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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Randy -

You are a gem.  Thanks for all your help and timely replies.  APM is a neat place to be in these exciting times...

I don't want to go OT in this thread, 5.5V is fine, but seems a tad high for a 5V BEC which should peak at 5.25V, replacing the MUX chip should fix it.

Another good damper - neoprene. Buy a chain guard protector for £6, and cut/layer to your hearts content. Waterproof too, even if your electrics aren't! :-)

well it rated as a 5.5v bec  not a 5v bec i figured that the absolute max is 6v so 5.5 should be ok.

I assumed you meant roughly 5.5V, in this case, NO!! DO NOT CONNECT THAT BEC to the input headers of APM 2.5 just asking for trouble, that is what has caused your MUX to fry IMHO, you can use that 5.5 BEC on the ESC headers with JP1 installed as the Diode and Fuse on APM 2.5 ESC Input will drop that 5.5V by 0.7 V down to 4.8V which should be just right to power APM 2.5. Remove the RED wire from ESC BEC.

oh well now there you have it .....guys don't believe the absolute max values ....

i will replace the mux as soon as mr rs components courier rocks up with them,

if i wanted to connect using uart 2 would i need to set somthing in the fw first or will the solder jumpers do the job,

maybe i can atleast connect via telemetry.....

Hi Rick,

I have some code that can work out what all the correct coefficients for roll, pitch, yaw, and throttle for any combination of motor positions relative to the CG. It is Matlab and I am happy to enter the positions and send you the results if you give me them or send you the code if you have access to Matlab.

Excellent post. Helps explain basics about GPS. Thanks for sharing.

So how many satellites are we all getting?

For me its 8 or 9 for 95% of the time, given a few minutes, with an average hdop of about 1.5. In one guys post I think he had 12 while hovering in the basement!

Are there fewer visible, on average, at higher latitudes?


Hi guys,

I need some help please.
My Quad is flying well, except for one thing. The trottle is way too jumpy. one click up and it rises, lower by one click and it falls. This makes for a very uneven flight.

What parameters do I need to tune?




You should never be trying to fly any multi rotor or helicopter with an airplane radio with a ratchet throttle. You want a helicopter radio that has a smooth throttle.

Most transmitters can be converted to a helicopter type throttle. The guys at the RCG radio forum had help with your particular brand of transmitter.


Where is your throttle? It your stick is about 50-60% during hover then it is just right. If it’s below 50% slightly smaller or lower pitch props will make it easier to fly.





The problem is not the 5.5V, that is fine. The issue is that when you connect the USB is supplies 5V (drops to 4.8v after the fuse). Two Voltage sources in parallel must be equal, or very close. Otherwise the higher voltage will try and pull-up the lower voltage. With the 5.5V BEC that will cause considerable current flow from the BEC to the USB. With a 5A BEC it can supply quie high current. That current is heating up the mux. If the BEC is connected to the outputs with JP1 in place. The voltage is dropped by a protective diode and fuse that limits the current in that case to 500mA.

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