Canopies? - I Sneer at Canopies!

Canopies? - I Sneer at Canopies!  Part 1 – Issue No 56 Summer 2001

Ian Simmonds shed a few tears in the Spring 2001 issue over fitting and finishing cockpit canopies, so I thought I might comfort him, or send him into paroxysms of uncontrollable laughter, with a few words about the ways I do mine.  I say ‘ways’ because there has to be more than one method of fitting a canopy to a fuselage, not least because of the alternative finishes that will be applied.  And of course, when I say ‘a few’ words I don’t necessarily mean a small number, so I have agreed with your editor that this might take the form of a mini-series.  We’ll treat what follows as Part 1 of an unknown number of parts (perhaps only one?!)  Send applications for film rights of this series to me c/o the PSSA.

Not being a complete idiot (no, I don’t know which bits are missing) I’ll start with one of the easier ones, for example Andy Conway’s Hawk.  This is often finished in film, and a large proportion of the canopy edge makes surface-to-surface contact with the fuselage.  It’s round the front where it’s an edge contact only, and the worst problem is to make this a reasonable fit against the top of the fus.  Here, I agree with Ian, it’s glass in the ar…. Sorry – it’s a window in the gluteus maximus.  There is no easy way, it’s all down to cut and try.  May approach, which has evolved over time and is not as difficult as some, is as follows.

Firstly, cover the fuselage.  Then trim off the back of the canopy to fit the rear cockpit line, and lay it on the model.  It will sit high, assuming that it remains over-length at the front.  Cut the front off – straight across, don’t attempt to shape it yet – at about 1/8” over the right length, and mark the edge across the centre with a felt tip pen.  This makes it easier to see where it is during the subsequent trimming process.  Pop it back on, and it will sit high at the front, held up by the corners.  These can now be removed bit by bit, angling or curving them as appropriate, until it can be made almost to fit by a little pressure on the top.  Now finish off the centre front by cutting it in a curve to match the fuselage top – those small nail scissors with slightly curved blades are useful for this.  This curve will bring the front of the canopy back a little (hence the 1/8” excess length to start with) and doesn’t have to be an exact fit, for reasons which will emerge later (provided I don’t forget).  If you find the canopy still slightly too long, trim a smidgen off the back; there’s no point ruining all that good work at the front by a careless slip.  That’s really all the hard bit done, and now that the canopy is sitting correctly, you can trim off the sides so that they overlap the fuselage by about 3/16”.

It’s now ready to attach, but don’t forget to complete the cockpit fitting first, or you’ll have to rip it off again.  Tape the canopy in position with four small pieces of low-tack masking tape, two along each bottom edge, making sure the canopy is pulled firmly down into place without distortion.  Cut a strip of self adhesive trim, for preference the same colour as the turtle deck, about 3/8” wide and long enough to go round the canopy back edge plus an inch or so.  Stick it on so that it covers the overlap of the canopy onto the fuselage.  Behold!  The rear is stuck down, and at the same time you have formed the rear canopy framing.  It’s a good idea to stick a bit of paper to each end of the strip before you attach it, so that the ends don’t cleave immovably to the fus, because later – not now – you’ll have to trim off the surplus length.

Unless you’ve built it wrong, you’ll have noticed before you started that this strip really should be curved to stop it wrinkling, and you can sort this out in one of two ways.  The easy way is to run a warm iron along it as you stick it down, pulling it into shape as you go, but first do check on a piece of scrap canopy plastic that it wont melt or go all out of shape if you touch it with the iron.  The less easy way (which you’ll need to use at the front end, so you might as well try it now) entails making a simple paper pattern.  Cut a strip of thin paper, around 1” wide and lay it in position to overlap the canopy onto the fus.  Borrow a hand from somebody, unless you’re a guitar player, because you now need to run a fingernail along the joint line between canopy and fus top to form a similarly shaped crease in the paper, holding down both ends of the strip during the process.  Now the crease can be used as a pattern for each edge of the trim strip, which should of course be parallel.

