Making vacuum formed canopies and cowls that are flush to the fuselage - RC Groups
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Oct 31, 2017, 08:51 PM
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Discussion

Making vacuum formed canopies and cowls that are flush to the fuselage


Hello guys...

One question has been killing me for so long...

I'm about to start building a scratchbuilt Extra 330 and got a question regarding making the plug for the canopy.

The traditional way of making a plug that can be used for vacuum forming a clear plastic canopy is well known: Fill the space with foam (using guide formers or not), shape it, cover it with wathever your prefer (fiberglass, wbpu, tissue, etc...), smooth it and use it in your vacuum forming machine.

That's ok for most of the models, but this approach will form a canopy that is slightly larger than the plug (depending on the thickness of the plastic used) and that won't allow the canopy to flush with the turtle deck for instance, instead, it will overlap it.

The same problem applies to cowls too, if you make a plug and vacuum form a cowl, it will overlap the fuselage. By the other hand, a traditional two parts mold taken out of that plug if used for a fiberglass cowl will make a cowl exactly with the same dimensions of the plug itself).

So, i got to ask: Once made the plug, is there any approach to form a canopy that has the same dimensions of the plug?
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Oct 31, 2017, 09:15 PM
Registered User
Canopy: trim thickness of plastic off bottom sides, squeeze sides til flush with fuselage edges and turtle deck and GLUE!

During WWII, one common technique for forming "identical"metal cowl panels was to stretch form up to seven layers of .040 aluminum sheet over the die, all at one time. Naturally those farthest from the die were larger than those closer in. However the idea was to get them close enough for final jigging and machine trim. Dzus fasteners would hold everything tight against formers or existing skin. etc,.
Oct 31, 2017, 10:11 PM
B for Bruce
BMatthews's Avatar
The width is the really big one. So make the plug from foam as you mentioned. Then take a slice out of the middle of it equal to the thickness of the two layers. That'll move all the widths inwards by just the right amount. As for the extra height it's not a big deal since you would pull the canopy to be a little longer to allow for trimming anyway. So just trim a little more off the bottom edges.
Nov 01, 2017, 02:33 PM
Registered User
Hello guys...

appreciate your replies.

Those approaches makes sense and may produce a good result, but it seems a little inprecise and a little of try and error...

Let's look at the opposite situation where we have a plug and we want to make it larger in such a controlled way.

What you usually do is vacuum form a piece of the plastic over the plug and let it there and vacuum form another one over the previous one.

Knowing the thickness of the plastic and how much of its thickness it looses after stretching it seems a much more precise way of controlling the variation of the dimensions for your requirements than triming the sides of the formed piece or removing a slice from the middle of the plug doens't it ?

Just remembering that both sugested ways (trimming the sides of the piece of removing a slice of the plug) will cause some degree of deformation and non conformity to it's original design, depending on the scale of the model, such a distortion may be more or less noticeable.

The only way that came to my mind to reduce the final piece's dimension and at the same time keeping its original form was to laminate some fiblerglass insided a vacuum formed piece. The number of layers and thickness of the fiberglass cloth will determiny how much final piece will shrink in relation to the vacuum formed one.

After laying those fiberglass cloth inside the plastic formed canopy you would fill it with plaster, after it dries you would have a plug with smaller dimensions than original plug.

Problem with this aproach is that after you pull the plaster, probally it would have either fiberglass weaves marks of brush marks that would requise some work on the new plug which in turn could cause variations of the original form once it involves sanding and filling...
Nov 01, 2017, 02:45 PM
B for Bruce
BMatthews's Avatar
I think you're failing to appreciate how flexible the plastic is. Particularly when the shapes have larger radii curves to them. On some scale models where you need hard sharp features the spacing becomes far more important. But for the well rounded shapes like on an Extra 300? Just make the plug up as previously mentioned and blended to the outer lines. Remove the plug and extend the edges smoothly by 1/4 inch or 5'ish mm so you have a little extra to worth with. Pull a "liner" over the foam so you are working with a smoother surface and pull the canopy you want over that.

TECHNICALLY it's a touch oversize. But with a thinner gauge liner it'll be minimal. And with the shape of an Extra's canopy unless you're using this as an entry into the World Championship Scale event the shape will hunker down just fine and fit tightly to the framework you make up.

For the framework around the edge to glue the canopy to just make the pieces smaller by the thickness of the plastic. Glue the plastic to the framework and trim the edges. You'll love the results and the canopy will simply drop down lower and squeeze in at the sides by the thickness of the material without an issue in the world. And the outer surface will blend wonderfully with the fuselage you've built.
Nov 01, 2017, 04:54 PM
Scott
Pylonracr's Avatar
Why not just make the plug smaller? Am I missing something here? If you have to make the plug anyway, make it smaller when you sand it to shape.

