Any amateur aircraft builder here ?

[Jemiba]

Acrylic glass, in aviation often called “Perspex”, although that’s just a brand name, is
the an often used material for aircraft canopies and windows since the early ‘30s.
Flat windows can be made of sheets, forming of more complex structures is done using
heat and pressure AFAIK. But the material has an inherent flexibility, can this be used
to form curved windows, just by sqeezing and fixing it into a frame, without affecting
the optical qualities ?
During my own tests with a piece of acrylic glass there appeared reflections at quite a low
bend angle, but of course I had no material certified for aviation ! But maybe more important,
than just the technical possibility, is the question if such a method would be admissible,
because it could decrease the materials resistance against cracking, I think. Or could the
overall strength of the windshield/frame combination even increase by this kind of prestressing ?

But my main question is, if bended windows made from acrylic glas were (and are ?) used in aviation.

 

[Just call me Ray]

Ummm...not sure what the question is, but I know transparent lexan is often used as a substitute though I hear it's on the heavy side.

 

[Retrofit]

I think the windshield of the Eurocopter AS-350/AS-355 are made of a flat "Altuglass" panel bended over the canopy polycarbonate main frame and hold by adhesive seals and screws.
The first helicopters were equipped with a single transparent panel covering both the RH-side and the LH-side. On the more recent ones, two separate panels are used.

 

[Jemiba]

Thanks for your answers!

Good clue with the AS-350, so at least this method isn't taboed principally. But both Altuglass,
as well as Lexan are more recent variants of acrylic glass, aren't they ? It would be difficult
with the more basic variants, I think.
Reason for my question is, that to my opinion producing a curved windscreen, or even a canopy
from this material is something out of the scope of an amateur builder even today, not to mention
those in the early fifties. But it's hard to find any quotable information. Googling just bring results
for modern materials and following the statements of the manufacturers, everybody can make
everything with ease .... :

 

[agricola64]

perhaps it would be better not to refer to tradenames .. but by the chemical shorthand names used in the plastics industry

plexiglas / altuglas is PMM (Poly Methyl Methacrylate

lexan / makrolon is PC (Poly Carbonate)

there are also grades of ABS (Acrylonitrile Butadiene Styrene) that are transparent and are used sometime to create "window material"

all these materials have different physical characteristics - with PC usually being the "toughest" one - and different needs when bein worked on ..

for creating cockpit windows i would start looking at raw material sheets intended for vaccuforming .. this probalby is the method of choice used by the early aircraft producers for complex 3D shapes of the cockpit window (bubble cockpits)

 

[mz]

You are already a member at the homebuiltairplanes.com website I presume?

 

[Jemiba]

Two more answers, two more good clues, many thanks !

The chemical names really brought much better search results and the link
to the homebuilder forum is great . Some clicks and there are sites about
blowing canopies ! For such methods, generally more modern materials seem
to be used.
Poly Methyl Methacrylate (Plexiglas, Perspex) was available since 1933,
Poly Carbonate (Lexan, Makrolon) since 1953, Acrylonitrile Butadiene Styrene
(ABS) since 1963 and one of the latest additions was PET-G (Vivak) in 1992.
In my fathers bookshelf I found an old table book with characteristics of materials,
like strength, specific weight, cutting speeds and general informations for handling.
Plexiglas is said to be prone of developing cracks around drilled holes, especially if
bending or torsional loads are applied.
So my conclusion is, that before the availability of materials like Lexan, three-dimensionally
formed transperencies for aviational use, would have to be made using heat and pressure.
Today, this can be done even by amateurs , but could it be done during the late '40s/early
'50s, too ?

 

[agricola64]

Two more answers, two more good clues, many thanks !

always happy to help ..

