3D Printing Applied to a Slope Soaring Plank

RCGroups user Gbreeze shares how he is using a 3D printer to print slope gliders!

Splash

Hey everyone,

I've recently got access to a 3D printer at work with a fair amount of free print time, so I thought I'd see if I could get a nice technique together for printing slope gliders. My first subject will probably be one of the planks developed at the thermik-board.de site, especially the BOW or Lilloo.

I have made some progress in wing construction, so I thought I'd start sharing in case anyone working in the same direction could benefit or if anyone had some good ideas to contribute.

This design uses ColorFabb LW-PLA and TPU along with carbon spars for the airframe. The LWPLA is a newer foaming material that can make extremely light prints. Printing a small section of wing gives roughly 2.5 oz/sq ft weight just for the printed material. Of course carbon spars and radio gear will bump that up in practice, but it's a rough figure to get an idea of how light this material is. It sands easily and can be carved with a razor similarly to balsa. The TPU material is similar to shoe sole and skateboard wheel rubber and my idea is to use it for the leading edge and forward fuselage. Most planes need noseweight anyways, so I thought I'd just make the forward part of the aircraft basically indestructible.

I've attached a few pictures that show how the TPU leading edge could be attached to the LWPLA wing. The carbon strip spar slots and the live hinge are printed into the wing. You can see I've got some surface issues in the trailing edge, which I am pretty sure I know how to fix (this is an intricacy of getting 3D printers do to what you want, which I am not sure if I want to digress too far into in this thread). Printing a nice TPU leading edge is a pretty simple matter and I imagine it could be useful for planes with otherwise more standard construction techniques.

One of my main issues now is that the LWPLA is pretty flexible at the extreme trailing edge. It would be fine for Alula style flight, but I'd like to be able to fly a bit faster and I can imagine some pretty bad failure modes if a high speed dive straightened the reflex out of a plank airfoil. I think I'll try soaking some CA into the trailing edge since the LWPLA is a bit porous. Any ideas on making a stiffer trailing edge are welcome. The other option I can think of is abandoning the live hinge and printing control surfaces with a heavier and stiffer material with tape hinges.

Another area I could use input is which radio gear to use; I know the x08's are well liked and would probably use them for any faster models, but alternative 8mm servos are welcome. I was also considering designing battery compartments around the zippy 700 LiFe batteries.

Cheers,

Gabriel

Last edited by Jim T. Graham; Jan 21, 2020 at 10:04 AM..
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Jan 10, 2020, 03:30 AM
Auzzie built planks
timbuck's Avatar
Just brilliant Gabriel , thanks for sharing

Cheers

Tim
Latest blog entry: More colours
Jan 15, 2020, 03:17 AM
3D-Printed Planes

About the Bow...


Hey there, sweet Idea
I already tried the BOW, it worked pretty nicely but I never developed it far enough to make the files public...
https://www.thermik-board.de/viewtopic.php?f=27&t=1872

Using TPU for the leading edge sounds interesting, but I never really liked LW-PLA. I tried it for a short period of time but always thought it was too soft and flimsy to build stiff planes out of it.
Also another benefit of 3D-Printed planes, is that (due to the harder Material) they do not "age" like foamies do. The LW-PLA is so soft that you can score and dent it with your fingernail, in use it will propably age quite quickly - but I have to admit that I am only speculating here.

I got some pretty good results using a 0,3mm Nozzle with a lot of infill - this stiffens the wing, and the stuff I am building now ist so solid that you can stand on it, with a structural weight (just 3D-Printed Parts) of about 18g/dm^2.
When spars and electronics are added, that ends up between 25-35g /dm^2.

Interesting project, keep us updated!

Lasse
Jan 18, 2020, 01:10 PM
Registered User
Thread OP
Thanks for the encouragement, Tim. I read through your Gizmo development and it is very inspiring - congrats on a great plane.

I did follow your thread with a lot of interest, Lasse. Great work! At least I followed as well as I could with Google translating the whole thing. I would probably post over there if it wasn't for the language barrier. You have definitely proven that you can make great planes with PETG. I'll have to see how bad the "hangar rash" gets for LW-PLA.

Attached is a pic that shows my current development. I prefer this LE attachment since impacts that get through the TPU will be transferred to the spar instead of damaging the main wing body. I'm considering making the forward spar round for that reason. There was a slight amount of warping due to printing in a cold space with no enclosure, but the fit is still pretty good. Ultimately I think I would smear a bit of flexible glue along the connection between the TPU LE and body surface to make a smooth aerodynamic surface.

