LANCASTER Plexiglass

Interesting to see metric dimensions on a wartime A.V.ROE & Co Ltd (!) drawing; I thought they were AVRO long before drawings for the Lancaster (and Manchester!) would have been drawn.

Also interesting that ‘Perspex’ was a trade name of ICI (who ‘invented’ PMMA) and that ‘Plexiglas’ is a trade name of German, and not American, origin (as I had previously thought):

https://en.m.wikipedia.org/wiki/Poly(methyl_methacrylate)

Yes, download it and you can zoom in and see it at the top right.

No, that is the only one I could find 🙁

Anon, thanks for your reply and confirmation of my thoughts
smirky, thank you also very much, but I can’t find the 4mm on this drawing. Is it in the top right corner? Do you have the drawings of all Perspex parts to evaluate the different thicknesses?

Here is the cockpit curved corner windows, as you can see the thickness is given as “4 m/m”. The flat front screens are safety glass.

I would think the engineering drawing for each piece would give a required thickness…or acceptable range.
Does anyone have the drawings?
Also, are the pieces being reproduced? (An interesting question. If the BBMF or restorer needed a new one would they have it built or are there still used or surplus units available? Certainly, with only two flyers and most preserved examples already restored, there isn’t much call for them.)
If so, the manufacturer would need to know the thickness.

There are definitely different thicknesses throughout the glazing areas and these are not random or stretched but are precise thicknesses according to the duty they have to perform.

Whilst stretching and thinning does inevitably take place it is fully accounted for and there is sufficient thickness where it is required by design, not accident.

Areas such as the front windscreens, whilst laminated glass is often used for flat areas, the main screens and curved sides are usually around 3/8 inch thick plastic. Blisters and other protrusions are also quite thick due to the fact that anything which faces the airflow will be subjected to greater loads (wind, rain, hail bird strike, etc) much more than, say, side screens and the thinning of the material can be more pronounced in such mouldings.

Usually opening panels such as DV windows are quite thick as they are subject to the loads imposed by opening, pushing, flapping about in the air stream, etc. and are not always supported by the main structure, as well as usually being a part of the front or front quarter glazing.

Sheet materials are heavy and perspex/plexiglas is no exception. So, in accordance with good practice to promote weight saving, those areas subject to less load and susceptability to damage are much thinner and lighter. In short: yes, there are several thicknesses apparent in any cockpit glazing, though there is more use made of the various gauges on larger aircraft types’ glazing structural area.

In an aircraft, there is little that is accidental – except, sometimes, the actions of the pilot!

Anon.

Dear batsi,
thanks for this Link – I have already looked for these tecknique in Youtube. It’s a dream how it works. And many thanks for your engagement. You are obviously the only one, that shows interrest!
My hobby, for about 25 years, was to built and fly remote controlled airplanes. I built some canopies for scratch built planes (Mustang, Focke Wulf 190, P47 Thunderbolt) using a self made mold out of wood or plaster. Heating the Plexiglas in mother’s oven and then by assistance of my son and wife pulling it over the mold using gloves. With this method the results are different thicknesses, but for this application it’s acceptable. The contact to the mold prevents the thermoplast to expand as much as the areas that have not yet contact. But with this professional pneumatic bubble technique you have all aroung the material the same temperature, the same pressure, and therefore a constant thickness.
Finaly, I am a retired mechanical engineer. And I know out of my job that each tecnical part – and this is extreme important in aeroplane industry – is calculated very detailed in view of strength, endurance or durability, weight and costs. Therefore I am convinced that the different thicknesses of these parts are result of defined requirements.
Greetings and have a good time

Take a look at this :

https://www.youtube.com/watch?v=ZlBQME0aB2s

I would think near the outer edge and possibly the centre would have close to the original thickness of material, whereas the sides where it has stretched the most would be quite a bit thinner. But it could easily be the case that different makes and models of aircraft used different thickness of perspex.

I found Plexiglass parts in thicknesses of 8,6 / 6,6 / 4,4 / 3,5 millimeters at different crash locations. This would be in inches 0,33 / 0,25 / 0,18 / 0,15
I do not think that this wide range results out of an unprecise manufacturing process!

As most of the shaped plexiglass mouldings are made by inflating/stretching a warmed up flat sheet by compressed air or by using a preformed mould to shape the sheet over, I would think the thickness of plexiglass varied a great deal due to stretching over the individual moulding ……making it very difficult to say where it came from.