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

You need to be a member of diydrones to add comments!

Join diydrones

Email me when people reply –


  • i want to say Bravo ...and that you are an inspiration for my future designs.....i have some ideas if you can find light materials to make the structure more solid.....i put some photos later

    • MR60

      Brilliant design. Interesting idea using a "mitered cap" to come down onto the joint from the top. Sexy and neat.

      Strength issues are the following:

      - try to make the joint so that it is slightly stronger than the raw tube

        ... Part II

        ... Part III

        ... Part IV

        ,,, Part V

      - that entails

        ... using carbon in the direction of tension/compression (parallel to the axis of the tube)

      . ... using as much cross section of carbon (calc the cross section of carbon in the tube; match that in the joint)

        ... bonding that cross section to enough length of the original tube to spread the strain

            o calc the surface area of the tube

            o make sure that the bondline between the "patch joint" and tube exceeds that

            o usually the length of the patch is 3.2x the diameter on each half or 6.4x the diameter 


      my idea is this and i think of putting a wood cube(37X37) with 4 holes 16mm to make strong

    • MR60

      Absolutely gorgeous!

      Sorry about the delayed response ... the site was down for over a week.

      In one of my blags on construction, I show how to:

      - wrap sand paper around one tube and sand the other tube into it so there is a perfect match between the side of the continuous tube and the end of the bisected tube.

      - join the bisected tube so it is as strong as the continuous tube.

      Aluminum is pretty, but heavy. Printed plastic can be 2x to 4x lighter. Bonding is 20x lighter. But people need to use what they have access. So sometimes metal is best.

  • Forrest: you test this batteries? LGDBMJ11865  looks higher c rate?, (slowly but I continue with my long flying copter, now I recived lighter esc and thinner wires to change I always remember 40 min flying time objetive to migrate to ion) )

    • MR60

      yes, 2.5C is about right for a high-performance Li-Ion battery.

      2.5C means that it would theoretically hover on a ship that is designed to stay in the air for 24 minutes or more (1/2.5 = 40%; 40% of 1 hour is 24 minutes).

      But in reality, the ship could not take off or turn a corner.

      In reality, it needs to at a minimum be able to stay in the air for about twice that long and even then you need to be careful.  For example, if i put the world record holding ship into a tight high-speed turn, it will fall out of the sky because the battery chemistry will quickly collapse when it exceeds 2.5C (a high-speed sharp turn takes about 3x the hover amps). I've done this twice, tracked the amp usage, and rebuild the broken ship. I guess 3 times is a charm.


      if this can be made  we can have more rigid strucyures or not???

      Zentralverbinder Quadrocopter
      Allgemeines:   Dieser universelle Zentralverbinder aus Carbon in Monocoque-Bauweise bietet in seinen verschiedenen Varianten eine Vielzahl an Eins…
    • Thank's Forrest, I hope when I finish lightweigthing my copter new more efficient batteries appears but I learned a lot in the process :D.

      I bought 20A hobbywing esc to change for the 40A esc and 18awg wires to change for 16 I estimate 100 grs less, If I can, Im going to try to flash them with BLHeli. not much but is another step. Thank's for all your help.

  • I'm aprox 18 A the four motors, perhaps I can go to the power module with 16 ones and take away all the millon connectors that I have, when I builded I though that it's easier to change esc or motors if I have connectors but the real thing is that I never had to change anything, perhaps one day I have to do it but isn't a frecuent work and allows to lighweight my copter, correct?

    I suppose that you have to take a margin from your common consumption and a high requirements one (ej wind goes up when you are far, etc.) how margin is convenient?   (sorry I'm a rock with Amps, W, V, etc etc.)

    First I'm going to test how low my Amps can go and re-check the tables to see how thin can be the wires; many thank's

  • What you do guys with power board to light weigh? better soldered wires or board?I have a mesh of wires and connectors under my two cuads and think's that there is a lot of weight there, on my next buy I'm going to ask for thinner wires (18 AVG) to change motors to esc ones (I enlarge with 16 but motors original are 18)) and wish to do something with that mesh, any suggestion? Thank's

This reply was deleted.