To setup to octorotor, follow the instruction for Adding a New Platform in the Developer Tutorials. You should not have the propellors on yet for safety. Specificly, these are the steps I followed:

1. In Eclipse, open Firmware–>ROMFS–>px4fmu_common–>init.d–>rcS and add the following code:

    	if param compare SYS_AUTOSTART 108
   	then
   		set MKBLCTRL_MODE no
   		set FRAME_GEOMETRY x
   		set MIXER /etc/mixers/FMU_octo_x.mix
   		sh /etc/init.d/108_io_octo_x8
   		set MODE custom
   	fi

   somewhere in with the other sysconfigs

2. in Firmware–>ROMFS–>px4fmu_common–>init.d create a new source file called 108_io_octo_x8 and add the following code:

   #!nsh
   # X8 octorotor
   echo "[init] PX4FMU v1, v2 with IO on Octo"
   
   #
   # Load default params for this platform
   #
   if param compare SYS_AUTOCONFIG 1
   then
   	# Set all params here, then disable autoconfig
   	param set SYS_AUTOCONFIG 0
   
   	param set MC_ATTRATE_D 0.0001
   	param set MC_ATTRATE_I 0.0
   	param set MC_ATTRATE_P 0.1
   	param set MC_ATT_D 0.0
   	param set MC_ATT_I 0.0
   	param set MC_ATT_P 5.0
   	param set MC_YAWPOS_D 0.0
   	param set MC_YAWPOS_I 0.0
   	param set MC_YAWPOS_P 3.0
   	param set MC_YAWRATE_D 0.005
   	param set MC_YAWRATE_I 0.2
   	param set MC_YAWRATE_P 0.3
   	param set NAV_TAKEOFF_ALT 3.0
   	param set MPC_TILT_MAX 0.5
   	param set MPC_THR_MAX 0.7
   	param set MPC_THR_MIN 0.3
   	param set MPC_XY_D 0
   	param set MPC_XY_P 0.5
   	param set MPC_XY_VEL_D 0
   	param set MPC_XY_VEL_I 0
   	param set MPC_XY_VEL_MAX 3
   	param set MPC_XY_VEL_P 0.2
   	param set MPC_Z_D 0
   	param set MPC_Z_P 1
   	param set MPC_Z_VEL_D 0
   	param set MPC_Z_VEL_I 0.1
   	param set MPC_Z_VEL_MAX 2
   	param set MPC_Z_VEL_P 0.20
   	
   	param set BAT_N_CELLS 6
   	param set BAT_V_SCALE_IO 17015
   	
   	param save
   fi
    
   #
   # Force some key parameters to sane values
   # MAV_TYPE list: https://pixhawk.ethz.ch/mavlink/
   # 14 = octorotor
   #
   param set MAV_TYPE 14
   
   set EXIT_ON_END no
   
   #
   # Start and configure PX4IO or FMU interface
   #
   if px4io detect
   then
   	# Start MAVLink (depends on orb)
   	usleep 5000
   	mavlink start
   	commander start	
   	sh /etc/init.d/rc.io
   
   else
   	# Start MAVLink (on UART1 / ttyS0)
   	mavlink start -d /dev/ttyS0
   	usleep 5000
   	fmu mode_pwm
   	param set BAT_V_SCALING 0.004593
   	set EXIT_ON_END yes
   fi
   
   # Set PWM values for typical ESCs
   pwm disarmed -c 12345678 -p 900
   pwm min -c 12345678 -p 1200
   pwm max -c 12345678 -p 1900
   
   #
   # Load mixer
   #
   mixer load /dev/pwm_output $MIXER
   
   #
   # Set PWM output frequency to 400 Hz
   #
   pwm rate -r 400 -c 12345678
   
   #
   # Start common for all multirotors apps
   #
   sh /etc/init.d/rc.multirotor
   
   if [ $EXIT_ON_END == yes ]
   then
   	exit
   fi

3. Double-click the upload px4fmu-v1_default and connect the fmu to the USB to program when prompted
4. Start qgroundcontrol, set SYS_AUTOSTART to 108 and SYS_AUTOCONFIG to 1, write the values and restart the FMU

At this point you should be able to perform sensor and radio calibration and arm the craft (long-press the safety, then both stick pulled down and in). Check that all motors are spinning in the correct direction (should not have props on yet) and you hear them respond to both radio stick input and attitude changes.

The plotting feature in the Pro tab is very helpful. Scroll down to the eight servos and select them to display. Slow down the update to ~1 minute. Then push the stick forward and confirm that servos 2&4 increase while 1&5 decrease. 6&8 should increase to a lesser extent and similarly 3&7 should decrease to a lesser extent. Rotating the right stick in a circle to watch all the motors rise and fall in sequence like below.

Also, when right-yaw is applied, you should observe motors 3,4,5,&6 should increase while 1,2,7&8 decrease. Finally test the sign of the servo feedback. With all sticks centered, tilt the craft forward. 1&5 should increase. Tilting left should result in 6&7 increasing in speed. Once all the motors are confirmed to be responding correctly it is time to attach the props and begin PID tuning.

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