Canopies

I've looked in on several posts which discussed canopies. I don't agree with much of what I've seen. Builders seem indifferent to the materials used to make the canopies they glue to their models.

My airplanes are typically a bitch to build and it just kills me when a canopy mounted on one of them distorts in use. the first photograph shows the classic Bearcat canopy distorted from sitting in the sun on a hot summer day. It was molded from .040 PETG.

The second photograph shows a test I ran on some different canopy materials.

The third photo shows three canopies which were exposed to heat. the left canopy was molded from butyrate and was severely damaged. the middle canopy was molded from Plexiglas (acrylic) and was undamaged by the heat and is still usable. The third canopy was molded from .040 PETG and was damaged but depending upon how much of it was needed, might still be usable.

The fourth photo shows before and after butyrate canopies.

The fifth photo shows before and after PETG canopies.

The last photo shows BBQB canopies and made from Lexan (polycarbonate). They are very heat resistant and will be unaffected by summer sun. The problems with Lexan is that it is hygroscopic (absorbs water which forms bubbles when heated unless pretreated) and it is a bitch to form because of its rather unbelievable strength. Still it makes a wonderful canopy if you can get some successful forms.

I still believe the ideal canopy material is plexiglas. It is sufficiently heat resistant to stand up to summer sun and is used on all of my airplanes prior to my quitting in 1979. It forms beautifully, much like PETG but is difficult to find sheets in thicknesses less than .060 (1/16"). Maybe I'll have better luck now that I've been tipped off to ask for .030 and .040 film. If so I'll be once again able to easily turn out beautiful, clear, heat resistant canopies.

Al,

How do you make your molds? As far as surface treatment, prepping it for molding.

Randy,

Photo one. This is the mold form. I make it by first, copying the canopy profile from the plans, then cutting the profile from ply and mounting it. Then I add bulkheads based mostly on half circles and straight lines. Then the front windshield is scaled and added to the form.

Photo two. The mold form is filled with scrap balsa. I buy my scrap balsa from Lone Star. They sell block cutoffs very cheaply. Don't use plaster. It works beautifully but always contains some moisture which will blow the mold apart when hit with the heat of a sheet of hot plastic.

Photo three. Carve to shape.

Photo four. Glass the mold with fiberglass and polyester resin and sand progressively to a very high finish. If you are going to drape form (pull hot plastic down over the mold) your mold is finished. The mold is held for drape forming by the mount extending from the bottom of the mold.

Photo four. I finished this mold to a very high finish and cut off the support because I intended to make a female mold for casting multiple male molds.

Photo five. This is the rubber mold I made to create a female mold.

Photo six. This photo is an optical illusion. It actually shows a canopy shaped depression. It is my female mold.

Photo seven. This shows two cast male molds and a canopy ready to install and paint. The mold with canopy markings was made so that I would only have to spend hours laying out proper canopy frame outlines once. To lay out more conopy frames, I simply place a canopy over the mold and add masking tape as indicated. With this reference, it is fast and easy to properly lay out canopy framing.

Photo eight. This shows five BBQB Bearcat canopies for the two airplanes completed and three under construction.

Al,

I would like to join with the others who have expressed their appreciation for your generosity in sharing with us.

As long as you are discussing canopies, can you explain how you "drape" the material around the form? I assume that you somehow capture the plastic in a frame of some sort, heat it, and then pull it over the male form.

Gary,

When drape forming, I use the clamps shown below. The screws are loosened and plastic inserted between the linked pieces of wood. The screws are then tightened to clamp and hold the plastic. The assembly is then hung in my wife's oven (preheated to 300-350 degrees) and watch as the plastic gets hot and begins to sag. If it doesn't sag enough, you won't be able to form it. If it gets too hot, bubbles will form in the plastic making it unusable. don't expect to get it right the first time. It isn't rocket science. I figured out how to do this myself, but with a considerable loss of material.

To mold canopies, things are hot and you have to wear gloves. I use two pair, one cotton and a second pair of workman's heavy, Grey gloves.

