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Canopy Plug


Andy J
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 Need to make a new canopy for my Seagull Sparrowhawk as when I acquired the airframe the canopy had a large crack running around the base which could not be hidden.

 

Not at all sure if I am tackling the manufacture of a canopy plug on which to form a new one is correct as I intend to use the old canopy as a mould.  Have made a simple cockpit conning to the same profile as the model and my thoughts are to simply attach the old canopy and whilst holding it vertical such that the open rear end of the canopy is at the top fill the void between the conning and the canopy with plaster of paris. Once the assembly is set solid will remove the old canopy and make any corrections to form the plug.  To avoid the plaster of paris sticking to the inside of the canopy will give it a liberal coating of Vaseline or possibly WD40.

 

Assume this process will work or will I be proven wrong?

 

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I've used plaster of Paris very successfully but it does take some days to dry out. If you do use plaster, you don't need to use anything to stop it sticking to the original canopy. Anything you do use may cause a rough surface on the plug. Every little detail will be transferred to the new canopy when you vac-form it. I once got a hair on the plug and it was perfectly replicated. 

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3 hours ago, Andy Joyce said:

 Need to make a new canopy for my Seagull Sparrowhawk as when I acquired the airframe the canopy had a large crack running around the base which could not be hidden.

 

Not at all sure if I am tackling the manufacture of a canopy plug on which to form a new one is correct as I intend to use the old canopy as a mould.  Have made a simple cockpit conning to the same profile as the model and my thoughts are to simply attach the old canopy and whilst holding it vertical such that the open rear end of the canopy is at the top fill the void between the conning and the canopy with plaster of paris. Once the assembly is set solid will remove the old canopy and make any corrections to form the plug.  To avoid the plaster of paris sticking to the inside of the canopy will give it a liberal coating of Vaseline or possibly WD40.

 

Assume this process will work or will I be proven wrong?

 

DSC01426 (Small).JPG

DSC01431 (Small).JPG

Yes, that should work a treat, if you have an existing canopy and can use it to make a male Plaster of Paris plug. Worked for my Cub and Beaufighter - pop bottle for the Cub and home made vacformer for the Beaufighter.

beaucanopyvacform.jpg.d853f7017d6e08f6c01ad4daa6d11e31.jpg

 

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Plaster of Paris arrived today so thought I would do a quick pour just to seal the edges.  Little came out so continued to pour more and more into the mould.  Thought the plaster would take hours to dry but found it set almost immediately which took me by surprise.  Tapped down the plaster as I poured it in so I hope the plaster got into all the corners.  Will leave it till boxing day just to be sure its fully set.

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Removed the old canopy today and found some minor damage to the plaster plug which I have attempted to fill using additional plaster. Very messy business working with Plaster of Paris which I assume will be the case when I start to sand the surface using wet and dry sandpaper.  Plug still feels very damp to the touch so will leave it a couple more days before I do any final sanding.

 

Interested to know if its worth adding say a skim of fiberglass wing finishing gel to harden the surface of the plaster as sure I will need to pull several canopies.

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Left the mould in the airing cupboard yesterday and now that the surface is dry you can easily see the defects, so will definitely have to do a bit of sanding.

 

Next job will be to make a simple vac form box. Trip required to B&Q to see if they sell peg board or similar as it will be a hassle to have to drill a ply plate.

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I used the old canopy as a mould outline but filled most of the void with balsa Martin. Then I back filled with 1kg of Plaster of Paris. Warning... its gets very messy working with Plaster of Paris but its very easy to sand and conversely its very easy to mark the plug.  If you don't have an old canopy to use as a mould then dont think PoP will be any good as its far too runny to mould into a free shape. So suggest you make a balsa plug to the outline shape and then use a car body filler to get a good surface into which you can carve or sand to the correct size. 

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29 minutes ago, martin collins 1 said:

What method do people use to scratch build a mould for a canopy, i have built a KI-61 tony mini fun fighter and just have the plan and the completed plane to work with, do you make a balsa plug then use filler over the top to make a smooth finish?

 

If you start at the other end Martin, with your ability to vacform. In schools, we could vacform a 4" deep canopy on an 1 8" square bed without creases, using a solid wood former that was finished with beeswax so as not to stick. 1 mm sheet with care of 2mm sheet more easily.

So how deep is the canopy?

3" - 4" deep, is best achieved with a wooden block, with canopy frames glued on made from stripwood.

 The block can be made up from thinner sheets, even plywood, and finished absolutely smooth, by any means, as every crevice finishes up on the final canopy.

 

A small Tony canopy, is more easily made on a stick of wood, up to 2"  deep, pushed into a trimmed pop bottle, heated carefully with a heat gun and pulled tight to the former.

Edited by Denis Watkins
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Well having completed the plug thought it would be fairly cheap to make a simple vacuum former however the price of materials, particularly the metal to form a frame to hold the PETG makes the whole thing impractical given the number of times I will ever use it.

Wondering therefore if the frame could be made out of wood and the PETG sheet attached say using staples. The heat would be provided by a heat gun before the whole arrangement is simply pushed over the mould which I pictured in previous posts on this thread. 

Another option is to make the plug itself into a vacuum plenum with holes drilled around the base onto which the plater shape sits.

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1 hour ago, Andy Joyce said:

Well having completed the plug thought it would be fairly cheap to make a simple vacuum former however the price of materials, particularly the metal to form a frame to hold the PETG makes the whole thing impractical given the number of times I will ever use it.

Wondering therefore if the frame could be made out of wood and the PETG sheet attached say using staples. The heat would be provided by a heat gun before the whole arrangement is simply pushed over the mould which I pictured in previous posts on this thread. 

Another option is to make the plug itself into a vacuum plenum with holes drilled around the base onto which the plater shape sits.

Andy,

If you contact [email protected] <[email protected]>, they can vac-form from a plug you send them.

Edited by Andy Stephenson
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2 hours ago, Andy Joyce said:

Well having completed the plug thought it would be fairly cheap to make a simple vacuum former however the price of materials, particularly the metal to form a frame to hold the PETG makes the whole thing impractical given the number of times I will ever use it.

Wondering therefore if the frame could be made out of wood and the PETG sheet attached say using staples. The heat would be provided by a heat gun before the whole arrangement is simply pushed over the mould which I pictured in previous posts on this thread. 

Another option is to make the plug itself into a vacuum plenum with holes drilled around the base onto which the plater shape sits.

Yes. it's entirely possible to make the frame out of wood rather than metal. Mine is made from a wooden photo frame attached to a simple wooden box, with a perforated plywood base plate, created by drilling a uniform pattern of holes in the ply. The plastic sheet is retained by staples and some strategically place aluminium tape with heat resistant adhesive, often used in theatrical lighting, as a belt and braces solution.

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