Homemade canopies

Model Airplane News, Oct 2004 by van Mourik, Dick

Vacuum-formed canopies are a must-have for many warbirds and modern-day aircraft models. In a kit, the formed canopy is a factorymade part that comes as part of the package. But when scratchbuilding a scale model, you must make your own. Sometimes for popular designs, specialty companies and/or designers of published plans offer formed canopies, but more often than not, the scratch-builder is faced with the task of "growing his own." Here's how it's done!

TYPES OF CANOPIES

There are two basic types of canopies: the flat-sheet plastic type that's either bent or folded into shape, and the more complicated, teardrop-shape canopy that's used on many warbirds and modern sport aircraft. The teardrop canopy requires the plastic to be heated and then formed over a plug or male mold with a vacuum to pull it into place.

PLUG BASICS

The quality of a canopy stands or falls with the quality of the plug you use to form it. To produce a really good-quality canopy, your plug has to be as perfect as possible. Plugs can be formed in several ways, including the methods that I described in my "How to make fiberglass parts" article in the March 2001 issue of Model Airplane News.

The canopy's shape and size must be accurate because it is one of the most noticeable parts of a model; any deviations will immediately make your model look out of proportion and spoil its good looks. Start by enlarging various scale views of your subject aircraft to the actual size of your model, and check them against one another to see which is most accurate. You can also enlarge a profile photo of the full-size aircraft and check it against the drawings.

By adding a spacer piece to its bottom, make the plug about ½ inch higher than the canopy you're making. This is because, as it is formed, the plastic sheet will form a radius at the bottom corners of the plug. The size of this radius depends on the type and thickness of the plastic you are using and how much vacuum you use to pull the material over the plug. Another basic requirement is that the plug must be tapered upward, i.e., be wider at the bottom and narrower at the top so that it can be removed after the canopy has been formed. Slight blisters, such as those on the rear of the Ju 87 and Fieseier Storch and the bulged sides of later Spitfires, are OK, but don't expect to make a scale Mosquito bomber canopy out of one piece of plastic; additional smaller parts will be required here. In multipiece plugs, adding an extra ½ inch or so to the front and rear of each plug to form tapers will also make them easier to remove.

Be sure to remove all dust from your plug, as even a speck will immediately be visible in the formed canopy. If any sharp edges must be reproduced, drill small holes (about 1/32 inch in diameter) along the edges to allow air to escape and the canopy to pull down completely in those areas. Once you have pulled the canopy, let it cool for a while before you remove it from the plug. If you remove it too soon, it may be deformed.

Don't be surprised if you have to form two or three canopies before you get the knack of using the correct amount of heat and vacuum to form a usable canopy. Like anything else worth doing, it takes practice.

For those who want to make one good-quality canopy but have no intention of making a competition-class model, using a balsa plug will work just fine. Be sure to use wood of equal density throughout its construction, and don't use plywood for cores or formers. Use glue that is easy to sand, and then give the plug a final pass with dry, 400-grit sandpaper. Don't be tempted to apply dope, epoxy, or anything else to fill the wood; bare wood will be just fine.

MORE THAN ONE

If you wish to produce multiple canopies, you'll proceed differently. For better results, commercial canopy makers use the original plug only to make a basic negative (female) mold in which a second production plug is made. After I have made the first plug, I cover it with resin and thin glass cloth. I then prime it several times to produce a good, smooth finish. Don't go crazy on the sanding; final-sanding with wet, 400-grit paper will be fine. No further treatment, such as polishing, is required. Don't be tempted to make a canopy from this plug; it will not work because of the materials used.

When you've finished making the first plug, use it to make a female mold. It is required for only one item-the production plug. The recipe for making a final plug is pretty much the same as the one for making a fiberglass part. After treating the inside of the mold with a release agent, I use a minimum of three layers of molding compound. This is brushed on after it has spent a few minutes in a vacuum chamber to remove any trapped air bubbles. Don't confuse molding compound with gelcoat; it's an entirely different product.

After the molding compound has cured to the "sticky" stage, I apply a mixture of milled fiberglass fibers, chopped strands and resin to the compound in an 1/8-inch-thick layer and allow it to stiffen. The purpose of this layer is not to flatten the wrinkles but to reinforce the resin without a weave. If you use a cloth, it will show through, no matter how well you sand it. Next, I make a mixture of dry, fine sand and resin and, wearing rubber gloves, I put it into the molding. The resin-to-sand ratio is about 10 percent by weight, which gives the sand just enough adhesion to stick together. This sand/epoxy mixture needs to be about ¾ to 1 inch thick to be able to withstand the forces acting on the plug during the vacuum-forming process. When this is all done and the resin gets to the gel stage, I pour fresh resin around the perimeter of the plug. The resin tends to be absorbed by the sand, and this can lead to brittle edges; you can prevent this by applying resin liberally around the perimeter at the end of the process. Then I remove the final plug from the mold and sand it carefully with wet 400-grit paper. It is from this production plug that hundreds of canopies can be formed.