This piece also tested my servo pocket fit and control horn design. You can see there is a 3x.2mm carbon strip going from the control horn back to the TE. I am considering putting a similar "stiffening vane" at the root and end of the control surface to keep it stiff. Anyone have input as to the aerodynamic considerations? I thought I would keep the lower surface flat and make the protrusion on the top since the flow is more likely to be laminar on the bottom.

LW-PLA takes CA well, but that does take away some of its durability. Normally it is flexible but has a great memory, returning to its original shape after deformation. After soaking thin CA into it, you can force it to deform and hold other shapes. Still, I think the stiffening vanes at the ends of the elevon along with CA in the extreme trailing edge is a good solution that should be stiffer than normal balsa TE stock.

This test print is the same airfoil and dimensions as the center section of a slightly modified Lilloo - I have increased the chord 10 percent and scaled the span to about 1m. This size makes it convenient to print the wing in 3 sections (I can print approx 30 cm sections diagonally across a Prusa mk3s). I am pretty sure this will be my first subject, but I like the idea of a BOW or other fuseless plank because the large rounded rubbery front should be extremely durable. I'm always tempted to fly fast and low around the turbulent lip and I like the idea of a really indestructible plane.

The aft section of the wing shown here is only 8 grams, and it really isn't even optimized to be as light as possible. The TPU leading edge is 6 grams. So, using this technique, before carbon spars or radio, it weighs around 10g/dm^2 or 3.3 oz/ft^2. And, since the weight is concentrated forwards, it should be pretty easy to get a plank to balance.

-G
Jan 20, 2020, 05:31 PM
3D-Printed Planes

Getting more interesting...


Hey,

thank you I assumed you were german because you mentioned the "Thermik-Board", but aparantly I was wrong
Never mind, btw. my EBOW ended up beeing PLA, not PETG - It just didnt print as nicely as PLA does...

Your method to construct the wing is really... different from what I have seen so far.
Not just because of the TPU / LW-PLA combination but also because of the direction in which it is printed,
I am just not sure if that is a good thing.

On a usual composite or balsa wood wing, the section in front of the main spar is thickened / stiffened to get a torsional stiff D-Box.
Because of your (Propably) rather soft TPU front section, your D-Box will propably not serve this purpose.
So all the rigidity will have to come out of the rear portion of the wing - This will mainly depend on the amount and form of inner structure you use - but still, I think it might work just fine. (It just came to mind that this also changes the virtual "center line" the wing will twist around, it will propably be further back than in a conventional wing - this might increase the flutter issue...)

I think the biggest difference (and maybe issue) to me is this: On most other wings I have seen the main spar is placed perpendicular to the direction of the printlines. This means that the main spar takes most of the load spanwise while the layer orientation of the printed part provides strength and rigidity in the direction of flight.
On your design there is nothing preventing a fail between the layers - To me this seems counterintuitive, but I also have to say that for the stresses occuring in such a small wing it might work just perfectly fine...


I guess I am just still trying to figure out if this is genius or stupid

But anyway, just try it! Take it easy on the first speed tests and everything should be fine!

Side note: Having the layer lines perpendicular to the direction of flight might change the characteristics of the used airfoil, especially on something like the Lilloo (I think the low Reynolds numbers there might make it really sensitive).
Last edited by Lasse C; Jan 20, 2020 at 05:32 PM. Reason: forgot a word
Jan 22, 2020, 12:26 AM
Registered User
Very interesting, I suppose I need to get my printer working better.

It would be interesting to be an articulated wing instead of control surfaces.
Jan 22, 2020, 10:18 AM
Old Master, New Grasshopper
toro's Avatar
https://www.rcgroups.com/forums/show...291909-Bouncer

Check this out, I suggest you PM him, he's pretty far into the design of this, and has had some good success with some really interesting techniques. His plan is a go fast DS plank, and so far it looks like he's on the right road.
Latest blog entry: Pics
Jan 29, 2020, 07:41 PM
Registered User
Thread OP
Progress has been slow, mostly due to lack of design and print time, but I thought I'd give a little update and keep the thread alive. I did make a full length test print of the hinge and flap, and another test print of a wingtip (see attached pic) . The tip print is made with a 75D hardness TPU (actually pretty rigid), but a little heavy at 10g. I think this material would be useful for leading edges and tips for any type of construction. I plan to use a softer 95A hardness material for leading edges and the nose because it will absorb more impact in a crash. The 75D material would be great for rocky landing zones.

I've gotten pretty handy with CAD and made a model of the Lilloo wing with 10% increased chord and span scaled to 1m, which is just about ready to print. I'm still not really happy with the elevon stiffness, but at some point R&D has to stop and I have to put a plane in the air. I guess it will be a fun challenge to see if I can get the plane going fast enough to flutter off a control surface. My max design speed will be a terminal dive when ballasted for 20mph wind conditions so I might be safe.