Now apply more strips along the bottom edges, removing the temporary tape attachments as you reach them.  The Hawk needs two pieces to cover each bottom edge as it’s not a straight line all the way.  We are now left with the front edge, which is anything but straight.  Fret not!!  Repeat the paper pattern trick, but use a narrower piece of paper or else it will refuse to go into place.  If you cut the paper to an approximation of the required curve first, it’ll be even easier.  The trim strip cut from this pattern will be a little more difficult to apply than the preceding ones, and its done by stretching it tight across the joint, half on the canopy and half on the fus, symmetrically about the centreline, and then working it to each side so that it forms the front frame.  This, coincidentally, also secures the canopy front to the fuselage, covering any small gaps, which there may be (I remembered!).  All that remains is to cut off the surplus ends of the strips of trim (easy to lift off with bits of paper stuck to them) and Robert is your parent’s brother: a framed canopy, securely attached to the model in a few minutes.  Well, alright, perhaps a few minutes more then.

It’s a good idea now to whip round the trim with a warm iron to make sure the edges are firmly stuck down, and to work it into the little gap where its held off the fus by the thickness of the canopy, to give that careful manicured look.  There may also be a little renovative ironing to do if your supposedly low-tack tape wasn’t; it may have lifted off patches of film, as may the removal of any misplaced or mangled strips of trim.  I don’t use ordinary masking tape on film, it always seems to stick too well.  Tamiya make some narrow non-crinkly stuff which is excellent for this job, but unfortunately rather expensive.  It’s also excellent when used as intended, and produces nice smooth edges on painted schemes.

The story doesn’t end here, though;  the canopy may be of more use later.  If you damage it in an accident (yes, I know – very unlikely) don’t just rip it off and bin it, remove it carefully from the model.  A warm iron is handy to soften the adhesive on the trim if it refuses to peel off easily.  You now have in your hand a template from which a replacement canopy can be cut.  Mark it up and keep it safe, you may even want it again!  If you should need to build a replacement Hawk, the same old canopy can be used as a template when you are shaping the fuselage, so that it will still be useful for cutting a new one for this model too.

I’ve arrived at the above approach after trying all sorts of other adhesion methods, including double-sided tape under the canopy edges, and it has proved to be the easiest and the best.  It forms both the framing and attachment mechanism at the same time, there’s no glue involved to smear all over the model or mist up the plastic, it doesn’t need to be put aside for hours to dry, and if you make a mess of a trim strip it’s no problem to bung it in the bin and cut another.  The canopy is held securely enough for all normal use, yet is easily removed without damaging the model.

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Canopies? - I Sneer at Canopies! Part 2 – Issue No 57 Autumn 2001

Let’s move along and look at fitting canopies on top of painted finishes.  Because my idiocy-level is constant I shall use an easy example, as before.  If we consider a model such as the P-51B (razorback) Mustang, trimming the canopy to fit can be done in the same simple manner as described for the Hawk in Part 1.  The differences come only in its fixing, and the addition of framing.  My preference is to fit the glazing first, as in that state it is less flexible and adding the faming is then an easier task.  Attaching framing to a loose canopy is a bit like tying a string around a jelly, unless the canopy is a very stiff one.  As I make most of my own (canopies, not jellies) from thin plastic (even thinner when stretched over a mould) they tend to be the other end of the stiffness range, akin to a certain body part after the consumption of a gallon of ale.

I’ve tried all sorts of adhesives for fixing canopies, and have eventually settled on one, which works pretty well for me.  This magic substance is labelled “R/C Modellers Glue” (so no good for free-flight, then?) sold under the ‘Deluxe Materials’ brand.  It’s a white PVA-type glue, and for our purposes has several advantages over other adhesives.  It doesn’t leave a white fog on the inside of the glazing as some cyano-acrylates do.  It is water based, so any excess can be wiped off (while still wet) with a damp cloth, leaving no trace, and anyway it’s stiff enough to avoid most splodges or runs.  It dries clear, leaving any excess inside the canopy barely visible, and can be used so sparingly that the frame hides it all; I’m still only half way down my first bottle, and I bought it when ‘stiff’ wasn’t just what I get after climbing up a steep slope.  On the downside, it can take a long time to dry when the airflow around it is limited, and it isn’t waterproof when set (although this can be turned into an advantage if you need to remove the canopy later, when a good soaking will soften it).

