Durability

[Quick-Flash]

Have you done any crash testing? I understand that it is more stiff than most 5 inch frames, but if any part breaks it'll require a complete rebuild. Have you considered using rolled carbon tubes instead of the pulltruded tubes?
This is a very interesting design, I'm just wondering how feasible it is to make one using carbon fiber nylon filament as in other testing I have done that is far, far more durable than PLA or PETG, but I do worry about how well it will glue.
I'm particularly worried about the motor mounts, I'd imagine its not too hard to get into a crash where the motors want to twist along the arm and end up causing the glue bond to break, or just break the print.

[janherich]

@Quick-Flash

Regarding tubes, the they are fairly thick-walled and even in pultruded versions, they have good crush resistance, for bigger dimensions like 8mm OD main arm tubes, I'm using pull-winded tubes.

I had a fair amount of crashes, and I updated the design multiple times, taking the lessons into account, resulting in changes like:

• Motor mount using long bolts going through the whole part which are hugging the arm tube + compressing the motor across the Z direction (layers). Previously, the motor mounts used regular short bolts + bolt channels, that was not very durable because the tall motor mount "tower" was loaded in bending in the worst possible direction and sometimes the motor mount snapped in relatively benign prop-strikes.
  In contrast, motor mounts are now super-durable and since that change, I never broke that part, even in serious crashes where the whole frame was destroyed.
  Downside is slightly higher weight from longer bolts and necessity to use separately sourced longer bolts not coming with motors (12mm M3 for the 5", 20mm M3 for 7").

• Beefier central node, as that was the weakest point after fixing design of motor mounts, sometimes in harder landings, it cracked and because it's normally hidden, it was hard to notice -> but frame was suddenly a lot more flexy in torsion and flew like shit with a lot of vibrations, this was problematic especially in cold freezing weather

So now I'm quite happy with crash resistance, quads are able to survive light crashes, harder landings, etc. just fine.
Of course, they will never be as rugged as classic frames, and things like fall from treetop height, collision with hard object at speed etc. will always result in destroyed frame.

That said, repairing the frame after crash is actually super cheap -> tubes almost always survive without any problems, same with motor mounts, so it's just mater of printing new central node + 4 corner nodes, cleaning tubes and reassembling the frame.

Regarding materials, PA12-CF15 works very well and glueing is not a big problem. Epoxy has good adhesion to (scratched/sanded) CFRP and poor to most plastics, but here it works more on a principle of interference fit -> gap between cfrp and inside of 3D printed socket is filled with epoxy and because inner texture of 3D printed is quite rough, it provides very good mechanical (if not chemical) bond.

[vaska1992]

So you're saying that basically carbon tubes never breaks in crash, its always the 3d printed parts?
With that, i'd say a strong enough filament needs to be found for it. I think nylon is perfect for it, it probable will never break, but its hard to print it.

[Quick-Flash]

So you're saying that basically carbon tubes never breaks in crash, its always the 3d printed parts?
With that, i'd say a strong enough filament needs to be found for it. I think nylon is perfect for it, it probable will never break, but its hard to print it.

Carbon nylon is easy to print. I've made a few frames with it, that were quite durable, but it always ended up working poorly as the nylon would absorb water and get less stiff to the point that it didn't fly well. These look like they won't have that issue.

[janherich]

@vaska1992 Yes, although that's expected and I would even say preferable. Nylon is fine, it's around the same strength as PLA/PETG, but less stiff with much higher elongation at break, so it would be able to absorb a lot more energy before breaking, theoretically improving the durability of the frame a lot.
There are couple of potential problems with that though:

• FDM printed nylon unfortunately doesn't exhibit as good elongation at break and toughness in Z direction compared to X/Y
• You would need to use matching epoxy/glue, which is also not brittle and stiff, but tough and elastic, otherwise glue bond would break/shatter
• Introducing elasticity to nodes would decrease the stiffness of frame (but maybe dampen/isolate vibrations from motors for example...)

It's an interesting idea and maybe frame with less stiff but much tougher nodes from nylon/hard tpu would work well enough.

@Quick-Flash I suggest you try PA12 nylon, it doesn't have moisture absorption problems associated with more common PA6/PA66 nylons

[Quick-Flash]

@Quick-Flash I suggest you try PA12 nylon, it doesn't have moisture absorption problems associated with more common PA6/PA66 nylons

I can't remember which nylon I used, it was one of polymakers, I can check later though. From this graph pa612 will give the best results. PET CF17 also looks interesting and fairly cheap https://us.polymaker.com/products/fiberon-pet-cf17?variant=44225304231993
However wet PA6 CF20 has very good impact strength when wet (supposedly 3 times greater than PETG). So if it is stiff enough while wet, then it would make for a pretty durable frame.

[Quick-Flash]

I've also been thinking of the possibility of using some sort of screw mechanisms to hold the frame parts together.

Maybe something like hose clamps to hold parts together? If not those there has to be something that could clamp the tubes well enough to not require glue, but maybe I'm overly optimistic.

[Quick-Flash]

I've thought of having two cross braces under the flight stack instead of just 1. You'd need to file down some CF tubes and glue them together to make it work.
Kind of like this guide. https://imgur.com/a/building-tube-frame-hEevI

[janherich]

@Quick-Flash good ideas with tube clamps, the tool I'm using to generate spaceframe nodes support many node types, including screw-types, I used them extensively for my 3D printers, where links are stainless steel tubes -> but it will always be a lot heavier for the same stiffness and more bulky/less aerodynamic compared to sleek joints with just epoxy bonds.

Regarding the diagonal tube, as it's solid tube which is very similar stiffness in both compression and tension, it's unnecessary to add a second diagonal tube, the base is fully triangulated as it is -> if it was wire brace which works only under tension, then yes, cross-bracing would be necessary (and it would save some weight, but require proper tensioning).
Not worth for small 5/7/10" quads, but could be very interesting for truly big quads -> it could actually work with no glue/adhesive at all, using wires with loops at the ends, notches in corners to secure the loops and dry sockets in corners for perimeter tubes (wire tension would push the 4 perimeter tubes into sockets), only central node would need tube clamps... Thanks for the idea !

[Quick-Flash]

@janherich you mentioned that the central mount where the FC goes was a weak point. What if there was a small piece of flat carbon (0.5-1mm) that you glued to the top and bottom of it, then when you put your FC on you can clamp the two carbon pieces in between the printed parts and get a piece that is much stronger in the Z direction.