My longer term goal is to make a very tough plane that I can travel with and fly in conditions from 8mph to 20mph. I think ultimately the LWPLA won't be suitable because it has a reputation for warping in hot conditions. I have a few other printed ideas to try, but I am leaning towards doing a hybrid design based on the BOW with leading edges and wingtips made from TPU.... either using composites or perhaps even 3d print a mold and use 2 lb/ft^3, 2 part PU foam.

Lasse - Ah interesting. Do you have any problems with PLA warping if left in a hot car or in the sun?
In this case I am not building a Dbox for stiffness, but relying on 2 or more spars. With LWPLA the layer bonding is so good there is just about equal strength in every direction, but with other materials printing in this orientation has some advantages: each wing piece can be significantly larger by printing across the bed diagonally, the filaments are oriented spanwise giving a stronger wing in the bending mode (probably the highest load in flight). The airfoil accuracy I think is fine since aft of the thickest part of the wing the curvature is low and slope is high... if that is a serious concern you can bring layer height down to as low as 0.05 mm. The more highly curved leading edge is printed in the normal orientation to take advantage of the printers very precise XY movement. We will see if the wing has enough torsional rigidity at my speed range, but I suspect it will be fine just compared to other planes I've built and flown over the years. It looks like you are pushing your designs to very high speeds (well above 100 mph), which I am pretty sure would destroy just about anything made from LWPLA.

rcsoar4fun - that is definitely a possibility. The challenge is going to be making the structure rigid enough internally while still easily moved by a servo in a way that won't flutter or crumple the skin surface. With flexible leading edge material, there is also the possibility of making adjustable camber airfoils...

toro - I've been following that thread, but thanks - I think he is using the very hard TPU throughout since it is pretty rigid and he wants to fly very fast. The Bouncer is coming along nicely.

-G
Last edited by Gbreeze; Jan 29, 2020 at 09:50 PM.
Jun 22, 2020, 02:46 PM
3D-Printed Planes
Hey,

what happened to this project?
Any news?
Jun 22, 2020, 03:44 PM
Gliders ain't drones!
Zenmaniac's Avatar
A bit unrelated (and I don't want to hijack the thread), but I recently completed a nearly 1m-long fuselage for my 2m version of Peter Wick's Wipe. The wing was standard vacuum-bagged fiberglass and foam.

My immediate thought was to print it out of PETG (pretty tough) and ended up covering it with a carbon-fiber and kevlar sock. It ended up a bit short in the nose to get a proper CG.

Before I redesigned the fuselage with a longer nose, I discovered polypropylene filament. Wow, do I like this! It's a bit like a nylon but easier to print, and the inter-layer adhesion is almost too tough to be believed. It's 25% lighter than PLA or PETG and a bit more expensive. I played with a few infill patterns and wall thicknesses to to achieve the combination of flex and rigidity that I liked. It seemed tough enough to fly without an external sleeve. The nice part is the fuselage is in 5 printed pieces and I could lighten the tail with less infill and layers in those sections. The fuse still needed nose weight, so I ended up with a heavy print of PETG for the nose, and PP for the rest. I used "natural" polypropylene which meant the infill was visible through the walls, and got a lot of comments of the "cool factor" of that look.

The maiden flight went well, here's the first two minutes or so (I got more comfortable with it after the filming stopped, so this video doesn't portray the best the plane can do).

-= Dave

June 13, 2020 (2 min 52 sec)


-= Dave
Jun 23, 2020, 10:08 PM
Registered User
Thread OP
Thanks for the interest Lasse (congrats on the Boomerang, BTW!) and Dave - my work for the last 2 months has been very consuming for my time and the printer, so the project was on hold. Now I have some more time for hobby work so I'm getting ready to develop some more...

My mind is going in the same direction, Dave (great plane, don't consider that thread crashing) - I got a whole roll of 3DXTECH CF PP filament sampled to me since I buy so much of their ESD filament for work, and I have very high hopes for it. On paper it has all the right qualities: doesn't warp in sun or hot car, lighter than PLA, durable, and the CF should make it plenty stiff. I have to print some test sections to see if I can make a live hinge and how brittle the CF makes it.

The reports of the Wipe wings are so positive I'm leaning towards making some variation on them instead of a Lilloo. Either the straight Wipe 1.1 or 1.5 or a version with a BWB center section or crescent sweep a la the XBOW/Spliff/Etto if I'm feeling more ambitious. The exact project is still a little bit up in the air...

-Gabriel
Last edited by Gbreeze; Jun 23, 2020 at 10:49 PM.


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