There are two main problems in attaching canopies with glue. One is that as they are placed onto the fus (which we will assume has already been painted), the glue, whether applied to the canopy or the fus, tends to smear all over the place.  The second is that this smearing is exacerbated (!!!) if the canopy is not placed exactly in position first time.  So the question is how to overcome these little snags, and the answer is that I have no idea.  No, that’s not true; nearly true, perhaps, but not entirely.

To assist in placing the canopy accurately, you will need masking tape, for preference the low-tack Tamiya stuff mentioned in Part 1, and a felt-tip pen (dry wipe markers are ideal as some of the permanent types will leave marks on your paintwork).  If you are going to use ordinary masking tape, rub pieces first onto a piece of balsa, and lift them off again, repeating the process until they are just sticky enough to stay in place on the fus.  Place the canopy in position and stick a piece of tape onto the fus so that it touches the back end of the canopy, but doesn’t overlap onto it, as a back marker.  Stick a piece of tape onto the top of the canopy right up to the back edge, and draw a line across the joint onto both pieces of tape as a lateral alignment marker, and then do the same at the front.

If you want, you can run 2 pieces of masking tape on the fus just below the canopy sides, to act as both depth markers (unnecessary, really, because of the marks on top) and to pick up any adhesive which might be smeared and squeezed out later (so leaving less to wipe off).  Take two pieces of masking tape 2”-3” long (and as sticky as you like this time), fold them in half (the easy way, across the width) leaving a half inch or so of the ends apart.  Stick them by these loose ends ontop of the canopy, one near the front and the other near the back, so you get two tabs sticking up.  The scientific name for these is ‘handles’ and they will save you from having to exert a precarious grip on one of the floppy sides of the canopy when you come to attach it.  In the meantime, you can practice lifting it off and popping it back into place.  Right, that’s you happily occupied for an hour or two!

The final preparation before gluing is to attach two or three spacers (or even more for a long canopy) to each side of the fus, holding them in place with masking tape.  Strips of 1/16” balsa, no more than 1/8” wide, will do, positioned vertically and extending upwards just above the highest contact point of the fuselage sides and canopy edges, make sure the rearmost one is close to the back edge.  Their purpose is to spread the canopy just enough to avoid smearing the glue as it is lowered into place.  You may feel like chamfering off the top ends or even curling them inwards a little, more practice in lowering on the canopy will enable you to decide how far to take this.  If they are too thick or too tall they will pull the canopy badly out of shape as it is placed onto the fus.

And lo! Gluing time is upon us.  Holding the canopy by its handles, run thin beads of glue on the inside edge of the canopy along the windscreen and quarterlights (much easier if the canopy is lifted off the fus first); along the fus sides between the spacers, just above where the bottom edge of the canopy will fit; and finally, either inside of the back edge of the canopy, or round the turtle-deck – whichever you prefer.  Narrow strips of double-sided tape are an acceptable substitute along the sides, where the contact is surface to surface.

Now, viewing from above, lower the canopy vertically into place, using the masking tape and pen marks to align it.  Proceed slowly, making sure the canopy edges slide down outside the spacers.  When it is fully down, use masking tape to hold the back and front edges in position and avoid slippage whilst you release and remove the spacers.  Unpeel the temporary tape, press down firmly all round and wipe off any glue which squeezes out.  If you have made an unacceptable mess, whip it off and quickly remove all the glue with a damp cloth.  Then start again, but work more carefully, otherwise you will have to stay in after class!  Further strips of masking tape are used to secure the canopy in place while the glue dries, a process which can take up to a couple of days if the canopy is not ventilated.  The glue sets by evaporation rather than by chemical reaction, so if the fus surface is very well sealed it will take longer than if it is not.  The glue is not fully dry if any milkiness remains in it, so don’t remove the tape until it is fully clear!

If you must use ordinary masking tape to hold the canopy in place, don’t stick it directly to the surface of the fus, or it may bring the paint off when removed.  It isn’t intended to be left on paintwork for more than a very few hours, and may bond rather too well after a couple of days.  Instead, wrap paper strips around the underside of the fus almost up to the canopy edges, and lap the tape onto itself over these.

