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Yet another thread on Stress Cracks --PSA on Bondic.

Discussion in 'Pro/Adv Discussion' started by jcknows0, Nov 29, 2015.

  1. jcknows0

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    Let me preface my story with my P3 stats:
    174 Flights since June 29 with almost 2 million feet.

    I developed stress cracks back in late August but I used Bondic and was able to seal the spreading stress cracks on the one arm. I had no further noticeable issues over the succeeding two months and figured the problem had gone away with a later plan to replace the shell during the winter. I haven't flown much the past two months due to family issues, maybe once or twice a week and was just about to buy a new shell instead of sending it to DJI.
    Today I decided to relieve some stress and took a brief flight in a little park in town. I did typical flight checks and noticed nothing unusual. After flying 2000 feet in either direction I started to bring it in with a controlled fast descent, forward throttle when I noticed the rotors making unusual sounds as it approached me. The sound went away as it hovered over my head and I could see nothing unusual by the stress cracked arm or any other. I did a short tornado like ascent to 75 feet around me to run the battery to 50% when the sound returned and I decided to bring it in as a preventive measure. It was then that the drone made a bee line towards some trees before righting itself 5 feet in front of me. I quickly killed the motors with a hand catch and took to looking at the stress cracked arm.
    The center part between the motor mount screws was gone as I have seen from others on this forum and the motor was hanging very loose in the mount. I believe the Bondic saved the motor from flying out but not the unsecured center between the screws and cracks.
    I will upload some pictures later before I start my surgery (new shell is on its way from B&H) sometime in the next few weeks but I thought I'd share my experience with others about the benefits and limitations of using Bondic for shell cracks.
     
  2. bobmyers

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    Did you have the strong arm back up plates and would they have prevented the motor from coming completely loose?
     
  3. jcknows0

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    They didn't make any sense to me so I never procured any.
     
  4. bobmyers

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    Do you think that they would have prevented the motor from coming loose?
     
  5. jcknows0

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    Despite being a philosopher I'm not very good with hypotheticals but as I stated I didn't think they would prevent stress cracks so I didn't procure any before or after developing cracks.
     
  6. Dounin Front

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    I finally bought reinforcement plates after waffling over it for a long time. I thought the system made sense, but I hadn't experienced any cracks in my P3. After seeing crack lines starting to form around the motor screws on one arm, I finally did the install.
    Here's why: The reinforcement plates fit into the recesses where the existing screws go into the motors, thus making the inherently weak motor mount recessed area very strong. Additionally, 2 more screws on each arm are similarly reinforced. I can't see how the plate system could ever allow a motor to come loose in flight and would most likely distribute the stress better to prevent cracking.
    There is an explanation and video on the Strong Arm website that explains it much better than I can. I have only read good things about them.
    I wouldn't bother with the glue when such an easy fix is available. If it's a money issue, the Strong Arm system costs less than Bondic.
    My philosophy in this ongoing problem is to be proactive.
     
  7. CactusJackSlade

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    I am astounded that DJI has not addressed this yet?.... or have they? I've had P2's since they came out and I finally caved with holiday pricing and ordered a P3P... I will be sorely disappointed if my new P3 develops cracks.
     
  8. smodra

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    Hi all,

    This is my first post - and not what I was hoping to post.
    Anyway, I bought my P3P last week and after only two flights, I noticed the dreaded stress crack on one of the motor mounts this morning.
    I have returned the P3P to the retailer this afternoon for repair/replacement, so will be grounded for two weeks :(

    It only had two flights - no crashes - just hovering about 10m above the ground. It has the new motors too.

    It's pretty disappointing for a brand new bird, but glad I discovered the crack before anything worse happened.
    I'll have plenty of time to trawl through the forum now ;)

    Cheers,
    Simon.
     
  9. Dounin Front

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    Yes good thing you sent it in for repair right away, because once there is evidence of any crash, your warranty would be over.
    If your P3 is new, you probably have the redesigned motors, shell and main board. There haven't been as many problems with the redesigned units.
     
  10. AlexSP

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    That's the reason I'm experimenting with cloth reinforcements instead of just Bondic, or any other "glue" agent. I'm an engineer/architect and we do the same with concrete, the steel works to add flexibility and structural integrity to the parts. In other words, to improve characteristics (yeld, etc.) that deal with the forces acting upon it.

    Even relatively flexible agents like epoxy, Plast Aid, Bondic etc. have a breaking point, can become brittle or may unglue at some point. Also, when you add strength to a part of a structure, you have to consider the result of such forces elsewhere as it won't just go away of get dissipated, at least not properly.

    So I've been using carbon fiber cloth and kevlar strips/patches to stop spreading on already-cracked shells and also reinforce new, non-cracked shells. Not only the motor area but also the arms and all the way to the core of the aircraft. Doesn't take much, but I'm sure it helps keeping it all together for longer and certainly if something comes lose during flight.

