perplex under pressure

[andyman64]

just a question from a non aviation indutry person how do the plastic canopies withstand the pressures imperted on them at supersonic speeds i remember reading something about the tail cones on p-61,s colapsing in a dive and this seams like alot less stress than being at the front of a supersonic jet aircraft??????????????/

[The Inspector]

' can I just say one thing to you Benjie, plastics' constant technological improvements and the need to do 'because it needs to be'.

Want to drive your eyebrows up into your hairline? Find still pictures of birdstrike tests on canopies and see how far they flex inward and distort without failing, it'll make you go out and kick your cars headlights.

[JDK]

Perspex (like Plexiglas) is a trade name for an acrylic plastic*, and gets used generically. Like any material, its not all the same capability - different types and weights need to be used appropriately to achieve the task required, and that requires the engineering analysis of the specification, design and use to be correct. The failures you refer to were an engineering / design problem, not a symptom of a general inadequacy of the material.

* Poly(methyl methacrylate) (PMMA) a transparent thermoplastic.
http://en.wikipedia.org/wiki/Poly%28met ... crylate%29

HTH!

[PropsRule]

Also, the materials used to make canopies or windshields can be quite thick, giving them greater strength. It's not uncommon to see warnings in aircraft maintenance manuals advising against lifting a windshield assembly by yourself, because they're so heavy.

It's also common for the canopy or windshield to be made of several layers of materials laminated together, which in turn increases strength.

[brucev]

wasn't there a ANG F-16 (from Nebraska or maybe Iowa) that had a pilot become incapacitated after a bird strike on the canopy that flexed the canopy enough to knock him on the head very hard? My memory sucks sometimes (like now) and I can't remember the outcome but I think he ejected after coming to.

[JDK]

It's also common for the canopy or windshield to be made of several layers of materials laminated together, which in turn increases strength.

Indeed. In W.W.II it was common for the windscreen (but not quarter-lights or main canopy) to be made of armour or laminated glass or multiple glass sheets (the latter evident on an A-20 I was looking at recently) instead of Perspex. As per the original question, this was because in that era the plastics could not be made tough enough or tough and flexiable enough to deal with bullets or birds.