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Using alternatives to balsa and ply

For those interested in the use of composite materials
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BrianF
Posts: 83
Joined: 19 Mar 2018, 09:36
Location: Brisbane AU

Using alternatives to balsa and ply

Post by BrianF »

Hi, there was a discussion earlier about availability of "wood" for modelling. That got me thinking about other ways to build what I want.
I've tried various build techniques over my time. I'm building a pair of models that uses a combination of construction processes.
A model like this would usually be built with formers, stringers and planking or ply sheeting, or perhaps be fully molded.
This one combines laser cutting, 3D printing, CNC foam cutting, glassing and vac bagging. A composite of composites.
The model isn't a glider, it is 3.5m span twin engine projected weight of 10kg and it will fitted with a tow release to tow a troop glider. Although I can't find proof that this aircraft type did do glider towing, similar aircraft of that era certainly did.
Once I refine the techniques used to build it, I'll use them on some upcoming sailplane projects.

The fuselage is 4axis foam cut sections, glassed, the wings and tail surfaces are vac bagged composite layups on foam cut blanks. The wing and tail joiner areas are laser cut structures with composite joiners. The nacelles are laser cut frames with molded skins from 3D printed molds. The canopies are made from 3D printed forms.

The composite bits. The fuselage was cut from medium density 100mm thick XPS insulating foam. That thickness has only recently been a stocked material here. Shipping costs made it prohibitive before and I could only get 30, 40, maybe 50mm locally. So the sections are 100mm wide and have a wall thickness varying depending on the area of load, 10 to 20mm. They were aligned, glued with polyurethane, sanded, filled and glassed with 100gm and 50gm cloth using peel ply.

The foam sands quite well and is tough. However it is easy to gouge, so glue lumps must be removed before sanding and the sanding bar/block and sanded surface must be cleaned with a brush every few strokes. Balsa filler works for small voids. I used epoxy and microballoons for larger damage.

The result is a strong light tough fuselage with a good surface to apply a finish to.

The wings, and tail use standard vac-bagging techniques.

The 3D printed negative molds for the cowling and nacelles are printed coarsely with PLA, 0.5 and 0.6mm nozzles as they are large items, finish off the printer isn't that important, layer bonding is. They were printed in sections, glued together, sanded, surface filled with epoxy and micro baloons and painted. The natural finish of a 3D print is not suited to molding, hence the extra work. The mold finish isn't mirror smooth, the parts will be finish painted afterwards so only needs to be good enough for a primer finish. The lay up process was wax, PVA release. Primer sprayed on the mold, 2 layers of 200gm glass. Large 3D printed molds aren't great for vac bagging as they are mostly hollow and can crush, so it was a wet layup with excess epoxy removed.

Anyway, this is another way to make a model and I think the methods will work well for a lot of scale sailplanes.
JU88 block.jpg
JU88 sections.jpg
JU88 mold.jpg
JU88 molds.jpg
JU88 tail.jpg
JU88 front.jpg
JU88.jpg
Der Himmel ist blau und die Luft ist gut!
www.seqsa.net

Dave Mackintosh
Posts: 43
Joined: 26 Dec 2020, 19:02
Location: Cambridgeshire

Re: Using alternatives to balsa and ply

Post by Dave Mackintosh »

Hi Brian.
Very interesting stuff,Where did you source the xps?

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BrianF
Posts: 83
Joined: 19 Mar 2018, 09:36
Location: Brisbane AU

Re: Using alternatives to balsa and ply

Post by BrianF »

Over here it was an insulation products warehouse.
Der Himmel ist blau und die Luft ist gut!
www.seqsa.net

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ian davis
Posts: 167
Joined: 18 Mar 2015, 12:33
Location: Bishopstoke UK

Re: Using alternatives to balsa and ply

Post by ian davis »

Hi Dave

You can get XPS foam in the UK from Sheffield Insulation.

Ian ;)

Dave Mackintosh
Posts: 43
Joined: 26 Dec 2020, 19:02
Location: Cambridgeshire

Re: Using alternatives to balsa and ply

Post by Dave Mackintosh »

ian davis wrote:
13 Mar 2021, 07:51
Hi Dave

You can get XPS foam in the UK from Sheffield Insulation.

Ian ;)
Many thanks Ian.

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