Molding a canopy.

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bmcj

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I have 2 experiences with polycarbonate to relate.

The first is my neighbor, who is meticulous with his truck's appearance. He has a product to make the crazed headlight lenses clear again. Standard Ford lenses (whatever they are made of). They still have a slight yellow tint. I don't think they are Lexan or Polycarbonate (blue tint), although they seem tougher than plexi.

Don't suppose you have a product name or supplier name for that, do you?

Thanks,

Bruce :)
 

topspeed100

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In addition to the Bob Walters article that I mentioned in a previous post there are also some pictures and descriptions in Ladislao Pazmany's "Light Airplane Construction." He had a group of builders with thick gloves ready to help push the plexiglass over the mold soon as it came out of the oven, which was a borrowed from a molding company. the molders brought the mold to a plastics company and used their oven.
This might solve the problem for one time builder.

My question is...if 2 mm plexi ( polycarbon actually ) would be enuf thick for a very lite ac with very curved windshield/canopy ?

I just bought a 1 m x 1.5 m piece of it. If not then I just use it for a model.

For now it is under the plants upstairs.
 

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Pops

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I had a large scratch about 10" long in the top of my canopy of my I-26 and worked it out with toothpaste.

Don't suppose you have a product name or supplier name for that, do you?

Thanks,

Bruce :)
 

goldrush

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May I suggest as an alternative to Lexan or Plexiglass, you look at PETG. (Polyethylene Terephtalate Glycol-)
Available in thicknesses down to a "few thou" and used for such things as "fizzy" drink bottles etc.
Much easier heat formed than Lexan etc and I believe Columbian is also now using it for his canopies.
Also cheaper than Lexan etc.
 

cavelamb

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Lexan is a really poor choice for a heat formed anything.

The reason is that it is hydroscopic (absorbs water).
When heated the water vaporizes and makes lots of pretty little bubbles.
Very attractive - but not exactly what one would want for a canopy.

The water MIGHT can be baked out, but it takes many hours and let the
temperature hit 212 even for a moment, and the part is ruined.
 

ultralajt

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I use Veralite plastic for my lates model canopy. It is very "moldable" and the required temperature for molding is not too high. It is from 12 to 200°C.. depending of the veralite type (Veralite 100 or 200).
One side of Veralite is protected with peel of clear plastic foil, an another with white painted foil printed with material name and type.
I am sure it could be use for full scale homebuilds. It is very easy to forming, and there are many ways to form according to maual.
There are manx thicknes.. Veralite 100= 0,60 - 0,75 - 1,00 - 1,50 - 2,00mm
Veralite 200= up to 10mm
Temperature stability up to 40°C (probably problematic if cockpit is standing on the sun unprotected and closed... Some reflective cover should be used over the canopy to avoid solar panel effect.)

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AVI

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Would you use 2 mm thick ?
While glancing through this post, I had notes concerning the canopy of the SIAI Marchetti SF260 right in front of me, purely by coincidence.I believe most of you know of this airplane and are familiar with the shape of its canopy.The windshield and canopy are Plexiglas, the windshield being 4.75mm thick.The canopy itself is constructed in three parts. There's a top-center section formed of 4.0mm material, and two individual side panels, respectively of 3.8mm material. Each side is joined to the top center panel with longitudinal 40mm wide strips, one glued to the inside, overlapping the joint, and another glued to the outside, again overlapping the joint.The 40mm wide strips are cut from the 3.8mm material.
 

topspeed100

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You should be able to calculate those loads pretty easy. Designer should know all loads that affect plane at that particular design speed area. Here you find modulus of elasticity and modulus of rupture for plexiglass: Plexiglass Properties - Typical Physical Properties of cast Plexiglas G acrylic sheet
I don't think designer really needs to know it..if you have bunch of engineers that does these calculations for living.

I have a 2 mm thick polycarbonate sheet...and I think it does the job...material seems to have very bad reputation here.
 

4trade

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I don't think designer really needs to know it..if you have bunch of engineers that does these calculations for living.
....so you basically mean that designer is person who draw pictures of aircraft, and engineer is other person who calculate all aircraft loads and tell if it can be done, what empty weight it will be....and thousand other important thing to know?

I ask one really important question: How anybody can design aircraft, if really cannot understand structures and strength of aircraft building materials? Guessing don´t work in this business, because designer need to know really close aircraft weight, strength, building materials properties and on and on....
 

autoreply

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May I suggest as an alternative to Lexan or Plexiglass, you look at PETG. (Polyethylene Terephtalate Glycol-)
Available in thicknesses down to a "few thou" and used for such things as "fizzy" drink bottles etc.
Much easier heat formed than Lexan etc and I believe Columbian is also now using it for his canopies.
Also cheaper than Lexan etc.
PET has come up before and if I recall correctly there were some excellent reasons not to use it on aircraft. I can't remember what those reasons were though :ponder:
 

