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Jan 26, 2019, 05:54 PM
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Thanks for the info and advice, I agree that this is basically a balsa wing with other bits on it. Thing is I have discovered 3d printing and like it so will continue with the general principle.
I appreciate the aluminium to dowel comparison.
A friend mentioned wood distorts depending on humidity but I think that is over thinking it a bit. I looked at ribbed dowel rods in a local diy store and a lot of straight ones. If embedded in polystyrene block and gorilla glued should be no problem. Their only purpose is to resist compression and tension. It may take a few weeks but I will give the spars, dowel and polystyrene a go and see how it performs. Cost is 4 dowels a bit of PLA and the rest I have to hand. When done I will post a photo and stress test results I can then try cladding it.
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Jan 27, 2019, 03:28 AM
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Wood warping with humidity? Definitely over-thinking given that RC planes have been made from balsa wood for decades!

I printed a simpler rib with just a single wall and 10% infill. In ABS the mass was just 1.7g. I also think that the torsion issue might go away with a full span wing. I added a coat of paint to the top skin and it looks very promising for making scale subjects. So definitely a wing can be made light enough with printed ribs and acetate covering.

However I think that a lattice wing rib structure may make life a lot easier - simply print the wing lattice and cover with acetate. Here is an example of the structure I have in mind: https://www.thingiverse.com/thing:2781182. This particular example is too small to cover in card film, but at around 1m span, I think this could be your answer.

Maybe one of the skilled CAD folk can point us to a video tutorial on how to create such a structure.

It will probably still need a regular spar, and will definitely be heavier than simple chord-wise ribs, but will be much easier to construct.
Jan 27, 2019, 05:22 AM
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This is my take on a 3 mm thick 'printed' rib.
Name: WingRib.JPG
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It uses a 'triangle' infill pattern with the density and position set so that it creates a 'warren' type bracing pattern along with a single bottom layer which acts as a 'web'. At that size the rib takes 13 minutes to print and weighs 1.3 g.
Also done this way means a double wall count only applies to the outer rib surface so adds local rib strength for only a small weight penalty.

Not used with acetate sheet but with 2 mm Depron, which I can get!
Name: LHwing3.JPG
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The rib spacing is 2" (50 mm).
If you look carefully you can see the tapered 2 mm thick hard balsa spar flanges that are glued to the top and bottom of the ribs. There is also a Depron shear web between them. Done like this means the spar ends up flush with the wing outer surface so giving the maximum possible spar depth.
The completed 52" (1320 mm) wing.
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Jan 27, 2019, 06:10 AM
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I love this hobby, seems like you have a really taken this idea up. Great to take ideas and adapt them until you have what you want.

Good spot on the flying wing, I have figured a basic way to do it, see photo. Tricky to print as may need supports on a thin edge but many possibilities. Basically I took a solid spar section 20cm chord and made it 20cm long laid flat on slicer build plate (prints in 1h40 at 16g). Then made the outer skin 0 and infill 2% (a trick used by Carlito on the Gasb jet). and there you go, double infill % is double weight. some work to do joining parts as they have thin edges.
I like playing in the slicer (Cura in my case) as I found I can double the infill line thickness easily, but also doubles the weight.

Here is another scary thought. If you look at the second photo it shows the same spar but sliced along the chord (ignore the small bit just to show profile). So you end up with a thin bottom bit and thicker top, advantage is no supports needed as flat. Then I thought, what if you could print it directly onto a piece of thin plasticard or acrylic (no idea on adhesion properties, possibly sanded to roughen) so an immediate rigid centre, then glue the top half of the chord. Also 3mm foamboard, paper covered may work as a centre to print on, that should stick at least, just take 3mm out of the chord cantre.

Probably getting too complicated here but just a thought. It may well stick enough to run some epoxy around the edges before removal from the build plate. Top and bottom could then be offset, like bricks, to overlap the joins. Once covered in sheet should be very rigid, possible to cover the structure by epoxy glue onto the infill so outer is fixed all round.
Just more thoughts, I think we have something here. For me I am not into scale so much but am looking for an easy to build light wing and this is certainly heading that way.
Jan 27, 2019, 06:32 AM
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You posted the Depron version as I was typing That is what I was trying to get to originally. Surprised the depron works at 2mm, but obviously does.
Could be some mileage in future for the printing onto papered foamboard centre using infill only, then depron covering. Should be light and strong, the depron could still be covered in thin acetate to give the scale effect extreme is looking for ?

Great to have started from, can acetate cover a wing, to some whole new combinations .

I like the ruler by the way, think I had one like that at school. Whatever happened to twefths of an inch eh
Jan 27, 2019, 09:03 AM
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Quote:
Originally Posted by charentejohn
Good spot on the flying wing, I have figured a basic way to do it, see photo. Tricky to print as may need supports on a thin edge but many possibilities. Basically I took a solid spar section 20cm chord and made it 20cm long laid flat on slicer build plate (prints in 1h40 at 16g). Then made the outer skin 0 and infill 2% (a trick used by Carlito on the Gasb jet). and there you go, double infill % is double weight. some work to do joining parts as they have thin edges.
I like playing in the slicer (Cura in my case) as I found I can double the infill line thickness easily, but also doubles the weight.

