Tricopter (Ycopter) Arm Length... the consequence.

by x3mperformance | March 3, 2013 | (4) Posted in Tips

Hi.

Here's a small info about my experiments with the rcxplorer.se tricopter.

I had it build as to the plans, but during a rebuild after e huge crash, i had to replace the arms, here i desided to do some testing, i streched the arms a bit, in my mind, this would create even more stability. I mesured the lenght of my wires, and added an extra 5 cm, (app. 1,5 inch) to the arms. No big deal.

But, to dial the settings in. Ohhh!!!!

 It had so much ocilation, wobble and unstability, i could not understand what there was wrong. I then got it dialed in at the best i could, and in a tail in hover, i studied the yaw and rear arm...

Surprice, BIG TIME.

A breif rudder input made the tail/yaw/arm go crazy. Wobble and shaking out of control, but yet the tri was maintaining it's hover??? Uhhh.

Some more investigation, and i realize that the flexing of the used pinewood is getting bigger and bigger as you strech out the arm.

Longer arm = more flexing

Shorter arm = less flexing

So the 48 cm (18,9 inch) is about the max lenght of pinewood arms.

You could though experiment with some reinforcement with carbon etc.

But then the entire plan of using wood goes down the drain.  

But her's the video.

 

I hope this answered some questions, and spared some experimental time and hair-pulling.

 

Rene'

COMMENTS

Supergoldfish on March 3, 2013
I tried to build one with carbon arrow shafts for arms. Same result. There was too much rotational movement on the arms.
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x3mperformance on March 3, 2013
Yes, so thats not the solution either.
I think the only way to strech the arms are by using Carbon Fiber rods.
So stick to the max 48 cm, unless you wanna "Rock 'N Roll"

That lenght is also enough to make a fairly stable platform.
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ddrueding on March 3, 2013
For my H-Quad I'm using carbon square tubes from a company called Dragon Plate (they specialize in structural carbon fiber for trusses and cranes; serious stuff). I can't get any deflection at all with my arms, and I'm a strong guy.
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x3mperformance on March 3, 2013
I know, but the idea about the pine Wood, is that Wood has a natural tendency to dampen vibration, where carbon, due to it ridgid nature, seems to transport them to the controllerboard / 3 axis Gyro's are not happy with that, and to any video equipment.
I know that carbon is stronger and more ridgid.
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ddrueding on March 5, 2013
I'm pretty sure everyone knows that CF is more rigid. And while that isn't a good thing for lengths where wood works, it still works at lengths where wood doesn't. In fact, I suspect there is an arm length where even CF moves enough to dampen motor vibrations sufficiently. No idea what that length is, though.
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Dirt Pilot on March 3, 2013
The mode of vibration appears to be mainly in the horizontal direction. Try gluing thin strips (may be 1/8" thick) of wood (full height/depth of the arm) on each side of the arm. The wood does not have to be full length of the arm. Start the wood strips at the base and extend it out about have length of the arm. You get much more bang for your buck by increasing the width of the arm near the base than you will from carbon fibre. If the mode of vibration is vertical, do the same thing except add the wood top and bottom of the arm. If there is a significant twisting/torsional mode of vibration, add wood to the top/bottom and sides. Let me know how it works out.

Also, to save weight in the future, you can use soft light wood in the core of the arm and use harder/stiffer woods on the outside surface of the arm. You can also vary the cross sectional area of the arm. Start with a thick section at the base and narrow it down as you get towards the motor. This will give you the highest stiffness to weight ratio. It also gives you the highest strength to weight ratio.
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x3mperformance on March 4, 2013
Like mentioned.
That would be an option, if you need the bigger reach/longer arms.
Mine was a "experiment" so it took only a few minuts after locating the problem, to move the motors inwards, and chop of the excess 5 cm.
Not that big a deal, the tri still flyes really stable and well.
But thanks Dirt Pilot for sharing your solution and thoughts.
Rene'
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Adib Vahedi on March 3, 2013
Thanks for the great info!

Keep up the great work!!!!!!!!!!!!! ;)
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x3mperformance on March 4, 2013
Thanks Abib
I will continue.

Rene'
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Cyberdactyl on March 4, 2013
My simple suggestion for exceeding your empirical 18.5" limit for wood booms would be to use a strut system, since, as was said, it does indeed appear most of the oscillation is in the yaw plane.

Ensure you use a non-stretch cable like wire or kite string such as 'Spectrum'. Also ensure you have balanced tension between all three. Don't over tighten the cables so you don't promote a catastrophic failure in another plane.

Here's a simple illustration of what I'm referring to.

http://img.photobucket.com/albums/v606/Cyberdactyl/tricopterstrut.jpg
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x3mperformance on March 20, 2013
I think this could eliminate some of the oscillations.
But some is also a rotation in the wood.
And the size it have now, the 48 cm arm length, is also a really great size for a Tricopter.
It almost draws a circle of 1 meter.
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Tricopter (Ycopter) Arm Length... the consequence.