AVI

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....so you basically mean that designer is person who draw pictures of aircraft, and engineer is other person who calculate all aircraft loads and tell if it can be done, what empty weight it will be....and thousand other important thing to know?I ask one really important question: How anybody can design aircraft, if really cannot understand structures and strength of aircraft building materials? Guessing don´t work in this business, because designer need to know really close aircraft weight, strength, building materials properties and on and on....
On the other hand, for simple structures, isn't it simply wasted effort to go overboard in analyzing each and every piece of material and each and every nut and bolt? After all, he's not forming a windshield for a space shuttle ... it's a 90 kt airplane.Richard Hiscocks, in his book, "Design of Light Aircraft" has this to say in the preface: "Examples are provided of the approximations and educated "guesses" required in the early stages of a project. It is shown that a detailed analysis to establish the precise strength of a component is a wasted effort when, for example, the minimum metal gauges that is is practicable to use will provide generous margins, or when simple tests are more effective than analysis, and when considerations of stiffness, as a safeguard against flutter or fatigue life override the strength requirements. Refinements are applied to initial rough estimates and assumptions as the work progresses."
 

Hot Wings

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PET has come up before and if I recall correctly there were some excellent reasons not to use it on aircraft. I can't remember what those reasons were though :ponder:

My memory is just the opposite. :ponder:

There is a difference between PET and PETG. PETG is far tougher than Plexiglass/acrylic, but it does seem to be easier to scratch. Based on my reading I'd probably describe PETG as a poor mans Lexan/polycarbonate that can be formed as easily as Plexiglass. Here is an article that gives a good overall comparison of PETG with other similar plastics.

I've decided to use PETG for the flat wrap canopy on my project, primarily due to it's much better toughness. Where I tend to fly encountering a barbed wire fence during an unplanned landing is a likely event. Given it's relatively low cost I'm willing to replace it if the UV additive isn't good enough to keep it from yellowing.
 

topspeed100

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On the other hand, for simple structures, isn't it simply wasted effort to go overboard in analyzing each and every piece of material and each and every nut and bolt? After all, he's not forming a windshield for a space shuttle ... it's a 90 kt airplane.Richard Hiscocks, in his book, "Design of Light Aircraft" has this to say in the preface: "Examples are provided of the approximations and educated "guesses" required in the early stages of a project. It is shown that a detailed analysis to establish the precise strength of a component is a wasted effort when, for example, the minimum metal gauges that is is practicable to use will provide generous margins, or when simple tests are more effective than analysis, and when considerations of stiffness, as a safeguard against flutter or fatigue life override the strength requirements. Refinements are applied to initial rough estimates and assumptions as the work progresses."

I recall Minisytky ( Pik-23 ) was made based on eyeball engineering ( hattuvakio ). I actually wanna make it a bit further..making certain that aerial flying qualities ( tail volume etc ) are state of the art and efficiency peaking...I leave happily the stress calculations for professionals.
 

autoreply

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My memory is just the opposite. :ponder:
My bad, I thought that the differences between PET and PETG were marginal.
I've decided to use PETG for the flat wrap canopy on my project, primarily due to it's much better toughness. Where I tend to fly encountering a barbed wire fence during an unplanned landing is a likely event. Given it's relatively low cost I'm willing to replace it if the UV additive isn't good enough to keep it from yellowing.
I'm not so sure that'll be safe enough. "Prikkeldraadstangen" were long mandatory here in the Netherlands (lots of sailplanes landing in barbed wire). Even those sturdy metal beams could just marginally withstand the impact in a fence. I'm afraid the PETG, even if it's very thick (quarter-inch thick) will just dent in and eventually break.
 

Hot Wings

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I'm not so sure that'll be safe enough. "Prikkeldraadstangen" were long mandatory here in the Netherlands (lots of sailplanes landing in barbed wire). Even those sturdy metal beams could just marginally withstand the impact in a fence. I'm afraid the PETG, even if it's very thick (quarter-inch thick) will just dent in and eventually break.
Probably hopeful thinking on my part.:depressed The PETG looks like it won't be cut by the wire but it is "soft". It could very easily bend enough to not provide the protection I'm looking for. I am prepared to add the prickly wire rods.:para:
 

briantompo

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...Somewhere, I have a copy of a magazine article on how the Zuni sailplane canopies are free-blown, with excellent pictures and explanation.
I know this is an old, dead thread but it still comes up when searching for info about canopy blowing. The article Bart mentioned nearly 10 years ago in post #13 was in the July 1982 edition of Homebuilt Aircraft, pgs 52-56. It's called, "Blowing Bubbles. The Applebay canopy-blowing method for flawless cockpit cover" by Tom Baker. As of June 2016 it"s available to read in the second half of this pdf at:
http://zoomaviation.com/design/fuslage/canopy/how2- canopy.pdf

There are much newer posts on this forum with links to more recent pictorials but after 40 years, it's still the same successful technique detailed in the article.
I hope this helps the next person considering making their own canopy.

Brian
 
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