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Structural limitations of 3D printing

quorneng

Master member
#1
In aero modelling it is not long before material strength to weight becomes significant both in tension and compression.
Why not use high strength materials to build an RC plane? You could but to keep the all up weight within bounds the material would have to be very thin indeed. Not a problem for the tension forces but very thin material simply buckles at a very low compression. It follows that one material all in the same form makes a poor light weight structural material.
As basic 3D printing can only produce the same material it suggests it is not a 'good' process to produce a complete plane. It will be hard to create a structure that even approaches equal properties in tension and compression. To withstand both tension and compression means the elements in compression will have to thicker & heavier than those in tension which means the weight penalty is going to be significant.
One way to improve the compression performance of a thin material is to support it in such a way to limit buckling. This can be done by creating an integrated "cell" structure using the same material. This is in effect what most printed planes use but it is complex to programme and slow to print which limits just how fine and structurally efficient the support element can be.
Indeed most printed planes are already a 'composite' structures using inserted carbon rod or tube reinforcing elements to handle much of the bending loads.
The end result is all printed plane that has sufficient strength but it is not using all the material efficiently and is thus heavier than it might otherwise be. It is worth noting that the power required to fly a plane is directly proportional to its weight.
Once I had used a 3D printer I decided that it would be best used where its strengths (accuracy and repeatability) would give the most benefit and any weight penalty would be acceptable. This view was particularly driven by the fact I tend to build 'lightweight' planes that used a lot of sheet foam!
My usual foam wing construction consisted of thin foam top and bottom skins over foam ribs with a balsa spar. Cutting a lot of ribs accurately can be a chore. A 3D printed rib looked promising, particularly as the process allowed a single rib design to be scaled down to allow a tapered wing.
Starting simple I used printed ribs on a 'plank' wing.
LHwing1.JPG

20 full ribs, 8 shorter to allow for the ailerons but all of otherwise identical section.
After quite a bit of trial and error each 3 mm thick rib is a warren brace but with a thin (0.2mm) 'web' overall on one side.
LHwing2.JPG

Each full rib weighed 1.5 g compared to 1g for the same in 3mm sheet foam but was considerably stiffer. Furthermore I did not have to cut them out, just watch the printer make them!
The completed wing.
WingJoin3.JPG

Light, stiff and remarkably simple to make.
If ribs work how about fuselage formers, particularly for an airliner fuselage?
Same sort of structure but as a 'ring', a circular I beam.
FuseForm1.JPG

In fact such a former of the same weight and strength would be hard to make in any other material!
Part of the completed fuselage.
Inside1.JPG

Needs some novel techniques to build but the end result is incredibly light and stiff.
There are of course other areas where the strength required means a full printed structure is acceptable, particularly if more than one is required.
PrintNacelle.JPG

These are some examples of how I have used 3D printing to make parts to build RC planes.
I have printed a sub 250g RC plane that uses only 3D printed parts and tissue but that is another story.
 
#2
If you really wanna get the full potential of 3d printer in the aeromodeling world, maybe consider making a carbon fiber, make the mould with your 3d printer(high resolution is required), it'll cost quite a lot of you have no experience with carbon fiber, but it'll worth the price if you wanna get into professional aeromodeling, btw sorry for bad English
 
#4
I've thought of doing this. I actually found a F6F structure I was going to use until the guy decided to charge for it.

However, my main though was for formers from master series builds and the such. The formers really just help with the shape, and not the strength of the foam board.

In my experience, it isn't even really the fuselage that is that big of an issue, it is the wings. Both of the eclipson models, and other models I have built, the structure required to support a fully 3D printed wing, and the wing skin itself, it ends up weighting half or more than half of the weight of the rest of the plane.

I've been thinking about hybrid building like this myself (and it could be something neat that someone could kit up in the future for a business idea). You can make some parts beautifully out of foam board, and just things like ribs and formers out of 3D printed material, combined with a carbon fiber or wood spar if needed. Build most of the plane out of lighter foam, and you could 3D print parts like cowlings that would be better printed.
 

quorneng

Master member
#5
I really went to town on the printed/foam and a touch of balsa with the AN 124:
20Jan20a.JPG

Wing ribs, engine nacelles and all fuselage formers printed.
The nose lifts off to reveal.....
20Jan20c.JPG

It is completely empty!
Everything is in the wing apart from the elevator servos and they are in the tail plane.
The only balsa is in the wing spar top and bottom flanges and even then only as far out as the outer motors.
2500 mm span. It weighs just 1560 g ready to go with a 2200 mAh 4s.
When I had finished I said to myself 'never again'.
It actually flies pretty well.
 
