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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Hi Chris
I've attached a photo of a roll of tow. Basically it's carbon fibre strand (It weighs next to nothing). I'm just going to wrap one strand either side of each motor mount and brush with resin. I've read that another way to use it is to soak it in resin then with a rubber glove on pull the strand of tow between your fingers to get rid of the excess resin, then wrap the tow around what you are reinforcing.
Your sketch looks fine. It'll be interesting to see how and if it morphs as you think of new ideas. One thing that struck me looking at your sketch is the 12mm tubes. Have you considered a H configuration with the 12mm tubes mitered on to the 16mm tubes then one could epoxy a Nomex platform on to all four spars to reduce flex, this may mean that you do not need to use the carbon plate and may reduce the length of the 16mm tubes.
I will need to put some words together to go with my construction photos but will do over the next few days time permitting. I must admit I am very impressed with the nomex honeycomb, especially the 3.2mm aerospace one, it's very flexible prior to putting skins on.
Is your camera the 1.5 crop sensor one?
Hi Jon,
Thanks for that, good idea.
I did consider going for an H but thought, from reading here, that the frame would be more rigid with an X. The X would also allow a non structural isolated EP so helping with the gimbal vibrations.
In reality I plan to build a spider, like you, to keep the props out the way.
The Ricoh GR is 1.5 crop APS-C sensor with a 28mm (25mm equiv) very sharp lens and weighs only 280g. It also allows analogue video out and remote trigger over USB port, so is great to set up a shot. It does do video, but its not so great. I have a modified Xaircraft tilt gimbal I plan to mount on the front of the 12mm rails, so landing legs will only need to be really short, if they are needed at all. The gimbal can certainly be lightened by removing some of the screws and bonding certain parts, so expect it to come in at about 80g.
The Ricoh GR sounds pretty impressive with a good weight. I looked on DP Review the other day I think the earlier models had a smaller sensor. having AV out and remote trigger is really useful. I must admit I didn't know about this camera when I was sourcing mine or I would have seriously considered it due to the weight, sensor size and image quality, it sounds really good.
I got my NEX 5N body only off ebay second hand for just over £100. I already had some NEX lenses. I was originally going to use either my 19mm or 30mm f2.8 Sigma lenses but they are quite weighty. I ended up buying a 20mm f2.8 lens. The camera with bare lens and battery come in at 334g added to that is a hdmi to av converter at 4.3g and a gentled to fire the camera (I've managed to set it on my transmitter so that I can take photos or video)
Impressive Jon that you do carbon ply layup.
FYI. When you fly and put your luggage in the upper bins that everyone tries to abuse with luggage that doesn't fit ... yes ... nomex core with glass or kevlar skins.
If you are using ply or braided carbon rods (versus extruded), then you only have to reinforce the ends where they can strike ground. i like your tow idea. i've been using fiberglass caps bonded to the ends. it give the diameter axis strength from compression hits. adding tow around the tube also does that.
I no longer use metal X mounts on smaller motors. Turns out that small motors throw small props that require smaller diameter motor masts that allow for a nylon bolt directly into the bottom of the motor.
You can take a look at the post I just did on building a 300 size FPV quad that elevates the EP off of the frame on Tarot GoPro balls.
Sketch looks good (you will revise this as you conceive the build). You will find that wrapping sand paper around the continuous motor mast and mitering the other two motor masts into it will be one of the simplest manufacturing elements of the process. It's amazing now easily carbon tube miters on sandpaper.
Bond everything using the Scotch Weld 2216. It doesn't take much. Thin layers are good.
Indeed, I was planning to bond the rails, but your previous post led me to that suggestion. Although I immediately see in my design that the mounting points will be possibly too close to the centre of the rails for the length of the rails. - as you say - will be revised !
Great tip on sanding, thank you !
I was suprised you directly attached the FC to the frame, rather than on the suspended EP, usually I see builders using some form of vibration absorbsion for the FC - I guess its because your frame design is so stiff.
Logically bonding to the motor mast makes perfect sense--you are trying to control the stability of the motor mast so any isolation from the motor mast hurts.
Logically isolating from the motor mast also makes perfect sense--200 Hz vibrations are not handled well by the APM (no problem for the Pixhawk), so put the FC on foam. The problem with this though is that neither foam nor gel have reached the technology level required by them to work (there must be 15% to 30% natural compression of the material to work). The FC is too light.
everyone that has tested before and after has given hard evidence that you never want to isolate the FC from the prop plane. direct bonding works best. the sayers of isolation never show hard data of the before and after. By bonding directly to the frame:
1) pitch/roll stability is always reduced (no latency)
2) in most all cases vibrations at the FC are also reduced (the FC is too light for foam or gel to help so foam or gel makes it worse).
And you a very correct. The solution to get the best of both worlds is stiffness of the motor masts.
P.S. I sand over top of a drawing of the angle of the motor masts. Then check the groove as you sand to keep it centered. It takes less than 3 minutes per mast using 150 to 300ish grit sandpaper.
Great thread.
@Jon - How is your quad coming along ? I'm planning on something similar, I want to make something as small and light as I can to carry a Ricoh GR and tilt servo gimbal of about 450g weight.
Here is a European link honeycomb elecronics plate:
http://www.airfighter.eu/epages/1545...roducts/902185
and a European link to engine mounts with the rubber washer:
http://www.pitlab.com/pitlabshop/accesories-for-multirotors.html
cheers
chris
Wow thx for the european links. The Pitlab link is great for carbon tubes and fixing accessories and for a good price too!
Yes, the sandwich panel is comparable (1-ply carbon). What the spec didn't say was the weight of the finished board (only the weight of the carbon ply). If you bought some, did you weigh it? Nomex comes in various densities.
The motor mounts look really sweet. They even show them with nylon screws, a great weight saver. Their downside is that the clamp motor mounts for a quad (as light as they are) would still weigh about 48 grams total? If instead, you cut them from single ply 1/8" carbon/balsa (a nomex bond isn't strong enough), and bond the mounts to the booms (they will never come off and won't get out of square), they only weigh 4 grams total (including the adhesive). For folks that don't have the ability/access, i can cut and ship motor mounts if interested.
Jim and I are trying to get a gimbal to less than 200 grams. If anyone gets there, please document.
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