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Nasty Crash with CS8 .. diagnosis?

Discussion in 'Cinestar 8' started by Tyler Olson, Oct 9, 2013.

  1. Shaun Stanton

    Shaun Stanton Active Member

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    Your Even motors are all running lower currents than the Odds, which means that there is a motor or motors twisted, causing an adverse aerodynamic yaw. I do not think that caused the crash though. Your BL temps are well within range. What may have happened is if your motor wires got exposed that could have caused disruptions in the motor currents preventing them from working correctly. The MK mixer is designed to compensate for a motor out but there are limitations. Such as weight on the system. Your Motor Currents are unusually high for a 5S setup so you must have been flying a sizable payload. It is possible that the motors experienced thermal issues even the BLs don't that could have caused several motors to go out of ideal operating range. The FC can handle one motor anamolly but not several. That would explain the abrupt yaw. It is designed to yaw as a last resort to keep the copter at least level. This is something that the 6 motor guys rely on in order to mitigate a total crash do to a motor out situation.

    My best assumption is that the motor wire shearing caused a arcing incident that threw the system off.

    Looking at the video and the information these are what I believe could be the chain of events.

    1. You loose a motor
    2. The others ramp up to compensate
    3. A twist in the motor ount caused the counter motors to work harder hence the screaming and the initialyaw
    4. The attempt to yaw back aggravated the system even more since it is already fighting to keep level and straight.
    5. Because of the yaw its only available maneuver is to bank the copter is trying to stay airborne as best as it can
    6. Probably motors and or BL's started to overheat loosing power in the entire system
    7 The chain reaction has already commenced until ground impact.

    That would be most logical assessment, without the remaining data it will be impossible to tell for sure.

    Shaun
     
  2. Dave King

    Dave King Well-Known Member

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    Tyler

    I have had similar experiences with the GPS around heavily dense trees. This year I have filmed golf courses and a lot of properties. It's happened to me about 6-7 times this year all in locations of lots of trees surrounding it from all direcitons. When I look at the GPX files they show everything normal and no abnormal solar activity. I've tried different GPS and Nav boards, different ribbon cables, different molex connectors, and tried changing the routing of the cables. Nothing I changed has prevented it. For whatever reason (at least for my copter) the GPS has the capability to flip out if you engage it around very tall and dense trees where it doesn't have a GPX signal or a very weak signal. I have it set so that it doesn't take off unless it has a GPX fix, but that doesn't help when the signal drops after initial liftoff.

    Best practice I have learned from hundreds of hours of flying this summer (I have over 1000 GPX files) is that I use PH only in clear areas and I never engage PH unless I am far enough away from everything where that if it flips out I can recover it. BEST PRACTICE, IF YOU SEE ANY FLIGHT ABNORMALITIES, IMMEDIATELY DISENGAGE PH. After I adopted this practice, it saved my bacon 2-3 times. I fly a lot, and 99% of the time I have no problems with my GPS, and my copter always flies great, its just that on rare occasion I will get a GPS glitch around heavily densed trees.
     
  3. Dave King

    Dave King Well-Known Member

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    IF the motor was off angle you would see a yawing condition.
     
  4. Dave King

    Dave King Well-Known Member

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    Well after I wrote my reply I see that from your video you were up higher in altitude where the problems I have seen are not the case here. I would have chalk it up to solor activity but did you deactivate GPS when things started going weird? That's rule number one for me as I have never experienced any flight abnormalities in manual mode. Even though this has nothing to do with your crash I strongly recommend you disable carefree. If you thilnk that you can get your bearings back by switching on carefree you are sure to crash again. It's extremely difficult for you as a pilot to maintain the same orientation and direciton for an entire flight where you are not turning direction. Carefree mode=highly likelyhood of crashing.
     
  5. Ozkan Erden

    Ozkan Erden Distributor

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    Are you %100 percent sure that you didn't loose the orientation?

