Hexa YAW Then Crash Please Help. Pixhawk:3.3-rc11

I was testing out my rebuilt hexacopter today. I was just hovering in alt-hold mode when the hex yawed to the right then crashed.

Could someone help me with the log to try and figure out whats wrong?








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  • Moderator

    Hey Tim,

    Unfortunately I can't work with your flash log, but looking at the tlog, it looks as though you took off with a imbalanced/bad battery. Your battery voltage starts out at a 12.5v and rapidly drops to 11.4v at takoff. At the moment of the crash it dropped to 9.86v. I'm assuming it's a 3cell battery which should start out at about 12.6v fully charged (no load). People will debate this issue, but I wouldn't recommend anything lower than 3.4v/cell for failsafe (3.4x3=10.2). However I think there's more at work here. Do you use a balance charger? Do you use it in balance mode? I think you may have one bad cell. I say this because the voltage of your battery was just over 12.5v when you took off and sagged heavily at takoff then it suddenly dropped to 9.86v. It's commonly accepted that 3.3v/cell is the lowest voltage a battery can safely be discharged to before damage can potentially occur. If you're just charging your batteries without using the balance function you can't tell what the individual cell voltage is, only the pack voltage. This can cause overcharging of some cells and undercharging of others, causing damage to the battery. Or it's possible that the cell is just damaged somehow and not capable of delivering current under load.

    You mentioned rebuilding the vehicle, had you crashed before? Is this the same battery, could it have been damaged in the crash?

    There may be more going on here but that's what I see.

    Close up of voltage during the crash

    3702094642?profile=originalWhole battery voltage chart.

    3702094580?profile=originalSorry about your crash,


    Nathaniel ~KD2DEY

    • Thanks for the reply. You are right I have issues with that battery. Its no longer holding charge.

      I dont think it has to do with the battery though. I'm noticing that the hexa will yaw when in loiter mode. There is something really up with the yaw. Ive also noticed that when i give the hexa yaw input it seems to over shoot when i let go of the sticks. The hexa will keep yawing for maybe a second-second and a half.

      2015-09-19 13-45-45.log

      2015-09-19 13-39-37.tlog

      • Moderator


        That battery is toast and dangerous to be flying with. From the time you hit failsafe at 10.2v to the time you "Landed" took about 40 seconds and the battery was at 6.8v ! Get rid of that battery and things will improve dramatically. You need some tuning to be sure. Yaw vs Desired Yaw overshoots, over-corrects, overshoots again and so on. But what are you to do about it until you have a good/safe battery you can rely on. You very well may have an issue that can be corrected, but you're going to have a very hard time tuning it without reliable power. Is this your only battery? Can you borrow a good one and try with a battery that doesn't sag so badly?


        Nathaniel ~KD2DEY

  • do you have the actual .BIN file?

This reply was deleted.