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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Thank you Marco, I'm sure looking forward to the 2.9.2 release.

Important to get use to leaving the throttle near 50% when flying.

Randy does this also affect flight behavior in those modes with throttle half way or just below half way? I had a lot of bouncing with my DJIF450Crashtest quad (2.9.1b AMP1) in loiter and alt hold and also sudden drop of a few meters on my big quad (2.9.1b APM2.5) I posted the tlog for the crash quad last weekend if you want to look at it. For some reason the APM2.5 didn't record any logs, not sure why.

Don't know if this helps, but I make my best landings by switching from Auto to Alt Hold.  With the throttle at minimum the hex makes a slow controlled descent.  When it is about 1-ft from the ground I switch to Stabilize and since the throttles are at minimum, it drops in the last few inches.

Speaking of landing technique, last Sunday's quad race presented a very challenging landing. I usually land in alt hold, and go to stab only to disarm, no problems. However Sunday the wind was so strong and gusty that alt hold proved to be 'too gentle'. I had 2 minor flipovers because alt hold wasn't decisive enough with the throttle.

Flipovers in alt hold @beginning and ending, @1:55 a barely successful landing/takeoff... sliding down the same hill:

You watch the part in the beginning with my friends flying fast FPV wings, hovering them one moment, 101mph the next... again very windy. Like you said, I think flipping in to stab mode right at touchdown is a smart idea especially in these type of situations where a flip is very likely. In fact I knew this after that first flipover, but my brain goes in a different planet while racing and I simply forgot. :P

In this case, it would be very cool to have the landing mode: alt hold, look at Zacc for a trigger spike, and automatically switch to stab/throttle off... reset to alt mode after throttle down/throttle up for takeoff. Nah... sounds way to complicated LOL! I accept that I just have to fly around this.

On a side note, I detuned my ratePID's ~10% before that day, in attempts to smoothen out the video. That proved to be a bad decision, and the quad flew very wobbly in high wind. Lesson learned... keep a top tune, there is no replacement for smooth thumbs. ;)


I have a suggestion you guys might want to add as an option in a future release. I dont really need it but it has been on my mind and I thought I would mention it, if you havent noticed it already.

I read somewhere, (I think on the esc calibration page in the wiki) was talked about the dji esc's, I like these esc too and have used them with the apm. I noticed in the dji 550 flamewheel manual under spec's for the esc's (30a opto's) the signal speed max is 450hz. In the MP there is a setting, RC_SPEED I think, for 400HZ and for 490HZ but there is no way to set it for 450HZ.

I had run mine at 490HZ for a while and did not seem to have a problem but for safety I set it to 400HZ. I dont know if 50hz or even 90hz makes that big of a difference but I guess I would of liked to be able to set it for 450hz.

Like I said, I am not currently using the dji esc's but I thought I would bring it up for those that are.


DJI ESCs are operated at 400hz by DJI. I switched mine to 400. Did not notice a difference. Left it at 400 just to be within spec.


I have a question, I know has been asked because I remember reading it somewhere. There is a parameter that can be set to zero (I think its zero) that controls if the gps is used for attitude. when turned off it stops the horizon line from jittering while inside.

could someone please let me know which param it is? I didnt make a note when I read it, and I am not exactly sure which one it is.



I'm not 100% sure either, but is this it

HI all,

Not to sure where to ask this or start new thread so I will start here.

DJI frame, HK20a (SK flashed) esc, 980kv, AC 2.9.1. Has been a very successful build and perfect missions and FPV with this setup.

I am not sure how to read the log to look for Vibration? 

I switched from Loiter to Alt hold and she reared back and flipped.I can see at 82% of the log it was instant. I went home put another arm on and it flies smooth as silk which is I want to post and see if anyone can check the log and see if there is anything obvious. 

Thanks if anyone has the time.



To get data on vibrations you need to get the .log file out of the internal eeprom on the apm. you do this on the terminal page. but first you have to enable RAW and make a short flight.

If you look on page 281 of this thread, you will see I asked a question kind of sounds like yours. anyway it tells you what to look for in log file and may help. the problems I had was I did not calibrate my Simonk flashed esc's correctly.  here is a link to the page with my question and the reply's

hope this helps


Thanks John.
Wasn't sure which parameter to click to see vibration. Your scenario sounds similar so I will go over the quad and make sure COG is still good. I take it a well balanced quad in zero wind should track along zero. It was pretty windy when it crashed. I am confident all esc are flashed and calibrated but I will do it again.


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