Scott
Nov 01, 2017, 05:09 PM
Registered User
Quote:
Originally Posted by Pylonracr
Why not just make the plug smaller? Am I missing something here? If you have to make the plug anyway, make it smaller when you sand it to shape.

Scott
Usually when building the plug, you sand it in place, using the walls of the fuselage as guides for sanding and flush the plug with the fuselage.

I consider very hard to make a precise plug which is slightly smaller that it should be.

You can call me very conservative, and actually i'm... i really got to learn to live with small problems in the model. That would let me live better and not getting obsessed with perfection...
Nov 01, 2017, 05:11 PM
Registered User
Quote:
Originally Posted by BMatthews
I think you're failing to appreciate how flexible the plastic is. Particularly when the shapes have larger radii curves to them. On some scale models where you need hard sharp features the spacing becomes far more important. But for the well rounded shapes like on an Extra 300? Just make the plug up as previously mentioned and blended to the outer lines. Remove the plug and extend the edges smoothly by 1/4 inch or 5'ish mm so you have a little extra to worth with. Pull a "liner" over the foam so you are working with a smoother surface and pull the canopy you want over that.

TECHNICALLY it's a touch oversize. But with a thinner gauge liner it'll be minimal. And with the shape of an Extra's canopy unless you're using this as an entry into the World Championship Scale event the shape will hunker down just fine and fit tightly to the framework you make up.

For the framework around the edge to glue the canopy to just make the pieces smaller by the thickness of the plastic. Glue the plastic to the framework and trim the edges. You'll love the results and the canopy will simply drop down lower and squeeze in at the sides by the thickness of the material without an issue in the world. And the outer surface will blend wonderfully with the fuselage you've built.
You're right Matthews...

By making the frame smaller (by the difference of the plastic thickness) i agree that it will flush with all the other surfaces and probally not cause any noticeable deformation, as you said, specially when using thin plastics (not sure if the same would happen with a 2mm or so plastic sheet).
Nov 01, 2017, 07:56 PM
B for Bruce
BMatthews's Avatar
Up to something around 1mm thick I think you'd be fine. And I doubt that most canopy materials are even 1mm thick. More like 0.5 is likely more standard and up around 0.7mm for what would be considered as "heavy gauge" stock.

Molding 2mm would not be something I'd want to do with regular vacu-forming techniques anyway.
Nov 01, 2017, 09:22 PM
Registered User
One thing I would add to Matthews' suggestion: After you narrow the mold and have it almost finished, add some depth to the bottom with some 1/4" balsa, maybe to 1/2" depending on the size of the canopy and plane. If the canopy ends up a little tight the extra depth will make up the difference.

I've participated in the fitting of a Pitts S2-B and an Extra canopy on full sized planes and they're a little long in depth. Lots of fitting and trimming was required.
Nov 01, 2017, 10:11 PM
B for Bruce
BMatthews's Avatar
Yeah, adding some extensions on and feathering them to extend the edges on the mold is a hellishly good idea. Pretty much needed in any case if you're going to vacuform it. Might get away with it if you hand pull it. But it's still better to have it than not.
Nov 02, 2017, 07:29 PM
Scale Builder
It takes a bit of work to set up but you can actually vacuum form by pulling the plastic into a cavity rather than over a plug. The result is a part that is precisely the same size as your pattern. Plaster can be used to make the mold from your original plug. I've only done this one time, on a large fairing that I formed for a friend's 1/4 scale Lockheed Vega, but it worked like a charm.
Nov 06, 2017, 12:27 PM
The "pro" in procrastination
Steve85's Avatar
Quote:
Originally Posted by Chad Veich
I've only done this one time, on a large fairing that I formed for a friend's 1/4 scale Lockheed Vega, but it worked like a charm.
Wow, must've been a REALLY good friend...

Steve
Nov 06, 2017, 02:47 PM
Scale Builder
Quote:
Originally Posted by Steve85
Wow, must've been a REALLY good friend...

Steve
Nigel Tarvin, owner of Tarvin's Model Products and one of the finest model builders I know. Here he is with the Vega.
Nov 06, 2017, 04:44 PM
Registered User
Quote:
Originally Posted by BMatthews
Up to something around 1mm thick I think you'd be fine. And I doubt that most canopy materials are even 1mm thick. More like 0.5 is likely more standard and up around 0.7mm for what would be considered as "heavy gauge" stock.

Molding 2mm would not be something I'd want to do with regular vacu-forming techniques anyway.
Yeah, the thickest gauges of clear plastic sheets Sig sells are .030" (around .76mm) and .040" (around 1.01mm) and an onhand Ziroli 92" span Zero canopy - a long, relatively high domed pull - used something around .7mm plastic.

Michael in Ontario, Canada


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