once it is flying better (hopefully) is to see how acro feels higher up off the ground. It seems mine 'feels' better up in the air, and I have contemplated if the RATE KI value needs some tuning to improve more turbulent air performance [it is my understanding this is where changes in the I value can be most easily witnessed]

    I only have 2 batteries right now for my hex, so my tuning is severely limited by relatively short flight times and lack of daylight when I get off work, but I hope to iron mine out some more as I get to it.... I have a case of the 'wobbles-on-descent' problem that I need to tackle next.

  9. #9
    Moderator Derek_S's Avatar
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