#6
I really went to town on the printed/foam and a touch of balsa with the AN 124:
View attachment 220141
Wing ribs, engine nacelles and all fuselage formers printed.
The nose lifts off to reveal.....
View attachment 220142
It is completely empty!
Everything is in the wing apart from the elevator servos and they are in the tail plane.
The only balsa is in the wing spar top and bottom flanges and even then only as far out as the outer motors.
2500 mm span. It weighs just 1560 g ready to go with a 2200 mAh 4s.
When I had finished I said to myself 'never again'.
It actually flies pretty well.
Thats amazing!
 

quorneng

Master member
#7
Following on from my printed ribs and formers with Depron planes I wondered if it would be possible to make a light weight RC plane in the style of conventional 'stick and tissue' construction but with printed parts rather than balsa.
I knew I could make the ribs but the spar would be a challenge both to achieve the required strength in what would have to be a multipiece structure limited by the bed size, For simplicity the fuselage would be a printed pod with a glass fibre boom.
The target would be 1000 mm span and below 250 g! :eek:
After quite a bit of trial and error the wing spar was made in 5 sections. A centre section that included the dihedral, a mid and outer. The outer spar section is tapered towards the tip.
SparParts.jpg

Each spar section uses a similar construction to the ribs but being 5 mm wide and with a thin web on one side as well as a denser cross braced structure. Additional thin plates top and bottom covers each spar join.
1000mmSpar.JPG

The ribs have an appropriate printed hole so they could be fed along the spar and glued into position with super glue.
WingRib.JPG

The leading and trailing edges are also printed.
LEstock.JPG

TEstock.JPG

The completed wing with the tissue covered ailerons. They are built up on the same way as the wing.
NewWing2.JPG

Note the added printed aileron servo mounts.
With the tissue complete, water shrunk and doped with water based EZE Dope it weighs 64g including the aileron servos.
Next the fuselage and tail.
 

quorneng

Master member
#9
Piotrsko
Do you means tissue cover the face of the spar rather than use a thin printed web?
You might but even shrunk and doped tissue is considerably more flexible than thin printed sheet and the spar is already rather more flexible than might be considered normal for its weight.
Or do you mean something else?
 
#10
I really went to town on the printed/foam and a touch of balsa with the AN 124:
View attachment 220141
Wing ribs, engine nacelles and all fuselage formers printed.
The nose lifts off to reveal.....
View attachment 220142
It is completely empty!
Everything is in the wing apart from the elevator servos and they are in the tail plane.
The only balsa is in the wing spar top and bottom flanges and even then only as far out as the outer motors.
2500 mm span. It weighs just 1560 g ready to go with a 2200 mAh 4s.
When I had finished I said to myself 'never again'.
It actually flies pretty well.

That is epic.
 

quorneng

Master member
#11
On to the 'pod' of the printed sub 250 g plane.
The intention was to have the wing held on with rubber bands to give a bit of crash resistance but apart from that the fuselage was sized to just contain the required components.
Fuselage2.JPG

Just three components. Wing mount that also holds the tail boom, battery compartment with the ESC under and the motor cowling.
The front face of the battery compartment is also the motor mount.
FuseMotor.JPG

Just about minimum everything!
The motor cowling cooling air intake.
NoseCone.JPG

There is a hole in the motor bulkhead low down so air also flows over the ESC under the battery.
The battery is an 850 mAh 3s. The prop is a 6x4.
The V tail is also a printed structure tissue covered with a single 'elevator only' servo.
20Aug20.JPG

It weighs 241 g ready to go.
More a "see if I can do it" than a practical plane but with care it flies pretty well.
Of course if built the same way but using balsa it would be about 30% lighter with a corresponding performance improvement. ;)
 
#12
To be honest I don't know from personal experience, but check out these links. Colorfab makes a light weight pla and Eclipson has planes design to be 3d printed. One is even available for free download. Looks like the FT guys played with it a bit.
https://colorfabb.com/rc-planes
https://www.eclipson-airplanes.com/

Quorneng-I love what you did with the pod. I'm working on a swappable mini pod that can hold all the electronics and have a battery hatch. We'll see how it goes.
 

quorneng

Master member
#13
Afbrian
The problem with 3D printing is it is not possible (yet) to emulate the really light materials which are of course some form of cellular structure. Foaming PLA is getting there but there is only so much you can foam with a fine filament.
As a result with something like a plane it is not possible to use the material efficiently in all situations. It is either too strong, too heavy or too weak for the duty required.
This is why I favour only using printed components where its properties are well suited to the duty required and using other materials elsewhere.
Many of my planes use a combination of foam, balsa and printing to achieve a strength to weight ratio that is hard to beat at smaller sizes, say a 1.4m span plane.
 
Last edited:
#14
I agree with and understand what you're saying. RC models are, scale wise, very over powered and that's why they work. I think that's especially true with the foam board we all like so much.
I'd love to scan in some old plans I have and print all the structural components. I think that would be the best use to actually get a scale power to weight ratio.
It's kind of cool to think about using the latest 3d modeling and printing combined with classic tissue covering.
 

