Scale Soaring UK

I really like hollow moulded flying surfaces on model aircraft.

They have that lovely solid look, are commendably strong and light, and can be more robust than wooden structures which is handy if your models have to deal with the rough and tumble of slope flight. Maybe it’s just me but whenever I pick up a traditional wooden structure, I’m worried I’ll poke my finger through thin ply sheeting or dent some balsa. I am clumsy like that.

On the other hand, composite models can be a heap of work if you do them properly…making plugs for every part and then making female moulds off them. It’s only really worth it if you plan a production run.

So I’ve been pondering for a few years how to make a hollow moulded item without all the expensive equipment and in a fraction of the time.

When I smashed the back end of my fave 4m semi scale glider last Summer, I decided I’d have a go making a hollow-moulded tail fin/rudder.

It’s actually gone really well…

It’s Monday morning and I sketch out a new fin shape on some 2” blue foam, checking that the surface area matched the original fin, but going for a more trendy fin shape with less sweep and higher aspect ratio.

Off to the computer to print out a top and bottom profile. NACA 0009 thickened to 10%. Glue onto a scrap of 6mm MDF and then 2 mins of scroll saw therapy.
This is fun. I am making stuff, seeing fast results and it’s all going well.

Cut the outline of the foam block, draw a datum around the edge, and glue on the profiles.

And now just to show you how low tech I can go, I cut around one side of the templates using a SAW. I could have used a hot wire cutter. But I’d have to make one first. And I’m in a hurry.
2 mins later I’m sanding the last few mm down to the templates using my long permagrit sanding block. It’s surprisingly easy. Especially with the low early morning sun casting helpful shadow across the surface.

I survey my progress, check my watch and I’ve been in the workshop just over an hour.
Often I can spend 2 hours in there and have essentially nothing to show for my time, so this is brave new territory for me.

I tape over the blue foam with parcel tape, so that I can remove my fin skin later (remember, this is supposed to be hollow moulded, so I don’t want a sold cored structure, no, that would be cheating).

Now to decide on a lay up for the first skin.

I did a lot of experimenting over Christmas with different cloth types and weights, and I learned a heap. Which is a euphemistic way of saying I made many, many mistakes.

Anyway, I had best results with 160gsm carbon/Kevlar as the main structural layer, with a top layer of 100gsm glass twill, to seal in the Kevlar and give some thickness to work up a good surface finish.

I cut out the cloths, waxed the parcel tape, and got ready to mix my first batch of resin…

Fabric layer onto the parcel tape, epoxy screened on with an old credit card, peel ply laid on top.

By the way, Terry White introduced me to peel ply a few years ago and it’s a perfect example of something I’d previously scoffed at as being unnecessary complication and waste of money, but which I would now never be without. When removed after the resin has set, it takes away most of the excess epoxy, reducing finished weight, and gives a textured surface perfect for adding further composites.

With the first skin laid up, I could have popped it into a vacuum bag, but then this wouldn’t qualify as a low tech method.
Also, I scoff at vacuum systems as being an unnecessary complication and waste of money.

Tuesday morning I enter the workshop early, but tentatively. Will the plain sailing of the first day run aground on the rocks of reality?

Nope - the first skin is pretty much perfect! Hurray! I made a thing!

Straight on with the second fin skin.

Out with my saw and cut off the other side of the foam block, then sand down to the template.

Now, just for the laughs, I decide I will make the rudder with a living hinge (thanks to the encouragement of others on this site to try this method).

I hesitate for a moment, troubled by the fact that the rudder, previously a very symmetrical thing, will be very asymmetrically hinged on one side. But no, this is ok. I fully expect this whole attempt at hollow moulding to fail at some stage, so I may as well take a risk with this new-fangled hinging method.

So the layup is 160gsm carbon/kevlar again, then a 4cm wide strip of peel ply to act as the hinge, then 100gsm glass on top.

Epoxy gets mixed and off we go…

While the second skin goes off, I realise I need to figure out how to attach the tailplane and fit servos to make bits wiggle.

I laminate a tailplane seat from 3 layers of 1/8” ply faced with carbon that happen to be on the workbench.

Late evening and the second skin comes off the foam plug and looks….











Fantastic!

In a wave of euphoria I decide now is the moment to score a hinge line and carefully file it until it starts to bend freely.

The whole process takes about 10 mins and it is a total joy. The peel ply layer, sandwiched in the middle of the layup, faithfully does the hinging thing. A few strokes with a needled file is all it takes.

