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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Replies to This Discussion

Hi Forrest,

Thx for the explanations. I still do not see what would be the final "cut shape" you are trying to reach. Do I understand that only one boom is cut while the other remains intact at the junction ?

Please a drawing would make it easier to visualize.

At step 3 what do you mean a blade thinner than 4mm ? you mean the length probably, not the width?

At step 5, what is the appropriate angle? (horizontal ?)

thx for your explanations

Here is the equivalent firewall using only inside radius cuts rather than outside and easier to make using drill bits and hole saws without a C&C milling machine. See instructions on this thread.   Jim

Here is the CAD drawing.  I would only use the particular firewall hole pattern that fits the BLDC motor mount but you could do a two nylon screw mount and choose two holes of each pattern. Notice that each radius cut is outside to the part so simply put the right sized bit on the X and drill.  There is one inside radius relative to the part but you can omit it as I did as you can see in the picture in an earlier post.


Apparently I didn't do a good job of explaining.  Words are tough.

One mast is continuous.  The other is cut in two and mitered, via sanding, into the continuous mast.

"blade thinner than 4mm"  in reference to when you cut one of the masts.  if you use a table saw with a large rotary blade to cut the mast, make sure that the blade isn't too thick and thus takes away too much material.  if you use a small hand saw, those are thin, so there isn't an issue.  

the angle of the cut is 90 degrees for a regular quad.  but if you are building a spider, the cross cut will be mostly likely between 70 degrees and 90 degrees per your design.

the resulting miter, after sanded, will be a complex S.  don't worry about the shape.  the shape just happens as a result of sanding.  

let's say you are doing a regular quad.  the mast that gets cut in two will be cut at 90 degrees.  the continuous mast (wrapped in sand paper) is held horizontally on a flat table.  the cut mast is also held on the flat table so that one end is perpendicular to the sand paper wrapped around the continuous mast.  the cut mast is rubbed (keeping it perpendicular) into the sandpaper.  after a few minutes the end of the cut mast takes on the proper mitered shape ... sort of a U.  to keep the U centered in the end, one can:

- also apply a slight up or down force as you sand

- glide the cut mast over paper that is the same thickness of the sandpaper so they line up.

in the next week i'll be making a vibration isolated FPV/gimbal ship that will fly for about 1.5 hours.  i'll try to make a video of the process.

I think I got it. Then do you glue the two sanded half booms to the perpendicular one and/or add reinforcement strip above and below the joint? Is the reinforcement strip a rectangle or a cross shape? What thickness for these strips? 

I've got a blog somewhere that explains the sizing of the gussets.  Since you are an engineer, I'll give you the principle.

- calculate the carbon/epoxy area of the end of your rod being used for a mast

- divide that by two

- that should be the minimum end area of each gusset (i use 0.3" x 0.03"; about OD/2 wide x pi x mast wall thickness thick)

- the length of each gusset is governed by mast wall thickness (thinner the longer), tube diameter (length > 2pi x ID), and shear load of the adhesive (the lower the longer)

- a 3" to 4" gusset length using the Scotch Weld 2216 works well for our size ships.

no need for a cross since the one tube is continuous.  just a straight extruded strip.  

put adhesive on the end of the mitered masts and press them against the continuous motor mast (handles axial torque) and adhere each strip to rejoin the bisected mast.  If you are bonding a plate to one side of the masts as an electronics platform, then you probably don't need a strip on that side.

Thx. So there is no reinforcement to counter torque forces in the booms plane, to keep the right angle between them. Isn' t a weak point in case of crash ( I' m as a bad pilot as you are...) ? I intend to build to carry 2kg payloads.

the reason to keep the total cross section of the gussets equal to the cross section of the tube is to maintain the strength of the tube.  a tube as you know is an inefficient beam.  but it also provides equal strength in all directions, which an I-beam does not. if you put a floor joist on edge and it has phenomenal strength.  lay the floor joist on it's side and it sags dramatically from it's own weight.  this is also true with the .3" x .03" extruded carbon.  the strong side is flat or planar with the ship masts.  so there is actually excellent planar strength.  that's the reason why it must be at least 1/2 the diameter.  otherwise it won't deliver enough planar strength.  

what's cool about this solution is that strength is also provided in the perpendicular axis because the two gussets, while thin, are separated by the thickness of the tube, in effect, becoming that thick.

How do you get motor cables out at the center of the cross? Or do you leave cables running outside the tubes?

run outside on top of the tube or ...

you can drill a small hole, run the cables, lay them flat on the tube surface for 4 mm and then rejoin the hole with adhesive coated or epoxy soaked carbon or glass cloth patch.  this is another advantage of magnetic 22 AWG wire.  it is so tiny that such a small hole can be ignored and just lightly filled with 2216 adhesive to keep the wire from rubbing and some of the loads moving along the fibers.

on my ships i tend to run outside only because i put the naked ESCs in the prop wash to keep the cool, making the electrical runs short. 

Wow thx for the european links. The Pitlab link is great for carbon tubes and fixing accessories and for a good price too!

i've seen your videos and they are amazing.  185g is great (should be able to lighten more with composite sandwich panel replacement).  how many axis is it?  where do i find a link to know i'm getting the right one?



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