Free-blowing a bulged bubble canopy?

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cluttonfred

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Does anyone have any prior experience with free-blowing (without a mold) bubble canopies, especially a bulged canopy that had to fit around an existing frame or windscreen? I am just curious how challenging it would be for an amateur to make something like these Spitfire and Mustang Malcolm hoods but large enough for a side-by-side two-seater.

spitfire malcolm hood.jpg mustang malcolm hood detail.jpg 1622737092549.png
 
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cluttonfred

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I am talking about fixing a flat sheet in a frame, heating the sheet, and then air forming (compressed air on the inside or vacuum on the outside) so it bulges into a bubble while still fitting the frame around the edges.
 

cvairwerks

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But the problem is that you asked about blowing a canopy to fit an existing canopy frame...That's a combination process of molding and blowing. Free form is blowing from a flat sheet with no mold at all. A Malcolm style hood is first formed using either vacuum into a concave mold, or drape formed on a convex form to get the basic shape and frame attachment areas correct. It's then clamped into a frame, heated again and blown into a concave mold. There are some canopies that are done similar, with the exception of the second mold. They are heated and free form blown by eye to the desired shape.

Here's a free form blowing video:

Here's a combination of vacuum and pressure molding:

Either way is a challenge to make it without optical distortion in the pilot's view. Ours at work are molded and come with a "Measles Sheet" noting every visible defect to be checked by our pilots.
 

cluttonfred

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Well, I think what I am describing is free-blowing much like that first example but with the sheet held in a curve to start. Here is a quick sketch of what I have in mind...a former box to hold the sheet and contain the air, the acrylic sheet itself (heated to the point of softening), and a frame to clamp down the edges of the acrylic sheet to the box. Not shown is the air connection to the box bottom for blowing the canopy. This is just a schematic, in reality the edges of the box and the frame would be much wider and I would probably have to radius the corners as you can see was done in both Malcolm hoods but is clearest in the Mustang example.

HBA concept sketches (24).jpg
 

challenger_II

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Matt, you are on the right track. You are going to need a rather large "oven" to heat your assembly. There was a write up in the EAA Experimenter several years ago, on the process, and on building the heating chamber. I will wager you will expend a hand-full of Lexan sheets before you get the desired results, but don't get frustrated.
 

Hot Wings

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I've pondered this as well. I kind of like the idea of being able to poke my head over the side to look down.

Your 3 examples look like they could be done by starting with a simple flat wrap, then heating and blowing? something like an EZ or KR canopy with a side bulge would be a lot more difficult.
 

cluttonfred

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As our longtime members already know or suspect, my interest in this Malcolm hood approach is actually driven by that doodled drawing on the third photo in my first post...enclosing an Evans VP-2 as lightly and simply as possible and, ideally, with a solution that could be easily removed whenever you wanted the wind in your hair. The Malcolm hood has the advantage of providing a little more elbow room while still keeping out the wind and, since there would still be a normal flat-wrapped windscreen, a little distortion would not be a deal-breaker.
 

plncraze

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Jim Marske used a method that would work for this. He used this to do the canopy on his Pioneer 4 and it was perfect. The secret is that nothing comes in to contact with the bubble while you are blowing it.
Build the usual oven and make sure you have equal heating throughout the box. Cut two pieces of plywood that are female versions of the forward and aft canopy. Put these in the box, one at each end, then push the plexiglass down into these. Put the lid on the box and heat. Then use a shop vac to provide suction to draw out your bubble. After the correct shape has been achieved turn of the heat and blow cool air into the box with the shop vac.
 

cluttonfred

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I am not so sure. If you look at the Mustang detail above, there are clearly flat sections of the transparency with the bubble emerging from it, like one blister from a blister pack of pills. It appears to me that the bubble section was either free blown or molded while the flat section was held in place by a frame that created those radiused corners.

I suspect that the Malcolm hoods were blown into a big oval like a glider canopy and then cut to size, so an actual Malcolm hood is probably 60% of the material used. Long Ez canopy comes to mind.
 

