Setting up "Throttle Mid Position" for Good Altitude Hold and Loiter Performance - Arducopter

Based on my experience of nearly one year involving in the development of hexacopter platform powered by Arducopter firmware, I have discovered that if the hexacopter is able to perform well in altitude hold mode followed by a good loiter performance, it wouldn't have much problems to be flown autonomously. If you have problem on that, probably one of the tricks to solve it would be try setting up the "Throttle Mid Position".

Initially, I was using six 2212-13 1000kV brushless motors with 10x4.5 props which gave me a decent thrust of 0.6kg each. During its first flight itself, the platform was able to hover nicely at 50% throttle. I only had to tune the altitude hold P value for better performance. After several crashes during following flight tests, eventually the motor shafts were severely bent. 

Then, I switched to Tiger Motor MT 2216-11 900kv brushless motors. That time, my air frame weight was a bit higher (nearly 2.6kg) with additional payload attachments to the air frame such as video transmitter, payload drop mechanism, landing gear, camera and some optional components. During its first flight it hovered at 70% throttle. When I switched to altitude hold mode instantly, it would just shoot up and when I position throttle at middle, it descends.

I tried re-calibrating accelerometer, compass and radio. No improvements. Puzzled by this behaviour, I went through the Arducopter manual again to check if I missed anything. Finally I found  the guide on 'Throttle Mid Position' setting and found out that hexacopter was 'underpowered'.  

Basically for setting up this you have to:

1) Get your quad/hexa into a stable hover in Stabilize mode for around 30s.
2) Disconnect Li-Po and connect the APM/Crius board to the Mission Planner
3) Go to "Terminal" tab and download the latest dataflash log file. (Refer to the next image which was obtained from

4) Next, click on "Log Browse" button and load the recent log file that you've downloaded/
5) Now, search for "CTUN" messages in the data log > Search for "ThrOut" column and click "Graph This Data Left". 
6) You can find out an approximation of throttle out value from the graph. In the following image which is taken from Arducopter guide, the value is roughly around 550

7) Knowing that, now go to Config/Tuning tab > Standard Parameters > adjust the value for Throttle Mid Position value approximated above.
8) Click on "Write Parameters" button. Now, try flying your platform and check how it performs!

Personally, I adjusted the value to 700(which is actually the maximum allowed) for my hexacopter platform. After the mid throttle position has been adjusted, I was able to notice better altitude hold and loiter performance. Basically you need to take off, get into stabilize mode at your normal hover stick position (myself at 70%), switch to Altitude Hold mode and quickly position the throttle at 50% position. It will start holding its altitude nicely. Try moving around. If the copter descends while it moves, try increasing the Alt_Hold_P value till it is really able to hold at the required altitude. 

Under Altitude Hold mode, if you raise or reduce the throttle stick position within the dead zone (40% ~ 60%), there wont be much effect on the hexacopter. But, if you move beyond 60%, it will raise up slowly and if moved below 40%, it'll descend slowly. 

And becareful when you switch back to stabilize mode. Make sure you raise/reduce the throttle stick position to normal again accordingly. If your platform is underpowered, it'll descend at 50% stick position in Stabilize mode and if overpowered, will ascend at 50% position.

The following graph shows relative altitude variation vs time for different flight modes during the flight test.

Throttle Out  value during altitude hold and loiter mode now is  

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