Stick and tissue with 3d printed formers and ribs

[CTho9305]

I’ve started experimenting with making a short-kit style version of an Earl Stahl Hawker Hurricane plan by printing the complex shapes (and probably the stabilizer outlines) then using sticks and tissue to complete it.  This is very much a learn-as-I-go idea, so I expect mostly failures here but wanted to share my experience.

Part of my motivation is to interest some of the 3d printer generation in stick & tissue, which means I will try to print some shapes that should probably just be cut (e.g. keels) in hopes of minimizing the barrier to entry.

I came across some sources suggesting that PLA is about 8x as dense as balsa, so I’m trying to mostly print a single 0.1mm layer web with just walls. A 1/16th balsa sheet is about 1.6mm thick, so that first layer eats up half of the mass budget, but unfortunately the single-perimeter walls are heavy.  The stiffness seems promising, but I’m still expecting this to turn out a bit like a brick

The biggest challenge so far has been that CA gluing PLA to PLA makes “instant” CA feel like Elmer’s white school glue in the rain in terms of drying time. Connecting flat surfaces is bad enough, but adhesion between the hollow side of the U-shaped structures is painful.

Some ideas from my progress so far:

• Ensure there are flat surfaces available in some spots that would be hollow frames
• Attempt thinner walls, and less infill, or manually-design in any infill that seems necessary
• Add hint markings to help locate forms along the keels

Excuse my messy surface.

-Chris

[ZK-AUD]

Chris that's a pretty cool concept.

Obviously the 2 issues are going to be weight and how to glue plastic to plastic and wood to plastic (or whatever the stuff is).  Also the weight of the glue if you must use Cyano.

A few things occur to me:

1. To help with the weight you could pare the formers back to little more than ribs - the plastic won't need the width that balsa needs

2. Why not mirror the formers and do both sides as one unit - saves glue / weight and the hassle of setting them up

3. You could print jigs to hold everything in place while drying

Out of interest have you compared your formers with balsa for weight yet? 

[CTho9305]

Where’s the face-palm smiley?  I absolutely should have printed the fuselage formers as full ovals instead of halves!  At that point I’d probably need a jig for assembly, but as you point out I can make those too.

As far as paring them back, I’m not sure exactly what that would look like shape-wise yet because I don’t want them to get too floppy pre-assembly, but I definitely plan to see about further lightening in the next round.

Plastic-to-wood seems to glue just fine.  When I pull it apart I end up with a very thin “ring” of balsa stuck to the plastic I have to cut that away with a knife.  It’s just the plastic-to-plastic that’s annoying.

I don’t have a scale that can measure small enough weights to really compare the actual weights yet, hence reasoning about them from the design perspective for now.

[ZK-AUD]

Hmm - I frequently get my best ideas after the event!!

If your formers need the width you could leave them like that and just put lightening holes all around - so keep the web effect but lose half the plastic.

I'll be interested to see how you go with this.

[DavidOWade]

I was playing with a foaming filament LW-ASA (Light Weight ASA) to make coupe propellers and found I could get it light, but not very strong. I set the temp to the highest of the filament's range and cut the feed to 50%. I was leery to try PLA because of its propensity to warp sitting in a car. As far as curved wing tips, rudders, and stabs go have you tried putting holes in the pieces that would be covered with tissue? I would try some but most of the curves I build are in old timers and I think they would run afoul of FAC and SAM rules. But, you have given me a thought...

[SP250]

Watching with loads of interest in the finished weights and structural stiffness etc.  Could you try polystyrene glue as per the plastic Airfix kit modellers for the pla to pla joints and just cyano for the wood to pla?
John M

[CTho9305]

David, were your props solid, or infilled?

John, I don’t currently have polystyrene glue or I’d give it a shot.

I built up the fuselage, and I think I have more infill than necessary in those formers, so I could’ve saved some weight just by reducing it.  The stiffness seems adequate for flight, but it doesn’t have much rotational stiffness between F4 and F5.  That could be fixed with a couple diagonal balsa sticks, and may also be fixed by the tissue covering later…we’ll see.

