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.

Multi-Copter Design.xlsx

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    • MR60

      1) yes on torque

      2) you only need one test criteria.  please don't overwhelm yourself. i'd pick one criteria and try it.  i'm guessing that the eye chart will be hard to discriminate between tests, but it might be easy.  If you choose the Pixhawk route, i can help with data crunching.

      2) with Mission PLanner

      o connect

      o Click the Flight Data icon

      o In the GPS map, bottom left, check Tuning

      o Double click the real-time graph that pops up

      o Check the box for the 2nd item, roll

      You will see the graph automatically start to waver as you take the Pixhawk and rotate it in roll. 

      3) should prove interesting

      Best of luck!!!

    • Glass bubbles

      10mm 0.5mm wall thickness tubing

      For all the good info I've received over the past year which I started with absolutely zero knowledge I don't mind a bit of postage.

      The post by Forrest on the 7mm tubing. Possibly the 10mm x 0.5mm tubing may be slightly over-engineered for the gimbal. The 7mm tubing on Forrest's quad is longer and has to deal with the thrust of the motor which may be in excess of the stress placed by the camera and gimbal.

      It maybe that you don't need a gusset at all and the epoxy in direct contact with the end of the tube has sufficient strength.

      One further point if you use a lighter motor (e.g. 1806) for the pitch, the mobius wouldn't need to be as far over to counterbalance the weight leading to a smaller length tube and therefore less weight. Not sure about my logic here.

      Forrest are the Rock West 4206 tubes 0.5mm thick? I know that your favourite tubing for a quad was  45525 tubing mentioned earlier in this thread, I think may be approx 1mm thick.

    • MR60

      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)

    • MR60

      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. 

    • I emailed and spoke to an employee at Easycomposites about whether they were going to make 16mm x 0.5mm tubing but unfortunately at the moment it's a negative. A case of supply and demand. But if demand goes up they may change their minds.

      Also contacted a Hongkong/Chinese business via ebay and they did not have any but did say they would make it but it's a minimum order of 30

      Extracts from two emails:

      'Thank you for ordered from our shop.
      but right now ,we are not have 16*0.5*1000mm in stock,
      The MOQ is 30pcs,we could do it for you.'

      '3K surface 16*0.5*1000mm $11.0/pc
      Plain weave surface 16*0.5*1000mm $8.5/pc
      As the better price is up to your order quantity.
      The shipping costs is according to your detail shipping address.'

      Not a bad price but I don't know what the quality would be. I haven't ordered any as there is no way I could use 30 pieces.

      I am though awaiting a couple of pieces of 7mm x 6mm x 500mm from them.

    • This is very useful. A significant reduction in weight.


    • MR60

      the key to tube stiff/weight is large diameter + thin wall.  the best tube i've found so far for the > 700ish copters is the Rock West 4206 15.3mm OD at 37.2 g/m.  For 500 and less size, Goodlwind Skyshark II P90 7.2mm OD at 13.2g/m. 

      we should create an excel worksheet showing stiffness and weight for the best tubes so we can select based on ship mass.

      on mass ratio, yes, gimbal mass / camera mass.  most new ones have a ratio of about 2.9.  it would be nice to get that to less than 1.  i may try to come up with a quality test based on taking a video of this or one like it.3702681732?profile=original

    • MR60

      Good approach to be systematic rather than picking parts randomly. How would you !easure tubes stiffness systematically?

      This grid is nice to compare picture resolution-sharpness. 

      Will try to find these light cf tubes.

    • MR60

      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.

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

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