long ago it was possible to visit the local model shop and select a suitable
celluloid canopy for that latest creation. The choice of shapes and sizes
was wide. These days, if canopies are available at all they are more suitable
for RC models which have the size for carrying small people. So very little
is available to suit our control line airplanes. We have to be our own manufacturer.
No problem! All we need is easily available; you may even find most of it
in your scrapbox.
The only real problem is to find a suitable plastic sheet. My preferred method of canopy pulling means that most types of plastic cannot be used. However, the advantage of my method is its extreme simplicity; female moulds are not required - nor is exotic gear such as vacuum pumps. Green and blue cellulose sheets as supplied by Graupner work extremely well, as does the clear SIG butyrate offerings. You'll have to test others brands for yourself!
How's it done? A heated sheet is pulled over a mould, which is made from a balsa block. This mould must be larger than the desired size to allow for trimming the edges. Several blocks may be glued together to avoid unnecessary expense, but care must be taken to avoid gaps. The tiniest imperfection will show up on the finished canopy. First the side view of the canopy is drawn on a block. This is roughly cut with a scroll saw, then precisely sanded to exact outline. Now the shape of the top view is drawn on a piece of paper. I usually draw only one half, fold the paper at the centre line, and then cut the folded sheet. This will guarantee a true symmetrical shape ( foto 1 ). From this pattern a baseplate of thick plywood is made. Glued under the balsa mould it helps to strengthen the bottom edges of the mold. This plate also takes a piece of thick plywood which serves as a handle, enabling the mould to be clamped in a vice. Now the mould shape is carefully finished, checking frequently with templates if you like. Finish with very fine sandpaper. Although some enthusiasts advocate lacquer resin, I prefer to work with the plain wood, because it allows the hot sheet to slide easily over the smooth wood surface.
|Both edges of the plastic sheet are clamped between two hardwood spars each. They are held together with small nails or srews. I prefer srews to press the wood spars together very tight ( foto 2 ). These spars have little holes on all ends. Thus the plastic sheet with the spars can hang freely from a wire frame. This wire frame is bent as shown in the photo, in a way that it cannot tip over ( foto 3 ). Of course maximum dimensions are dictated by the size of the oven ( and this is the size limit for my method!). Now it’s time to think over the procedure. Everything will have to happen very quickly or the sheet will cool down before it’s completely pulled down, preventing satisfactory forming. Work out which hand will do what. Now clamp the mould in the vice. Preheat the oven; I work with 175 degrees Centigrade for about ten minutes. Use this as a basis for your own experiments, every oven works differently. The right moment is easily judged - the sheet goes floppy and starts to smoke ( and smell !). Don't wait too long. If the sheet gets very soft it will be difficult to handle the frame, and even if you succeed you may get a thin canopy. This may not be a problem with tiny canopies. But bigger canopies are prone to shrink when exposed to the sun too long ( foto 4 ).|
|Now work quickly. Use thick gloves since everything is very hot ( foto 5 ). Open the oven; pull out the frame by holding the upper beam first, then grab the lower beam and hold everything horizontal ( the wire frame will hang down freely ). Place the plastic sheet centrally over the mould and pull down with equal pressure on both beams - lightly at first, then harder as the sheet begins to cool. You have about five seconds time ! Once you get it right, hold everything in place for about one minute while the sheet cools, for it will shrink slightly, forming itself around the mould for a perfect shape ( foto 6 ).|
much depends on the style of the canopy. If the curvature is gentle, shaping
is very easy and only light pressure is needed. If we have sharper bends
on deep moulds ( high canopy ) especially on compound shapes, it may be
necessary to enlist help to grab the soft plastic sheet in front and behind
the canopy form, thereby pulling it down softly . Whatever you do, never
release the beams once the main shape has been created. You will immediately
destroy all your efforts ( foto 7 ).
If you haven’t heated the plastic sheet long enough you will instantly notice that it will not lay down easily around the form and resists pulling, because it will cool too quickly. No problem; just put it back into the oven and heat again.
|Once the canopy has cooled, be careful to not simply pull it from the mould. Cut around the mould until the canopy can be released easily. The surplus will be much thicker and harder than the actual canopy shape, thus resists bending open for easy removal from the form. If you force it, it will crack. Now the canopy can be precisely fitted to the fuselage ( foto 8 ). Once you have cut and finished the actual final shape of the canopy and consider to make another one in the future, a line can be drawn on the balsa mould to outline the exact shape. This helps cutting out the next canopy. It’s wise to keep all the moulds. Even if your next design has a different fuselage shape, the existing mould may serve to pull another canopy, and with some cutting here and there it can be fitted to another fuselage. Actually I have drawn several airplane shapes with an existing canopy mould in mind ( foto 9 ). As can be seen this is a very easy and practical way to produce that glasshouse for your new airplane. There’s only one little drawback.|
|Your pizza next evening may have a slightly strange taste.|