Under pressure

Okay, confession time. In the decade or so that I’ve been designing and scratch-building sport-scale models, one of the key factors that’s helped determine my choice of subject has been the complexity (or rather the lack of complexity) of the cockpit area. I’ve quietly tended to plump for models that feature simple, flat-screen windshields or those that are easily fabricated with single-plane curves. Whenever I’ve come across more complex canopies I’ve fudged the issue by carving them from foam or balsa, or utilising a portion cut from a plastic soft drinks bottle.

There are times, though, when a recycled pop bottle just won’t do. Take the 60” (1524mm) span Clean Sweep that appeared as a free plan in the May issue of RCM&E, for example; this features a bubble canopy some 9” long and 5” wide, i.e. way too big for any of my usual dodges or half-measures. There was nothing else for it, a different approach was needed, so I started some research on the internet for ways of manufacturing a suitable, bespoke canopy for the Sweep.

KEEP IT SIMPLE

I really didn’t want to get involved in building a ‘full house’ vacuum-forming system, considering that the required canopy was actually just a simple blob without any awkward concave curves or extraneous detailing. I was delighted, then, to find a method that uses not much more than a modified mastic gun and a domestic heat gun. Pioneered by Steve Bage, it seemed to tick all the boxes and, accordingly, I was keen to give it a go.

The idea behind Steve’s scheme is simple in that a carved balsa plug is gently forced into the middle of a warmed sheet of clear acetate, using said mastic gun as a force generator. The acetate duly yields to the pressure and heat and, as a consequence, wraps snugly around the balsa plug to form the canopy. It’s so simple it’s brilliant. Really!

PHOTO SHOP

The accompanying photos detail the process step-by-step and should, hopefully, see you through the method without any problems. Before we start, however, here’s a few notes about my own experiences that may help you on the way. To begin with, then, I used a domestic Black & Decker heat gun as my weapon of choice, with the temperature set to about half power. 

There’s no point in using prime balsa for the plug; rough blocks that you can buy for pennies at shows are ideal. This plug measures 9” (229mm) long and 5” (127mm) wide, and is formed from several blocks glued together with aliphatic glue. Aliphatic is better-behaved under the sanding block than cyano or PVA as it doesn’t leave so many lines or protruding hard spots.

Mark up the plug with cross-sections and the side elevation of the canopy (taken from the plan) before removing off all the sharp corners with a fine saw. I prefer to mark things up with a pen for clarity – it’s a lot easier than trying to find a faint pencil line.

The shoulders of the plug were shaped with a razor plane. Careful marking with a pen prior to shaping helps to avoid going too far.

My trusty Perma-Grit block came to the rescue when giving the plug its final shape; a few coats of sanding sealer then saw it ready for action. As this plug is likely to be used a number of times I toughened up the ends with liteply facing plates; not necessary if you’re only going to pull one or two canopies.

A standard (metal) mastic gun has its front end cut off. The residual stumps are then bent at 90° and drilled to take a couple of fixing bolts. I cut a 10mm ply plate, just a tad smaller than the base of the plug.

Note the additional four holes around the periphery of the circular plate (at the end of the plunger) to allow the plug to be screwed into place. Always remember to swap the ply plate to suit the canopy width.  

And there it is, the tool assembled and ready for action. 

I have no idea as to whether a gun designed for shrinking covering film will be adequate, as I haven’t tried it. It’s certainly worth a go, though. I lightly clamped my gun’s handle in a vice that was attached to the edge of my workbench, which left both hands free to manipulate the rather unwieldy mastic gun and plug assembly. I suspect you’ll need to do the same, or else find a willing volunteer to provide an extra pair of hands.

Further internet-based research has highlighted a couple of other things. First, it appears that I somewhat overdid the filling and sanding of my plug; apparently acetate sheet is very forgiving in this application, to the point that any minor faults in the plug won’t show in the finished canopy. Second, if your first moulding isn’t up to standard, spray it with a little bit of polish (this acts as a good release agent), replace it on the plug, then pull a second moulding over the top of it.

Finally, emulating the canopy framework of a full-size is easily achieved by adding strips of thin card or ply to the plug before moulding the canopy. 

I hope you find the system as successful as I have and no longer find yourself avoiding plans and own designs that have non-standard canopy mouldings. Good luck!

With the plug screwed to the end of the plunger, I found it convenient to angle it back a little to reduce the amount of shrinkage required.

The acetate is wrapped over the top and securely pinned to the edges of the ply plate (I tried duct tape at first, but the adhesive didn’t withstand the heat of the shrinking process). Warm the acetate with the heat gun, then concentrate the gun onto the area you’re working on, whilst gently squeezing the trigger to extend the pushrod. This gradually pushes the plug further into the acetate.

Finished! Note that the plug hasn’t actually moved all that much in total, just an inch or so in this example. Warm the sides of the moulding to ‘lock’ the shape, then allow it to cool before releasing the job.

The finished moulding is clear and free of unwanted marks. Trim to size then re-check the fit against your model.

With a 1/4-scale pilot in place the canopy seems just about right for the Clean Sweeps racy appearance. Very satisfying.

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