Need help investigating a crash of my F450 Quad [solved, motor failure]

Hi Friends,

I am new to quadcopter (arducopter 4.1.3 with FMUv3 Firmware) and using F450 frame (Pixhawk v2.4.8 chinese clone). When I tried top mount battery and I placed the flight controller at the edge of the quadcopter lower rack, my quad didn’t crash but it was bit struggling to hover (especially it is shaky nose and up when enabling CIRCLE flight mode). I think Its due to the incorrect placement of the flight controller to the CG and also i didnt add any vibration dampener that time. (Attach the image… Red arms are the front)

So this time, I tried setting up with bottom mount battery (5200mah) and mounted the Pixhawk on the top using proper anti vibration plate and flew for two days… All was well. But on 3rd day, when I was moving the quad towards right for 50 meters, all of a sudden, it felt from the sky.
Refer the below quad placement pic (bottom mount battery) where the crash is observed:

  1. I need your help finding the root cause. I am sharing the dataflash log. Can anybody help me with this?

  2. Any body can please tell me how to find the CG for the F450 quad? Due to the lack of room, I am unable to place the pixhawk (due to large size) exactly at the middle and inside the quad room.

  3. Is it okay to place the flight controller somewhere towards front of the QUAD? (Please refer the below pic) Or Do we need to place exactly at the center to the quad?

  1. Is it mandatory to place the flight controller at the top of the quad or its okay to place at the bottom (inside room) of the F450?

  2. Top mount or bottom mount battery (belly mount) which is recommended for F450 quad to achieve stable flight -Just aerial photography? (FYI, I cant place the battery inside the F450 frame due to the size constraint)

It experienced a thrust loss on Motor 4 (commanded output goes to max). Prop, ESC or motor failure.

You can place the FC on any plate but on the top with the battery on the bottom will have the best isolation from power interference.

Thanks a lot for your help Dave. The crash has caused only arms broken and a dented battery. Rest all intact… I will mount as per your suggestion in a new f450 frame and I will perform ground test and analyse the data flash log again and look for any abnormal behaviour on those motor or esc.