In my fathers bookshelf I found an old table book with characteristics of materials,
like strength, specific weight, cutting speeds and general informations for handling.
Plexiglas is said to be prone of developing cracks around drilled holes, especially if
bending or torsional loads are applied.

i believe many of the early window designs using PMM / plexiglas did not use screw holes for this reason, but used a clamping system instead (clamping the PMM sheet between the window frame and a metal band)

you might also want to be careful with some cleaning solutions and solvents - they can induce tension cracks in the plastic material (using alcohols on PC is a special no-no in this regards)

So my conclusion is, that before the availability of materials like Lexan, three-dimensionally
formed transperencies for aviational use, would have to be made using heat and pressure.
Today, this can be done even by amateurs , but could it be done during the late '40s/early
'50s, too ?

i think so ... building a vacu-form rigg is not so complicated .. and there are also many professional companies doing large scale vacu-forming .. it might have been possible to use their equipment at low cost, if you went there with a well prepared positive mold

btw - thanks for the information about PET .. i did not know about it being used for cockpit windows ..

Markus

 

[Jemiba]

Perhaps I should add, that I'm not going to build an aircraft on my own (there
are already so many projects waiting to be finished, that an own aircraft would really
be presumptuous : ), I'm just trying to understand an older construction, initially
intended for amateur builders, but in the event built only by a professional manufacturer.

If PET is used in any professional construction, I cannot say for sure, but it was used at
least for homebuilds.

And .. vacu form is another good key word !

 

[Just call me Ray]

Is it an aircraft of your own design or just a kit?

 

[Jemiba]

My question is directly linked to this one :
http://www.secretprojects.co.uk/forum/index.php/topic,6533.msg54704.html#msg54704.
Would be called an ULM today and should be sold just in the form of plans for amateur
builders. As I couldn't find a way, photos and 3-views would match, my thoughts were,
that for homebuild examples a flat windscreen was intended, just to ease construction.
But for the factory built (one and only) prototype), a curved one may have been used,
as the manufacturers facilities were readily available.
Writing this, I just had the idea of checking headroom, could be another argument ...

 

[gery]

Hi jemiba,
i saw your project and i think you can make your canopie with a flat sheet of polycarbonate....your surface is a developpable surface with just one curvature you don't need "heat ", pressure or vaccum systeme just your hands...it's depending of the Thickness of your sheet....
a plyed or curved sheet have got a good resistance...but warning at the weight plexi or polycarbonate are very heavy.....i think

gery

 

[Jemiba]

Thank you, Gery,

with the drawing shown in my older post, I just tried to find out, if the windshield had a curvature at all, as this wasn’t easily recognisable on the the photos. Judging those photos (and to my opinion !) the windshield of the prototype in fact was curved, so differing from several published 3-views. Of course, those 3-views could be wrong, but to blame several draughtsmen of making the same error seemed not really plausible to me. So my explanation is, that the prototype, built in a factory with all facilities, differed from an aircraft, as built by an amateur on his “kitchentable”. There are some other minor differences, too, like a pair of auxiliary struts, not evident in the prototype, which I would regard as an attempt to make this aircraft as simple, but nevertheless sturdy and safe, for someone with just a limited set of tools and maybe only limited experience. The case of the early “Pou de ciels”, which suffered from a dangerous design fault probably was well remembered at this time!

 

[gery]

Hi jemiba
I zoomed your picture, i think that your airplane have got a transparent roof or not roof...i can see the tube of wing root it isn't curved it's right....
canopie is probably just curved on back....i'ts really an old picture...i think (just look at the man..) 30' or 40'..plexy is very rare at this time...
and the forming technics too...just for military aircrafts and very expensive...

 

[Jemiba]

Correct Gery, it's the MS.660 from 1946. I think, in post-war France the available
facilities for private use weren't much different from those during the pre-war years.
And in fact, even a beautiful formed canopy wouldn't have improved performance
very much, so the motto probably was "keep it simple".

 

[gery]

what "brand " exactly....of ms 660..???

 

[gery]

just look at that

 

[Jemiba]

Well, that's what's possible today with a modern materials like Lexan
or Makrolon, I think. With "old fashioned" Perspex it probably would be
difficult. Sorry, I forgot to mention the producing company of the MS.660:
Morane Saulnier, of course ..
Here's the last version of my own drawing and one I've go via deltafan from
the MAE.