    Can you also check the GPS voltage?
     
  6. Brad Meier

    Brad Meier Active Member
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    From the video Id say everything was ok and somewhat normal (although a little out of control) until 28-29 second when there was a drastic change. From the copters sound and action it seems that was the same point at which you turned GPS hold on. As for why it didn't level but turned towards the ground instead? Still thinking..
     
  7. Shaun Stanton

    Shaun Stanton Active Member

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    Without data to back it up, looks like the Position Hold is on at the start of the video. The Aircraft is in a 10 degree bank to the right for the GPS to compensate for the wind. At this point stuff looks normal. 10 degrees of bank is about is about 15 mph of airspeed. It does not seem to go out of control until the right yaw is induced. The right bank seems to be more exacerbated when the a/c continues to yaw right. Once the aircraft's heading is about 180 degrees away from the initial heading just prior to impact, the aircraft pitches hard over beyond 40 degrees, from the looks of it at impact the front boom is 60 degrees down.

    During the video you can see as the plane yaws to the left it makes a few attempts to counter the bank. I don't think GPS error caused this to go hard over that aggressively, unless the GPS angle limit was changed to allow for that pitch and roll to go to 40 degrees or more. The default value is set to 140 which equates to a GPS maximum commanded pitch/roll or the resultant combination of 28 degrees.

    There are a couple of reasons why the the copter will do uncommanded attitude changes one is if an over current situation occurs in the BL's or an Over temp. The Copter will dip in the direction of the over exerted motor to get rid of the strain on that BL. Uncommanded yaw by a twisted motor is usually compensated for well by the system. You might see a "nose hunting" effect or on initial power changes but it usually corrects itself fairly unless max throttle is applied.

    However, if there are multiple issues the system is trying to compensate for then the system may not be able to physically handle them. Because the uncommanded yaw occurred prior to GPS engagement, suggests that something else was occurring in the system.

    I don't think it was the GPS
     
  8. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Which direction was the wind coming from? Was it blowing over the hills towards the aircraft?

    If the surface wind was 10-15 mph, the upper wind could have been considerably stronger.

    Gary, as a former sailplane pilot, probably knows where I'm going with this -- but if you had, say a 15-25 mph wind blowing over those hills, you might be getting some "curl-over" and/or "rotor," albeit in mild form. That is wind that is roiling over the hills and forming eddies like ocean waves breaking. One minute the wind is from the hills, next minute it's blowing *towards* the hills....

    This is pure speculation, but it might account for some of the sudden changes in motor noise as the FC/NC boards struggle. I wonder whether there is a "rate limiter" in the firmware that prevents over-rotating the aircraft into a "graveyard corner" of the flight envelope where it is so severely banked that the vertical component of lift falls below the AUW of the aircraft, but it's still struggling to maintain position?

    Given that the NC and FC boards survived, you can connect them up just by themselves, one at a time, using the jumper on the MK USB board and see if they are still functioning, or whether they have failed in some way. Might be worth a test just to see if the ACC etc. are all working normally?

    Andy.
     
  9. Shaun Stanton

    Shaun Stanton Active Member

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    Andy

    I believe there is. It sounds like you can put in a value for the max angle that the copter should ever go to to compensate for wind. If the number is too small it will drift downwind if the wind is strong enough. I found with altitude hold and position hold on the aircraft only needs about 10 degrees of pitch to maintain level in place with about 20 mile winds.

    This is from the MK manual site.

    • GPS Angle Limit
      • Limits the maximum control influence of the GPS. A value of 100 is equal to an oblique position of 20°.
        Is the value to small it could be possible that the MK can't work against the wind and drifts away.
        Is the value too high the approach to the target will be too fast.

        [​IMG]

        In this example the number is 140 which in theory means that the Aircraft will only bank 28 degrees to compensate for wind. He would have to put in a value of 225 or greater to get a angle that the copter achieved prior to impact.