Geraldc

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The exact procedure you want is explained here.

Site is in Russian but google chrome translates on the fly.
The ones that say it can't be done have not done it.
 
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Matthew you are on the right track. A few notes, plexiglass forming is an art, you will scrap sheets fine tuning it.
It is extremely EXTREMELY time consuming ( read impossible) to make a mold that doesn't have optical distortion.
Any setup for molding or blowing will take time, but that time will pay back in success.
Free blowing creates virtually no distortion.
You can just blow a sheet, up against 2 bulkheads to match the frame, and cut out what you need, but it wastes a lot.
Your 2 stage idea of drape forming on a simple flat wrap mold (thin sheet metal covered with flannel), then installing on a frame and blowing will work. If you are blowing after drape molding, the drape mold doesn't need to be perfect, the 2nd process will remove any defects if using cast acrylic.
Use cast acrylic, not extruded. Cast has an elastic memory and if your shape is wrong, you can reheat and try again.
It is much easier to make a blow frame and pressurize it than to create a vacuum chamber.
PID controllers are cheap, ~$50 and work very accurately to control oven temp.
Ovens need to be big to allow for airflow to even out the heat.
Ovens need fans inside to circulate air, they can be on a shaft through the oven wall with the motor outside.
I disected a small house electric furnace, placed the heater element grid and squirrel cage fan inside the oven with a shaft out to the motor. That's how important circulation is, a squirrel cage fan in a corner for a 5x7x4 foot oven.
Metal house building studs (3 1/2") and fiberglass insulation work great for oven building.
THIN sheet metal works good for inside walls, thick takes longer to heat up. Outside walls are not critical.
Window glass from old kitchen ovens work good for molding ovens.
Provide lighting inside, I like porcelain sockets, fiberglass insulated oven wire, glass bulbs.
As a bonus, your oven will also do powder coating.
Have fun, don't get frustrated.
 

patrickrio

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You are essentially just making a big convection oven, with PID temperature control. It is SUPER simple to do the PID circuit. You can buy all the parts off ebay or amazon and Home Depot.

I have converted a countertop convection oven to PID control for doing zinc coated metal testing in high temperatures (insulated inside of oven with kiln insulation so it could reach 900F). I think that the lower temps required for acrylic molding can be maintained with the guts of one of these convection ovens on only 120vac if you build your "oven" box out of 2 inch thick double faced polyisocyanurate insulation or a double faced rigid fiberglass insulation. Don't use XPS rigid foam insulation, it will melt at about the same temp as the acrylic.

The fan will already be situated next to the heater which works better for creating even convection temps. you can add more fans to the fan circuit if you need more circulation to keep temps even.

I think that for most users, building a box out of taped together rigid insulation pieces will be cheaper and easier to store collapsed between uses. make the box at least 6 inches larger in all directions than the object you are going to heat so that air can circulate around it.

In any case, if you search amazon for "used" convection ovens and sort price low to high you can likely find a suitable convection oven to covert for $30-50. When I searched just now I found a 1700W large volume countertop convection oven with a dent that cost only $40. amazon used convection oven search link. They are so cheap that you could buy 2 ovens to get extra high temp fan capacity if needed.

You might be able to use the thermostat circuit from the convection oven even. You would need to test to see how accurate it is. I would feel safer with a separate PID temp controller though, and they are not very expensive. PID temp controllers with auto calibration ability will be the most accurate but may not be necessary in this case.

Additional parts needed are:
1. PID controller. can be found for as low as $5
2. k thermocouple or pt100 or pt1000 temp probe (pick what will work with selected PID) for as low as $6
3. 20 amp or better solid state relay around $10
4. might need heat sink for relay depending....
5. Electronics "project box" big enough to hold relay and controller so you won't shock yourself using it....
6. Misc electrical wire and a plug pigtail (or re use the plug and wire from the oven...)

There are also full PID kits on amazon already, but I have never used one as I already have all the PID parts in my misc electronics box....

Here is a complete PID controller kit with relay and thermocouple on amazon you just need to add a project box and wiring.