Yep, I’ll be using living hinges again.

It’s Wednesday morning and I have a wobble. Flexing the two fin skins in my hands, I think how incredibly floppy they are. When joined at the leading and trailing edge, will they really make a rigid fin? Maybe I should just sandwich the blue foam core in the middle after all, to be on the safe side. I mean, the foam fits perfectly, and it only weighs 26g. Mmmm.

In the end I cut a big lump out of the middle, but leave an inch band along the leading edge to ensure the glass joining tape gets pushed tightly against both sides. I decide to leave the foam section inside the rudder.

I also finalise where the rudder servo will go, and cut a slot for it.

Show time!

Time to close up the fin, joined at the leading edge with a 2” wide strip of 200 gsm glass cloth (cut on 45 degrees) and 4 carbon tows pressed into the very leading edge. I think a really strong join is essential here to create the structural integrity of the whole fin. If it cracks open at this seam, the whole strength of the skins is totally wasted.

I be mixing epoxy again.

The whole sticky mess needs remarkably little clamping (unlike the sheeting of a traditional wooden wing). The skins seem to naturally cling to the foam core. I “hinged” the two sides together at the leading edge with parcel tape, and after that it only needed clamping at the trailing edge.

Today, 8am and the workshop door swings open to reveal an absolutely gorgeous fin.

A quick clean up of the edges and a test fit onto the tailboom (which I moulded over a male plug at Christmas).

I’m especially chuffed with the leading edge which is immaculate. No filler/bodging required.

Total weight is just over 120g, less than the original fin which was a traditional balsa/spruce structure covered in 50gsm glass cloth.

The fin feels unbelievably strong in both bending and twisting.

With nothing else to do, I cut along the non-hinged side of the fin to release the rudder, opened it out then sanded out some foam to allow it to swing both ways.

The last job to do is to line the inside of the fin/rudder recesses with carbon cloth.
For the rudder, this is more or less cosmetic as it is amply strong. However for the fin, this is essential for structural integrity as it completes the “tube”. How many times have we seen a plywood fin post break away from the sides of a fin moulding?

I may post photos of this if I get a chance but I’ve basically achieved what I set out to: a quick and cheap hollow-moulded structure, and my first living hinge.

Thanks to all who’ve offered tips/encouragement.

Rog

Hi Roger

Very interesting reading (for me), particularly the construction of the live rudder hinge. I am sure that your approach could be used in repairs too.

Thanks for sharing!

Mike

I've only just spotted your post Rog, but it's really great!
If I were ever to do another build I would definitely go hollow moulded and it's brilliant that your method worked so well (like you, I've always been put off by all the work making moulds etc when you're just making one).

Is it attached and flying again now?...

Simon

Great to hear from you Simon.

Yes funnily enough I just finished spraying it last week, and for once the finish came out almost perfectly (thanks to some top tips from CW). Trouble is, now the fus and tail are so pristine, the wings look a bit shabby!

I test flew it a few weeks ago before spraypainting and it flies as well as ever. I’ll recheck the CG now it’s painted, but it should be good for a fly at the Eden Valley event on Saturday.

Rog

come on then Rog, for us incapable of achieving a professional finish, what knowledge did the great one impart?

Elliot Howells wrote: ↑30 May 2022, 22:05 come on then Rog, for us incapable of achieving a professional finish, what knowledge did the great one impart?

Probably, "Get someone else to paint it"



BC

Barry I did seriously consider paying someone to spray it…but I live in Yorkshire so parting with money is always a challenge.

I think everyone’s probably found their own way to get a decent spray finish, but for me the key was…

1. Brush on, rather than spray the primer, to avoid pinholes.
2. Rub the primer down to almost nothing using 240,400 then 600 grit sandpaper.
3. Make a spray booth then Hoover it to get it forensically clean.
4. Set up really good lighting at chest height - I used 2 x 500W halogen site lights which had the other plus of heating the booth which helped the paint flow and dry quickly.
5. Build up 3 coats of paint, drying for 2 mins by turning in front of the Halogens
6. start with a light coat to get a key, second coat builds up the density of colour, third coat is heavy enough to let the paint flow but not so heavy it runs - use the lights to check when it glosses over.

Of course next time I spray it will be a disaster!

Rog

Rog, thanks - this is good info!

I'll save this for future reference and report back, although knowing me, it may be some time...

Ell.