TOPIC: 3D printed model for Jetex/Rapier/Tender

In the last few weeks I've been experimenting with a new light weight technology for 3D printing.





3D printing most commonly uses a plastic known as PLA, and while there have been a few 3D printed R/C models PLA is just too heavy for the kind of stuff we want to fly. (PLA Stands for polylactic acid. It is made from renewable resources (i. e. corn starch, tapioca roots, or sugarcane)
But recently that has changed. A Dutch company Colorfab have a new PLA variant, LW PLA. It is made from the same ingredients but with the addition of a chemical to cause foaming as the plastic is extruded. 3D printing is of course a process of laying down layers of molten plastic, often as thin as 0.12mm, which cools to form a solid part.

With LW PLA a somewhat higher temperature is used which causes the additive, which I understand is something like bicarbonate of soda, to vapourise and greatly increase the volume of the deposited plastic. It is essentially 'foaming' the plastic as it extrudes.
Indications are that the same size part can be made for only 30% of the weight.

With this in mind I took one of the 3D models I had and have started developing it to make a flying Jetex type F/F model. I'll try it with a catapult first.

The basic 3D model of the Eurofighter Typhoon was not made by me. (Here model refers to computer model not physical model.) It's actually derived from a 3D graphic model used on a flight simulator. And these are professionally made by draughtsmen with access to manufacturers drawings. The model is rather more detailed than we really want for free flight and includes all the stores hard points that the actual aircraft has for tanks, missiles, optional sensors and other good stuff. I've printed several versions previously in standard, heavy PLA for friends and colleagues as desktop models.

My first attempt printing with LW PLA, which you can see being printed in the first picture, has a wing of scale thickness and the stores points. I decided that might be too much so I imported the model into into a computerised drawing package called Fusion 360. (Fusion 360 is a very capable drawing package which can be downloaded FOC.) I modified the original model to have a Jedelsky section. I'll try and add a picture of that version later.

Once I had the STL file into Fusion 360 I was able to modify the wing aerofoil section to be a plate about 1mm thick and conforming to the upper part of the original model. I lost a couple of grams doing that and I may do the same with the canards. The cockpit too may be modified. Also I may delete the opaque plastic canopy and replace it with a blow molded transparent canopy.

The model is then printed in two halves which I stick together with cyno. What I need to do is add to a couple of registration holes to the model so that when the two halves are glued together they are positively lined up.

There's some way to go with this yet and thus far I haven't even had chance to test it with the catapult. We are in lockdown here in Bavaria until April 19th. Although I can go to the grocery store and work I'm not ready to risk a substantial fine, even for Jetex! We'll have a while to wait until I discover if I'm barking up the wrong tree with this.

3D printed EFA model with Jedelsky wing.

Hi Terry,
this is a most interesting project, and I hope we will see a model like this in the air at Old Warden .....
Way back in 2016 Andre Bird, an idiosyncratic modeller if ever there was one, brought a modified moulded commercially available 'chucky' jet.



Above: this 'F-22' is just one of a number of these sorts of toy jets that are out there, and, as My grandson can attest, they fly well from a good (like, a really good) throw. They weigh about 30-35 grams .

Andre had modified his to Rapier L-3 power:



I'm not sure if I ever saw it complete a flight, but the potential is there. Note, however, the extra nose weight (ugh!) and some melting of the plastic - something Terry may need to address.

I remember Andre also had an F-18, which had two Rapier L-2 motors mounted on the top of the model. He said it did fly, and a rather more extensively modified F-16 - note the trough.





These models are still available, see for example:
F-18 Chucky


and I'm sure they can be made to look much better than a lump of plastic!



More later ...

Thanks for that Roger, and the pictures.

Yes, and there are some interesting variants on what we call foam plastic. I've made a couple of models with that insulating foam that is sold in building supply houses. That stuff can be cut and carved much like balsa (and hot wired) and has the advantage over balsa of being totally homogeneous. I've not try powering the foam toys you show but I know they are strong and light. They really ought to work.

Regarding the LW PLA I've been printing with. It has a different 'character' to the building foam and the 'toy' model foam used in these models above. Moreover this character can be somewhat modified by extrusion temperature. Hitting the sweet spot of best volume and strength takes a bit of 'tweeking'. After printing the stuff can even be sanded. I've not tried painting it yet but if this lockdown last much longer I will!

Hi Terry,
all this sounds interestingly 'high-tech' and beyond what most of us aspire to. Me, I struggle with '2-D' templates and ordinary printers!

It also appears to be harder work than sticking bits of balsa together!
On the theme of 3D printed structures, there's a picture of an 'open' structure (think formers and stringers, an 'F-86', I think) ) in the latest Aeromodeller . I remember the fellow showing me an example at Old Warden last year. Clever, but (a) not at all what I want to do, (b) seems to combine all the disadvantages of both methodologies!



Howard Metcalfe sent me this copy from the AM. Hmmm ... Not sure what to think about this one - your comments and observations welcome!