Inq

Well-known member
#15
I really went to town on the printed/foam and a touch of balsa with the AN 124:
View attachment 220141
Wing ribs, engine nacelles and all fuselage formers printed.
The nose lifts off to reveal.....
View attachment 220142
It is completely empty!
Everything is in the wing apart from the elevator servos and they are in the tail plane.
The only balsa is in the wing spar top and bottom flanges and even then only as far out as the outer motors.
2500 mm span. It weighs just 1560 g ready to go with a 2200 mAh 4s.
When I had finished I said to myself 'never again'.
It actually flies pretty well.
Incredible airplane. Do you have a link to this 3mm foam board? I don't believe I've ever seen anything like that in my travels. Does it have any 3D curving ability... say with a little heating? It looks like you are planking the sides when the 3D curves get too tight.
 
Last edited:

Inq

Well-known member
#16
I really went to town on the printed/foam and a touch of balsa with the AN 124:
View attachment 220141
Wing ribs, engine nacelles and all fuselage formers printed.
The nose lifts off to reveal.....
View attachment 220142
It is completely empty!
Everything is in the wing apart from the elevator servos and they are in the tail plane.
The only balsa is in the wing spar top and bottom flanges and even then only as far out as the outer motors.
2500 mm span. It weighs just 1560 g ready to go with a 2200 mAh 4s.
When I had finished I said to myself 'never again'.
It actually flies pretty well.
Incredible airplane. Do you have a link to this 3mm foam board? I don't believe I ever seen anything like that in my travels. Does it have any 3D curving ability... way with a little heating? It looks like you are planking the sides when the 3D curves get too tight.

Following on from my printed ribs and formers with Depron planes I wondered if it would be possible to make a light weight RC plane in the style of conventional 'stick and tissue' construction but with printed parts rather than balsa.
I knew I could make the ribs but the spar would be a challenge both to achieve the required strength in what would have to be a multipiece structure limited by the bed size, For simplicity the fuselage would be a printed pod with a glass fibre boom.
The target would be 1000 mm span and below 250 g! :eek:
After quite a bit of trial and error the wing spar was made in 5 sections. A centre section that included the dihedral, a mid and outer. The outer spar section is tapered towards the tip.
View attachment 220471
Each spar section uses a similar construction to the ribs but being 5 mm wide and with a thin web on one side as well as a denser cross braced structure. Additional thin plates top and bottom covers each spar join.
View attachment 220472
The ribs have an appropriate printed hole so they could be fed along the spar and glued into position with super glue.
View attachment 220474
The leading and trailing edges are also printed.
View attachment 220476
View attachment 220477
The completed wing with the tissue covered ailerons. They are built up on the same way as the wing.
View attachment 220475
Note the added printed aileron servo mounts.
With the tissue complete, water shrunk and doped with water based EZE Dope it weighs 64g including the aileron servos.
Next the fuselage and tail.
Following on from my printed ribs and formers with Depron planes I wondered if it would be possible to make a light weight RC plane in the style of conventional 'stick and tissue' construction but with printed parts rather than balsa.
I knew I could make the ribs but the spar would be a challenge both to achieve the required strength in what would have to be a multipiece structure limited by the bed size, For simplicity the fuselage would be a printed pod with a glass fibre boom.
The target would be 1000 mm span and below 250 g! :eek:
After quite a bit of trial and error the wing spar was made in 5 sections. A centre section that included the dihedral, a mid and outer. The outer spar section is tapered towards the tip.
View attachment 220471
Each spar section uses a similar construction to the ribs but being 5 mm wide and with a thin web on one side as well as a denser cross braced structure. Additional thin plates top and bottom covers each spar join.
View attachment 220472
The ribs have an appropriate printed hole so they could be fed along the spar and glued into position with super glue.
View attachment 220474
The leading and trailing edges are also printed.
View attachment 220476
View attachment 220477
The completed wing with the tissue covered ailerons. They are built up on the same way as the wing.
View attachment 220475
Note the added printed aileron servo mounts.
With the tissue complete, water shrunk and doped with water based EZE Dope it weighs 64g including the aileron servos.
Next the fuselage and tail.
I remember doing the same kind of tests with the spar... I found using Cura's "Cubic" Infill style to be far stiffer and stronger in bending tests at +/-45 than the plain "Grid" pattern and was the same weight.

I really want to get back into the design of these things using techniques like you've shown once I can get to where I don't crash on the maiden flights. :cautious:
 

quorneng

Master member
#17
The 3mm foam is sheet Depron. Its is slightly more rigid than the 3mm foam that Hobby King supplies and comes in much larger sheets 1250 x 800 sheets. Its is an expanded polystyrene foam so absolutely needs foam safe glue and paint! My local model shop stocks it but delivery is expensive.
Using it is very much like you would using sheet balsa where the skin provides the majority of the strength and stiffness. To do it successfully does require a good understanding of structures along with plenty of experience of what no to do from failures!
 

Inq

Well-known member
#18
Thanks @quorneng

Hmmm... I live in "The Sticks". I can just imagine the shipping of an item that weighs about an 50 grams, is over a square-meter and only 3 mm thick. Buying/shipping a case lot would probably be cheaper than buying/shipping one. My luck, it probably comes in a 100 sheets/case and cost $1000. :p

I guess I'll have to stick with things I can buy at Dollar Tree, and Lowe's.