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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test results on this motor so far as compared to the most efficient T-Motor MN3508-29:
- more efficient when compared at the same voltages (means that the T-Motor is generating more thrust)
- as efficient as the T-Motor when driving a 16" prop at identical PWM. But needs to be run at about 1.4x the volts to generate the same lift.
- it can throw a larger prop more efficiently even when compared at identical thrust and PWM (the perfect apples to apples comparison; hold thrust and PWM constant, vary volts, compare the watts)
i'm exploring the limits of the later now.
Thats interesting news, look forward to hearing your findings.
Jon - keep the posts coming, looks like you are doing a great job there !
I'm playing around with Frantz's spreadsheet at the moment, probably going for a slightly smaller frame, with a 15" prop max size spider style - must admit coming from a non-engineeering background finding there is as awful lot of information to process - still trying to work out what some of it means, but its a great tool - thank you Frantz
Well I said I would put some words together on my current build so here is the first part:
Electronics platform
This is a photo of the EP template and the carbon fibre material -Carbon Fibre Plain Weave 1K 90g per sq metre
This photo is where the resin has been applied to the carbon skin
I obtained some very helpful advice from Easycomposites on the process
I had a spare sheet of glass which I cleaned and then applied easy-lease release agent by wiping it on with a cloth, six coats leaving 15 minutes between each one. Final coat left for an hour before the next step.
Taped down the carbon fibre.
Used my weighing scales to measure 10g Epoxy laminating resin and used a syringe to measure 3ml of hardener and mixed them together. Couldn't find a plastic spatula to spread the epoxy so I used my decorators flexible filler knife and spread the epoxy. Then went to work removing the excess. The flexibility of the knife helped a lot here. Next time I would spend more time on this so that I could scrape more epoxy out of the carbon fibre. Excess epoxy means more weight. Left it to cure overnight and gently peeled it from the glass the next afternoon. Then repeated the process.
As regards the weight I calculated that two sheets of the carbon fibre material 20cm x 30 cm should weigh 10.8g plus epoxy weight.
For the 10mm Nomex based on 48Kg per cubic metre I calculated the 20cm x 30cm should weigh 28.8g. Stabilised compressive strength is quoted as 348psi and stabilised compressive modulus 20.3 KSI
Prepreg carbon 1.5mm thick 20cm x 30cm is approx 150g
I measured the thickness of one of the skins and it was between 0.15 to 0.18mm and weighed 8.3g
The 10mm Nomex weighed in at 20.8g
I weighed out 10g of epoxy + hardener and spread it on a plastic sheet just a bit bigger that the EP.
I then put the Nomex on the plastic sheet so that one side would receive the epoxy. I moved it slight from side to side to make sure there was enough epoxy on. I think this was a mistake I think I should have just placed it down and them lifted it of to reduce the amount of epoxy build up.
Place the Nomex on one of the skins Place a couple of large heavy tiles on top and left overnight before repeating for the other skin.
The Nomex with one skin weighed 30.8g with 2 skins 41.2g (I take it that when one knows where everything is going to be placed a lightening exercise can be performed.
The photo below is the completed EP
I also made an EP out of Woven glass ultra light plain weave 25g per square metre and Nomex aerospace honeycomb using the same technique.
The aerospace honey comb is a lighter weight at 29Kg per cubic metre. Stabilised compressive strength of 131pse and stabilised compressive modulus of 8.7ksi.
I calculated that a piece of 20cm x 30cm should weigh approx 5.22g and one piece of woven glass 20cm x 30cm 1.5g before any epoxy applied.
The EP with one skin weighed in at 7.6g and 12.1g with 2 skins attached. The thickness of the skin was 0.02 to 0.03mm
The calculated weights of 20cm x 30cm for the Nomex and material is just to give a feeling of size and weight. I find that so many Kg for a cubic metre is difficult to relate to.
The aerospace EP didn't come out as well as the heavier one possibly due to my dipping process of the honey comb into the epoxy. It didn't 100 percent stick to all parts of the woven glass. However, it is the first time I have done anything like this.
Initially I was looking at the EP to add rigidity to the frame but using Forrest's technique of joining the carbon tubes the frame feels very rigid just by itself. So I am tempted to just use the 12.1 g aerospace EP which will only have to hold the electronics.
I read somewhere that Nomex has some vibration absorbtion qualities. If it has then that is another plus for using it as an EP.
I also made a battery carried using the aerospace nomex and a combination of the woven glass and carbon material. I'll put some words together for the next installment.
I hope someone finds this useful.
Fascinating, I have lots to learn !
So are you keeping the arms as two pieces above and below the EP, or going to use 3 sections mitred together ?
I think we all have lots to learn and by everyone sharing their ideas and their build process helps create new ideas/solutions.
I've already done the basic layout photos below. Used Forests' calculation for the size of the joining brace but didn't have the right size carbon strips. So the top strip is made up of 3 x 1mm x 6mm strips epoxied together. This will also allow a slot to be cut into the 3.2mm EP platform and the EP should not have the bracing strip protruding above it. The 10mm x 10mm box section was epoxied to the bottom to make up for the undersized top strip. It feels pretty strong with no flex. Overdid the epoxy a bit
really nice work!
Regarding engine mounts:
Frantz, I take your point about the boom blocks and nylon screws adding excessive weight.
The larger motorsseem to generally be mounted directly onto the mounting plates rather than with the extra metal X bracket used on the smaller ones. So how about using half a boom block on either end of a mounting plate, all bonded to the arm. This would give the mounting plate the required offset from the arm and save some weight by mounting the motor directly ?
Mounting plate:
http://www.pitlab.com/pitlabshop/accesories-for-multirotors/motor-m...
That would come in at about 8g per motor
the plate is stiff enough (i also used to use 0.060" fiberglass). doing what you suggest would work. what i've also done with a thin motor mount that doesn't have enough thickness for the protruding motor axle on the back is to lightly drill the center out (going into and sometimes through the motor mast tube itself) after it is directly bonded/cured to the motor mast. But either way works. Also using a double plate might give you the required thickness.
Interesting points to consider, part drilling the motor mast is a great idea, that would halve the weight without sacrificing any strength
Thanks for the updates Jon and Frantz, all very interesting.
Jon, I would love to see some pics when you are ready, it does sound like you have been applying a lot of thought to your build. I wasn't sure what you meant by 'tow' ? And how do you plan to attach the motors, many of the large ones don't appear to come with the metal mounting plate Frantz uses with smaller motors.
I've been having some thoughts for a build:
The main X frame would be one full length tube and one tube cut and mitred to be bonded either side as Frantz has done on the mini quad with bonded carbon plate to reinforce - Frantz you discussed this method earlier in the post thread. One of my concerns is how easy is it to profile the cut tubes to butt up tightly against the uncut tube...
I'm not sure on how to attach the 12mm rails, bonding or on compression dampeners.
I suppose the electronics platform would sit on the rails.
The rails would allow secure mounting of gimbal and battery with scope for balancing depending on battery size and gimbal, as well as adjusting the camera FOV depending on the lens - so basically some flexibilty in the payload.
I've attached a hand-CAD sketch of what I'm thinking about, would love to hear your thoughts.
frame.jpg
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