I haven’t assembled the wing yet.  The ribs don’t have much bending strength; I’m hoping that’s ok once they have the main spar and stringers to take that load, or they’d need a little more (or better-placed) infill.  The single 0.1mm layer “web” is almost like paper but as long as it has some support it does seem surprisingly strong.

One annoying thing I’ve done is using unusually-light balsa.  It’s an unmarked scrap sheet I’ve been cutting into strips, and it’s very light and more brittle and easier to crush than other balsa I’ve used from kits before.  That means any final weight I give will be skewed favorably by the balsa.

[che]

I've tried hard not to reply to this thread but when I read statements like this :-

The ribs don’t have much bending strength; I’m hoping that’s ok once they have the main spar and stringers to take that load, or they’d need a little more (or better-placed) infill.  The single 0.1mm layer “web” is almost like paper but as long as it has some support it does seem surprisingly strong.

I really do get concerned.

CHE

[Nigel M]

The ribs don’t have much bending strength; I’m hoping that’s ok once they have the main spar and stringers to take that load, or they’d need a little more (or better-placed) infill.  The single 0.1mm layer “web” is almost like paper but as long as it has some support it does seem surprisingly strong.

Hi Chris
have you checked out the structure of a built-up rib on a full-scale aircraft that has wooden wing ribs? To my view, your placement of the cross braces inside the rib profile will do a good job of stabilising the 'web' but it may be more useful and/or important to stabilise the rib outer profile (and probably tie-in the corners of the spar-slots to that structure ).

The chap at our club who 3D prints some of his models uses lightweight PLA which is significantly lighter than conventional PLA. I'd guess the manufacturer websites will give relevant densities. Also, this chap uses lightweight PLA as a single skin - no need for outer skins and % infills. Because the lightweight PLA is a foaming material, it is self stabilising, although his wing panels do have the style of 'star' reinforcement that your ribs do.

Hope that makes sense. Good luck. Keep going.
Nigel M

[fred]

Good try... But in my experiences printeds are HEAVY .
 Seriously unlikely any of those printed bits will even get close to the weight of their balsa versions.
Plus... It takes far more effort to  cobble up a usable .stl then babysit the printer .. for each part,  than simply cutting everything out of wood in the traditional route.
Possibly Using thin diy Cut Foam sheets  (to cut parts from) rather than Balsa might be a better experiment .. if determined to reinvent the wheel :-)

[PB_guy]

Model building should be seen more as art than as work. Yes, balsa is a beautiful material given to us by God (and foam is a terrific substance as well). But experimentation in any genre should be encouraged as much as possible. I see beauty in this construction. Keep up the good work!
ian

[DavidOWade]

[David, were your props solid, or infilled?]
They were infilled at the thickest areas but pretty much they were solid. I was using a 0.25 nozzle with two perimeters and a .015mm layer height. I am having trouble getting my CAD (DesignCAD) system to overlay a skin on top of the framework so I have been doing other things for a while. I need to put it in an enclosure again and start trying some more.

[CTho9305]

Hi Chris
have you checked out the structure of a built-up rib on a full-scale aircraft that has wooden wing ribs? To my view, your placement of the cross braces inside the rib profile will do a good job of stabilising the 'web' but it may be more useful and/or important to stabilise the rib outer profile (and probably tie-in the corners of the spar-slots to that structure ).

Hi Nigel, because of the stiffness of thin PLA (or especially PETG) vs “real” materials, my concern is that a pure truss would be worse at stabilizing the outer profile. I’m using the web to stabilize the profile, and then the truss to stabilize the web. In the directions that matter it seems to work quite well; in a future revision I might add “strengthening holes” (that will look like, but not work like lightening holes) so I control the truss position myself. These trusses are just the slicing software’s triangular infill, and their positioning is tolerable but imperfect.

The chap at our club who 3D prints some of his models uses lightweight PLA which is significantly lighter than conventional PLA. I'd guess the manufacturer websites will give relevant densities. Also, this chap uses lightweight PLA as a single skin - no need for outer skins and % infills. Because the lightweight PLA is a foaming material, it is self stabilising, although his wing panels do have the style of 'star' reinforcement that your ribs do.

