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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  • An issue using round tubes (my preferred choice) is mounting the motors securely at the end of the tubes.  Typically the BLDC motor's shaft sticks below the thin firewall plate mount and the obvious tempting solution is to drill a hole through the tube so the shaft spins freely without rubbing against the tube.  Bad idea though!  

    In crash studies, this hole at the end of the tube weakens the tube and it splits from the end in hard landings with future torsional rigidity compromised.  Better method is to make the motor firewall thicker.  The lightest way to do this is by using balsa core CF panel available from a number of sources.  For most motors, 1/8" panel is thick enough and works well.  When this firewall is bonded onto the tube's end, the motor firewall is on for good.  And provided the motor is secured to the balsa firewall using nylon screws, in a crash the motors separate without damage to the firewall or the tube.  That is unless you fly like my other brother Marty, who can break anything and was the inspiration for the "Don't Fly Like My Brother" instructional videos.

    See the two pictures attached.  Note the 3M Structural Adhesive application which was applied using a wood tongue depressor modified to the proper fillet radius.  And as my brother Forrest and others have mentioned, be sure to use spiral wound tubes because the less expensive pultruded tubes just do not have the torsional stiffness we all require for a durable build.





    • MR60

      Your bonding looks really great (much better than my jobs).  I put tape over the holes; put adhesive on the motor mount and press it onto the motor mast; align the two by taping them onto a full-scale drawing; let it cure; peel off what I can of the tape, then carve out where the head of the screw goes when adhesive ran too thick.  not always pretty and the last part is a pain.

      Jim - Could you talk a bit more about your tongue depressor technique?  Do
      you put adhesive on the motor mount first, press it onto the motor mast,
      and then wipe away excess with the radius fillet tongue depressor?  Or do
      you press the two parts together first and then apply the adhesive with the
      tongue depressor?

    • I use Generic or Auto CAD to design the parts but any CAD will do.  Since I don't have a C&C milling machine, I avoid outside radius parts like Forrest makes instead sticking to inside radius designed parts especially for those smaller parts like BLDC motor firewalls.  All hole or radius centers are marked with an X.  I print the part drawing full scale using my laser printer onto regular bond paper.

      Then photo mount the paper drawing onto the plate or panel to be cut.  Use about a #40 bit or smaller to create a guide hole for all the later holes.  I use Forstner bits to cut out the larger holes first. A hole saw could work too if the radius is really big.   Then enlarged the smaller holes for the nylon motor mounting screws in increments using regular jobber bits.  This assures nice round holes.  Then use a band saw to free the part from the plate or panel.

      Voila - we have a part that looks like it was milled via C&C.  I can hold tolerances to less than +-.010 and that's plenty close enough.  I have a nice vertical milling machine with a digital indications in x,y and z but is as easily done on a simple good quality drill press.

      Regarding bonding, yes use a generous amount of structural adhesive.  Remove the excess with a wooden tongue depressor sanded to a small radius on one end.  I have several of these tongue depressors with crooks in the ends to get into difficult places.  Every bit of unnecessary adhesive removed is less weight and actually results in a stronger bond.  I would guess that the radius pictured in the earlier post is 1/16" or less.

      I have some experience here having built a full scale composite racing aircraft and flight testing it myself.  There is no pucker factor like putting all the body parts where the mouth is and landing all those parts at a minimum 100 mph. A building or pilotage mistake at double the speed will result in twice as many bones broken my laymans rule of F=1/2mV^2 made understandable.

      So, to avoid injury, lightly rough sand the parts to bond together using 120 to 80 grit where ever the structural adhesive will make contact.  Wipe the contact clean with MEK or acetone.  Apply 3M structural adhesive and mate the parts together.  Use the tongue depressors made earlier to carry away the excess epoxy. Let cure without movement and follow the 3M directions exactly for the proper environmentals.  Using the 3M applicator tool is a great assurance that the two parts are mixed properly and I highly recommend this even though more expensive than hand mixing. 

      A clean shop with proper tools makes for a good build.  And combined with good piloting skills, the results are successful safe flights.

    • MR60

      Jim - Sometime you should show folks how you can do such a beautiful job without a CNC.

      I'm not as good as Jim (especially piloting) at hand cutting these shapes so I cheat and use a 3-axis CNC. This is what i've settled on for the motor mount (cut out of the 1/8" Balsa Core Single Ply Sandwich Panel that Jim recommends).  They are not as light as what are shown in Jim's photos.  Subjective and definitive advantages are discussed below.


      Definitive Advantages:

      - As Jim pointed out, they are thick enough to handle the protruding aft side of the axle (about 1/8").