Here is a fully assembled temperature controller that may not be PID but I suspect it will be accurate enough for an acrylic oven.

when you wire the components of the convection oven or ovens, be sure to wire the fan or fans without the PID circuit. You can re use a switch on the convection oven for a fan on off switch. Do not wire them through the relay because they need to stay going all the time.
.
Wire the heating element through the SS relay. Only use the heating element from ONE convection oven.
 
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geosnooker2000

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You are essentially just making a big convection oven, with PID temperature control. It is SUPER simple to do the PID circuit. You can buy all the parts off ebay or amazon and Home Depot.

I have converted a countertop convection oven to PID control for doing zinc coated metal testing in high temperatures (insulated inside of oven with kiln insulation so it could reach 900F). I think that the lower temps required for acrylic molding can be maintained with the guts of one of these convection ovens on only 120vac if you build your "oven" box out of 2 inch thick double faced polyisocyanurate insulation or a double faced rigid fiberglass insulation. Don't use XPS rigid foam insulation, it will melt at about the same temp as the acrylic.
....
Awesome information here. Couple questions, you speak as if you have done this before, did you make the whole oven box out of 2" foam, or did you apply the foam to a substrate like plywood? If it is just out of foam, what kind of tape did you use to create the oven box? These questions have to do with what can withstand 300 degrees F. I could see where you could glue the insulation to plywood, plywood is the outside structure, the insulation protects the plywood. But if you just used the rigid insulation, what type of tape would be fireproof?
 

Vigilant1

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Awesome information here. Couple questions, you speak as if you have done this before, did you make the whole oven box out of 2" foam, or did you apply the foam to a substrate like plywood? If it is just out of foam, what kind of tape did you use to create the oven box? These questions have to do with what can withstand 300 degrees F. I could see where you could glue the insulation to plywood, plywood is the outside structure, the insulation protects the plywood. But if you just used the rigid insulation, what type of tape would be fireproof?
None of the polystyrene foams (XPS or EPS) can be used at temperatures that high. The most appropriate commonly available (relatively cheap) foam that you could use is polyisocyanurate foam rigid insulation which can have service temps up to 350F (check the individual foam spec sheets).

Examples of "polyiso" foams include Dow Tuff-R, John's Manville AP Foil Faced, etc You can get it at Menards, HD, Lowe's, etc.

All these foams will burn at some point. You'll want to protect any of them from any possible ignition sources.
 
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geosnooker2000

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None of the polystyrene foams (XPS or EPS) can be used at temperatures that high. The most appropriate commonly available (relatively cheap) foam that you could use is polyisocyanurate foam rigid insulation which can have service temps up to 350F (check the individual foam spec sheets).

Examples of "polyiso" foams include Dow Tuff-R, John's Manville AP Foil Faced, etc You can get it at Menards, HD, Lowe's, etc.

All these foams will burn at some point. You'll want to protect any of them from any possible ignition sources.
Uhhh..... Yeah. That's what we were discussing using.
This is the one I found after reading his post: https://www.menards.com/main/building-materials/insulation/foam-board-insulation/johns-manville-foil-faced-polyiso-foam-board-insulation-4-x-8/w-n5150/p-1444438916099.htm
 

Vigilant1

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That's readily found and inexpensive. If you can find Thermax polyiso, you may find it to be slightly preferable. It has somewhat better fire properties than some other types (contains fiberglass fibers that help keep it in position despite some melting, and a tougher facing material). Some building codes allow Thermax to be exposed in some situations where other foams need to be covered.
 
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geosnooker2000

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Next question is where to source the UV filtering cast acrylic to form windshields and side windows. My hour-long google search has yielded a bunch of sneeze-guards and some companies that don't understand how to organize a website for choosing purchase options (one of them only allowed the purchase of a sample). The one site I found that I knew what I was buying was what I wanted was $180 for a 2x4 sheet of 1/4". Heck, I can come real close to buying actual Piper Cherokee windscreens for that kind of money...
 
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