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Rear arm shell cracks and active braking

Discussion in 'Pro/Adv Discussion' started by UnknownCaller, Sep 17, 2015.

  1. UnknownCaller

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    I notice alot of people are having cracks on the rear arms near the motor mounts. I have the same problem but I have never crashed and I have never had a hard landing. I was thinking maybe active braking could be the culprit. It's just an idea just wondering what everybody thought.
     
  2. bbfpv

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    I believe it's been determined that there was a bad production run of certain shells and/or the assembly of certain birds, which has been addressed in more recent runs. DJI is replacing cracked shells at no cost (but of course you have to send the bird in).
     
  3. N017RW

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    The consensus seems to be that the arms aren't up to the stress by the motors.
    Lot's of 'sub'-theories about heat, prop balance, etc. but stress seems to be the root cause. Braking must certainly increase this stress.
     
  4. UnknownCaller

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    Yeah I got mine in early May. I guess that's one of the problems you have buying a new product.
     
  5. atki

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    I believe it's exactly what you think it is OP combined with a bad batch of p3 bodies.
     
  6. toorboy

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    iv had my phantom 3 pro for 2 weeks
    and had about 10 flights & checked it & have not seen any cracks yet. is active braking related more to flying in gps than it would in atti?
     
  7. atki

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    Yes as the p3 uses gps and active braking to try to hold an immediate position when you release the stick controls. Best way is to dial back the active braking in the advanced settings. Just know the p3 will drift a little further before coming to standstill in p-gps mode due to the more relaxed braking setting. It's not by much and well worth the adjustment in my opinion.
     
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  8. AL T

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    If you have time, can you tell me the correct way to get and lower the active braking feature? Thx
     
  9. D-Dog

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    I am no expert, however, I believe it is the setting in MC Settings ---> Gain & Expo Tuning --> Brake

    I have mine set at 80 now and will see how I like it - 100 was the default I believe
     
  10. emederos

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    I have this same problem and have concluded it also may be overtightened screws as well as braking effect, unbalanced props may have an affect to a degree as well i backed mine out a bit they were VERY TIGHT FROM FACTORY
     
  11. phantom13flyer

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    I lowered my active braking to 70 since the first week. I've got about 1.7 million ft on her and no cracks. I'm not saying that's how you prevent cracks but I think it's one of the factors for sure.
     
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  12. Rocket_Aus

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    Bad batch of shells and over torque settings when putting the bird together where the problem.Tho even with the new shells you can still suffer from the over torqued screws which can also lead to cracking. No use blaming the active braking sure you can lower the braking down but it still wont prevent either of the two aforementioned issues.
     
  13. AlexSP

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    Just like everyone else, I have my theories. For once, I believe that temperature is a factor but not the heat of the motors - quite the opposite in fact, as low temps do have a worse effect on polymers in general (of course considering both to a certain point). Plastic becomes more "glassy", more brittle and that could lead to the start of fractures in certain structures and the P3 shell is a monocoque structure.

    Second, I am not sure the active braking has any more bad effect than any other sudden change in torque, at lest not in a significant degree. The motors act in controlled and concerted manner due to the individual ESC and that does mean they change torque and there are severe twisting going on on the AC during certain (and different) maneuvers. But not to the point of causing enough stress to crack, it should withstand that and much more as it does on seemingly most P3s. Besides, they motors are projected and built on a quad to adjust and compensate to drive the AC in ways that no single motor gets overloaded or overburdened in comparison to others for too long. And if that occurs then it might be caused by some imbalance in the motor/prop. Yes, prop balancing is that important, if you come from RC you know what I mean.

    In aviation it's usually a sum of two or more commonly more causes that leads to failure, like cracks. Actually in engineering in general, no single factor can cause or explain cracks in any dynamic structure. It might be the case here.

    Considering most P3s (either from newbies or experienced, bold or conservative pilots) perform similar moves, but in different conditions - and not all of them crack - IMHO these problems are caused by a wrong or bad mix in the plastic used to manufacture the shells in conjunction with low temperatures. Of course from different batches, I'd guess not even DJI or its supplier may be able to tell exactly because they're seemingly overwhelmed with the success of P3s and even though DJI stuff in general seems to the manufactured to a high standard, QC issues can be a factor when there are big outputs involved.

    Now, I am aware that some crack even in the summer or warm climates (like where I live), and some don't. The apparent randomness of the cracks leads me to believe that the root of the problem is indeed in the plastic. Not the design (otherwise a vast majority would crack and doesn't seem to be the case), not the brake (some set at 100 and still don't crack, while others set at 60 or even lower do crack), and certainly not the motors being new or old, as both crack - though no one knows at what rate on each.
     
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  14. dirtybum

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    Active breaking is definitely hard on the arms and motorsmounts, and I agree that over tightening on poor plastic is also part of the problem but any of you P2 fliers know the stress cracks on the rear arms especially the rear driver side was a common issue
     
  15. alokbhargava

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    Wow such a detailed analysis.

    The batch problem theory is a baseless theory as we have seen cracks in different batches with old and new motors.

    No doubt there would be difference in material from batch to batch but a good design should take care of such tolerances.

    Cause of the cracks is definitely continuos pulsed stresses on plastic parts.

    In the current design, motors sit on mounts and then screwed to shell. This design creates radial as well torsional stresses on mounts which over period of time create shells to crack. A better design would have been with flat motor base with flat shell without raised mounts. People have used external flat plates to strengthen the shells. Those shells exactly create artificial flat mounting of motors and reducing torsional stresses and thus improving the overall structural design. People who are using the external quick release prop guards plated will have the same benefit.

    Why rear shells crack more? Cause most operations are forward motions and in such cases rear motors work little faster than front motors and thus take more load.

    In short, I see lot of room to improve the motor mounting design. So my recommendation to users would be to use either shell strengthening plates or quick removal prop guard mounting plates. Second seems to be better for those who want to have prop guards too. Will it violate warrantee? No idea. But we should take it to DJI to allow such use of external plates officially as this would be a win win situation for all.

    These additional plates will also take care of over tightening to some extent though I believe DJI use torque adjusted screw drivers.

    Happy flying.

    (DJI owes me for the suggestions to improve the design :) )
     
    #15 alokbhargava, Dec 2, 2015
    Last edited: Dec 2, 2015
    nickyb, ronbo and Reed L like this.
  16. thormanf

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    Can someone give us the correct torque for the screws.
     
  17. Fourprops

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    Just cheap plastic !!! Being used
     
  18. alokbhargava

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    What's cheap plastic? I don't understand material science.
     
  19. AlexSP

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    Again IMO it´s more like "wrong plastic" than anything else. Maybe a mistake in project/engineering, maybe in the mix/injection/curing phase (or all of them)... It´s hard to tell for sure. Call me naive but I for myself doubt they´d deliberately use cheap plastic. It´s just too risky from a business standpoint, and even more so when dealing with advanced aricrafts like the Phantom or drones in general. Yet it´s a fact that this tech is in its infancy (at least for consumers and in such scale), so anything is possible.

    But it´s indeed curious that this cracking problem is somewhat "old" and hasn´t yet been definitely fixed. Really weird. Then again what do we know eh...
     
  20. N017RW

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    One example is a plastic comprised of a large % of 'regrind' or scrap material mixed with virgin.