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
Comments
They are AXI 2814/22 motors with Graupner 11x5 props. I added the tubing to every motor and the vibrations are gone now. However it looks hideous :( It's absolutely what @R_Lefebvre said, the motor's frequency is the same as the natural frequency of the motor mount. Just like this: https://www.youtube.com/watch?v=JUVgouE_sg0
the motor mount you are using is simply a aluminium plate..... and presently the c bar you are using are good to handle this vibrations, but they are not coming to the motor base they just fade out early.........! just extend it...... and i think you will solve the problem...... :-)
All I could think about when looking how close to the propellers it will be is something aerodynamic, But a tube structure is pretty close or the next best thing I guess if you could not get it perfectly aerodynamic.
Just out of curiosity What is your prop and motor combo , ive had some of the same issues attempting a downward facing prop
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 ; )
@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.
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 :(
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.
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.
How balanced is the motor and also the prop and prop adapter?
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