Fully 3D Printed Glider

[telnar1236]

I was inspired by @Gadeirus beautiful hybrid 3D printed and foam gliders to try and make my own 3D printed glider. With all the fast stuff I've been building and have built recently, it would be nice to be able to slow down for a bit too. I also wanted to see if I could fully 3D print the thing without even having a carbon fiber wing spar and it does seem like it will be possible. The design is a hybrid of ABS or PLA for the fuselage and LW-PLA for the wings and tail surfaces to keep weight down. The wingspan is 1.8m and flying weight will be around 650g so it should be very slow and floaty.

I actually tried to throw something together very quickly over this past weekend (so quickly I forgot to get photographs of the thing once it was built) and it was an absolute disaster. I have never built a glider remotely similar to this before, and it really showed so much so that the best thing I can say about it is the wings didn't fall off despite having no spar whatsoever.

There were four big issues. The first and simplest is that it was nose heavy - I CGed it like a jet and it wasn't really right for this plane. Next up is that the wing didn't have enough dihedral in flight. I was relying on the wing to flex and give it some polyhedral but it didn't flex as much as I expected and the plane was borderline impossible to steer on 3 channels. The vertical stabilizer and rudder also weren't big enough - again contributing to the turning issues. And finally, and most significantly, the tail of the fuselage wasn't stiff enough. I printed it in LW-PLA and I think a large part of both my elevator and rudder inputs went into bending the tail instead of turning the plane. Combined, these resulted in a plane that barely listened to any control except throttle and maybe 30 seconds of flight before a crash. It ended when I tried throttling up a bit to give me more authority and I think I overstressed the tail so it completely stopped responding any control inputs whatsoever.

Fortunately all easy fixes. Changing the fuselage material to ABS for the whole fuselage instead of just the nose both moves the CG a bit more aft and fixes the stiffness problem. The vertical stabilizer is 40mm taller on the version I'm working on. And the wings were completely redesigned to make them stronger, stiffer, and lighter, and to add dihedral

 

[Gadeirus]

I am flattered!

What you're doing looks good so far, I wish you much success!

Regarding stabilizer sizing, I can share with you a simple spreadsheet I made that I use for basically all of my aircraft designs.

MAKE IT FLY public

docs.google.com

It just uses established formulae, I just got tired of doing things by hand and I can iterate quickly. I am sure there are a thousand similar ones floating around the web.

 

[telnar1236]

I am flattered!

What you're doing looks good so far, I wish you much success!

Regarding stabilizer sizing, I can share with you a simple spreadsheet I made that I use for basically all of my aircraft designs.

MAKE IT FLY public

docs.google.com

It just uses established formulae, I just got tired of doing things by hand and I can iterate quickly. I am sure there are a thousand similar ones floating around the web.

Thanks, looks useful. Yeah, I'm familiar with tail volume coefficient calculations - this was mostly me rushing and not even calculating the CG before flying it much less trying to do any calculations for lateral-directional stability. There was a lot wrong with the v1

 

[telnar1236]

The stiffer fuselage, dihedral wings, and bigger tail are big improvements. It was now fully controllable and flew fairly well. It turns out the original CG was actually pretty good, so it was a bit tail heavy this time around. However, my wings weren't quite strong enough. They held up fine in flight, but on landing I dragged one of the wings on the ground a bit and a bit of the upper skin buckled. It still seemed fine, so I tried to put the plane up a second time, and within maybe 10 seconds, the wing folded right where the damage was. I could add PLA spars, but carbon fiber spars are much lighter for their strength, so I'm planning to keep things simple and use those.

 

[Gadeirus]

I think you're spot on, CF spars should be the way to go, especially since they're so inexpensive and easy to get.

There's a fellow with a YT channel that does a ton of 3DP sailplanes, and he actually epoxies unidirectional CF rovings into his wings, and some of his planes get quite large.

 

[telnar1236]

I think you're spot on, CF spars should be the way to go, especially since they're so inexpensive and easy to get.

There's a fellow with a YT channel that does a ton of 3DP sailplanes, and he actually epoxies unidirectional CF rovings into his wings, and some of his planes get quite large.

That's a smart idea using CF rovings - probably far lighter than using whole spars. I wonder if I could get make a bargain basement version using packing tape and achieve many of the same benefits.

Is that SoarKraft? I think one of his dynamic soaring designs might also hold the current 3D printed speed record - something like 235 mph.

 

[Gadeirus]

Take a look here:

 

[telnar1236]

Take a look here:

Don't think I've ever seen someone hand launch a 1/4 scale plane before - those are some impressive builds he does

 

[Gadeirus]

Agreed!

For your stuff, what do you usually print at in terms of walls and infill?

