[PSUBS-MAILIST] Non developable surfaces

[Marc de Piolenc]

Amateur airplane builders often "roll their own," if they're making a 
"one of" design or if they can't afford to buy a canopy from a kit 
supplier or a parts-and-materials house like Aircraft Spruce.

And you'd be surprised at the primitive nature of a lot of "industrial" 
production, as aircraft production runs are often too small to justify a 
large investment in permanent tooling. And of course the big 
manufacturers' prototype shops routinely use the same methods as amateur 
builders, as their production runs are typically one to five units.

If you want to amuse yourself, there are videos on YouTube about the 
beginnings of Bell's helicopter division under Arthur Young... but he 
only refers briefly to the Model 47 "bubble."

The simplest molding method for thermoplastics is so-called drape 
molding, where a "minus-t" male plug is made and placed under the 
plastic blank, and the plastic sheet gradually heated until it sags onto 
the plug, at which point the temperature is GRADUALLY reduced. Obviously 
this only works with shapes that allow the plug to be withdrawn after 
molding (though you can sometimes cheat by making the plug so it can be 
disassembled and withdrawn in parts). If you're willing to have a 
slightly less-than-perfect contour, the male plug can be an eggcrate 
structure instead of a solid plug. There will be ripples in the finished 
product of course, but in your application, with water on both sides of 
the MBT shell in the submerged condition and a close match of refractive 
indices between the acrylic and water, that might not be a problem. 
Here, besides the plug, the only equipment required is an insulated 
plywood box and an air heater.

A more complicated process is vacuum molding, which is popular for small 
parts but gets awkward in the larger sizes because the size of the 
required vacuum reservoir grows with the part size.

After that comes blowing, and that's something that some amateur 
builders have managed that I for one would rather not try. But I suspect 
that your application can made do with drape molding.

Best,
Marc

On 2/16/2014 9:41 AM, Joe Perkel wrote:
> Marc,
>
> Those canopies of course, are made by industry with industrial capital and support. I was thinking more of a homegrown method.
>
> Is it then possible to heat a flat sheet in a conventional oven to x degrees for x time, then form over a plug or mould, that kind of thing.
>
> Hank,
>
> Standard k MBT thickness no bigger than say Gamma's window.
>
> For the curve complexity, no more than to keep lines fair and or tangent transitions.
>
> Joe
>
>
> On Feb 15, 2014, at 7:49 PM, Marc de Piolenc <piolenc at archivale.com> wrote:
>
>> It can't be too difficult. Compound-curved Plexiglas (British: Perspex) cockpit canopies have been made for decades, and the full-surround "bubble" cockpit of the Bell 47 helicopter dates to the late 1940s. The problems we run into with acrylic pressure hulls are caused by thickness and the consequent need to use casting technique instead of blowing, but that's not going to be a problem for you in making a transparent ballast tank shell.
>>
>> Do you want me to dig into the aeronautical literature?
>>
>> Marc de Piolenc
>> Aero-geek
>>
>> On 2/16/2014 8:18 AM, Joe Perkel wrote:
>>>
>>> How much trouble is it to form acrylic in two directions? As in following a complex hull form to make an integral window.
>>>
>>> In other words a transparent section of MBT with a bidirectional curve.
>>>
>>> Joe
>>>
>>>
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