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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    • For your LiIon batteries there is simply a two step charging profile as follows:

      Constant current - The charger current is limiting set to about 1C or as per the battery specifications.  The voltage increases to maintain a fixed current as the voltage of the battery and the voltage of the charger differential decreases (increasing internal resistance).

      Once the voltage reaches a set voltage per cell (usually 4.2V per cell) the profile is switched to the Saturation charge.  

      Saturation charge - is where the voltage is held constant and the amps continue to be pumped into the battery although at an ever decreasing rate.  Some others call this the absorption mode.

      There are tricks to get more energy into the battery but all these methods shorten the LiIon battery life.

      Once the battery capacity has been nearly reached, the charge is cancelled to prevent over charging.

      As you add more energy to the battery, it's weight remains the same unlike other forms of powering aircraft using fuel tanks.


    • MR60

      Thx for the explanation.

      As I was usigng the "Lipo" profile of the charger (does not have a Li-Ion" profile) I was afraid it would not stop on time before overcharging and I thus stopped it manually when it read 16,8volts (4,2 per cell). Now I know that the charger automatically drops the amps in the absorption phase and stops when the charging current diminished enough and/or voltage goes over 16,8V. So the Lipo profile just works fine to my satisfaction. We are never cautious enough with these batteries...

    • MR60

      you were smart in doing that. you will want to get a charger with Li-Ion profile.

      Li-Po charges i think 0,1 volts more per cell than Li-Ion.  Thunder AC has it.  It's not in the menu per se.  By changing the cell voltage, to 3.6 (?) the label for the battery changes from Li-Po to Li-Ion.

    • MR60

      Ah thx, that makes it much clearer. I like the idea of the individual wires to have a practical mean to balance charge each of th 28 cells per group of 5 (faster than do this individually for each of the 28).

      However all of these little wires add weight and soldering complexity (I'm not an expert at soldering and have not spot weldering tool. I have just got a classical fine tip 48W solder station).

      I have glued the 4S7P together with CA for plastics. It seems to hold pretty good:


      So I guess, I will just wire the pack to have the two main leads. For linking in parallel the cells, I was thinking of using copper desoldering wick. It looks light enough but will ti bear the nexessary amps ?:

      3702833666?profile=original(see picture for relative size of solder wick versus battery). As current skin effect does not apply to DC currents, I could more simply use a solid copper wire too I guess.

      And total weight so far is 1300g (23800 mah at 14.4V). to compare with 1900g of similar capacity on the lightest Lipo technology (about 30% lighter but lower C discharge rate)

    • MR60

      really cool!  beautiful craftsmanship.

      for the gnd (-), bare wire is OK (but insulate it when the bare wire is even near the + terminal in case crash forces moves it to that terminal).  i use insulated wires for both.

      for the +, recommend using tough plastic coated wire.  during a crash, wire can scrape the plastic cover around a cell and contact the metal grounded casing.  the crash can also expose lightly insulated wire. if the wire is a neg, no problem.  but if a + ...

    • MR60

      Here some soldering and final result:

      3702903200?profile=originalI decided to follow your advice of soldering the main leads to the middle. It is 17 awg wire and 4mm bullet connectors. I chose these for their small volume (allows me to attach any adapter, like a XT60 afterwards)

      3702903570?profile=originalAnd some hot glue to protect these cables:

      3702903488?profile=originalI then decided to build a thick neoprene protection box against crashes. However I am still wondering how to attach this to the frame,


      On the picture you can see I added some plastidip (liquid rubber) around some sensitive points, to ensur good isolation.

      I am now charging the pack. The charger pumped in more than 10AH and displayed 16,8V. So I disconnected it from the charger and measures the cell values : 4.08V

      I then replugged the pack and reduces the charging amps a bit. It still injects MAH but the last part of charging to 4,2V seems to be tricky. Should I wait for the Voltage to go over 16,8V to have them fully charged ?

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

    • MR60

      You do beautiful work!

      You might be in a gray area on charging.  Went through my logs of Li-Ion Battery charges and saw the following:

      - 16.4 V  (so cell of 4.1)

      - 16.4 V

      - 16.4 V

      - 16.5 V

      When charging, i use a Thunder AC6.  It has a Li-Ion charging mode.  It automatically drops the amperage at the end.  So when it says it is done charging, i fly.  the world record was set that way.  voltage meters can also vary a bit.

      The charge voltage is 4.2 so it wouldn't surprise me if the voltage of the batteries at the last moment of charge is close to 4.2.  Haven't measured it while charging.  I've only measured after the charge was done.  It wouldn't surprise me if there is an immediate fall-off.

      Whatever you do if you try to push the envelope, experiment on a lone cell, not your big mother.  Let me know if you find out anything interesting.

      P.S.  During the first record (only 80 minutes), it as the mid of winter and -2C outside.  So i had to add the weight of a foam box like yours (heated the battery prior to flight to 120F and hoped for the best).  The foam/batteries strapped to the bottom of the ship just fine.

    • "

      The charge voltage is 4.2 so it wouldn't surprise me if the voltage of the batteries at the last moment of charge is close to 4.2. 


      better replace your charger to constant current vs. constant  voltage, monitoring cell voltage at the same time,

      as advised in case of Li-Ion chargers ( charging curve plot - voltage vs. time)

      If your charger has no upper limit set on current you can easily overcharge or destroy your cells.

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