ACRO bug (fixed in 2.9.1b): while doing flips in ACRO mode, if you switch to Stabilize while inverted your throttle will go to minimum.  To regain throttle control you need to switch back to ACRO then back to Stabilize again (i.e. switch to stabilize twice).  You never lose control of roll/pitch/yaw.

Loiter/AltHold/Auto/RTL bug: if you switch into these modes with throttle at zero motors will go to minimum until you raise the throttle.

Auto mode altitude bug (fixed in 2.9.1b): setting a waypoint altitude greater than 320m over home altitude may wrap around and instead be interpreted as a low altitude.

ArduCopter 2.9 is now in the mission planner and the downloads area!

The major improvement is we use inertial navigation to improve altitude hold.  This increased reliance on the accelerometers means you must do some additional set-up before flying:

1. Perform the new accelerometer calibration in the mission planner (video).  The auto-trim metho has also changed (video).

2. Add vibration dampening foam between your frame and the APM.  Some suggested materials: DuBrogelhk foam.

 3. If upgrading from 2.8.1, modify the throttle and altitude PID values:

  • Increase your Throttle Rate P, reduce I to zero, increase D
  • Increase Altitude Hold P, reduce I to zero
  • Tune Throttle Accel P and I terms but try to keep P about 1/2 the size of I


Here is the list of major changes (a more detailed list can be found in the release notes):  

  • Alt hold using inertial navigation (Leonard, Randy, Jonathan)
    • AUTO_VELZ_MIN, AUTO_VELZ_MAX parameters control the max climb/descent rate for the autopilot (cm/s)
    • PILOT_VELZ_MAX controls max climb/descent rate for the pilot (in cm/s)
  • Landing improvements (Leonard/Randy).  Copter will descend to 10m or until an object is sensed with the sonar.  Then slows to 50cm/s descent (speed can be adjusted with LAND_SPEED parameter). (video).
  • Surface tracking with sonar (Randy/Leonard).  Copter will attempt to maintain current distance from objects in front of sonar regardless of altitude.  Only used in alt-hold and loiter, not used for missions.  Sonar can be enabled/disabled with CH7 switch. (video)
  • Failsafe improvements (Randy/Craig/John Arne Birkeland) including bug fixes, additional check for PPM encoder failure and implementation of battery failsafe.  Set-up instructions are here.
  • Mediatek gps driver accuracy improvements and use of SBAS [Craig].  Instructions on upgrading your mediatek to firmware 1.9 are here.
  • Traditional Heli improvements (Rob) including (a) bringing heli code back into the fold, (b) enabled rate controller (previously only used angle controllers). (c) fix to rotor speed controllers - now operates by switching off channel 8.  (d) allow wider collective pitch range in acro and alt hold modes vs stabilize mode  (e) bug fix to allow collective pitch to use the entire range of servos
  • Acro trainer (Leonard). Copter will return to be generally upright if you release the sticks in acro mode.
    • ACRO_TRAINER : set to 1 to enable the auto-bring-upright feature
    • ACRO_BAL_ROLL, ACRO_BAL_PITCH : controls rate at which roll returns to level
  • Camera control improvements (Randy/Sandro Benigno):  (a) AP_Relay enabled for APM2  (b) Trigger camera with CH7 or DO_DIGICAM_CONTROL command  (c) Allow pilot override of yaw during missions and fixed CONDITIONAL_YAW command.
  • PPM sum support for transmitters with as few as 5 channels (Randy/Tridge/John Arne Birkeland).
  • Performance and memory useage improvements (Tridge).


As per usual PIDs are optimised for the 3DR/jDrones quad with 850 motors and 10" props. If you're using more powerful motors/props and are seeing bad flight behaviour in stabilize, start by turning down Rate Roll P in 25% steps.

Special thanks to our testing team lead Marco and the dedicated bunch on the 2.8.1 release thread who put their copters at risk while testing the pre-release version.  Some of their videos are here: 1 2 3 4 5 6 7 8

Please feel free to report issues you find in the discussion below and/or add them to the issues list.


Tags: ACM, APM1, APM2, ArduCopter, Heli, Quad

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I think I am below the deadband at hover and that makes sense.

How do I adjust the sticks to be center at Hover?

Thanks :)

** cancel that - just saw Randys post updating the wiki. Will give THR_MID parameter a try.

Well done on 2.9 guys.

By the way, this question about slow-descent has been raised so many times that I've added it to the trouble shooting guide along with the other common questions re 2.9.1.

Look for "My copter slowly rises or descends when I enable Alt-Hold or Loiter"


I am seeing the dreaded tilt after upgrading 3 of 3 of my tricopters. I can't seem to get rid of it, any secret sauce here. I tried the accel calibration per video and all suggestions in the thread, but I am still showing a left tilt on take off, I can trim it out and fly but it is annoying as all heck! My 3 quads and 1 Octo and 1 Hexa are fine, so I am wondering is this possibly something in the tricopter firmware for 2.9.1 ?

In the troubleshooting guide it refers to THR_MID is this the same as Trim_Throttle (I cant see THR_MID)

ok. 2.9.1 feature - time to upgrade.

I am running Simonk firmware - whats the bug fix actually fix?


     The simonk related change was to allow the simonk escs to be calibrated like nearly all ESCs.  the procedure for doing the calibration is on this wiki page (look for "Automatic ESC calibration (all at once)").  It wasn't a bug in the arducopter code though, SimonK firmware apparently uses the update rate of the incoming signals for something that it should have used it's internal avr timer for so this is a work-around for it's slightly bad behaviour.

Cool - thanks for that. I did have to calibrate my simonK off the apm. Now I know why.


     They're different.  THR_MID allows you to adjust the manual throttle stick's mid point.  So if your copter currently hovers at about 40% throttle stick, you can set the THR_MID to 400 and then you will find that it hovers when you throttle is in the middle.

     The TRIM_THROTTLE is the autopilot's guess as to what throttle is required to maintain a stable hover.  It's updated automatically so you shouldn't adjust it yourself.

     We may rename TRIM_THROTTLE parameter in a future release to make it a little less confusing.

     Note that since 2.8.1 I've gone through an updated the descriptions for all parameters.  you can see them in the Adv Parameter list in the mission planner.

Thanks Randy,

Your time is appreciated to explain this :)

I have modified the THR_MID to 40% but still have some drift. I have moved the Accel P rates up and down but still happening. 

I have also changed the dampening of the APM board with foam.

Attached logs if anyone cares to take a look



     So like most of the others having trouble with the new alt hold your issue is vibration.  You can try reducing the INS_MPU6K_FILTER to 20 or maybe even 10 and/or you should revisit the vibration dampening.

     Maybe it's not causing you any troubles at the moment but your compass offsets are a little bit bit on the x-axis.  Probably nothing to worry about but just fyi.

     On the other hand, your roll and pitch axis looked to be tuned to near perfection!

Again. thanks so much for spending the time to do this  :) :)

This gives me something to work on now I have a benchmark.


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