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.


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Hi, Randy!

Thanks for looking into this, it's just a little MP inconvenience. I will flight test "42Hz" vs "0" lpf tomorrow :).


Kraut Rob

hi all

testing cli over 3dradio telemetry all is ok except log download .. it remain with with empty dialog box ... is it my fault?

Marco, thanks for that, I was looking for Alt_Hold_RTL!



Randy, not an objective test, but I've flown with and without, and I think it "feels" better flying with GPS_Use = 1.  It really only matters when doing high speed, banking and turning, dynamic flight.  

As I said before, the problem with the HUD moving around when disarmed is a simple fix.

First of all thanks for that great Software.

It's a shame, I cannot test it right now. We have lots of snow and, what is the bigger problem, lots of wind.

So I use the timne to build a new Trikopter for my APM2.5.

When I read the release notes, I see a note that the RTL will not be enabled when throttle is 0.

Does this mean, that even the throttle failsafe will not work, when throttle is at 950 or something like that?

Marco, you should try it.  It feels better in fast flight.  It's really hard to describe.  Just a little more accurate in turns.  Maybe it's because I was pulling 2-3g turns in the heli.  

Thank you for your efforts developers.

Hex now uploaded with the new 2.9 and GPS 1.9, look forward to testing when this snow settles in the UK.

Keep up the good work :-)

Good work dev team!

SBAS -- is HUGE win guys. Will start updating our stuff here.

Ok Robert, i try to enable it.

Yes Mui, see my parameters file at line #72: "LAND_SPEED,40".

So far seems stable - the only prob I have is under sonar, alt hold works fine the first time it engages, but then as the flight goes on it starts to rise in spurts and keeps going up. Sometimes if I disarm and rearm it settles down but then starts it after a minute or two. I have the log wit RAW, seems like vibration not a prob. The sonar is very reliable under 2.81.

I tried lowering Throttle Accel P and I and Alt hold P down - still about the same. Is some term winding up as it goes on?


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