The best results in forming the hot plastic will be had if you first drape and make an initial forming pull down of the plastic. Then have a helper to physically grab the hot plastic, front and rear and finish the pull, tucking the plastic under the front and rear of the mold.

Lexan is too hard to drape form. Use PETG unless you can find acrylic FILM either .030 or .040. Trim with Dremel cutoff disk, not scissors.

I made the Millennium canopy with these clamps with the help of Bill Rutherford. For the Millennium, I was able to find acrylic. I had a buddy vacuum form the BBQB canopies from Lexan. I may use PETG for the Snaggletooth 2 if I can't find acrylic. I already have PETG canopies for Snaggletooth series airplanes.

Al,

Thanks for the information. I really like the progression.

There's a full-scale pilot in the neighborhood who brought his plane out to our local field for a small private "airshow". His plane is unique in the absolute definition of the word. Fairchild built 2 prototypes of a Navy trainer to compete with the T-34 and lost the contract. One prototype was destroyed and the other became a derelict. This gentleman took on the task of rebuilding this plane- I'm sorry, I don't know the designation, and had to come up with a bubble canopy- about eight feet long! It resembles closely a T-28 canopy. As a matter of fact the plane itself looks like a small T-28 with a tail-dragger gear. To end the story, he made a huge mold using the old canopy and using Al's methodology, draped a hot piece of plexiglass over it and came out with a very clear, one of a kind canopy! Where there's a will, there's a way. I imagine someone with a Jane's aircraft book can identify this plane. best regards,

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That one! How do I get one of those?

Has anyone tried a quick and dirty technique I've only read about? You take a clear plastic soft drink bottle and cut the bottom off. You make your canopy form with a deep base so that you have to force it into the bottle. Then you heat the bottle with a heat gun or in the oven until it shrinks down on your form.

Jim,

This is the reason I made this post. Not all simple solutions yield satisfactory results. Personally, I'm not going to put a pop bottle canopy on one of my airplanes and I wonder why anyone else would if they stopped to consider likely results from poor decisions resulting from lack of information about materials and techniques. I supplied the information.

>Jim,
>
>This is the reason I made this post. Not all simple
>solutions yield satisfactory results. Personally, I'm not
>going to put a pop bottle canopy on one of my airplanes and
>I wonder why anyone else would if they stopped to consider
>likely results from poor decisions resulting from lack of
>information about materials and techniques. I supplied the
>information.
>
>Al

Al,

I have made multiple canopies for my Classic Era ships using the two liter coke bottle method. Idiot-Proof. With a good male mold of the quality that you espouse, the result is as good as what you get. And the plastic is impervious to all solvents. Plus it withstands the sun, yes, direct sun light.

Cheers and Tally Ho,

Brad,

That BBQB canopy is 11 inches long and 3 inches wide. Will that fit your airplane? It is possible to cut the canopy down but it can't really be narrowed. Check the first photo showing the canopy plywood form. The bubble is 3 inches wide with straight sides below. You can't narrow the canopy much by pulling the sides in. the top of the canopy would resist being narrowed.

Wonderful post, Al!

And what impressive results.

Thank you for sharing this material with us. When it comes to innovation, you're the MAN!

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"Creativity is the sudden cessation of stupidity." -Edward H. Land

The vicat softening point of a thermoplastic material is a pretty good indicator of how it will perform under heat and load. It is also provides a good indicator of temperatures that will be required to fabricate. It's a temperature at which the material will start to deform (actually it's a penetration with a needle but that's more info needed) with some stress applied.

The materials you are working with have Vicat softening numbers as follows: (Varies to some degree by Mfgr)

PETG - 170 to 190 F.
ABS - 210 to 220 F.
Acrylic - 240 to 260 F.
Polycarbonate (Lexan) - 280 to 300 F.

It's obvious that PETG will deform in conditions that you would encounter in normal use like direct sun or the trunk of your car.(Your first post). Remember these are thermoplastic materials and soften as the temp. goes up.

The problems you are encountering with drape forming can be fixed with some minor tweaking.

