Unstable at takeoff

Hi All.

I built a 3dr frame + motor kit last week, and started testing. After passing all the standard tests, I tried a launch. The copter is not even close to being flyable due to some very strange behavior.   It wants to take off to the aft left.  With some aggressive right stick action, I can kinda get it into the air, but not for long.  Perhaps someone can point me in the right direction.

While holding the copter in my hand and advancing the throttle to 60% (or so), I can kinda feel the copter react to pitch/roll movement, but not exactly what I expect.


So on the ground I did some static tests with the motors disconnected.  Looking at the AMP Flight Data screen, the HUD responds perfectly to all copter movement.  When I look at the “Status” tab, looking at the ch1out – ch4out values, I see very strange behavior as follows:

(Frame is in X configuration)  If the front left motor = 3, front right = 1, rear left = 2 and rear right = 4, throttle advanced to ~50%:

At level  (all 4 values should be equal, right?)

3=1556    1= 1485 

2=1485     4=1556


Pitched up 4 degrees (front motors up)

3=1600    1=1467

2=1500    4=1552


Pitched down 5 degrees (front motors down)

3=1600    1=1500

2=1461    4=1552


Rolled Right 4 degrees Pitched up (right motors down)

3=1523    1=1520

2=1480    4=1564


Rolled Left 4 degrees  (left motors down)

3=1626    1=1428

2=1462    4=1576


As you can see, those results are not even close to correct.  But the the “motors” command in
CLI works fine.  The radio values in status are what I expect too.    


One other clue – the IMU takes 2+ minutes to warm up (able to arm).  The manual says it should take
5 seconds. 


The copter is bone stock. 

3DR frame + motors kit. 

Turnigy 9x in acro mode with ele and thr reversed,

rev H oilpan with filter pads soldered,

APM 1280. 

Sonar and magnetometer enabled.

Running the latest software/firmware:  2.0.49 and AMP 1.0.86.

I’ve erased and reset several times, reloaded software several times and set up several times in both the mission planner and the CLI. (it’s a pretty cool system by the way!).  I’ve tried leveling it in every conceivable way.   


So I’m at the point where I need help with what to look at next.  What am I doing wrong?  What tools can I use to investigate the problem.  Its acting like the motor outputs are not connected right.

Thanks for reading…..

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    Success!   Another flying arducopter!    It’s an amazing thing.

    My copter had a bad tendency to pull to the back and to the left on takeoff. If I
    could get it into the air without crashing, I could fly it -sort of. So I built
    a great test rig and ordered spare parts,   Then, I balanced it to high precision, adding
    small weights where needed – all with NO effect, by the way.

    Finally, I figured out what the problem was. A bad prop on the # 2 motor! There is NO visible or
    otherwise detectable issue with the prop, but if I install it on another motor,
    the copter tends toward that side. Replacing it with a new prop today resulted
    in gentle takeoffs and stable hovering in the backyard. yep-eee.

    It still fluxuates in altitude, but I guess thats normal.  And it still takes 2 minutes for the IMU to
    allow me to arm it. But once it arms, all is well. Not sure what’s going on there.

    Anyway, on to testing the other modes this weekend.

  • Jason,  Yawing is a clue.  I reloaded all software and erased the eeprom and re-setup the copter.  Now, looking at the test sensor page on the wiki, its starting to make sense.  It helps to understand how APM wants to control yaw, even if there is only a pitch or roll change.


    After a new test flight this monring, if I can get it into the air (say 2 feet up), it seems to be stable in attitude, but widely unstable in altitude.  I was loking at the sonar test output on the bench.  It is stuck at 33 cm until 23 cm above the ground, when it starts going up.  So it looks like sonar reads 10 cm higher than actual and only starts produces valid readings after 23 cm above the ground. 

    Is ths normal?. 

    Is there an adjustment I need to make?

    Can this cause unstable altitudes or should I be looking at somthing else?


    Thanks for looking at this - coming up to speed is a long slog. 




  • Developer

    Imu not starting up right away indicates a hardware failure. Maybe a gyro chip has not been soldered properly.

    There is an IMU and an ADC test in the CLI. Try those. You may need to return then IMU, but I'm not sure without the test data.



    Your PWM numbers could be correct considering the copter may try to Yaw back to a desired heading.


    At level  (all 4 values should be equal, right?)  <--- No, it is "Yawing" now.

    3=1556    1= 1485 

    2=1485     4=1556


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