        Shaun
     
  10. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Good find! I knew I should have done an Advanced Guide to MK Tool then I would know this stuff! :)

    Andy
     
  11. Tyler Olson

    Tyler Olson Member

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    Thanks for all the thoughts guys.

    Here is what we know for sure - just for clarity and arguments sake.
    At the start of the video all 'helpers' are off (PH, AH, CF). At 29 seconds into the video I switched on the GPS (possibly went to Come Home) and it took off at the strong angle.

    @Dave - I still think CareFree is a reasonable panic button (feel free to argue more otherwise) but when the copter is, say, 200 or 300meters away it is terribly hard to tell which way it is pointing. At that point I really don't want to drive another 50meters further away. It is very hard to tell when I pull the nick if I am indeed coming towards me or if the copter is backwards and going away from me. If I put on the CareFree at this point I can be guaranteed that when I pull back on the nick it will come back to me.. even if it rotates a little in flight. When it gets close enough to me to get proper orientation I turn off care free. I'm not proposing that one should use care free all the time and it is nearly impossible to shoot video with care free on, but for orientation in difficult situations it has saved by but a couple times.
     
  12. Tyler Olson

    Tyler Olson Member

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    .. adding to what we 'know for sure'
    The copter has been flying well for a long time so it isn't something simply wrong with the build.

    I was shooting with a 2 axis gimbal and carying a 5Dmark II and 24mm IS lens
    I have had a motor go out before (actually just a wire came unplugged) and the copter didn't have a problem flying and landing with the 5d mark II.
    I am using 750kv motors
     
  13. Dave King

    Dave King Well-Known Member

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    Hi Tyler

    Each pilot has their own preferences of how they like to fly. To me its very easy for a pilot to change standing orientation which can lead to dangerous situations with carefree. I think this is why 99 out of 100 pilots won't use it. If I lose orientation I prefer to use my OSD to regain my orientation or I will use my video feed off boom to regain orientation. If I lose my video feed I would come home as a last resort. Carefree is too dangerous IMO, and its much better to rely on solid aids to regain orientation. Another way to help orientation is to have really good orientation lights and I try not to fly more 150-200 meters away from the copter. With all the LED's I have on my copter they are still visible to me at 175 meters in daylight.
     
  14. Shaun Stanton

    Shaun Stanton Active Member

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    When we dont have data present we have to figure what we do know. I looked at the start of the video and the last recorded position of your copter. Your last last recorded position was heading 015 flying a 250 course at 190 feet in a descent of 9.6 ft/s.

    It appears that your system stopped recording data coincidentally right when the uncommanded movements start. So that suggest something more catastrophic occurred in the avionics and control system. I know there is a buffer, but I don't believe its 30 seconds. It is almost as if your navigation system went offline or partially and possibly sending garbage signals to your FC. Its hard to tell. But it is quite possible that you no longer had control of your aircraft.

    I wont speculate further, but to suggest plugging in your FC and NAV boards on the bench and see if there are any errors. You might want to plug in everything including the BL's to your Lipo on the bench and connecting copter tools to see if the NAV system comes online. This is not a 100% test. Damage in the crash could have been significant to permanently render them useless but it may be a clue to what happened.

    Shaun
     
  15. Shaun Stanton

    Shaun Stanton Active Member

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    Tyler,

    Quick question, did you properly configure your FC to handle 5S batteries? They are designed out of the box for 4S. The BLS can handle 5S with no mods. The FC needs a different Voltage Regulator and higher power capacitors.

    Shaun
     
  16. Tyler Olson

    Tyler Olson Member

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    Yes, the copter was properly configured for 5S batteries. I purchased the copter used from a another pilot who has had a lot of experience building and flying. I am using an Aerodrive 8 power board.
     
  17. Shaun Stanton

    Shaun Stanton Active Member

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    Fair enough I figured I would ask, since that is a non standard setup from out of the box Mikrokopter setup.

