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Motormount to prevent resonance

Posted by Mike de Landgraaf on June 21, 2012 at 2:53pm

So i'm working on a prototype helicopter. The motorhount however seems to resonate when the throttle is between 50% and 70%. Here is the video:

 

 

So i made a extra stability-rod, giving the motor mount no room to twist:

 

 

So it works, but it's not a charming solution. What kind of motormount should i make, preferbly with 4 arms for stability, that wont have this problem (and wont weigh more than 50 grams :)

 

Thanks

 

Mike

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Comments

  • Barrett Ames June 21, 2012 at 11:25pm

    This might help with selecting a proper shape: (http://mathworld.wolfram.com/TorsionalRigidity.html). You may not need a circle, but certainly something better than half of an annulus. 

    From the math you can tell that Greg was spot on with the idea of a tube. 

    Torsional Rigidity
    The angular twist theta of a shaft with given cross section is given by theta=(TL)/(KG) (1) (Roark 1954, p. 174), where T is the twisting moment…
  • Greg Fletcher June 21, 2012 at 7:23pm

    Use the tube alone and you will have more thrust and no resonance.

    Resonance is a very powerful yet subtle phenomena. You know when you got it, like getting poked by a pin!!!

  • Rob_Lefebvre June 21, 2012 at 4:33pm

    Well, the problem is not the number of arms, but the fact that that arm design is... well it's terrible.  Sorry.  I've said the same thing about similar arms that people have designed, but you're the first to actually see the problem first-hand.

    C-channels like you've used have almost zero torsional stability.  A tube is 10,000% better.

  • Tian June 21, 2012 at 3:30pm

    well, it should be fairly stable with 3 arms also. Try making the bars out of carbon fiber. They are light and extremely rigid. You can just buy premade carbon fiber square tubes. They are easy to work with in my opinion.
    Also make sure that your prop is balanced and aligned perfectly. It is like a car wheel, when there is some alignment problem, it will vibrate the whole car at a certain speed.

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