Building Copters with Round Tubes - Stronger, Lighter, Easier to Mount Motors than Square Tubes

Round tubes offer the following pros:

  • handle twist better than square tubes.  Thus for the strength, are far lighter.
  • are more readily available (pipes, tubes, arrows, kite frames, golf clubs and sticks of all materials).
  • easier to cut (square tubing can twist and be damaged by cutting forces)
  • easier to peg (fits into a drilled hole and easy to find extenders)
  • less expensive

Round tubes are thought to have the following cons:

  • difficult to mount motors
  • difficult to join

None of the cons are real if you know how to work with tubes.  Mounting round tubes to motors is easier and faster than with square tubing.  Round tubes are also faster and easier to join together.  The resulting joins are also far lighter and better.

In the H-frame forum, I was asked to share these build techniques so have decided to demonstrate the methods on the most complex multi-copter one can build, an Octa-V.  I'll do this step by step.  The result will be a multi-copter that reduces frame, screw, gusset, and motor mount weights by more than 40%.

The steps will be Design, Assembly, Charmin Test, and Flight Test

Installment 1:  Design

First, both simple and complex multi-copters share something in common.  When using round tubes for arms, there is no reason to cut a perfectly good tube in half for each arm.  And then add a bunch of weight and fasteners to hold the halves together.  How this is done will become evident in the third installment, the Charmin Test.  For now, just know that each tube is continuous (no breaks, no joins).  Opposite rotors share the same boom in a quad, hexa, or octa.  All of those fasteners are gone.  The cross beams on a V or H are also continuous.  Assembly and disassembly is quick.

Most quads are so simple that one grabs two pieces of wood the same size and slap them together.  Done.  If the angle of view isn't good, just move the camera forward a bit.  But, if you want to know the exact length of a quad boom based on prop diameter and platform size so you can minimize weight to get longer flights, then I've attached a worksheet that does the math.

An Octa V is a bit more complex.  It is specifically used for camera work.  So you need to optimize the motor boom angle and aspect ratio of the frame to achieve the desired Field-of-View for the camera (void of propellers),  It also uses 8 motors so that if one dies, the copter can return to the ground with the $12K of camera/lens in tact.  You also need to minimize platform vibration, so the platform needs to be large enough for the electronics, gimbal mount, and at least 1.2" (30mm) from the prop radius.

I've attached an Excel worksheet that does all of the calculations for optimizing weight.  There is an instruction sheet if you want to ever build one and calculations for a Quad X, Quad +, Quad Spider, and Octa V.

The next installment will be Assembly.

P.S.  I'm not experienced nor am I an expert.  I'm just a tinkerer like many of you. There are builders out there with far more experience and wisdom. I'm hoping that this blog will allow us all to share ideas on building strong, fast, and light not only for initial build, but also for crash repair.

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one can use two methods to classify tube thickness.

1) Theoretical - estimate by a simple factor.  pick one tube and call its stiffness 1.  then calculate relative stiffness of all other tubes to it by using the following equation.  Use theoretical to select tubes to try when they might have a good stiffness to weight ratio.

w2/w1 x (r2/r2)^3 where w = wall thickness (OD-ID)/2 and r = OD/2 - w/2

2) Actual - create a standard span.  place the tube over it.  add a standard weight (enough to bend the strongest but not so great that it bends the smallest to much) at the center.  measure deflection by the thickness of the end spacers holding the tube off of the table less the distance to the table top under the tube at the center.  doing this at the edge of a table allows calipers to access that distance easily.  once again, standardize back to the standard tube as an index of 1 with simple proportions for the other tubes - d2 / d1 where d = deflection

'tell me more about the .5mm bond line.  something isn't right.  what are you bonding together?'

It was in relation to the 0.5mm thick tubing where it is joined/butted to the other tube at right angles. Thinking about it part of the tube will be chamfered  so this will be slightly wider than 0.5mm.

my brother Jim made up a simpler one. i've got full print outs but have been trying to find the full-scale jpg Jim created for printout.

Hey Jim - need your help here.  Can you find the original file.  It would be great is we all used the same metric and knew the size to print out and video.

tube favorites vary depending on stress requirements.

45206 - 0,6mm thick x 15.3mm OD great for > 700 size ships

45525 - 45% stiffer.  2x thicker.  1.8x heavier.

i'm using the 45206 now on all builds.  but if i go more up in size, will have to use the 45525 or develop a truss. 

Oh thx a lot for the links to easycomposites. I had anyway to order silicon @easycomposites, so I will order this 0,5mm tube (I did not know they had so thin ones).

on adhesive, keep in mind the physics.

adhesive has a

- shear of about 2000 psi with carbon to carbon

- peal of about 25 pounds-force or .8 g-forces

so the math is as follows.  bond the end of a mitered tube to a tube without a gusset.

- area in shear = wall thickness x OD x pi x 1.3 so for a .6" OD tube about .05 sq in or about pounds.  so a 16 g-force crash is OK for a 5ish kg ship.

- bondline in peal = OD x pi / 3 x 1.3 = 0.8 or 20 lbs of peal force or a .2 kg gimbal could survive a 3ish g-force landing.  Not a lot.  So the weakness of the adhesive, isn't shear, it's the peel number. Thus the need for gussets at end connections.

(the calcs were done quickly making assumptions so might be in error for your situation)

yes ... use a gusset to bond the tube to the tube to increase bond line ... adhesive just on the end does little in peel.

edge view (with the gusset the outer element and the tube the thicker inner element

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This is very useful. A significant reduction in weight.

Thanks

Another option is to use Tow to strengthen the join.

Various types at http://www.easycomposites.co.uk/search.aspx?keyword=tow

Is there a way to upload a pdf?  We need something that is printed to a scale.

Have you foreigners :-) ever tried to order from Rock West?  I think they ship internationally.

Their 45026 will be 2.5 x stiffer for only a 25% weight gain relative to the easycomposites tube.

Here it is.  Thanks Jim.  This is what it looks like, but print from the attached pdf at full scale on black and white on 18 x 24 inch bond paper.

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