Shinobi | A Home-Grown Moulded Fuselage
Part II: Fairing the Wing Stub Block into the Fuselage
Readers who have not already done so may want to read the previous parts of this series before continuing with the article below. — JH
If you were following along at home in the first article in this series, you will now have finished sanding your fuselage plug to shape. However, before I continue, may I offer:
Advice: Check that plug one more time. Is the final shape 100% what you want?
Satisfied? OK then, carefully ease out the tacked-in temporary wing insert and check the wing stub block alignment in the fuselage plug. If you did a good job carving and sanding it will be a nice fit. So, now the final wing stub block can be glued in, but first check the alignment. Now check it again. Apply just a couple of tack glue spots and check it again. Is it 100% aligned, and the faces parallel with the centre line? If you are happy, then apply more glue to make the insertion permanent.
Advice: Take time aligning the wing stubs. A few extra minutes to check here will result in a perfect wing fit later.
Time to Think About That Fairing
To kick off the fairing, glue a couple of triangles made from 1mm or 2mm G10 glass or some spare carbon sheet at the trailing edges of the wing stubs, and flush with the fuselage to form the beginnings of the wing fairings. These don’t have to be shaped before attachment as it will be easier to do it — and to get both sides the same — after we apply the resin that will make up the fairing curves; but make sure to get them straight with the centres of the wing sections. The same can be done with the front of the fairing at the leading edges. These small triangles when embedded in car body resin, will provide strong, hard points on the plug and prevent the sharp edges of the fairings from breaking.
After the reinforcement triangles are hard bonded to the plug, use a piece of dowel and abrasive paper to sand them to the shape you want. Again, take time with this job. The shaping of these fairing edges will have a great effect on the eventual organic look of the fuselage.
Advice: Try to make the radii blend in as much as possible with the lines of the fuselage and the trailing edges of the wing stub block in a smooth curve.
Now it’s time to make the actual wing stub fairings, so the first thing we need is a nice body filler fairing tool, and the very best fairing tool that I have ever used is the good old humble teaspoon. You can take, nick, steal, borrow, re-purpose, or appropriate one from the spouse’s silver heirloom family tableware case, but don’t tell them I told you. Or, alternatively you can just find an old teaspoon from somewhere. Either way, the compound ovoid shaped curves of a humble teaspoon work fabulously well as a fairing tool.
Shaping the Fairings with Car Body Fillers
I guess everyone who makes models has their own favourite filler. It really depends on the application. For a repair you’d need to use a hard, strong compound like epoxy and a powder fill, maybe Cabosil or loose fibres, but for our fuselage plug, what we need is something that is reasonably hard but easy to sand. My favourite? Car body polyester resin type fillers. There are many brands, and the name Bondo springs to mind for those in the USA, while Plastic Padding was a popular product in the UK. They all share some common properties: quick setting, easy sanding, good feathering performance and good adhesion. We are not worried about weight so any good car filler resin fits the bill.
This Is a Stick Up
Like always, plan what you want to do, prepare well, and then work slowly and carefully. If you do a lot of filling work you will probably have a special wood or plastic platter — like an artist’s palette which you use for mixing and application of fillers. Actually, I have seen some tear-off disposable palettes that look good, but I have never tried them, preferring my own old but familiar square of plywood. Scoop out a tablespoonful or so of resin, and then close the product lid. Put your scoop — normally a short length of thin wood — in a safe place, away from your work area with the product can. Now take the hardener — normally a tube of paste, and squeeze the correct amount onto the resin blob.
Advice: It’s better to use too little hardener and have to wait than it is to use too much and have the batch go off too fast.
Advice: ALWAYS keep the resin and hardener separate and never have both open at the same time.
Advice: Aim to do only one part of the fairing at any one time — that means you would normally mix and apply four batches to do an entire fairing.
Mixing and Applying the Resin
Using a putty knife or something similar (an antique Onyx-handled silver cake knife, stolen from that same spouse’s cabinet?) mix the resin quickly but thoroughly until it’s a uniform colour, and then using the same tool, apply the resin to the first side of the fairing, working it into the angles as much as possible. Now while the resin is still very soft, take your trusty teaspoon and using the back side, run it along the filled gap from the front to back, removing the excess resin and leaving a nice concave curve. Changing the angle of the spoon to the resin changes the radius. That is, sharper at the leading edges for a smaller curve, and more oblique at the trailing edges. You should now have a nice radius in there — but don’t worry, you should have time for a couple of tries if you are not happy the first time. You can, and probably will almost always apply more resin later. Now repeat the same process on all the corners of the fairing.