 

[telnar1236]

Depends on the design - there are 3 main techniques I use and I mix and match them depending on the design.

The lightest and strongest for most things is a fully modeled ABS structure where I model everything as I intend to print it. However, it takes the most work to design and surface finish can be an issue without a perfectly tuned printer so I typically need to do some light sanding. For that I typically use 2 walls (0.8mm) and 10% infill, but the majority of the structure is only 0.4-1.2mm thick so that only has an effect in a very few places as there just isn't enough space for that many walls or any infill. I used this for the front of the glider's fuselage and the entire fuselage of my speed challenge entry.

Then there are vase mode parts which have thin cutouts to create the ribs instead of having the internals modeled. They're a bit heavier than fully modeled parts in some situations, but they have a better surface finish and you print them in LW-PLA or LW-ASA and make them even lighter. Those are 1 wall and no infill. The tail of the glider is designed like that.

And then finally for some parts I just let infill provide the structure. For parts with large ratios of volume to surface area like fuselage parts this tends to be weak and heavy, but for wings it works pretty well since they're pretty flat/low volume and need more internal structure regardless. It also gets the best the best surface finish and lowest warping. Sometimes I'll mix in thin cutouts like in vase mode to force the slicer to create additional structure in a particular place like around holes for screws or to better connect an internal spar to the skin which carries most of the load. I used this for the wings and tail surfaces of both my glider and speed challenge entry. Generally, I'll use 1 wall and 3-5% infill for this. For some LW-PLA designs like the prototype for my speed challenge entry I'll go to 2 walls and deal with the added weight in exchange for a substantially stronger and easier to print structure.

 

[Gadeirus]

Wow, thanks, I appreciate the thorough response!

It's also interesting, I seem to build heavier than most, though I've not (yet) built an entire airplane from 3DP parts. My fuselage pods are all modeled to exact dimensions, but I never go under 1mm wall thickness.

Likwise, for parts that use infill, I've never done anything less than 2 walls, the thought of a single wall concerns me.

I was actually trying out some LW-PLA today, the prefoamed kind, and I have to say that I did not like the results very much. Yeah, it's lighter and the texture is probably great for sanding and gluing, but man does it seem flimsy. Prints like crap, too.

 

[telnar1236]

Wow, thanks, I appreciate the thorough response!

It's also interesting, I seem to build heavier than most, though I've not (yet) built an entire airplane from 3DP parts. My fuselage pods are all modeled to exact dimensions, but I never go under 1mm wall thickness.

Likwise, for parts that use infill, I've never done anything less than 2 walls, the thought of a single wall concerns me.

I was actually trying out some LW-PLA today, the prefoamed kind, and I have to say that I did not like the results very much. Yeah, it's lighter and the texture is probably great for sanding and gluing, but man does it seem flimsy. Prints like crap, too.

It definitely felt wrong going down to a single wall at first when I was designing these things, but it's the only way to keep the weight under control when building a whole airframe. The biggest plane I own is a 90mm EDF F-106 and the entire 71" fuselage is only 1 layer thick which holds up great, so for pretty much any reasonable application, seems like single wall is the way to go.

LW-PLA is definitely a mixed bag. It's far lighter than normal PLA but also very much weaker by an even larger percentage. It's the best option for small airplanes where other filaments are just too strong and heavy, but for bigger airplanes, I think it's actually heavier than ABS because of how much more material you need. And like you said, it prints poorly - the reason a lot of planes use vase mode is to make it so the whole part prints with no retractions or travel moves to prevent stringing which is otherwise very severe. But with careful design of the parts, the results can be excellent.

I don't really like the prefoamed stuff - it feels like you get a lot of the disadvantages with few of the advantages. I bought a spool of it a while back and did use it, but I wasn't a big fan. And more recently, I bought a spool of a different brand by accident and it was unprintable

 

[Gadeirus]

This one (super duper sabre?) looks super clean, going to have to keep my eye on that one

 

[telnar1236]

This one (super duper sabre?) looks super clean, going to have to keep my eye on that one

50mm prototype for the big one. It flew reasonably well and was able to hit 93 mph which is pretty good for such a small jet, but the ducting wasn't really optimized for a plane this small so it lacked static thrust and using purely the stabilators for roll meant it didn't have quite enough authority. I may revisit at some point and give it a higher lift wing, ailerons, and a better duct, but for now it's been retired.

 

[telnar1236]

Redesigned the wing to include 3 carbon fiber spars, two inboard and 1 outboard as well as a conventional PLA joiner to connect them together where the dihedral angle changes.

 

[Gadeirus]

But it did fly, and it looks superb, so still a win! The Sabre, I mean.