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Jan 25, 2019, 05:37 AM
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Plasticard as a wing covering for 3d plane ?


I am thinking about various ways to do a wing and am looking to print internal airfoil shapes (can be as little as 5g each) then skewering them onto rods.
Basically will look like a balsa wing but in plastic, covering is something to consider. I am looking at 4 or 5mm foamboard but a friend suggested 0.5mm plasticard.

Anyone tried this as an alternative to heat shrink wing film ? Just a thought as this could provide a solid 'monocoque' structure as when glued the sheet will provide good strength.
Just an idea for a quick and easy wing build.

Edited to add, also anyone used Palight ? Says it is like Plasticard but lighter
Last edited by charentejohn; Jan 25, 2019 at 07:43 AM. Reason: Add info
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Jan 25, 2019, 07:57 AM
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nope


Quote:
Originally Posted by charentejohn
I am thinking about various ways to do a wing and am looking to print internal airfoil shapes (can be as little as 5g each) then skewering them onto rods.
Basically will look like a balsa wing but in plastic, covering is something to consider. I am looking at 4 or 5mm foamboard but a friend suggested 0.5mm plasticard.

Anyone tried this as an alternative to heat shrink wing film ? Just a thought as this could provide a solid 'monocoque' structure as when glued the sheet will provide good strength.
Just an idea for a quick and easy wing build.

Edited to add, also anyone used Palight ? Says it is like Plasticard but lighter
plasticard is high impact polystyrene "hips" and as far as i know also used for vacuum forming.
Have not seen it used as load bearing in rc planes,
The thinner sheets should be stronger as 3d printed wing skin at about equal weight.

One way to find out is to try so go for it

Jan 25, 2019, 09:09 AM
My planes plow into the field
farmertom's Avatar
Quote:
Originally Posted by cmulder
plasticard is high impact polystyrene "hips"
I have used HIPS to vacuum form fuselage skins. The problem I have encountered is that I can not find an acceptable adhesive that will not deform the HIPS. I have tried all of the commonly used adhesives. Not much sticks to HIPS. If someone has knowledge of a good adhesive, and proof, please let us know.
Jan 25, 2019, 09:29 AM
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foam save ca?


Hips is styrene just like polystyrene/ depron and as far as i know the roofmate style foams (blue and pink dense foams)
Could there be a mold release wax style chemical left on the sheet that resists adhesion?

Also revel (and likely airfix) static model kits are made off styrene so those glue's might work though most dissolve the plastic for a "weld" style joint?


just some thoughts
Jan 25, 2019, 10:27 AM
My planes plow into the field
farmertom's Avatar
Quote:
Originally Posted by cmulder
just some thoughts
It has been a while since I tried this so my memory maybe a little foggy. I did clean the HIPs with denatured alcohol,as it had release agent from vacuum forming. I was surprised that CA did not work on HIPS to HIPS. I like a little more open time to work with than CA allows so I was looking for something else. Too bad E6000 did not work either.
Jan 25, 2019, 10:38 AM
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Extreme Sports's Avatar
Definitely worth a try! I've considered trying plastic sheet in the past, but since there was limited to no weight saving over my traditional correx skin technique, I never felt the extra effort of cutting ribs would be worth while. But 3D printing could change that.

My only suggestion is to look for thinner plastic sheet. Locally I can get a .1mm acetate sheet from an art supply store. This weighs only 140g/m^2, so in the same ballpark as 3mm Depron and glues well with simple contact adhesive. Would be plenty strong for a large wing with 3D printed ribs and CF spar system. There are other advantages too - it is quite simple to score panel lines and add quasi rivets to this plastic, so scale subjects might be possible. Plus it will naturally do some of the 'oil canning' seen on old warbirds

