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Check your props!!

Discussion in 'Pro/Adv Discussion' started by BarryM, May 25, 2015.

  1. BarryM

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    In the P3 help section some forum members may have read my posts re fitting a Mobius cam to have some audio feedback....well after playing back the last video from the Mobius I could hear an unusual "thrashing" noise from the props. So I put my P3A up to eye level to check the props and sure enough one prop was feathering while the other 3 were smooth. I am changing that prop and will post again when I've completed the next flight. I also now have a tachometer and will check the motor speeds relative to each other whilst idling. These actions are as a result of Adams unfortunate experience just recently. Edit: feathering is the wrong description, better to have said spinning out of true, one blade higher/lower than the other. Sorry for any confusion.
     
    #1 BarryM, May 25, 2015
    Last edited: May 26, 2015
  2. packerman8

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    Do you need to remove anything to use the tachometer or can you just shoot it at the motor?
     
  3. flyNfrank

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    I have an idea what you mean by feathering, but could you give a little more detail on what you're referring to?
     
  4. Hasan Ahmad

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    [​IMG]


    meaning the props might be facing downwards more than they should
     
  5. BarryM

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    The tachometer comes with adhesive strips, cut off a small piece and stick on a prop or side of the motor and point the tacho at the prop/motor and the tacho gives a reading on its screen. Repeat on the other props/motors to see any variation. Bought mine on ebay, probably available Amazon etc.
     
  6. BarryM

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    You're sort of correct, but I meant looking at the props from a side on view you will see if the blades are not spinning true. I would also advise wearing eye protection whilst doing this, just in case...I probably should have said vibrating out of balance. Mine was caused by contact with a soft but large leaf on a bush. At the time didn't think much of it. Pays to check any contact.
     
  7. F6Rider

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    Feathering is a flattening of the pitch thus moving less air, what you describe is more like a warped blade, not spinning true, or in the same path as the other blade. As these are a composite material with some memory I am not sure how a light and soft impact caused that, although not saying it couldn't, I could see localized heat warping a blade, or one that was subjected to constant pressure like in a case (one reason I take blades off when using a case) If you find out a definitive cause please share.
     
  8. BarryM

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    Just got back from another flight...there's a sweeter sound from the props now, no thrashing sound like before. Waiting for the battery to charge and I'll put the tacho on to check idle revs. It's a real pain waiting for spare batteries to arrive....
     
  9. BarryM

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    Update....the tacho works good, shows opposite pairs of props running the same revs 1770 and 1990, but obviously all four are not the same? Still I cannot complain about my P3A and the way it flies, it responds exactly as it should. Thank you DJI, and long may it continue!!
     
  10. leekiya

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    Can you send a link to the tacho meter you have got please.
     
  11. BarryM

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    Sure can...it's called a digital laser photo tachometer, I bought mine on ebay, item no. 281580527787 from Hong Kong, cost about $16 odd Australian, should be available elsewhere like Amazon I reckon. Tomorrow I'll run the motors faster without lifting off and see if all four motors are the same rpm. HTH.
     
  12. flyNfrank

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    I'm pretty sure with the propulsion system on the P3 you will find there is variations that will display differently between each motor.

    When you adjust your IMU you are tweaking each motor to spin the speed it needs to in order to get the aircraft to be as lever as possible

    You would be better off to mount a spare motor on your test bench that spins the same rpm all the time without a prop on it. Then add your prop and check for whatever weaknesses.
     
  13. Jeff48920

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    Make sure you setup a hold down rig if you intend to high speed rpm test with props on.
     
  14. N017RW

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    It sounds like your experiencing tip flex and observing possible 'tracking' error where one tip is raised above the other.

    RPM is based on the FC trying to level the craft.
    It would not be uncommon for different RPMs at each corner regardless of idle, held-down, or in-flight conditions.
     
  15. BarryM

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    Thanks for the reminder, I'll be careful.
     
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  16. BarryM

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    Agreed, I am just curious that's all, and the good thing is the P3 has always lifted off perfectly, if it tried to tip over there would be an issue with a motor or an ESC. Thanks for your input.
     
  17. CactusJackSlade

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    I do not think you will ever see the same speed on all the motors... the motor controller is always trying to adjust and compensate for something. Sure, theoretically in a perfect hover maybe they will be close, but before takeoff and just idling up I'm not sure you will see the same rpm's before actual flight occurs - just my thinking anyway.
     
  18. Jeff48920

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    Trust me in a static test you will see seriously impacted motors reflected in both rpm and temp differentials regardless of actual flight
     
  19. ctp

    ctp

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    That a good suggestion. I have a Seek Thermal infrared camera that attaches to my phone. I'll try to set it up so I go out for a flight, come back, and check the temperatures of each motor to make sure they are reasonably close. If nothing else I'll have some interesting infrared video to splice into a flight video.
     
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  20. yorlik

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    I forgot I had a non contact tach at work, so checked our p2v+ at idle: 1780rpm on both back two props, 1150rpm on front two props. This is just motors on, sitting on table top.

    Turning off and on a few times we saw pretty consistent 1780rpm on back and while front 2 varied between 700-1300rpm....

    I agree that until more is called for - like flight - the rpms of front and back will be sorta random since no special command is being given yet. A 9.4" dia prop at 1800rpm only produces 1.3oz of thrust, so it really doesn't matter what slow speed they idle at.

    Been too many hands/arms sliced lately for me to rev it up and check. I have even stopped hand catching after seeing the blood freely running! Suffice it to say, calcs shows full static thrust of 33oz happens about 7500rpm on the p2v+....