Round or Square tubing?

I've been trying to decide on which aluminum tubing to go with for my quadcopter motor arms. Looking here and elsewhere there appears to be a mixture of square and round tubing being used by builders and companies. The question is, which would be a better choice?

From previous experience I guessed the round tubing would be stronger. One thing I did find from researching online is: although round tubing is stronger than square when compared by weight, the square tubing will generally be stronger when compared by size. For example, a 20mm square tube would be stronger than a 20mm round tube.

Another interesting factor is described here. Apparently square tubing loses rigidity advantages when the force is applied from a non-square direction.

This leads to the question of: how often will the motor arms be subjected to non square forces?

Here are some discussions and data about square and round tubing within other uses:

-http://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/3372-round-vs-square-tube-strength.html

-http://answers.yahoo.com/question/index?qid=20070313140244AAbtLLE

-http://www.steeltubeinstitute.org/pdf/brochures/dimension_brochure.pdf

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Replies

  • Thank you both for the input.

    -Davey, good point about using the motor arms as the breaking point rather than the frame. I'll keep that in mind as I was worried about using tubing with too thin of walls.

    -Dave, I agree with the square tube being a bit easier to deal with. Although, I plan to use my 3D printer to print out the frame and motor mounts so the tubing could be either type.

    I'll have to go check Ace hardware tomorrow to see what tubing they have. That may sway my decision, unless I have to order tubing online somewhere.

  • Either square or round should be more than strong enough.  However, don't use U - it's not strong torsionally and twists easily.

    I used square for mechanical simplicity and 19mm because it was available at the local hardware store.  I'm not a mechanical genius and it's much simpler with square tubing mounting the motors and attaching the arms squarely to the frame.

    If I had more choice, something around 15mm would be my pick.

  • Another thing that I'm not sure of is the needed size of the tubing. I would guess something bigger than an arrow shaft. But how big is over-kill?

  • Round! Definately. It looks much better :)
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