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Too much flex in the arms, exactly.  The airframe  is plenty strong enough otherwise.  I tried it with the following electronics: arduino mega + WMP/NK sensors running aeroquad firmware, then with multiwii firmware, then with baronPilot firmware.  Then I tried it with a teensy++ 2.0 running multiWii, same IMU.  Then tried with a KK2 LCD flight controller from hobbyking.  While I did manage to get the copter in the air a few times, it proved impossible to get the PID control tuned to the point where it was able to keep it stable.

I'm pretty certain that it would have been fine if I had printed it from PLA...