Please post progress - I definitely think this could be a viable technique for some 3D printed designs. In fact I was just looking Lynxman's little pylon racer and thinking it would work well with the wing structure printed and covered with acetate, and just the curved tips fully printed!
Jan 25, 2019, 01:41 PM
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Thread OP
Thanks for the comments. I had assumed epoxy would work but seems probably not.
A friend mentioned acetate sheets he had used in overhead projectors. Just looked an these are 0.25mm (ish) but you can buy rolls of cake wrapping stuff at 0.12mm. I will take a look at some as it may well work, just needs to brace the wing.
Photo of airfoil starter for 10 attached. This would be 9g per airfoil with 20cm chord, printed with two outer walls but no top/bottom layer so the infill supports it all. 6mm thick part.
I think there is some mileage in this system but needs thinking about.
I was just looking for a simple build system, I have tried printing a skeleton wing section and foam filling but a lot of sanding required and heavier than I would have liked. I will have a think and maybe we can start a thread dedicated to 3D+cover sheet if it works. Need to experiment a bit, the beauty of 3d, I can print a couple of airfoils, stick some acetate on and see what happens.

The following info is from modelshop.co.uk when I asked them what sheets weighed. They had no actual figures but weighed what they had, worth noting.
PVC FOAMED 'PALIGHT' 210 297MM Weight: 45gr
FOAMBOARD, WESTFOAM WHITE 5.0MM 210 x 297MM Weight: 30gr
Plasticard We stock styrene plastic in white at 0.5. x 220 x 340MM. Weight of styrene: 43gr
Jan 25, 2019, 02:51 PM
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Extreme Sports's Avatar
Any reason why you have the ribs 6mm thick? I'd have thought 3mm (1/8") would be more than enough given balsa ribs are seldom more than 1/8". Maybe drop the infill % a little too?

So that could be ~4g per rib, or ~45g for a 1m wing (assuming 100mm spacing). Then say 20g for the CF spar and ~60g if your cake wrapping acetate weighs much the same as the thin stuff I get. That totals 125g for the full wing, which is on par with a simple balsa and film wing (no D-box, just LE, TE, spar, ribs and iron on covering). Add another 40g/m^2 if you want a coat of rattlecan paint.

Definitely merit in giving this one a go.
Jan 25, 2019, 03:25 PM
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that rib design is overkill.


There is no need for 2 walls and the additional walls around the holes.

A honeycomb infill (without a wall on the printbed) and a single wall in the center of the rib would be more then enough.
No need then either for the holes.
Depending of the material chosen for skin a leading edge support might be needed like a carbon rod for example so that would require a matching cutout at te front of the rib,.

The front 1/3 of a wing is the part that generates the lift and carries the load. All after that is just to keep the shape and to mount the control surfaces to.
Based on that the center wall in the rib might not even be needed after the front1/3 saving even more weight/ print time.
The skin does need to be supported at the thickest point of the rib ; usually that is the spar.

take a look at this

https://www.airfieldmodels.com/infor...raft_wings.htm
With a "stressed skin" design this will be "shear webbing" that can also be printed and if you include slots that match slots in the ribs then that also acts as spacers while assembling the wing.
Jan 25, 2019, 06:01 PM
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Thread OP
You have some good points, a thinner rib without holes would print quicker than one with holes. Just did it in a slicer and 4g and 30 mins to print at 3mm thick with a 0.6bottom layer, no top layer. this gives a solid structure. Adding a top layer is 6g and 50 mins. I have printed such things before and very rigid.
I like 3d printing for it's accuracy and ability to mass produce.

Hadn't heard of shear webbing, the airfield models site looks interesting. Simple enough to produce a printed piece that plugs into another through a slot in the rib. Overlapping and glued all round should be strong.

