dirk8037 wrote: ↑Sat Sep 19, 2020 8:43 pm
Hi,
Does the printed core will have in your calculation take any statical load?
I wonder what you want to use for the print, since all the for mortal accessable filaments seems to me too week.
Also the bond between composite and print is a thing that, from what I know, none of them do realy do that.
Maybe not immeadiately but over time it might be a factor.
Just PLA+
It's easy to sand and almost as hard as regular PLA with a small amount of flex. At 60% fill I would say it has more density than pine.
It's mostly intended as a former with the strength being in the composite fabrics wound around it. I'm now thinking that some short prickles on the surface would facilitate winding with carbon tow which is much more reasonably priced than in woven form.
Could it be a solution only to print the Infille without toplayer and fill the spaces in between with a mis of cottenflakes, glassbubbles and epoxy.
Relativly easy to sand, good bond and most likely much more solid. (quite some work though)
Or substitue PLA+ for PLA and print at 100% fill.
For big wings and rider wight I would also opt more for 3mm through out the fuse and solid around the cup.
I have been trying arround with carbon tubes for that that had 2mm wall thinckness. The might seem stiff enough but seem still very fragile. If you have unwanted ground contact tha cause a small fracture the whole thing might fail suddently. Wich is maybe quite essential - to have a lot of impact resitance.
Basalt is rated at 2/3 the strength of carbon but much more impact resistance
At this area you have, at least a I envision it, the highest stress.
Torque from the board and rider through the mast by stearing on the yaw axis and due to the inertia of the wing on the role axis.
In addition to that you have the upward force of the wind focusing there, too.
If you look at the Gong wings with big chord then you'll notice the fuselage is actually not attached right at the back, it starts a few cm in. It's leaverage we need to be most mindful of as a stress. Particularly during a wingtip breach during a turn. These stresses are at an angle to both mast and wing and best handled by braid weave or double bias cloth. Or hand wound tow.
Since due to the cup the structure is not "solid" anymore, but having a whole, the forces will spread arround and the walls outward.
If the cross-section is round there will have on the top corner hardly any material to cope with that.
In one of the upper posts someone was talking about failurs of fuses in forced, loaded turns. I guess that could be one reason.
You have the spreading force from one direction in combination with torque on the yaw with lots of momentum behind it.
Having it square on the top side you have more material to take those forces.
I thought of making that part out of G10 plates on bottom and both sides that are rejuvenated on bothe ends and surrounded by the laminate. (because I have small plate rotting in the celler.
Or since you are anyway into making moulds, something to pour in a mix of epoxy and glass cuttings.
Dirk
Maximum compression and tensile forces are carried near/at the surface of objects. That's why 3D printing is still successful with low infill ratios. The 3D printable insert doesn't carry those forces because it's designed to be completely wrapped with composites excluding the 12mm tail rod hole. Ideally with braid that can be pressed down through the fuse hole in the insert to bond with the braid underneath. In this way a double tube is formed surrounding the inserted mast for continuous fibre paths. Before assembly, the pressed down braid can be stitched with kevlar or polyester thread to itself where it lies under the insert. For stiffness of the wing tips from yawing during impact and from winglift at the front, it needs unidirectional along the same axis as the fuselage. I see this as best achieved with an inner layer of braid, a next layer of thick unidirectional and 3 or more outer layers of braid.
If hand wrapped with tow, this forms the primary controlled path for tension and compression forces but to prevent tow splitting apart from each other (weakest direction) it still needs an inner braid layer which can be kevlar. It's exactly the same for the outer layers except that these are also impact prone, I'd use basalt braid for the outer most layer.