Sketching out an idea for framing. It revolves around the idea that the fuselage frame is 3D printed as a shell (spiral print). The structural strength comes from the internal structure which should be light but stiff in bending and twist and also strong. Obviously these look like solid boards but they would be strategically hollowed for lightness and to hold hardware bays. Bulkheads for sure can be 3D printed, but the length-wise walls may need to be stronger. CNC’d ply?
First problem is, these bulkhead will have to be inserted and fit within the printed fuselage shell. If the fuselage is printed in sections, the seams could be chosen at or near the bulkheads, possibly resolving this problem.
You folks have ideas? Let’s kick some around.
I’ve never designed/built an aircraft of this size, or construction type. I’m doing this for the challenge, fun, to sharpen engineering skills (professional development), and to explore and encourage community participation and influence in mechanical hardware design. Plus, it’s collaboration with @Brook_Drumm .
I’d print each of the wings and tail stabilizers separately, then get the internal frame lasercut from acrylic for initial experimentation, maybe FR4 or something lighter / stiffer for real version. As far as installing, if you insert the skeleton within the fuselage and use a syringe with needle tip do dispense superglue or similar, do the same for the wings/stabilizers, bond them on and your set!
I’d make sure your internal cage has cutouts to accommodate a receiver, pixhawk (?), servos for driving flaps (be really cool if they were internal to wings, mount servo to skeleton insert in wing, then poke actuator wire in and attach to arm.), battery, and motor mount.
Painting the sprial vase printed parts with epoxy or that smooth-on print coating (basically a fancier epoxy) could prevent layer separation from point forces - the biggest risk to a single wall print. Also, I’d print with a pretty fat extrusion width / nozzle - it should stay fairly light as long as it’s one perimeter.
@Anthony_White , yup, wings, and stabs are separate parts. Internal ‘cage’ will be hollowed out for hardware and to lighten the frame. Thanks for sharing your ideas so far :).
I hear ya, that’s why you see me moving at a snail’s pace. We started in December.
You mean split lengthwise?
We can try anything. I’m open. The benefit of substructure approach, is there is no need for bulkheads, the internal frame can be a simple rectangularish tube.
There is merit in a carbon fiber reinforced shell. My gut tells me that it would be heavy for the equivalent strength of internal framing. If it’s easier to fabricate, that would be a reasonable reason to go that way.
You’d still need some form of framing to handle the loads from the wing spars and the loads on the stabilizers.
@Trung_Nguyen I think that is very constructive. We are open to finding the best solution. I haven’t been much help lately. Business gets in the way. He is doing the design, my role is to figure out a way to print it. And I’ve fallen behind. So I’m up for anything that works.
The model being designed is huge, 1/4 scale. It has a 2.5m wing span and a fuselage length a bit under that. It will weigh a number of kilos. The loads on an airframe of this size and mass will be significant. My instinct tells me 3D printed material alone, even with a helical type framing, will be insufficient to sustain these loads. Particularly if the 3D printed layers are sliced along the length of the fuselage. Structural strength will have to come from a stronger and stiffer material.
My thoughts so far have been to use some strong/stiff plywood (or other composite sheeting) walls, held up by 3D printed formers. We’d CNC out a truss like shape in the lengthwise walls to increase the strength to weight ratio. These walls would provide most of the required structural strength, and will absorb the large loads coming from the wing spar.
The formers or bulkheads would be 3D printed and mostly hollow and are mainly there for the purpose of holding the 3D printed skin to the frame walls.
I haven’t decided yet and the pic above is just a sketch to convey the idea. Ideas are welcome :).
This is a full sized aircraft’s bulkheads. They are mainly hollow and resemble the helical design, only orthogonal:
Agree that FDM printed skin - even with helical structure - will be weak in the bending direction due to layer adhesion. (assuming z-axis is longitudinal axis of the plane) I was picturing integrated bores in the helix/bulkheads for CF shafts lengthwise. Also, while I’m daydreaming, I would like to see a structure that varies in density depending on the concentration of loads in the area - like variable meshing for FEA. (I know, getting into ultra complex modelling here.) Back to reality, what I’m suggesting would probably be more feasible in a simpler air frame like a flying wing.