Just amazing the info and tools available now. I have started again in my twilight years and 40 yrs ago this would have seemed impossible.
Example shear web photo below, took 10 mins to design, something along those lines, easy to print (no overhangs or supports).
Jan 25, 2019, 06:51 PM
Registered User

tension vs compression


many materials are a lot stronger in tension then they are compressed.
So in the case of a wing (in normal flight) the bottom spar is in tension and the top spar is compressed.
What happens is that when the compression force reaches a threshold when the material starts to flex it will fail.
Supporting the spar will increase the amount of force needed for the top spar to fail.
Since you already have a spar in tension you can use that to absorb the forces of the compression
Materials in tension can take much higher side forces then without the tension present; the cables from a suspension bridge are a example.of this
All that is needed is to transfer those forces and that is what shear webbing does.

You might already have noted that when wood is used the direction of the grain is chosen for a reason ;
Wood can transfer higher forces when those forces are in line with the grain and even a thin 1.5mm "wall" makes a big difference in how strong a wing will be.

With 3d printing there is also a "grain" and if possible follow the same principles as with wood ; a single wall might be all that is needed.
Jan 26, 2019, 02:10 AM
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Extreme Sports's Avatar
When I was tinkering with this idea, my thinking was to replicate the elegant rib structure used in many full size kit planes and the old WW1 biplanes - see the attached photo (of a Kitfox homebuilt). I printed a 250mm chord sample in ABS and it came to 6g, but could no doubt be lightened to under 4g. Making ribs this way in balsa is a pretty time consuming undertaking; with 3D printing it is relatively simple.

For the spar, I was thinking of just using a CF tube, much as the kitfox frame in the photo. Except that I recon one can get away with just a singe tube at the normal spar position and no LE or RE rods. The acetate sheet when curved round the LE should be pretty strong without a LE rod - I use it to form decent LE's on my correx planes and it is plenty stiff. It just needs a little pre-bending, and light sanding on the areas to be glued.

The acetate wing skin on its own provides a lot of longitudinal strength, but as pointed out, the top skin will be prone to buckling in compression. Several easy options to resolve this:
  • Use a CF spar as suggested above. The CF then provides all the necessary longitudinal stiffness and the acetate is just dealing with torsion. Best to run the spar on the top of the wing, attached to the upper skin.
  • Use two thin spars (~2mm CF), one attached to the top skin, the other to the bottom. This might still have some tendency to buckle if the ribs are too far apart
  • As above, but run a 6mm thick Depron shear web between the ribs, next to the spars
  • Just use the Depron shear web glued to the top and bottom skins. Needs to be tested, but I think it could be more rigid than you'd expect
  • 3D Print a shear web, much as in your last post. However, that one is an overkill and way too heavy. The main shear web loads are supposedly at 45 degrees, so a simple lattice spar such as in the diagram below should be OK and a LOT lighter. Worst case the flange section of the spar can be made thicker than I've shown.

Personally, I'd just do the single CF spar and print a matching 'hole' in each rib for the spar to pass through as in the last picture.
Jan 26, 2019, 07:07 AM
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Patience not being my strongest point, I quickly constructed a 100mm test section. It has an MH30 profile with a 180mm chord. The total mass of the section is 15g, which would translate to ~135g for a 36" (0.91m) wing). A close comparison is the simple Q-Tee balsa and film wing which invariably comes out at 100g.

Each 5mm thick rib weighs 2.8g. Could be reduced to 2g if made only 3mm thick. I used an ordinary wood dowel rather than waste CF. That was 3g, whereas a CF tube would no doubt be a gram or two less. I also needed a foam spar web and a foam wedge to get the skin to behave. A similar piece of foam at the LE would be a good idea - if you look at the photo, you will see where the LE distorts a bit in the center. It can also be fixed by creasing the LE slightly.

I don't know what an equivalent fully printed section would weigh, but this is not too heavy and almost certainly tougher than a single skin print. However, there was more play in torsion than I'd expected, so a bit more thought might be needed to have a spar system that provides more torsional strength (mine adds zero - the dowel is not even glued in). I'm thinking that a balsa top and bottom spar flange with a balsa or foam web, plus a rear spar might be a better option than a CF tube. Or one could print the just the interior 'X' rib structure that many 3D prints use, add a CF spar and cover with acetate.