Here is another scary thought. If you look at the second photo it shows the same spar but sliced along the chord (ignore the small bit just to show profile). So you end up with a thin bottom bit and thicker top, advantage is no supports needed as flat. Then I thought, what if you could print it directly onto a piece of thin plasticard or acrylic (no idea on adhesion properties, possibly sanded to roughen) so an immediate rigid centre, then glue the top half of the chord. Also 3mm foamboard, paper covered may work as a centre to print on, that should stick at least, just take 3mm out of the chord cantre.
If I understand what you are doing, this can only work with a flat bottomed airfoil, since you will otherwise have extreme overhang where the front cambers up. For me the main attraction of 3D printing is to accurately print good airfoils. I can't see any merit in going to all this trouble for an airfoil that not only is cr*p, but which can be made far more easily and lighter by other techniques (assuming you can find a reason to use it in the first place )

EDIT: But what you've done did spark a thought: I like that the print thread now runs in a way that makes the wing structure strong, rather than chordwise as in normal prints (i.e. breaks occur along the seam lines, which are unfortunately also along the plane of highest stress).

Is it perhaps possible to cut up a wing along a horizontal plane - i.e one that runs from the center of the TE to the center of the LE and from root to tip?. So you'd end up with a top and a bottom half. You can't print these normally because 99% of the skin is an overhang....but if you are just printing a rib and spar structure (be it 'X' shaped or conventional), then the overhangs can be kept short or eliminated entirely (ribs are printed solid, but only 2-3 perimeters thick, so are kept light enough).

I'd have thought this would be easy enough to do in CAD:
  • Create a wing surface by extruding the airfoil to the desired span (and taper)
  • Create a separate lattice structure and intersect the two bodies
  • Delete all but the now airfoiled lattice structure (this might be tedious). Include a LE and TE if required
  • Cut the wing up into sections that can fit on your printer
  • Slice each section along the LE-TE plane - now have a top and bottom part for each section
  • Print flat on the print bed (care needed for warping no doubt)
  • Glue sections together to form the wing and cover with acetate/depron (adding spars if needed)

I think this could work in Sketchup, which is hardly the most sophisticated tool. Anyone know of a simpler way to do it?
Last edited by Extreme Sports; Jan 27, 2019 at 09:52 AM.
Jan 27, 2019, 10:15 AM
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Thought it was time I did some work and printed a test piece. 150mm wide section single wall and came out well, I took a photo before removing from the printer but all ok I have tried leaning on it and it does not give, sure I could break it if I tried but once attached to a covering surface should be strong, and light (6g). So a 1m wide wing would be about 40g, call it 60g with the lower bit. In this case + foamboard and should be very light.

I will be interested to see if it needs any other bracing, maybe not if I tried the 'print on foamboard' option. Reckon if I tape a piece of foamboard onto the print bed in place of the current glass, print on it then glue the grid on again just to be sure. To get round the flat bottom bit this will work. As the bit below the chord is a very small section (see photo) I could print these as is at 5mm wide gives 2g. Just space as needed on the underside of the foamboard centre, then wrap in whatever. The X part of the grid is very strong, as you would expect, can be racked sideways but not if stuck to a board.

Again to get around the flat bottom bit just print as I just did, and the bottom as photo, then glue them together without the centre board. May be a little floppier until covered. Once I can source some paper covered 3mm foamboard I will give this a try. If I need to brace along the length of the wing I can use a soldering iron to burn away the PLA and glue a length of something against the central foamboard.
Just to note the squares in the grid are 3.7cm square at 2% infill. If I had to glue it I think perhaps a tube of 'pseudo epoxy' stuff put on the grid and the foamboard folded over it till it sets.
When printing the grid it may be as well to take the 3mm for the board out of the top part, so less printing of the grid.

This will help with a current project where the wing I ended up with is really too heavy, no heavier than all printed equivalent but I was trying for lighter . This should be no heavier than the stepped wing these guys used
Flite Test - Airfoils - PROJECT (9 min 47 sec)
(6 mins into video) as actually less foamboard.
Jan 27, 2019, 10:27 AM
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We are playing leapfrog I am part way to what you are suggesting, I think.
I use freecad which creates the airfoil that I normally save as 10mm thick. I can then cut it along the chord into top and bottom also in freecad. Freecad can do lots of clever stuff but I don't, I usually use the Cube or Cylinder options and hack them about, does all I need.

I like slicers too, more versatile than people think. I also have ideamaker slicer but prefer Cura. I have ideamaker as it can cut up and join STL objects easily. It has a cutting plane. So load a spar with chord horizontal, cutting plane along the chord and you can save to top and bottom separately. In Cura I just put the 10mm wide spar in the middle of the bed and use 'scale' to make it 200mm wide, then remove skin etc etc.
This should work with an acetate skin as I described previously, the foamboard core should add to strength I would think, the usual of the object being greater than the sum of it's parts
Jan 27, 2019, 10:29 AM
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Not sure I understand how you get around the problem of the normal non-flat bottom of an airfoil, but definitely think that you are on to something. The X grid printed flat on the bed should deal with loads in all major directions, including torsion, and, as you are demonstrating, should be sufficiently light.

