Comment by Tian on 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.

Developer Comment by R_Lefebvre on 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.

Comment by Greg Fletcher on 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!!!

Comment by Barrett Ames on 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. 

Comment by Not Sure on June 21, 2012 at 11:53pm

How balanced is  the motor and also the prop and prop adapter?

Developer Comment by John Arne Birkeland on June 22, 2012 at 12:01am

Like many others have already said. Square tubes and C bars are terrible when it comes to vibration harmonics. A nice 3K twill weave carbon fiber tube cost a bit, but is worth every penny if you want a vibration free copter.

Comment by Melih Karakelle on June 22, 2012 at 12:19am

Use a prop balancer and fine tune your prop. Use perfect tuned motor and motor mounts or tune them your self, most of time a piece of tape enough for balancing them. you can find hundreds pages about how to.

Comment by Mike de Landgraaf on June 22, 2012 at 12:36am

Hi guys,

Thanks for commenting. I've balanced the props, they were nearly perfect from the factory. Couldnt find any extraordinairy vibrations from the motors too, those AXI's are great (used them before).

I like the idea of a 3k woven carbon tube, however i need the propellor to exactly sit in the middle of the frame, thus protecting it. I could have a go laminating the tube in the upper part of the frame, that might work. 

And then using a droidworx motormount for clamping the motor on the tube. Don't think it will save weight though :(

Developer Comment by R_Lefebvre on June 22, 2012 at 6:22am

@John, actually, square tubes are not to bad in torsion.  They are obviously not as efficient as a round tube (stiffness/weight), but they're not terrible.  In fact, most people assume that round tubes are stronger than square tubes in bending as well, but that's actually not true at all.  Square tubes are stronger than round tubes in bending.

@Mike, do I understand that your design problem is that you need the support to be "arched" in order to move the prop disk into the center of the frame?  If so, I would suggest aluminum tubing, and bend it, or cut and weld it to get the arch you need.

Your current design won't work.  What has happened is that even though your props are balanced, the rotational frequency has lined up exactly with the natural frequency of the motor support.  So at that speed, the whole thing "lights up", and actually amplifies the small imbalance that will always be there.  In fact, if your props had been unbalanced, it's entirely possible the whole thing could have catastrophically failed!

You could also try moulding arched supports out of CF, similar to how you made your beautiful frame there.

Comment by Ron Perry on June 22, 2012 at 12:32pm

Does it resonate without that carbon chassis(protective ring)? Not sure if you're fighting the right battle - stiffening will only transfer the vibration to the rest of the copter. Your chassis (specifically the protective ring) might be inducing an effect like the Tacoma Narrows Bridge fail. Wind going past the bridge caused a periodic frequency that matched the resonant frequency of the bridge itself. Because so much air is being forced through your protective ring it makes a great candiate for this scenario... and I could be totally wrong, just trying to help troubleshoot ; )

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