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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the excitement is building!

sorry to hear about DHL. no problem on the Pixhawk data.

this will be an interesting test.  my data shows that the 18" won't fly as long as the 16", but reality will be a good lesson. i'm rooting for the 18".  surprise improvements are welcome.

thanks for the timely reply.

just went onto e-bay via your link and made the order.  no problem with the changes.  if they work on one or more of the axis for the GoPro gimbal i'm building, wonderful.  If not, it will be a valiant try.

... and the lots remaining drop to one :-)

the flight time are coming out of eCalc estimates. It could be very wrong and only real trial will tell...

by the way, i saw you took the pixhawk out of its case. Did you cover the barometer ? how ?

In the area where the Pixhawk has a foam mini-block, look for the chip with a breathing hole.  bond the piece of foam to the chip without accidentally sealing the hole.  then lightly spray with 3M electrical sealant (the foam has large open cells so the sealant won't close up the pores).

And what about light ? The baro foam does not block sunlight well enough and has proven to be a cause for bad readings causing flyaways. I have to think of a small pprois cap which shall keep this baro in the dark.

Regarding above comparison, Just sharing my test result yesterday.
With quad setup, MN3508 kv380, RangeVideo li-Ion 3S 18.300mah tested hover with string at all 4 axis to the wall, just to make sure it's not running away.. But still freely move around garage.

The result below:

A. Prop 15" CF Foxtech supreme lite, AUW 2.150gr, consume 18A with ~75% throttle.
B. Prop 18" CF Foxtech supreme lite, AUW 2.210gr, consume 20A with ~65% throttle.

it's possible that light is a problem.  but more likely the issue is UV converted to heat.  if the chip gets warm, it can cause the gas inside the chip's tiny chamber to expand.  That causes the ship to think that it is higher in elevation than it actually is.  crash.  it flies forward quickly and cools and whoosh, up it goes.

in the Pixhawk, the chip is wisely located under the board, fore, port.  so there is little UV.  it is first to see cooler air pushing away insulating air during flight.

so what about indirect heat.  the top of the board gets hot from the sun ... won't that ... ? possibly.  but think about a black semi-enclosed case that is getting hit with UV and has heat from electronics warming up the semi-enclosure from the inside.  both will do the same thing. change the inside temperature of the chip's wee chamber and cause it to think that elevation is changing when in fact, it's just temperature.

actual experience.  in most flights it is rare when the landing elevation is the same as the takeoff elevation even though it's the same place and the flight has been short enough that local air pressure hasn't changed.  i'm thinking that the difference is internal chip temp.

would putting a little tent over the top of the Pixhawk help?  yes.  on my mini-me FPV ship, the battery/camera platform is on a VDEP that is directly above the Pixhawk.  but on the latest design, no such complete cover.  so don't know what the impact might be.

would a bara with a built-in temperature probe help?  don't know if the baro has this or not.  if it has one, don't know how well it's calibrated or how close it is to the actual chamber (it should be in it).  if it doesn't have a good one, then it should.

great comparison.  well done.  love the tie downs to the garage wall!

what was the voltage?  trying to figure out watts.

what was the AUW of the ship with batteries?

are you going to try the 16"?  i'm guessing that's the sweet spot.

do you have a gram scale?  if so, what are the weights of the FoxTech 15" and 18" Supreme Lite?

if further testing of props is too expensive, please send me a message via the friend route.  i haven't tested these props.  i'm a tad curious.

P.S.  If you do a two minute hover, then our Hover Analysis excel sheet will tell you everything (hover throttle, watts, etc.).

I've been looking your battery build process. Very impressive.

Thanks Forrest, although its faaar from scientific, but it satisfied my need and I only focusing on volt recording so forgot to record the watts.

It was on 3S fully charged up to 4.2 v per cell ~ 12.6 v total, will test with 4S soon.
The above AUW was with the battery that weight 806gr alone, and yes I scale it with my wife's cooking gram scale :)

Unfortunately don't have 16" prop in hand now, the weight of 15" is 25gr and 18" is 40gr without their prop-shaft adapter on the middle.

I though that we are friends already ;)

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