A Little General Advice on Resin Use
- Don’t try to do too much in one shot unless you are really familiar with the process.
- Car body filer is normally a polyester resin compound so don’t allow the resin product and the hardener paste to be near each other unless both are firmly closed.
- Use one component at a time and while using it, firmly close and remove the other component from the bench.
Warnings for Polyester (Styrene Monomer) Type Resins
- Be careful — Using too much hardener will cause a catalytic reaction where the resin will set too quickly and may get hot. This is an exothermic reaction and can cause enough heat for the mixture to actually ignite.
- Be careful — Keep all your mixes under observation until they harden. It’s safer and can also let you know more about the mix.
- Be careful — The slightest amount of hardener that mistakenly gets into the Resin paste can and will cause the whole contents to harden within 24 hours. Even sanding dust can do this.
- Be careful — Don’t use an old can of resin that you have had hanging around for years — or your Granddaddy has. It’s a recipe for disaster, and anyway your lovely new plug deserves the best, right?
I did mention that you should be careful, right?
Nicht Fur Gefingerpoken
Don’t be tempted to start poking, stroking, carving or sanding the filled fairing before the resin has cured hard. Leave it at least 12 hours and preferably for a whole 24-hour day. Despite avowals to the contrary and swearing on the last resting places of numerous deceased relatives from the resin manufacturers, it’s still likely to shrink, and unless it sets pretty hard it can flake off and ruin all your beautiful handiwork if attacked too early.
Sanding, Sanding and More Sanding
When the Resin is hard you can begin to refine the curves. Use tools like a piece of dowel of about the right radius with the abrasive paper wrapped around it. Work slowly and carefully, as usual, taking care not to accidentally round off the sharp edges of the wing stub faces or dig grooves into the rounded fuselage profile. Remember that its highly likely that you’ll need at least one or more applications of resin.
Advice: Use different sizes, of sanding aid — it’s amazing how many different sizes of round implements and how many implements with round handles, you have in your kitchen drawer.
Advice: Always remember that it’s easier to ask forgiveness than it is to seek permission. And that’s because you ain’t going to get no permission. Whoops I’m going straight to hell for that one…
Advice: If you are worried about over sanding or accidentally sanding the wrong parts, you can put some tape on the edges or surfaces that you do not want sanded to protect them.
Advice: Leave the plug to harden thoroughly before attempting to shape the fairings.
Advice: Use sanding aids to attain the correct curves and radii.
Advice: Protect any critical faces or surfaces with tape.
Finishing
All that remains at this stage is to finish the plug. I use a couple of coats of clear epoxy, thickened with a dash of colloidal silica (Cabosil) to give it some thixotropic properties so its doesn’t just all run off. The reason I use clear coating is because I want to see the centre lamination line which will be really useful when lining up the plug on the parting board later. Colour paints can be used but it’s a good idea to keep track of the nice centre line. Whatever you do please don’t use any kind of cellulose type paint, because let me tell unto you that if you do, your plug will stick in the mould — whatever you do. Rub the coating down to a smooth finish, but note that it doesn’t have to be mirror finish. About 400 grit will be good. This will provide a good ‘key’ for your parting wax to get a good hold on the plug surfaces. The finished mould will be polished later.
Advice: Never, never, never use a cellulose based paint to coat to finish your plug — whatever you do, it will eliminate your release agent’s effect and the plug will stick in the mould.
Advice: Break the surface of the plug by rubbing the surfaces down to about 400–600 grit — this will provide a key for the wax to thoroughly coat and infuse the surfaces thus providing a good release.
Advice: Use a little soap or dish washing liquid to provide a lubricant and keep the abrasive paper clean if done wet.
Next time: What about that nice slip-on nosecone? Until then, if you have any questions please don’t hesitate to post them to the Responses section below and I will do my best to answer them. Putting them here also means others will benefit from both your question and my answer.
©2021 James Hammond
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