The thermal conductivity of the form is very important. If at intial contact the hot plastic sheet starts to chill on the form then the problem of thick tops and thin sides will happen.

I would think you would have fewer problems with the glass covered balsa than the rubber mold. Thermal conductivity at work. But you can take advantage of this by using you monokote gun to warm the form as much as possible without damaging it to reduce the chill rate as you start the forming process. (probably would work better on the rubber form)

Seems like you are using the same oven temp for all materials. Not the right thing to do. Your sucess with PETG is pretty obvious when you look at the Vicat softening point and compare it to your oven temp.. You are well above it and remain above it during most of the forming work.

Try increasing your oven temps based on the Vicat temps. above. I would not be afraid to go up to around 375F to 400F for polycarbonate. Good starting point would be set the oven 50F to 70F higher than the vicat temp and don't be afraid to go higher. You will know when you've gone to far.

I used to make canopies on our Thermal vacuum formers where I worked. We could selectively heat areas to different temps. to reduce necking. Talk about easy! (Wish I had one)

Dick, It isn't just the chilling of the plastic sheet, the problem is partly if not mostly mechanical. Particularly when vacuum forming. When the hot plastic hits the top of the mold, the pulling force is perpendicular to plane of the hot plastic sheet, so there isn't much tendency to stretch when the plastic first hits the mold. By the time the plastic sheet has pulled farther down the mold the direction of pull is much more efficient at stretching and thinning the sheet. I've found that .040 canopies may very well have approximately .035 thickness on the top and less than .015 on the sides. Drape forming isn't quite so critical because there is a greater downward wrap before much pressure is applied to begin to stretch the plastic.

And yes, I do use a hair dryer to heat the mold before canopy pulls. I also use the hair dryer directed at the metal frame of our vacuum forming machine because the metal frame is a heat sink and absorbs heat and leaves a cooler band of plastic around the perimeter of the frame when heating. This is about all that I can do with home grown equipment, and, is enough. I have always been able to achieve a crystal clear, relatively blemish free canopy, even when drape forming my myself. Not without a lot of waste but its really a matter of how bad you want that custom canopy and what lengths you will go to to get it. Material selection improves the durability of the result and the difficulty of the molding. I consider it very important to be able to sit my airplane down in the sun in Dallas on a hot summer day with reasonable assurance that my canopy will stand up to the effects of the direct sun. Acrylic works fine and is easy to form. Lexan is even better if you can exert enough force and are able to get it hot enough without hydroscopic bubbling. PETG is OK but don't let it sit long in direct sunlight in Dallas when the local temperature is above 105 degrees. For what its worth I consider Lexan too difficult for home drape forming (unless you really need to).

My results are displayed in my initial post above and below. The Bearcat material is Lexan and dangerously thin on the sides but, so far, holding up to stress and vibration. The Millennium canopy is acrylic and drape formed in my kitchen using my wife's oven and the clamp/hangers shown above, I'm not worried about sun on these canopies. Home made canopies of acrylic and Lexan can be as clear and are much more heat tolerant than commercial canopies (usually PETG).

Roger Wildman used to work for Hi Johnson and also in the molding dept of Kenhi. He told us of a very nice wood for carving canopy molds called Jelutong. Carves easy, requires minimal finishing and has long lasting qualities with heat and pressure. They used this for molds of some of the first polyethylene glider fuses produced in the 60's.

Keith Bryant has the mold.. for anyone wanting a Hawker Hunter canopy.

John

I have used that wood, Jelutoung? It is an easy carving, close grained wood available at specialty wood stores and some lumber yards. I was using it to carve fuselage and stab molds for a Spitfire I never built in 1979

Which is great until you are trying to carve a very specific shape. To make a scale canopy (or semi-scale), I find it works best to make the plywood form which translates directly from the plans. It is possible to make outside templates, but much harder to carve and shape than using the plywood forms which show as you carve. Jelutong is used when you have no intention to put a finish on the mold and are relying on the wood's close grain not to show. For canopies, I prefer finished molds. Anyway, you pays your money and.......