When the glue is completely dry, framing can be applied, after removing all the masking tape, of course: for this, some brown paper and paint are needed.  Most frames are different colours inside and out, so start by painting one side of a piece of brown paper with one of the colours you want.  When it’s covered satisfactorily, and dry, paint the other side with the other colour, and after drying, cut the paper into strips the width of the framing; you can of course also cut out curved strips as are often found at the top edges of windscreens.  Use small patches of masking tape to mark the edges of the frames on the canopy, and stick a strip on (inside colour inside!) using the old favourite, R/C Modellers’ glue, smeared across the full width of the strip.

Continue with all the other strips, butting against or overlapping other strips as you choose, but being careful not to dislodge any already in place.  If joints are complex or in any way difficult, leave earlier strips to set for a couple of hours before applying more.  When all the frames are set, a final coating of varnish applied with a fine brush will finish them off.  Getting the varnish right down the edges of the frames (without marking the glazing) reduces the chances of moisture softening the glue later.  For this, water based acrylic varnish is a godsend, as any overspill can be wiped off the canopy with a damp cloth before it sets.

One thing which I intend to try, but haven’t yet got round to, is masking fluid which sets into a thin rubbery film that can be peeled off later.  It should be possible to cover the canopy with this (after attaching the handles) fix the canopy in position and then paint the fus.  Any glue residue, which would otherwise be visible, and any handling marks or damage caused by too sticky masking could then be obliterated.

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Canopies? - I Sneer at Canopies! Part 3 – Issue 58 Winter 2001/2002

We’ve dealt with fitting canopies on a film covered model where much of the canopy has surface contact with the fuselage, and with fitting them on a painted one, where there may be edge contact only.  What we are left with is fitting the second type of canopy to a film-covered fuselage.

The shaping, trimming and fitting processes are exactly the same as before, but the drawback comes in finding a totally satisfactory adhesive.  R/C Modellers’ glue is generally acceptable, and although it adheres rather better to polyester film than to Melinex, it is not perfect on either.  Its one big advantage is that surplus can be wiped away leaving no trace.  On Melinex, you can try masking off and painting the intended glue line with an etch-primer such as Prymol, and this does improve adhesion somewhat.  However, besides this minor adhesion issue, there are two other concerns to be addressed.

Firstly, the fact that both the film and the canopy are impervious to water means that the drying time of this adhesive will be extended, which, coupled with the almost total lack of airflow through the canopy, means that it may not be fully hardened (when it goes clear) for several days, perhaps even a week.  During that time the canopy must be protected against movement, as whatever bond is forming will easily be broken by careless handling.  Now its not normally any great hassle to put the fus aside for a few days, except when you are burning the midnight oil to get the thing finished for a session the next day, which I know is not an unusual occurrence.  For this job, you need to gear up to do the job well before the model needs to go out for a wheeee!

The second problem comes rather later: as mentioned before, the glue is not waterproof, and seepage of moisture under the canopy edge will eventually soften it.  Normal atmospheric moisture isn’t really a worry, its when you are caught out in a downpour on the slope, and the model gets thoroughly drenched, that the trouble starts.  There are two solutions I can offer.  You can varnish the joint line, very carefully (and after the glue has set completely, of course), with a waterproof varnish, immediately wiping off any misplaced blobs from the film and canopy.  Depending on how good a pilot you are, this seal may outlast the life of the model.  Alternatively, use a waterproof adhesive.

The alternative glue may also provide better adhesion, and the obvious choice is epoxy.  This can be thickened with colloidal silica so a stiffer bead may be formed along the edge of the canopy, reducing any tendency for runs to occur.  Epoxy sticks to film at least as well as does any other adhesive, and gives you ample time to position the canopy accurately.  Alternatively, foam friendly (low odour) cyano is supposed not to fume and leave a white deposit, but is a little more difficult to manage unless you can find some which is a bit thicker and slower setting than the usual stuff.  You don’t often get two chances with cyano, and while a canopy stuck in the wrong place can sometimes be removed and re-fixed (usually with a bit of effort), there always seems to be lumps and bumps of the stuff left in places where they wont come off – and scraping dried cyano off film generally leaves some marks.

So, you’ll gather, I don’t sneer quite as much at canopies in this situation as in others.  The fitting process is no more difficult than before, but there are these other little snags to live with.  I regret the series of articles not having a happy ending, but that, as they say, is life!

Glossary of Terms :

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