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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Looking forward to the next installment. I just built a symetrical octo but want to do an octa V for my next one so I will be following your progress.

Nice!!!

When they are back in stock, if sizing is right, I'll buy a set and test their vibration system with accelerometers to see how it works.  It's amazingly difficult to isolate using conventional materials.  This is my primary focus after the assembly and flight test.

Any experience with these?  What does each bracket weigh?  

Great post! I also am looking forward to the next post.

I've got a bunch of these in stock as spares at www.steadidrone.ca and I'm sure the other dealers do too - http://www.steadidrone.com/where-to-buy/

They weigh around 28g per motor (within a few grams anyway, I don't think my digital scales are super accurate).

And they're not a magic bullet for vibration damping, but in a chain of things to optimise everything helps. I was just suggesting them as a neat solution for mounting on round tubes (if your tubes are 20mm)

Installment 2 - Assembly Using Round Tubes

Today, once design was completed. I completed the frame assembly.  This only takes a few minutes on a quad and less than an hour for an Octa V.  Each step is documented with a photo.  First though, a discussion on motor mounting plates.

Why use motor mount plates? Why not just screw the motor into a wide piece of plywood? If you are trying to build a high performance machine, it needs to be light weight.  Motor plates are required even with square tubes.  Why?  See photo A - Motor Mounting Plates.  If the boom is weight efficient, it will be smaller in size than the motor diameter.  If the end of the boom is flared to be wide enough to mate to the motor, then the flare is your mounting plate (just embedded).  The other approach is motor brackets that clamp onto the tube.  but those weigh too much for an octa (8x the weight) and typically adds more than 100 grams (a motor) than the approach shown below.

A1 - Find a flat spot large enough to build your copter.  Flat is key.  Don't care about level.  Flat ... no twists.  This is critical for all of the motor mounting plates to end up on the same geometric plane and the four crossbar mounting plates to end up on the same two parallel planes.  Print out your drawing and tape it to the flat spot or tape down paper and draw where your plates go.  Tape the corners of the plates to the paper.  See photo A1 - Full Size Drawing on Flat Surface.

A2 and on continue below

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Installment 2 (continued)

A2- Apply adhesive to the plates where the boom crosses (make sure you haven't taped that spot).  I over kill this with 3M EC Scotch-Weld 2216 and get a typical 500+ pound load.  Other less expensive adhesives should work. Keep in mind that the motor base gets to about 100C on some motors so make sure the adhesive works when hot. Try your choice of adhesive on your choice of materials.  Let it cure to 90% or more and then give it the Charmin test. For me, applying adhesive takes far less time than screws, which weakens structure and weighs more.  Dab the glue, place the parts.  Done.  Start working on something else.

A note.  Not all tubes, square or round are perfectly straight.  Square tubes, of course, have the additional disadvantage of having the potential of twist.  In any case, find the subtle warp in the tube by placing it on the flat surface.  It will roll onto its side.  That is how you want to place the boom over the plates.  Mark the top of the tube so you place it correctly.    

A3 -  Place the booms onto the lower plates (see photo A3).  Give each boom a slight forward and aft glide and slight roll to increase the surface area being bonded (get the bond area nice and muddy but don't use a lot of glue; in most cases a thin layer spread around is best).  Tape the booms in place so they don't move while curing.

Remember, we aren't going to cut perfectly good beams in half and then add a bunch of fasteners and beefy center plate to heal the cut.  We are going to offset the two cross beams or in the case of a quad, the two motor arms and create a perfect slot for the electronics platform (explained in the later assembly steps after cure).

So, then put adhesive on the cross bar upper plates (one per boom) where the boom crosses.  I mark this line first with a pencil and double check the pencil line before adding adhesive.  The upside down backwards thing screws with my head.

A4 - Place the Upper Cross Bar plates onto the boom.  See photo A4. 

... more follows

 

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Installment 2 (continued)

A5 - Tape the just added crossbar plate to the boom.  Do so in a way so that there is a single layer of tape over the entire upper surface of each plate.  Place a rectangular and straight bar (I knew there was a good use for rectangular/square bars) on top of the upper cross plates (see photo A5).  Now not only are the lower plates perfectly flat and on the same plane, the upper plates are now also perfectly aligned to each other and also parallel with the lower plates.

A6 - Weight the entire assembly so it cures well.  See phot A6.

The next installment will continue assembly of the entire frame and electronics platform with light weight zip ties.  Because we are using continuous cross beams and round tubes, we've eliminated the need for high strength attachment systems.  Plus no more fasteners and holes through structure!  The result is a right-strength-stiff ship with easy repair.  Brittle is out.  A high performance auto is one that is stiff but collapses on impact around your body, not into it.  That is what zip ties will do when properly designed and implemented.

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Thanks.  I hope this generates best practices from everyone who builds.  I have a lot to learn from everyone else.

Dang.  My tubes are half that diameter.

SteadiDrone builds beautiful ships.  And bullet proof.  Thanks for the tip.

Nice job of explaining the assembly process.  Could you also mention suppliers for the stuff that is not available at my local HomeDepot, Ace or Lowes.  Who is a good supplier for the glue and dispenser? 

Thinking about your design I see one downside. It lacks modularity. In other designs with screws and separate pieces, after a crash you can replace damage pieces. In your design since everything is glued into one piece, crash and you have replace everything.

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