So it works, but to get it to work well may take some construction time....probably not what the typical 3D print enthusiast is looking for

PS: If cake wrapping is that crinkly plastic that you can scrunch up, then it won't work. You need a sheet that is more card-like. My 0.1mm acetate is similar to an OHP transparency, just a bit less rigid.
Jan 26, 2019, 07:58 AM
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Thread OP
Well done that man Basically always worth a quick mock up if possible. While you were doing that I was, and still am, pondering some of the other points you were trying to prove. Great to see something visual that highlights problems, to fix.

So far - I have a general spar based on previous posts but with 4mm rods in the 4 spaces, see photo. Doing this will give a solid structure but still needs shear webbing. Being a fan of hybrids I thought of what I have to hand, so imagine this. Spars threaded onto rods with spacing as required. Make blocks about 1" (2.5cm) wide ext polystyrene to fit in between spars bracing the top and bottom rods, quicker and lighter than printing. Very light material, the rods provide tension / compression strength and the polystyrene provides resistance to movement in all directions ?

I would gorilla glue these in place and just sand the polystyrene to be level with the spars. I have done this before and polystyrene sands easily but PLA does not so a short time with a sanding block and you should have a rigid structure. with the right shape.
For covering that can be anything that suits, I wondered if acetate covered with wing tape (bit of colour) would work. The structure would be the thing.

I have been looking for weights of aluminium tube (4mm) and people will give everything but the weight of it, even model suppliers. Seems about 2x CF but for me I can buy this locally (6mm anyway, hopefully 4mm) rather than ordering CF online. Embedded in polystyrene and gorilla glue should be strong enough.

The people from Modelshop uk suggested the following - 'The best way would probably be to either use glassfibre cloth and laminate it with epoxy laminating resin, which means it will maintain very light weight but it will harden in shape, or use a suitable fabric with cellulose dope, which is going to make the fabric shrink in shape and harden, maintaining again very light weight. Those two are the most common ways in creating light weight model airplane bodies and wings. '
I know the dope method, not the fibreglass one.

Covering in 3mm foamboard and tape may also work.
Jan 26, 2019, 08:42 AM
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Extreme Sports's Avatar
Aluminium has a density of 2700kg/m^3 so you can work out the mass of a rod/tube. Wood is about 1/3 the density, so would be a much better choice - wood dowels are usually available in various small diameters at the hardware store, and strong enough for this construction technique at the sizes we have been discussing. Balsa strips would be even lighter.

You are heading towards a hybrid of 3D printed ribs, spar tubing and a covering film and this has been done before - google a bit and you will see the previous efforts. My view is that this soon becomes quite a complex construction with the only advantage being printing the ribs rather than cutting them from balsa or foam. If you have a laser cutter, there really is no advantage over conventional built up wings.

If you use foamboard, there are established techniques that don't require ribs, so you'd be re-inventing the square wheel again. The fiberglass technique referred to is normal vacuum bagged composite construction. Lots of work, expensive and can be heavy, but great results are possible. Google 'vacuum bagged wing'. But now we have drifted away from 3D printing completely...

For me the attraction of the card like sheet is that it should could give a strong and reasonably rigid skin, analogous to the aluminium skin on a full size semi-monocoque wing, while remaining simple to construct. The remaining challenge is that, since the acetate cannot be ironed taut, the card skin is not in sufficient tension to provide the required torsional strength. That might be resolve just through better construction technique and not need a change in structure.

FWIW, I realize I over-estimated the mass of a 36" wing - should be closer to 110g. You only need 1 additional rib for each additional 10cm section. So the mass is (15-3) x 8 + 15 = 111g. So comparatively light.
Last edited by Extreme Sports; Jan 26, 2019 at 08:48 AM.


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