In my mind I'd still plan for a proper spar. But this is easy to do - just leave the appropriate cut-outs in the rib and they will end up in the print as well (or just glue on spars the same thickness as the Depron, as Quorneng has done).

EDIT: just seen your last post. I don' t think Slic3r has this functionality

EDIT2: I've just downloaded the latest version and it might be able to cut along a plane

EDIT3: Eureka! It looks like it will work. If I can somehow get the grid to match top and bottom then the mass will be ~ 8g for a 100mm section. That becomes 13g with the acetate, so 130g for a 1m section. Maybe 150g if spars and a TE plus ailerons are added. That is quite acceptable
Last edited by Extreme Sports; Jan 27, 2019 at 10:52 AM.
Jan 27, 2019, 12:22 PM
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Ah, I knew I'd seen the lattice structure used before. However, the idea of printing it flat on the bed and just a single perimeter wide should make for a lighter and stronger structure.

KRAGA Maripi - 3D printed RC acrobat plane (2 min 53 sec)
Jan 27, 2019, 02:09 PM
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I like the Kraga plane but tricky print I reckon, possible for me but a few failures en route
Talking with a friend about this and I think the single skin grid will work, if your estimate is right 150g ish will be great. The spar was also discussed, he suggested a styrene tube, this could be gorilla glued to a central chord foamboard to allow fixing to the plane.
Also thought of how to make sure any skin in held as the grid walls are only 0.4mm. Most glues would just run off. One thought is to put 4 1cm square 'sugar cubes' in all junctions of the X grid. Glued they will give a surface area we can be sure is glued at little extra weight.
For trailing edge (think I will end up with 3mm paper covered foamboard all round) gorilla glue should work on the overlap.
I say someone make a filler of gorilla glue, mixed with water in a plastic cup till it started to foam then applied to fix a damage foam leading edge.
Jan 28, 2019, 05:53 AM
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Been playing around some more and this will work I am sure. Also realised a few things about it.
I was messing in the slicer to see where the infill (no skin) ended up. In my case, you may need to play around, 162mm wide meant the infill corners all met up at the sides of the print, so attaching more pieces is easy.
On that subject, I also realised that this design does not really need a single large piece of this grid the full length of the wing, it can be done in sections. Advantage is that you can print a huge box full of them (if you like) and just cut a piece of foamboard the size of the wing you want and start sticking. Just cut the last one when you reach the end. I have done a shortened one to use for ailerons, also one with notches to take 6mm diameter rods, see photo. Based on previous info I will probably use dowels.
Rods don't need to run the full length of the wing due to the grid strength but needed to attach the wing to the plane. I think the wing will need a solider end to hold the rods and brace against the foamboard. Not much just enough to glue all round, example photo is 25mm high and uses gyroid infill. Could be 40mm high maybe. Just to give a solid base to the wing otherwise the stress would all be on the rods alone. Only 11g weight.
Jan 30, 2019, 08:41 AM
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ok, just catching up and it's late and I havn't read all. When you extrude, remember you can either add or remove material. For example you can extrude a lattice box with no airfoil and then extrude away material leaving the airfoil . The tricky part is setting up datum planes at the correct angles to allow for tapered airfoils. Tricky, but not impossible.

The issue of flat bottom airfoil is solved by printing a top half and a bottom half and bonding them together to give symmetrical or semi symmetrical. Again, anything is possible.
Jan 30, 2019, 09:56 AM
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I think I have the top / bottom half bit figured out bu fitting a 3mm foamboard sheet down the chord and gluing to that, just need to finalise some details. I have a CR-10 printer and don't think it can remove parts.

I tried a tapered wing and can do it in Freecad, sort of. It is the Loft command where you do a small chord and a larger chord as distance apart then tell it to Loft them. You end up with a solid figure which tapers, or even curves between the different chords. If you Loft between 3 chords and they don't line up along one edge freecad curves it. So you can make a curved tapering wing but not easy.
I have made wing tips using this but haven't figured how to get a second airfoil in the same model. Just haven't asked around yet, sure it is possible.
Jan 30, 2019, 10:03 AM
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More or less figured out a way to do all of the above in Sketchup, but it required deleting all the extra bits item by item....only to find that the final mesh had holes and other problems (possibly related to adding some washout twist in a tapered wing with lightning holes). So either my Sketchup skills need help (definitely true regardless) or Sketchup is not the best tool for doing this type of thing. I also wanted to keep the tips as full prints, but Sketchup really seems to struggle with such complex parts (test piece is a close to scale FW190 wing).

At the risk of coming across as a Luddite, I can't help noting that I could have built several conventional wings in the time I've tinkered with this

My thinking is gravitating towards simple printed ribs and CF reinforced balsa spars, a depron Dbox and finally a skin of acetate covering everything. While there is definite appeal in the idea of printing a lattice and then just covering it, the combination of CAD and print time kinda evens out the scales.

That said, if someone has figured out how to do the lattice wing quickly and reliably in Sketchup, I'd definitely give it a try.


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