    Ok well here is what we can eliminate

    1. Can solar activity cause a bad GPS fix causing a fly away? Yes
    2. Can it cause a flyaway to an assumed position to home to a non flying attitude, causing a crash? Not unless the GPS parameters were wrong, this wold have shown itself earlier.
    3. Can it cause an uncommanded control movement when the GPS modes are not activated? Not likely.
    4. When return home is selected will it rotate the nose? Nope, not supposed to.
    5. Will a twisted motor cause a 180 degree uncommanded yaw? Not unless there is something else aggravating the physical forces for flight.
    6. If the copter flew fine once on a failed motor, can it fly differently the next time? If a different motor fails that was critical in countering yaw induced by another motor that has a twist in it. This may prevent the system from adequately countering the moment produced by aerodynamic thrust.

    What would cause an uncommanded yaw followed by an aggressive flight attitude change?
    1. Flight Control Error
    a. Bad Gyro
    b. Bad Accelerometer
    2. Nav System Error
    a. Magnetometer Error
    It would have required both systems to fail in order for the magnetometer and gyro/acc to fail, Not likely.
    3. RC Controller Issue, Possible but should be easy to rule out.
    4. Data error from one of the avionics systems? Coincidentally, the NAV board stopped recording last 30 seconds of the flight. Could it be just coincidence? Worth investigating further.

    So that is the breakdown of possible events that can be considered and those that most likely can be ruled out.
     
  18. Brad Meier

    Brad Meier Active Member
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    The recording stopped 30 sec prior to the crash but the crash didnt occur until GPS hold was turned on in the last few seconds of the flight. If the issue began simultaneous with GPS hold on then I would conclude something wrong in the nav board or connections to it. Flys fine up til that the point of GPS hold on, but stopped recording well before that.
     
  19. Shaun Stanton

    Shaun Stanton Active Member

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    It starts uncommanded yaw and roll at arround 14 to 15 seconds in the video. I would agree with something being wrong with the NAV board. The chain of events started about 15 seconds prior to GPS engagement, according to Tyler. At around 14 seconds the aircraft is yawing to the left which is not being commanded by the pilot. There is a roll to left that eventually turns to a pitch down once the front boom has yawed 180 degrees. The video is showing uncommanded yaw coupled with a pitch and roll. These aggressive attitude changes well before the last 2 seconds. The reason he stated why he turned on PH was because of the uncommanded movements. Those events can't be ignored. If the chain reaction has already started, turning on GPS is probably not solely causal to the mishap. Obviously the last few seconds it got more aggravated, but its hard to say with any certainty if the GPS was the contributing factor.

    The way I look at it if I stall an airplane 200 feet above the ground, then turn on the autopilot prior to ground impact. I would not be able to blame the autopilot for crashing the plane when in an unrecoverable attitude that was already induced.
     
  20. Brad Meier

    Brad Meier Active Member
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    I just reread the thread to look for: "The reason he stated why he turned on PH was because of the uncommanded movements. "

    All I found was: "when it was getting close it drifted to the side (there was a little wind.. max 20km/hr) and rotated a little and I lost my bearings as to wear front was. I decided to switch gps hold on again so I could make sure it would stay in one place and I could rotate back to how I wanted it, then would turn gps off and land. "

    Maybe Tyler can help but it sounded to me like it flew fine (although windy and somewhat erratic), he just began to get disoriented and switched on GPS hold. But either way Nav became disconnected or out of the loop early on and thus unreliable or possibly unavailable for the duration of the flight. It seems the copter did react in some way to the turning on of position hold or maybe that was altitude hold coming on and no position hold due to the nav board out of commission. If he had alt hold on and thought he had gps but didn't, maybe whatever stick inputs he did have in caused an angle of flight that the altitude hold couldnt correct for thus a high power impact.

    Tyler, did you also switch altitude hold on at the same time as GPS hold?

    Im no expert.. basically just streaming thoughts
     

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