I’ve seen the single-wall foaming PLA designs, but I enjoy stick-and-tissue building so that’s why I’m going down this path.  I don’t own any foaming PLA and unless it retains solid material’s stiffness I am concerned it wouldn’t work as well with this approach.  The other factor is as mentioned in my original post: minimizing the barrier to entry for others who might also not have any foaming PLA.

As far as Fred’s concern on the design effort, that’s fine, it’s a one-time effort and then the STL exists forever.  Much like Earl Stahl’s effort in drawing up this plan in the first place.  I certainly would use balsa for more parts if my interest were solely in building a single aircraft for myself just once. I have been working on the wing leading and trailing edges and they’ve been a bit of a nightmare… but 1. now it’s done and 2. now I know how to do the next one, so it’ll be much easier to design.  For my own builds I’d rather cut and sand balsa for those parts but as I’ve said, that isn’t the point here.

A few more progress pics…and yes I’ve built the wings in the “wrong” sequence this time (I had reasons; they were bad).  More learnings for next time.  The bending strength of the horizontal stabilizer isn’t great, so I may have to revisit that if it remains so floppy when covered.  The wings and vertical stabilizer feel fine.  The sticks I used for the stabilizers were heavy; I should have used the light stuff there and the heavier stuff on the front of the fuselage.  We’ll see what balancing this thing looks like.

[PB_guy]

Looks great! I think that you will have to use pre-shrunk tissue on the tail surfaces to avoid warping.
ian

[CTho9305]

The vertical stabilizer (4 1/4” tall) weighs 1.1 grams and the horizontal stabilizer (~ 8 1/4” wide, ~3 3/16” max chord) weighs 2.2 grams, both covered (undoped, shrunk with water) with tissue paper received at a recent birthday party.  The paper used on the horizontal stabilizer either didn’t shrink or shrunk minimally… I had planned to let it warp and then re-cover it but now I don’t know whether it would’ve tolerated tight paper.

The uncovered fuselage weighs 11.25g with no canopy or cowling.  The light balsa used on it is miserable; I break stringers frequently.  That said, I’d lighten the formers significantly in a future build, so the weight of better balsa would be offset somewhat.

Each wing weighs about 4g uncovered; the center wing section doesn’t have leading or trailing edges yet but weighs 3.5g.

[CTho9305]

Total weight without rubber, prop, or spinner for initial test glides, with ~12 grams added to set CG: 48 grams (so 36 grams for the covered structure). Covering was a mix of random tissue paper from party gifts.

The test glides were remarkably good, especially given that I did no trimming whatsoever.  The balance weights fell off on the last test, so the next update probably won’t come until I build a proper support for the weights.

Over all, it seems like for anything more planar than linear, PLA works very well.  An 0.1mm single bottom layer, light infill, a single wall, and no top layer gives light parts that are plenty strong (the ribs, once glued to the trailing edge, became rock-solid).  For long-thin pieces, PLA works but the stiffness of balsa can be better.  The major downsides of PLA are:

• gluing sucks
• workability sucks. Balsa is soooo much easier to sand, shave, build up (e.g. adding a bit when you took too much off), etc.

The strength and weight did not give me any trouble so far (we’ll see if the wing leading edge gets destroyed after more crashes, but so far it survived a crash into a tree unscathed).  To be precise I’m using some “PLA+” for a bunch of the parts, which is a bit less stiff and less brittle.

With some more tweaks to the plan, I could make assembly quite a bit easier (e.g. by creating rib notches/guides in the wing leading and trailing edges); depending on my level of motivation I may or may not make those improvements for this plan.

[Nigel M]

Hi Chris, well done for getting this far. I'm not familiar with the plan and can't see the wingspan anywhere in the thread or on the plan. The plan appears to print on a single A4 suggesting about 13" span? Apologies if I've missed it.

The latest AM has a model by Brian Spencer which includes some 3d printed parts, though I haven't read beyond the headline yet (50" wingspan Horsa).
thnx
Nigel

[CTho9305]

Hi Nigel, I think it’s 26 inches.
Edit: mine is 23”