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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@Andreas: motors too powerful for the total weight of the quad, or propellers wrong for that motors (too large or with a lot of pitch).
When the quad as weight is perfectly balanced must stay in hover to 50% of stick, the optimal rpm for all the motors.
You can remove the "click" in the throttle stick of your tx (you have to open it to do), a linear control is better for rc quad\heli.
However it's normal that it is difficult to manually find the perfect hover point, experience helps.

I get a minimum of 9 all the time, even indoors.  When reviewing my post-flight logs, I see it climb to 12 along the way.  It seems to go up and down during the flight, perhaps due to altitude, although I haven't gone higher than 30m and I'm usually around 7m.  Doesn't seem to vary with cloud cover.  I'm using ubox and I'm in Ohio, USA.

Looks like 8 or 9 is the norm on this animation for 45N.

This is a great site for sat predictions

I agree Jim.. This elevated GPS lowered the hdop, raised sats in the logs..

PS: I also use a 5.5V BEC on the inputs, but I have been disconnecting it when connecting USB because if its not powered the output impedance of the regulator is very low (short) so it just causes the APM to reset.

Also you should be able to connect using an FTDI cable see this link for info

Thanks for your reply's guys.
I dont acctually have the rachet in place, I just wanted to use that to explain my problem.

My Quad has very large props for its size and still I hover at +- 50% throttle. I will change the props from 12 inch down to 10 inch and see what it dose. I also have some 11 inch to try.


Kind regards


Leonard, thanks for the offer, I'd like to get the code.  I'll PM you my email address.

Stop and consider why the TS5A23157 MUX chip gets fried and not the other components and why it doesn't happen to others who only apply a 5V BEC to the input headers of APM 2.5 - the reason is a 5V BEC outputs 5V +/- 5% so the maximum voltage is 5.25V whereas a 5.5V BEC can output more than 5.5V and that is what causes the mux to fry in that case.

If you examine the TS5A23157 datasheet you can see why, red arrows

The advice that it is safe to apply 6V supply via APM 2.5 Input headers is simply WRONG!! absolute MAXIMUM voltage on Input headers is 5.5V.  A 5.5V BEC assuming it is rated at 5.5 +/- 5% can exceed that when it outputs 5.775V

I'm wondering if the props (especially carbon fiber) can cause multi-paths for the GPS signals and thus bad mathematics to happen?

@Joe: Thanks for the info on the datasheet. That would explain why the chip is paticulary sensitive.

That said, I still don't think you will see a failure by just adding a 5.5V on the inputs. (And I haven't with my setup)

The problem is the diffrence in voltage supplied from the 5.5V BEC and the 4.8V from the USB connected at the same time creating a current flow that is too high for the MUX. The serial signal swicthed through the MUX is very low current, so point (2) in the data sheet applies. "2) The input and output voltage ratings may be exceeded if the input and output clamp-current ratings are observed"

The 0.7V difference is going to pull more than the max current (50mA-100mA) through the MUX and that is going to cuase it first to heat up and then fail.

Even if you powered at 6V you still are in operating power for the MUX, unless you connect the USB, since then you have the same problem.

Solution for now: Do not power the board using a BEC that is outputting more than 5V on the inputs and then connect a USB cable.

RTH tuning question.

I have RTH tuned fairly well but I get a slight wobble on the descent at the end. What is the main gain parameter for RTH? Or do just have the basic rate gains slightly too high?


RTH descends very much vertically, which demands a lot from your tuning. I'd probably start by increasing Roll and Pitch Rate_P by 10%. Using RTH and waiting for the descent to see if it wobbles might take a while; if you can manage to get vertical descent wobbles in stab mode, you can tune it a lot faster that way.

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