      - Best strength to weight material for the high peel and torsional forces (single ply carbon skin balsa core; Nomex core does not have the required peel strength).

      - Holes for motor cooling (in Jim's design the area is completely cut out).

      - Holes not located where bonding surface area is required to adhere to the frame motor mast. 

      - Holes (slots) for 19mm or 25mm standard mid-size motor mounting.  The slots are sized/located so that only 2 nylon motor mount bolts are needed in either the 25mm, 19mm, or on the MultiStar that uses a combination of both.

      Subjective (compared to Jim's design) Pros/Cons:  the mounts shown above form a complete circular base of adequate diameter for mid-size motors using the 19-25mm hole pattern.

      - adds weight (a few grams so slight) thus requires a harder working motor, higher vibrations, lower flight times.

      - provides an improvement (not likely measurable if at all) in motor mount stability.

      - increased protection of the motor from dirt/grime upon impact.

      - decreased airflow (not likely measurable if at all) causing hotter motors.

      Attached are two dfx files for those working with CNCs.  One if for the 19-25mm mounts.  The other is for the smaller class of motors using a 16mm hole pattern with base diameter sizing to match.

      Motor Mount 4 19-25mm in Inches.dxf

      Motor Mount 4 16mm in Inches.dxf

    • This is all very helpful, I like the idea of the balsa core which allows for the depth of the centre part of the motor.

      One thing that has slightly confused me:

      "The slots are sized/located so that only 2 nylon motor mount bolts are needed in either the 25mm, 19mm, or on the MultiStar that uses a combination of both."

      Are you holding the motor on with two or four nylon bolts? The reason I'm asking is that as well as building a large quad I'm also building a smaller one and have Multistar 2213 motors with spacing 16mm & 19mm. I'm using 10mm x 0.5mm tubes and have made and fixed the motor mounts. To enable me to get the bolts on I added two strips of 6mm x 1mm strips to add some height to the motor mount. If two bolts have sufficient holding power it makes things a lot easier for future builds. (I got a bit carried away with the Tow winding)


      I know you've done a lot of motor testing, do you put any lubricant on the bearings? I'm not sure whether to add some or not.



    • MR60

      Great photo.  If one doesn't have access to 3M 2216, the tow is a great solution.  With nylon bolts, the motors always shear away before the motor mount when the adhesive is properly mixed and applied.

      With 18" props, one can use 4 nylon bolts without washers so you can tuck them pretty close to the tube.  With 12" and below, you can probably get away with just 2 nylon bolts (size 4-40).  I also use 0.6" (15mm) mast tubes for up to 18" props and 0.28" (7mm) mast tubes for up to 8" props.  Thus when going to smaller 16mm motors, the tube doesn't interfere with the bolt pattern.  If it does, one can always add a X plate to the bottom of the motor.

      thanks again for the photo.

      I've added oil to BL motor bearings (at the axle top and bottom) but don't know if it is needed.  Good BL bearing are sealed so they only need oil after they take an unfriendly visit to the ocean.  I'm not an expert on this, so look for other opinions.

    • Thank you. It makes fitting the smaller motors on future builds a lot easier with only two bolts.

      I had looked on the web on whether to lubricate motors but there seems to be a split of opinions. I've just added a bit of oil to the 2213 motors.

      On another subject have you considered using ESC32 on your long flight quads to make them even more efficient (That's if I understand it right). You can also control the max current that the esc uses.

      7.2g weight of bare esc. If I understand it right the calibration matches required amps for your motor/prop combination.

      Last night I calibrated one of my Multistar 2213 via ftdi and Qground control with the ESC32 tab, then saved the calibration to the ESC32

      I've attached the results - one for the current limiter calibration and one for the volts/rpm.

      I must admit I don't fully understand it but it may be worth having a look.



    • Here is the CAD drawing.  I would only use the particular firewall hole pattern that fits the BLDC motor mount but you could do a two nylon screw mount and choose two holes of each pattern. Notice that each radius cut is outside to the part so simply put the right sized bit on the X and drill.  There is one inside radius relative to the part but you can omit it as I did as you can see in the picture in an earlier post.


    • Here is the equivalent firewall using only inside radius cuts rather than outside and easier to make using drill bits and hole saws without a C&C milling machine. See instructions on this thread.   Jim3701891910?profile=original

  • MR60

    Hey guys,

    It has been a while since I posted in here but we also come back on the crime scene.

    I tried to catch up with the tens of pages you exchanged, very interesting.

    In particular this new crossing the cf booms method. I have a question about it. Do you have detailed pictures or schematics about how the crossing booms are joined? How do you practically cut a middle hole? How do you ensure they remain perpendicular?

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