Hi, I bought these very cheap hobbyking rectangular carbon tubes. I made this not so serious experiment with them as they could potentially be great to build your own multicopter frames. Tridge, by the way, used those recently in his quadplane build he posted about on diydrones.
And, this is a not so serious contribution to coming Easter chickens...
Enjoy Easter everyone!
Hugues
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@Starwalt, thx. Yes plywood is great for prototyping and I find it much better to dampen vibration than 3Dprinted parts in PLA.
I love it Hugues... you are using plywood!
When 3D printers are being sold in yard sales for $25, hobbyists will still be using plywood.
-=Doug
I used to have a description of the double O-ring motor and frame mounts in the vibration page of the DIYDrones wiki along with a picture of the SteadiDrone motor mounts which clearly showed their double O-ring construction, but that seems to have been deleted from the wiki.
I believe their current ones feature it as well and they showed a clear reduction of vibration from the use of them.
Basically they are a typical round tube type doubled U clamp with a groove in the middle portion for the O-ring which is held in compression in the U clamp.
Silicone O-rings are preferred over Buna N because their damping qualities, lack of heat softening and cold hardening and their aging characteristics are better and they provide considerably superior stiction to the tubes preventing unwanted rotation.
Steadidrone no longer sells that particular drone or motor mount (which was available separately, but I believe their current motor mounts still incorporate this form of vibration control.
I am currently working on an H copter design which will actually use a cast split block of silicone in the middle (connecting the arm to the frame) and a silicone O-ring incorporating U clamp type mount at each end of the motor mounts.
To illustrate the use of these rectangular carbon fiber tubes, Hereunder some pictures of the anti-vibration camera gimbal mount prototype I am working on. In this application, to reduce vibrations as much as possible i am using the mass of two batteries on the "clean side" , that is where the camera brushless gimbal is attached to.
Doing so I had the problem, each battery weighing 1 kg, that the 2mm carbon plate holding them was flexing too much (and might even generate a flapping effect in flight).
So to make the whole battery mount rigid, I used a 30 cm long rectangular carbon fiber tube, which is sticked to the battery plate. Two attachment element are 3D printed and plugged to both ends of the tube.
The fact that they are rectangular facilitates alignment of everything you need to be "squared",
I definitely think twice before drilling holes in my structural units, Gary. Especially the fibre-based ones.
I try to drill the sides, not top/bottom (most of the stresses are taken by top/bottom, and the sides are less important. though not unimportant.)
In fact, for this reason, I think the omnidirectional strength of our current solutions is probably inefficient.
Try not to drill larger gauge than necessary, and try to brace the tube from the inside, tightly. Wood braces better than metal (ask me how I know that!).
I suspect, too, that filling the hole with epoxy and re-drilling through the epoxy might be worthwhile.
However, sometimes, a bolt adds more than it subtracts.
My 'airframe philosophy' is, roughly - if the frame breaks because of a crash (as opposed to the other way around), the frame isn't the problem. Something more serious is.
And as Andrew indicates, sometimes breakage is better than no breakage.
In fact, for us Canadians at least, Transport Canada seems to be headed toward requiring airframes with 'frangible components' - ie, components that will break/shatter on impact, rather than remaining intact.
George
Hey Gary,
Can you show an example of your o-ring clamps? For the quad frames I have (tarot fy650, HK talon v2), they don't have any rubber between the aluminum clamps and the carbon fiber tubes.
Hi George,
Yeah for a lot of construction it is easier to incorporate square tubing, but for omnidirectional strength, cost and if applicable air resisance, round tubing can't be beat.
Round tube is the same strength and flex in very direction and when you are "crashing or performing some other unknown or not recommended maneuver, that can be a great characteristic.
Also typically with "real" CF laid tubing, square or rectangular tubing is a lot more expensive to make and subject to wider strength variance in manufacture.
I try to work with CF or G10 Plate and round tubing and mill round adapters where the joints are made out of either aluminum or G10.
For multicopter Arms I often use double silicone O-ring compression mounts at both the frame plates and the motor mounts which provides impact flex and which serves to provide a bit of vibration damping as well.
This is a very well proven system and is more or less used by the SteadiDrone multicopters.
If you build the arm mounts in such a way as they do not allow any flex, when it crashes they will almost inevitably fail right at the outer edge of the joint.
At first glance it is certainly easier to incorporate flat tubing into designs, but if they are mounted in such a way as to alleviate stress, it is often not much more difficult to incorporate round tubing with the above listed advantages.
CF in general has the disadvantage that it tends to be brittle, basically it can withstand a huge amount of bending or slow force, but hit it with a hammer and it can shatter like glass (also when crashing).
This is why I like to use fiberglass either G10 or FR4 its flame resistant equivalent whenever possible, it is not as strong or rigid per unit weight as CF, but it is a lot more forgiving of impacts.
It is a pain to machine and really requires microcrystalline diamond tooling for a proper finish and decent tool life.
CF is actually somewhat easier to machine than G10.
BTW, drilling holes in CF can easily introduce crack starting stress points, it needs to be drilled thoughtfully with the right drill and deburred if at all possible.
Benbojangles> Tubes vs Squares
Any thoughts as to which is better? I can't help but think squares are easier to use with 3D printed parts for easier, more accurate alignment and less chance of slippage or rotation during flight
In my experience tubes are generally unbeatable in terms of strength to weight ratio and stiffness, although this greatly depends on expected shear force and bending moment direction, i.e. a square or rectangular tube of equivalent weight may have better characteristics in some directions. And when it comes to ease of assembly, alignment, and slippage, square or rectangular tubing are definitely great. Less or no need for clamps, fewer additional parts needed, etc ...
Short lengths of wood glued into the ends of tubes will go a long way to protect against crushing them by over-tightening motor mounts or attachments of arms to centre plates, or while drilling holes in them. Even balsa is helpful.
I agree on the epoxy taking energy out of crashes - I think it's better to let it break (and focus on why we're crashing instead!). But, if you want, the bonds can be re-inforced with zip-ties if needed, at virtually no weight penalty. (yes, zip-ties!).
Square tubes are just easier to work with - attach things. Wind drag is a concern, definitely, but on the other hand, they lend themselves much better to gluing balsa leading edges onto (not much weight), for a more aerodynamic profile than round tubes (yes, I tried. I just can't be bothered so much now).
If you're worried about things sliding off round tubes, one solution is to seek out square tubes that your round tubes nest very closely into (as I mentioned above), then epoxy short lengths of the square tube just onto the spot you want it. I would also put a narrow-gauge bolt through the two (horizontally rather than vertically).
George
Hi Andrew,
If you want epoxy to actually hold on carbon fiber (or G10 for that matter) the 3M ScotchWeld 2216 family beats pretty much everything else.
You can find a link to their PDF here on my drones are fun link page.
http://dronesarefun.com/InformationAndLinks.html
Forrest recommended this first and it is way better than everything else I have tried.
If you rough up the surface first and mix it and cure the epoxy properly, it is very unlikely that the glue joint will let go first.
On the tubing in this article, I would expect the "foil" coating surrounding the pultruded tube to fail first.
Usually you can find a good deal for medium viscosity gray surplus on Amazon - it is not cheap.
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