    Just a quick disclaimer: That's a tip to the DIY'ers out there of course, maybe the best (in more ways) thing to do is to get back to DJI and get it fixed.
     
    alokbhargava and Dounin Front like this.
  11. alokbhargava

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    Every time I read such threads, I feel so sad for the members facing such issues. There are two major unresolved issues (I'm not aware of other major issues):

    1. Stress cracks on shell arms
    2. Unexpected battery low voltages

    We want to hear now directly from DJI about the technical explanations for the above and what measures have been taken to resolve them. We don't want a prolifically answer like only few have faced such issues etc.....

    We, hobbyists, who have been flying RC gliders and planes earlier always repair things and go ahead with the fun. We don't go back to manufacturer and ask to repair damages. But we deserve an explanation from you about the design shortcomings and DJI's plan to solve them.

    From the listed two issues, first one is related to mechanical design and the second is related to software implementation of battery characteristics. This is our understanding but has any analysts been done by DJI on these issues?

    Apple had a Antennagate issue and promptly distributed free cases to circumvent the problem. DJI also can issue strengthening plates costing just 1-2 dollars to who so ever needs them. That would solve the issue temporarily until P4 is released.

    Second issue seems to be related to SW representation of battery characteristics and doesn't need any hardware change. It is very much in the hands of DJI. We hobbyists can only restrict ourselves to start flying with fully charged batteries.

    DJI, we want hear from you here in this forum.
     
    #11 alokbhargava, Nov 30, 2015
    Last edited: Nov 30, 2015
  12. Dounin Front

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    Well said, alokbhargava
    Dji should work on a battery firmware tweak and should absolutely make available either for fee or warranty, certified reinforcement plates to customers.
    Dji--please read and respond to the above recommendations from your loyal customers.
     
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  13. jcknows0

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    Here is the damaged arm. Shell should be here tomorrow but it might be a few weeks before I get a chance to watch the videos and perform the surgery. Bought a hubsan again (my others need motors replaced) to keep me entertained in the interim. I thought about getting a backup p3 (even settle for a standard) but I think I'll wait as I would want a better camera at this point anyway and don't want yo invest extra in P3 platform at this point
     

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    #13 jcknows0, Nov 30, 2015
    Last edited: Nov 30, 2015
  14. Dounin Front

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    That's really bad! From the looks of it, one would guess the propeller was stopped by an oak tree or a brick wall. That or the motor was torqued down when it wasn't properly seated. I know you said that was not the case--that's just what you would expect from that much damage.
    Queue up the video and follow it step by step. They say 3 hours, but you can probably add a few more if it's your first.
    Two more things:
    • If you decide to replace the motors on your Hubsan, don't unsolder the wires from the board. Just cut them and splice to the new motor wires. I ruined the board trying it before someone told me about just splicing. (Same with the P3)
    • Remember if you buy a backup P3, the Standard is different. It's a wifi system. Actually depending on the age of yours, even the same model may have different Motors, shell, controller and main board so there may not be too many interchangeable parts. What about a bind and fly P3 without controller?
    You have a good workshop day ahead of you.
     
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  15. jcknows0

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    So I finally got to finish the surgery. Part 1 was last Thursday and I was little leery about leaving the drone apart with my two little boys running around. After about 3.75 hours taking it apart and reassembling, the drone powered up indoors with no errors and perfect video feed. I will have to recallibrate and reset IMU when I get a free chance but I need to buy some new gimbal dampeners before as they were disintegrating in my hands. Glad I got the experience under my belt but for anyone thinking of completing a similar exercise, the P3 is far more complex than the P2 and requires much more finesse and good torque sets (you will need at least 6 different screw head shapes and sizes from my count).
     
  16. Dounin Front

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    That's really great. I'm glad you were successful. You give me inspiration to start on mine, which runs fine but has a small arm tweak from a crash early on. It might be a good time to add faux CF decals to the shell while it's apart. :)
     
  17. jcknows0

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    If you have any familiarity with P2 shells and innards than it will at once both welcoming and foreign. Interestingly enough the tutorial I followed was also about changing the shell exterior while replacing it. I really only needed it for the removal of the lightbridge antennas and the tricky part of removing the battery terminus while the board was still attached. The one thing I wish I had done that the video demonstrated was put each set of screws in a cup with a label so there is no guess work on reassembly.

    Thanks for the tip on the Hubsan motors. Now that I know to splice instead of soldering the new motors (thanks to incorrect information on the internet) I can do those lickety split and should have 2-3 little drones to go with my P3 monster.
     
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  18. Dounin Front

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    Re: the Hubsan, I'm glad some good will come from my mistake. Avoid soldering on the board (the itty bitty board).